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				22	<title>The design and implementation of Valgrind</title>
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				26
				27	<a name="title"> </a>
				28	<h1 align=center>The design and implementation of Valgrind</h1>
				29
				30	<center>
				31	Detailed technical notes for hackers, maintainers and the
				32	overly-curious<br>
				33	These notes pertain to snapshot 20020306<br>
				34	<p>
				35	<a href="mailto:jseward@acm.org">jseward@acm.org<br>
				36	<a href="http://developer.kde.org/~sewardj">http://developer.kde.org/~sewardj</a><br>
				37	<a href="http://www.muraroa.demon.co.uk">http://www.muraroa.demon.co.uk</a><br>
				38	Copyright © 2000-2002 Julian Seward
				39	<p>
				40	Valgrind is licensed under the GNU General Public License,
				41	version 2<br>
				42	An open-source tool for finding memory-management problems in
				43	x86 GNU/Linux executables.
				44	</center>
				45
				46	<p>
				47
				48
				49
				50
				51	<hr width="100%">
				52
				53	<h2>Introduction</h2>
				54
				55	This document contains a detailed, highly-technical description of the
				56	internals of Valgrind. This is not the user manual; if you are an
				57	end-user of Valgrind, you do not want to read this. Conversely, if
				58	you really are a hacker-type and want to know how it works, I assume
				59	that you have read the user manual thoroughly.
				60	<p>
				61	You may need to read this document several times, and carefully. Some
				62	important things, I only say once.
				63
				64
				65	<h3>History</h3>
				66
				67	Valgrind came into public view in late Feb 2002. However, it has been
				68	under contemplation for a very long time, perhaps seriously for about
				69	five years. Somewhat over two years ago, I started working on the x86
				70	code generator for the Glasgow Haskell Compiler
				71	(http://www.haskell.org/ghc), gaining familiarity with x86 internals
				72	on the way. I then did Cacheprof (http://www.cacheprof.org), gaining
				73	further x86 experience. Some time around Feb 2000 I started
				74	experimenting with a user-space x86 interpreter for x86-Linux. This
				75	worked, but it was clear that a JIT-based scheme would be necessary to
				76	give reasonable performance for Valgrind. Design work for the JITter
				77	started in earnest in Oct 2000, and by early 2001 I had an x86-to-x86
				78	dynamic translator which could run quite large programs. This
				79	translator was in a sense pointless, since it did not do any
				80	instrumentation or checking.
				81
				82	<p>
				83	Most of the rest of 2001 was taken up designing and implementing the
				84	instrumentation scheme. The main difficulty, which consumed a lot
				85	of effort, was to design a scheme which did not generate large numbers
				86	of false uninitialised-value warnings. By late 2001 a satisfactory
				87	scheme had been arrived at, and I started to test it on ever-larger
				88	programs, with an eventual eye to making it work well enough so that
				89	it was helpful to folks debugging the upcoming version 3 of KDE. I've
				90	used KDE since before version 1.0, and wanted to Valgrind to be an
				91	indirect contribution to the KDE 3 development effort. At the start of
				92	Feb 02 the kde-core-devel crew started using it, and gave a huge
				93	amount of helpful feedback and patches in the space of three weeks.
				94	Snapshot 20020306 is the result.
				95
				96	<p>
				97	In the best Unix tradition, or perhaps in the spirit of Fred Brooks'
				98	depressing-but-completely-accurate epitaph "build one to throw away;
				99	you will anyway", much of Valgrind is a second or third rendition of
				100	the initial idea. The instrumentation machinery
				101	(<code>vg_translate.c</code>, <code>vg_memory.c</code>) and core CPU
				102	simulation (<code>vg_to_ucode.c</code>, <code>vg_from_ucode.c</code>)
				103	have had three redesigns and rewrites; the register allocator,
				104	low-level memory manager (<code>vg_malloc2.c</code>) and symbol table
				105	reader (<code>vg_symtab2.c</code>) are on the second rewrite. In a
				106	sense, this document serves to record some of the knowledge gained as
				107	a result.
				108
				109
				110	<h3>Design overview</h3>
				111
				112	Valgrind is compiled into a Linux shared object,
				113	<code>valgrind.so</code>, and also a dummy one,
				114	<code>valgrinq.so</code>, of which more later. The
				115	<code>valgrind</code> shell script adds <code>valgrind.so</code> to
				116	the <code>LD_PRELOAD</code> list of extra libraries to be
				117	loaded with any dynamically linked library. This is a standard trick,
				118	one which I assume the <code>LD_PRELOAD</code> mechanism was developed
				119	to support.
				120
				121	<p>
				122	<code>valgrind.so</code>
				123	is linked with the <code>-z initfirst</code> flag, which requests that
				124	its initialisation code is run before that of any other object in the
				125	executable image. When this happens, valgrind gains control. The
				126	real CPU becomes "trapped" in <code>valgrind.so</code> and the
				127	translations it generates. The synthetic CPU provided by Valgrind
				128	does, however, return from this initialisation function. So the
				129	normal startup actions, orchestrated by the dynamic linker
				130	<code>ld.so</code>, continue as usual, except on the synthetic CPU,
				131	not the real one. Eventually <code>main</code> is run and returns,
				132	and then the finalisation code of the shared objects is run,
				133	presumably in inverse order to which they were initialised. Remember,
				134	this is still all happening on the simulated CPU. Eventually
				135	<code>valgrind.so</code>'s own finalisation code is called. It spots
				136	this event, shuts down the simulated CPU, prints any error summaries
				137	and/or does leak detection, and returns from the initialisation code
				138	on the real CPU. At this point, in effect the real and synthetic CPUs
				139	have merged back into one, Valgrind has lost control of the program,
				140	and the program finally <code>exit()s</code> back to the kernel in the
				141	usual way.
				142
				143	<p>
				144	The normal course of activity, one Valgrind has started up, is as
				145	follows. Valgrind never runs any part of your program (usually
				146	referred to as the "client"), not a single byte of it, directly.
				147	Instead it uses function <code>VG_(translate)</code> to translate
				148	basic blocks (BBs, straight-line sequences of code) into instrumented
				149	translations, and those are run instead. The translations are stored
				150	in the translation cache (TC), <code>vg_tc</code>, with the
				151	translation table (TT), <code>vg_tt</code> supplying the
				152	original-to-translation code address mapping. Auxiliary array
				153	<code>VG_(tt_fast)</code> is used as a direct-map cache for fast
				154	lookups in TT; it usually achieves a hit rate of around 98% and
				155	facilitates an orig-to-trans lookup in 4 x86 insns, which is not bad.
				156
				157	<p>
				158	Function <code>VG_(dispatch)</code> in <code>vg_dispatch.S</code> is
				159	the heart of the JIT dispatcher. Once a translated code address has
				160	been found, it is executed simply by an x86 <code>call</code>
				161	to the translation. At the end of the translation, the next
				162	original code addr is loaded into <code>%eax</code>, and the
				163	translation then does a <code>ret</code>, taking it back to the
				164	dispatch loop, with, interestingly, zero branch mispredictions.
				165	The address requested in <code>%eax</code> is looked up first in
				166	<code>VG_(tt_fast)</code>, and, if not found, by calling C helper
				167	<code>VG_(search_transtab)</code>. If there is still no translation
				168	available, <code>VG_(dispatch)</code> exits back to the top-level
				169	C dispatcher <code>VG_(toploop)</code>, which arranges for
				170	<code>VG_(translate)</code> to make a new translation. All fairly
				171	unsurprising, really. There are various complexities described below.
				172
				173	<p>
				174	The translator, orchestrated by <code>VG_(translate)</code>, is
				175	complicated but entirely self-contained. It is described in great
				176	detail in subsequent sections. Translations are stored in TC, with TT
				177	tracking administrative information. The translations are subject to
				178	an approximate LRU-based management scheme. With the current
				179	settings, the TC can hold at most about 15MB of translations, and LRU
				180	passes prune it to about 13.5MB. Given that the
				181	orig-to-translation expansion ratio is about 13:1 to 14:1, this means
				182	TC holds translations for more or less a megabyte of original code,
				183	which generally comes to about 70000 basic blocks for C++ compiled
				184	with optimisation on. Generating new translations is expensive, so it
				185	is worth having a large TC to minimise the (capacity) miss rate.
				186
				187	<p>
				188	The dispatcher, <code>VG_(dispatch)</code>, receives hints from
				189	the translations which allow it to cheaply spot all control
				190	transfers corresponding to x86 <code>call</code> and <code>ret</code>
				191	instructions. It has to do this in order to spot some special events:
				192	<ul>
				193	<li>Calls to <code>VG_(shutdown)</code>. This is Valgrind's cue to
				194	exit. NOTE: actually this is done a different way; it should be
				195	cleaned up.
				196	<p>
				197	<li>Returns of system call handlers, to the return address
				198	<code>VG_(signalreturn_bogusRA)</code>. The signal simulator
				199	needs to know when a signal handler is returning, so we spot
				200	jumps (returns) to this address.
				201	<p>
				202	<li>Calls to <code>vg_trap_here</code>. All <code>malloc</code>,
				203	<code>free</code>, etc calls that the client program makes are
				204	eventually routed to a call to <code>vg_trap_here</code>,
				205	and Valgrind does its own special thing with these calls.
				206	In effect this provides a trapdoor, by which Valgrind can
				207	intercept certain calls on the simulated CPU, run the call as it
				208	sees fit itself (on the real CPU), and return the result to
				209	the simulated CPU, quite transparently to the client program.
				210	</ul>
				211	Valgrind intercepts the client's <code>malloc</code>,
				212	<code>free</code>, etc,
				213	calls, so that it can store additional information. Each block
				214	<code>malloc</code>'d by the client gives rise to a shadow block
				215	in which Valgrind stores the call stack at the time of the
				216	<code>malloc</code>
				217	call. When the client calls <code>free</code>, Valgrind tries to
				218	find the shadow block corresponding to the address passed to
				219	<code>free</code>, and emits an error message if none can be found.
				220	If it is found, the block is placed on the freed blocks queue
				221	<code>vg_freed_list</code>, it is marked as inaccessible, and
				222	its shadow block now records the call stack at the time of the
				223	<code>free</code> call. Keeping <code>free</code>'d blocks in
				224	this queue allows Valgrind to spot all (presumably invalid) accesses
				225	to them. However, once the volume of blocks in the free queue
				226	exceeds <code>VG_(clo_freelist_vol)</code>, blocks are finally
				227	removed from the queue.
				228
				229	<p>
				230	Keeping track of A and V bits (note: if you don't know what these are,
				231	you haven't read the user guide carefully enough) for memory is done
				232	in <code>vg_memory.c</code>. This implements a sparse array structure
				233	which covers the entire 4G address space in a way which is reasonably
				234	fast and reasonably space efficient. The 4G address space is divided
				235	up into 64K sections, each covering 64Kb of address space. Given a
				236	32-bit address, the top 16 bits are used to select one of the 65536
				237	entries in <code>VG_(primary_map)</code>. The resulting "secondary"
				238	(<code>SecMap</code>) holds A and V bits for the 64k of address space
				239	chunk corresponding to the lower 16 bits of the address.
				240
				241
				242	<h3>Design decisions</h3>
				243
				244	Some design decisions were motivated by the need to make Valgrind
				245	debuggable. Imagine you are writing a CPU simulator. It works fairly
				246	well. However, you run some large program, like Netscape, and after
				247	tens of millions of instructions, it crashes. How can you figure out
				248	where in your simulator the bug is?
				249
				250	<p>
				251	Valgrind's answer is: cheat. Valgrind is designed so that it is
				252	possible to switch back to running the client program on the real
				253	CPU at any point. Using the <code>--stop-after= </code> flag, you can
				254	ask Valgrind to run just some number of basic blocks, and then
				255	run the rest of the way on the real CPU. If you are searching for
				256	a bug in the simulated CPU, you can use this to do a binary search,
				257	which quickly leads you to the specific basic block which is
				258	causing the problem.
				259
				260	<p>
				261	This is all very handy. It does constrain the design in certain
				262	unimportant ways. Firstly, the layout of memory, when viewed from the
				263	client's point of view, must be identical regardless of whether it is
				264	running on the real or simulated CPU. This means that Valgrind can't
				265	do pointer swizzling -- well, no great loss -- and it can't run on
				266	the same stack as the client -- again, no great loss.
				267	Valgrind operates on its own stack, <code>VG_(stack)</code>, which
				268	it switches to at startup, temporarily switching back to the client's
				269	stack when doing system calls for the client.
				270
				271	<p>
				272	Valgrind also receives signals on its own stack,
				273	<code>VG_(sigstack)</code>, but for different gruesome reasons
				274	discussed below.
				275
				276	<p>
				277	This nice clean switch-back-to-the-real-CPU-whenever-you-like story
				278	is muddied by signals. Problem is that signals arrive at arbitrary
				279	times and tend to slightly perturb the basic block count, with the
				280	result that you can get close to the basic block causing a problem but
				281	can't home in on it exactly. My kludgey hack is to define
				282	<code>SIGNAL_SIMULATION</code> to 1 towards the bottom of
				283	<code>vg_syscall_mem.c</code>, so that signal handlers are run on the
				284	real CPU and don't change the BB counts.
				285
				286	<p>
				287	A second hole in the switch-back-to-real-CPU story is that Valgrind's
				288	way of delivering signals to the client is different from that of the
				289	kernel. Specifically, the layout of the signal delivery frame, and
				290	the mechanism used to detect a sighandler returning, are different.
				291	So you can't expect to make the transition inside a sighandler and
				292	still have things working, but in practice that's not much of a
				293	restriction.
				294
				295	<p>
				296	Valgrind's implementation of <code>malloc</code>, <code>free</code>,
				297	etc, (in <code>vg_clientmalloc.c</code>, not the low-level stuff in
				298	<code>vg_malloc2.c</code>) is somewhat complicated by the need to
				299	handle switching back at arbitrary points. It does work tho.
				300
				301
				302
				303	<h3>Correctness</h3>
				304
				305	There's only one of me, and I have a Real Life (tm) as well as hacking
				306	Valgrind [allegedly :-]. That means I don't have time to waste
				307	chasing endless bugs in Valgrind. My emphasis is therefore on doing
				308	everything as simply as possible, with correctness, stability and
				309	robustness being the number one priority, more important than
				310	performance or functionality. As a result:
				311	<ul>
				312	<li>The code is absolutely loaded with assertions, and these are
				313	<b>permanently enabled.</b> I have no plan to remove or disable
				314	them later. Over the past couple of months, as valgrind has
				315	become more widely used, they have shown their worth, pulling
				316	up various bugs which would otherwise have appeared as
				317	hard-to-find segmentation faults.
				318	<p>
				319	I am of the view that it's acceptable to spend 5% of the total
				320	running time of your valgrindified program doing assertion checks
				321	and other internal sanity checks.
				322	<p>
				323	<li>Aside from the assertions, valgrind contains various sets of
				324	internal sanity checks, which get run at varying frequencies
				325	during normal operation. <code>VG_(do_sanity_checks)</code>
				326	runs every 1000 basic blocks, which means 500 to 2000 times/second
				327	for typical machines at present. It checks that Valgrind hasn't
				328	overrun its private stack, and does some simple checks on the
				329	memory permissions maps. Once every 25 calls it does some more
				330	extensive checks on those maps. Etc, etc.
				331	<p>
				332	The following components also have sanity check code, which can
				333	be enabled to aid debugging:
				334	<ul>
				335	<li>The low-level memory-manager
				336	(<code>VG_(mallocSanityCheckArena)</code>). This does a
				337	complete check of all blocks and chains in an arena, which
				338	is very slow. Is not engaged by default.
				339	<p>
				340	<li>The symbol table reader(s): various checks to ensure
				341	uniqueness of mappings; see <code>VG_(read_symbols)</code>
				342	for a start. Is permanently engaged.
				343	<p>
				344	<li>The A and V bit tracking stuff in <code>vg_memory.c</code>.
				345	This can be compiled with cpp symbol
				346	<code>VG_DEBUG_MEMORY</code> defined, which removes all the
				347	fast, optimised cases, and uses simple-but-slow fallbacks
				348	instead. Not engaged by default.
				349	<p>
				350	<li>Ditto <code>VG_DEBUG_LEAKCHECK</code>.
				351	<p>
				352	<li>The JITter parses x86 basic blocks into sequences of
				353	UCode instructions. It then sanity checks each one with
				354	<code>VG_(saneUInstr)</code> and sanity checks the sequence
				355	as a whole with <code>VG_(saneUCodeBlock)</code>. This stuff
				356	is engaged by default, and has caught some way-obscure bugs
				357	in the simulated CPU machinery in its time.
				358	<p>
				359	<li>The system call wrapper does
				360	<code>VG_(first_and_last_secondaries_look_plausible)</code> after
				361	every syscall; this is known to pick up bugs in the syscall
				362	wrappers. Engaged by default.
				363	<p>
				364	<li>The main dispatch loop, in <code>VG_(dispatch)</code>, checks
				365	that translations do not set <code>%ebp</code> to any value
				366	different from <code>VG_EBP_DISPATCH_CHECKED</code> or
				367	<code>& VG_(baseBlock)</code>. In effect this test is free,
				368	and is permanently engaged.
				369	<p>
				370	<li>There are a couple of ifdefed-out consistency checks I
				371	inserted whilst debugging the new register allocater,
				372	<code>vg_do_register_allocation</code>.
				373	</ul>
				374	<p>
				375	<li>I try to avoid techniques, algorithms, mechanisms, etc, for which
				376	I can supply neither a convincing argument that they are correct,
				377	nor sanity-check code which might pick up bugs in my
				378	implementation. I don't always succeed in this, but I try.
				379	Basically the idea is: avoid techniques which are, in practice,
				380	unverifiable, in some sense. When doing anything, always have in
				381	mind: "how can I verify that this is correct?"
				382	</ul>
				383
				384	<p>
				385	Some more specific things are:
				386
				387	<ul>
				388	<li>Valgrind runs in the same namespace as the client, at least from
				389	<code>ld.so</code>'s point of view, and it therefore absolutely
				390	had better not export any symbol with a name which could clash
				391	with that of the client or any of its libraries. Therefore, all
				392	globally visible symbols exported from <code>valgrind.so</code>
				393	are defined using the <code>VG_</code> CPP macro. As you'll see
				394	from <code>vg_constants.h</code>, this appends some arbitrary
				395	prefix to the symbol, in order that it be, we hope, globally
				396	unique. Currently the prefix is <code>vgPlain_</code>. For
				397	convenience there are also <code>VGM_</code>, <code>VGP_</code>
				398	and <code>VGOFF_</code>. All locally defined symbols are declared
				399	<code>static</code> and do not appear in the final shared object.
				400	<p>
				401	To check this, I periodically do
				402	<code>nm valgrind.so \| grep " T "</code>,
				403	which shows you all the globally exported text symbols.
				404	They should all have an approved prefix, except for those like
				405	<code>malloc</code>, <code>free</code>, etc, which we deliberately
				406	want to shadow and take precedence over the same names exported
				407	from <code>glibc.so</code>, so that valgrind can intercept those
				408	calls easily. Similarly, <code>nm valgrind.so \| grep " D "</code>
				409	allows you to find any rogue data-segment symbol names.
				410	<p>
				411	<li>Valgrind tries, and almost succeeds, in being completely
				412	independent of all other shared objects, in particular of
				413	<code>glibc.so</code>. For example, we have our own low-level
				414	memory manager in <code>vg_malloc2.c</code>, which is a fairly
				415	standard malloc/free scheme augmented with arenas, and
				416	<code>vg_mylibc.c</code> exports reimplementations of various bits
				417	and pieces you'd normally get from the C library.
				418	<p>
				419	Why all the hassle? Because imagine the potential chaos of both
				420	the simulated and real CPUs executing in <code>glibc.so</code>.
				421	It just seems simpler and cleaner to be completely self-contained,
				422	so that only the simulated CPU visits <code>glibc.so</code>. In
				423	practice it's not much hassle anyway. Also, valgrind starts up
				424	before glibc has a chance to initialise itself, and who knows what
				425	difficulties that could lead to. Finally, glibc has definitions
				426	for some types, specifically <code>sigset_t</code>, which conflict
				427	(are different from) the Linux kernel's idea of same. When
				428	Valgrind wants to fiddle around with signal stuff, it wants to
				429	use the kernel's definitions, not glibc's definitions. So it's
				430	simplest just to keep glibc out of the picture entirely.
				431	<p>
				432	To find out which glibc symbols are used by Valgrind, reinstate
				433	the link flags <code>-nostdlib -Wl,-no-undefined</code>. This
				434	causes linking to fail, but will tell you what you depend on.
				435	I have mostly, but not entirely, got rid of the glibc
				436	dependencies; what remains is, IMO, fairly harmless. AFAIK the
				437	current dependencies are: <code>memset</code>,
				438	<code>memcmp</code>, <code>stat</code>, <code>system</code>,
				439	<code>sbrk</code>, <code>setjmp</code> and <code>longjmp</code>.
				440
				441	<p>
				442	<li>Similarly, valgrind should not really import any headers other
				443	than the Linux kernel headers, since it knows of no API other than
				444	the kernel interface to talk to. At the moment this is really not
				445	in a good state, and <code>vg_syscall_mem</code> imports, via
				446	<code>vg_unsafe.h</code>, a significant number of C-library
				447	headers so as to know the sizes of various structs passed across
				448	the kernel boundary. This is of course completely bogus, since
				449	there is no guarantee that the C library's definitions of these
				450	structs matches those of the kernel. I have started to sort this
				451	out using <code>vg_kerneliface.h</code>, into which I had intended
				452	to copy all kernel definitions which valgrind could need, but this
				453	has not gotten very far. At the moment it mostly contains
				454	definitions for <code>sigset_t</code> and <code>struct
				455	sigaction</code>, since the kernel's definition for these really
				456	does clash with glibc's. I plan to use a <code>vki_</code> prefix
				457	on all these types and constants, to denote the fact that they
				458	pertain to <b>V</b>algrind's <b>K</b>ernel <b>I</b>nterface.
				459	<p>
				460	Another advantage of having a <code>vg_kerneliface.h</code> file
				461	is that it makes it simpler to interface to a different kernel.
				462	Once can, for example, easily imagine writing a new
				463	<code>vg_kerneliface.h</code> for FreeBSD, or x86 NetBSD.
				464
				465	</ul>
				466
				467	<h3>Current limitations</h3>
				468
				469	No threads. I think fixing this is close to a research-grade problem.
				470	<p>
				471	No MMX. Fixing this should be relatively easy, using the same giant
				472	trick used for x86 FPU instructions. See below.
				473	<p>
				474	Support for weird (non-POSIX) signal stuff is patchy. Does anybody
				475	care?
				476	<p>
				477
				478
				479
				480
				481	<hr width="100%">
				482
				483	<h2>The instrumenting JITter</h2>
				484
				485	This really is the heart of the matter. We begin with various side
				486	issues.
				487
				488	<h3>Run-time storage, and the use of host registers</h3>
				489
				490	Valgrind translates client (original) basic blocks into instrumented
				491	basic blocks, which live in the translation cache TC, until either the
				492	client finishes or the translations are ejected from TC to make room
				493	for newer ones.
				494	<p>
				495	Since it generates x86 code in memory, Valgrind has complete control
				496	of the use of registers in the translations. Now pay attention. I
				497	shall say this only once, and it is important you understand this. In
				498	what follows I will refer to registers in the host (real) cpu using
				499	their standard names, <code>%eax</code>, <code>%edi</code>, etc. I
				500	refer to registers in the simulated CPU by capitalising them:
				501	<code>%EAX</code>, <code>%EDI</code>, etc. These two sets of
				502	registers usually bear no direct relationship to each other; there is
				503	no fixed mapping between them. This naming scheme is used fairly
				504	consistently in the comments in the sources.
				505	<p>
				506	Host registers, once things are up and running, are used as follows:
				507	<ul>
				508	<li><code>%esp</code>, the real stack pointer, points
				509	somewhere in Valgrind's private stack area,
				510	<code>VG_(stack)</code> or, transiently, into its signal delivery
				511	stack, <code>VG_(sigstack)</code>.
				512	<p>
				513	<li><code>%edi</code> is used as a temporary in code generation; it
				514	is almost always dead, except when used for the <code>Left</code>
				515	value-tag operations.
				516	<p>
				517	<li><code>%eax</code>, <code>%ebx</code>, <code>%ecx</code>,
				518	<code>%edx</code> and <code>%esi</code> are available to
				519	Valgrind's register allocator. They are dead (carry unimportant
				520	values) in between translations, and are live only in
				521	translations. The one exception to this is <code>%eax</code>,
				522	which, as mentioned far above, has a special significance to the
				523	dispatch loop <code>VG_(dispatch)</code>: when a translation
				524	returns to the dispatch loop, <code>%eax</code> is expected to
				525	contain the original-code-address of the next translation to run.
				526	The register allocator is so good at minimising spill code that
				527	using five regs and not having to save/restore <code>%edi</code>
				528	actually gives better code than allocating to <code>%edi</code>
				529	as well, but then having to push/pop it around special uses.
				530	<p>
				531	<li><code>%ebp</code> points permanently at
				532	<code>VG_(baseBlock)</code>. Valgrind's translations are
				533	position-independent, partly because this is convenient, but also
				534	because translations get moved around in TC as part of the LRUing
				535	activity. <b>All</b> static entities which need to be referred to
				536	from generated code, whether data or helper functions, are stored
				537	starting at <code>VG_(baseBlock)</code> and are therefore reached
				538	by indexing from <code>%ebp</code>. There is but one exception,
				539	which is that by placing the value
				540	<code>VG_EBP_DISPATCH_CHECKED</code>
				541	in <code>%ebp</code> just before a return to the dispatcher,
				542	the dispatcher is informed that the next address to run,
				543	in <code>%eax</code>, requires special treatment.
				544	<p>
				545	<li>The real machine's FPU state is pretty much unimportant, for
				546	reasons which will become obvious. Ditto its <code>%eflags</code>
				547	register.
				548	</ul>
				549
				550	<p>
				551	The state of the simulated CPU is stored in memory, in
				552	<code>VG_(baseBlock)</code>, which is a block of 200 words IIRC.
				553	Recall that <code>%ebp</code> points permanently at the start of this
				554	block. Function <code>vg_init_baseBlock</code> decides what the
				555	offsets of various entities in <code>VG_(baseBlock)</code> are to be,
				556	and allocates word offsets for them. The code generator then emits
				557	<code>%ebp</code> relative addresses to get at those things. The
				558	sequence in which entities are allocated has been carefully chosen so
				559	that the 32 most popular entities come first, because this means 8-bit
				560	offsets can be used in the generated code.
				561
				562	<p>
				563	If I was clever, I could make <code>%ebp</code> point 32 words along
				564	<code>VG_(baseBlock)</code>, so that I'd have another 32 words of
				565	short-form offsets available, but that's just complicated, and it's
				566	not important -- the first 32 words take 99% (or whatever) of the
				567	traffic.
				568
				569	<p>
				570	Currently, the sequence of stuff in <code>VG_(baseBlock)</code> is as
				571	follows:
				572	<ul>
				573	<li>9 words, holding the simulated integer registers,
				574	<code>%EAX</code> .. <code>%EDI</code>, and the simulated flags,
				575	<code>%EFLAGS</code>.
				576	<p>
				577	<li>Another 9 words, holding the V bit "shadows" for the above 9 regs.
				578	<p>
				579	<li>The <b>addresses</b> of various helper routines called from
				580	generated code:
				581	<code>VG_(helper_value_check4_fail)</code>,
				582	<code>VG_(helper_value_check0_fail)</code>,
				583	which register V-check failures,
				584	<code>VG_(helperc_STOREV4)</code>,
				585	<code>VG_(helperc_STOREV1)</code>,
				586	<code>VG_(helperc_LOADV4)</code>,
				587	<code>VG_(helperc_LOADV1)</code>,
				588	which do stores and loads of V bits to/from the
				589	sparse array which keeps track of V bits in memory,
				590	and
				591	<code>VGM_(handle_esp_assignment)</code>, which messes with
				592	memory addressibility resulting from changes in <code>%ESP</code>.
				593	<p>
				594	<li>The simulated <code>%EIP</code>.
				595	<p>
				596	<li>24 spill words, for when the register allocator can't make it work
				597	with 5 measly registers.
				598	<p>
				599	<li>Addresses of helpers <code>VG_(helperc_STOREV2)</code>,
				600	<code>VG_(helperc_LOADV2)</code>. These are here because 2-byte
				601	loads and stores are relatively rare, so are placed above the
				602	magic 32-word offset boundary.
				603	<p>
				604	<li>For similar reasons, addresses of helper functions
				605	<code>VGM_(fpu_write_check)</code> and
				606	<code>VGM_(fpu_read_check)</code>, which handle the A/V maps
				607	testing and changes required by FPU writes/reads.
				608	<p>
				609	<li>Some other boring helper addresses:
				610	<code>VG_(helper_value_check2_fail)</code> and
				611	<code>VG_(helper_value_check1_fail)</code>. These are probably
				612	never emitted now, and should be removed.
				613	<p>
				614	<li>The entire state of the simulated FPU, which I believe to be
				615	108 bytes long.
				616	<p>
				617	<li>Finally, the addresses of various other helper functions in
				618	<code>vg_helpers.S</code>, which deal with rare situations which
				619	are tedious or difficult to generate code in-line for.
				620	</ul>
				621
				622	<p>
				623	As a general rule, the simulated machine's state lives permanently in
				624	memory at <code>VG_(baseBlock)</code>. However, the JITter does some
				625	optimisations which allow the simulated integer registers to be
				626	cached in real registers over multiple simulated instructions within
				627	the same basic block. These are always flushed back into memory at
				628	the end of every basic block, so that the in-memory state is
				629	up-to-date between basic blocks. (This flushing is implied by the
				630	statement above that the real machine's allocatable registers are
				631	dead in between simulated blocks).
				632
				633
				634	<h3>Startup, shutdown, and system calls</h3>
				635
				636	Getting into of Valgrind (<code>VG_(startup)</code>, called from
				637	<code>valgrind.so</code>'s initialisation section), really means
				638	copying the real CPU's state into <code>VG_(baseBlock)</code>, and
				639	then installing our own stack pointer, etc, into the real CPU, and
				640	then starting up the JITter. Exiting valgrind involves copying the
				641	simulated state back to the real state.
				642
				643	<p>
				644	Unfortunately, there's a complication at startup time. Problem is
				645	that at the point where we need to take a snapshot of the real CPU's
				646	state, the offsets in <code>VG_(baseBlock)</code> are not set up yet,
				647	because to do so would involve disrupting the real machine's state
				648	significantly. The way round this is to dump the real machine's state
				649	into a temporary, static block of memory,
				650	<code>VG_(m_state_static)</code>. We can then set up the
				651	<code>VG_(baseBlock)</code> offsets at our leisure, and copy into it
				652	from <code>VG_(m_state_static)</code> at some convenient later time.
				653	This copying is done by
				654	<code>VG_(copy_m_state_static_to_baseBlock)</code>.
				655
				656	<p>
				657	On exit, the inverse transformation is (rather unnecessarily) used:
				658	stuff in <code>VG_(baseBlock)</code> is copied to
				659	<code>VG_(m_state_static)</code>, and the assembly stub then copies
				660	from <code>VG_(m_state_static)</code> into the real machine registers.
				661
				662	<p>
				663	Doing system calls on behalf of the client (<code>vg_syscall.S</code>)
				664	is something of a half-way house. We have to make the world look
				665	sufficiently like that which the client would normally have to make
				666	the syscall actually work properly, but we can't afford to lose
				667	control. So the trick is to copy all of the client's state, <b>except
				668	its program counter</b>, into the real CPU, do the system call, and
				669	copy the state back out. Note that the client's state includes its
				670	stack pointer register, so one effect of this partial restoration is
				671	to cause the system call to be run on the client's stack, as it should
				672	be.
				673
				674	<p>
				675	As ever there are complications. We have to save some of our own state
				676	somewhere when restoring the client's state into the CPU, so that we
				677	can keep going sensibly afterwards. In fact the only thing which is
				678	important is our own stack pointer, but for paranoia reasons I save
				679	and restore our own FPU state as well, even though that's probably
				680	pointless.
				681
				682	<p>
				683	The complication on the above complication is, that for horrible
				684	reasons to do with signals, we may have to handle a second client
				685	system call whilst the client is blocked inside some other system
				686	call (unbelievable!). That means there's two sets of places to
				687	dump Valgrind's stack pointer and FPU state across the syscall,
				688	and we decide which to use by consulting
				689	<code>VG_(syscall_depth)</code>, which is in turn maintained by
				690	<code>VG_(wrap_syscall)</code>.
				691
				692
				693
				694	<h3>Introduction to UCode</h3>
				695
				696	UCode lies at the heart of the x86-to-x86 JITter. The basic premise
				697	is that dealing the the x86 instruction set head-on is just too darn
				698	complicated, so we do the traditional compiler-writer's trick and
				699	translate it into a simpler, easier-to-deal-with form.
				700
				701	<p>
				702	In normal operation, translation proceeds through six stages,
				703	coordinated by <code>VG_(translate)</code>:
				704	<ol>
				705	<li>Parsing of an x86 basic block into a sequence of UCode
				706	instructions (<code>VG_(disBB)</code>).
				707	<p>
				708	<li>UCode optimisation (<code>vg_improve</code>), with the aim of
				709	caching simulated registers in real registers over multiple
				710	simulated instructions, and removing redundant simulated
				711	<code>%EFLAGS</code> saving/restoring.
				712	<p>
				713	<li>UCode instrumentation (<code>vg_instrument</code>), which adds
				714	value and address checking code.
				715	<p>
				716	<li>Post-instrumentation cleanup (<code>vg_cleanup</code>), removing
				717	redundant value-check computations.
				718	<p>
				719	<li>Register allocation (<code>vg_do_register_allocation</code>),
				720	which, note, is done on UCode.
				721	<p>
				722	<li>Emission of final instrumented x86 code
				723	(<code>VG_(emit_code)</code>).
				724	</ol>
				725
				726	<p>
				727	Notice how steps 2, 3, 4 and 5 are simple UCode-to-UCode
				728	transformation passes, all on straight-line blocks of UCode (type
				729	<code>UCodeBlock</code>). Steps 2 and 4 are optimisation passes and
				730	can be disabled for debugging purposes, with
				731	<code>--optimise=no</code> and <code>--cleanup=no</code> respectively.
				732
				733	<p>
				734	Valgrind can also run in a no-instrumentation mode, given
				735	<code>--instrument=no</code>. This is useful for debugging the JITter
				736	quickly without having to deal with the complexity of the
				737	instrumentation mechanism too. In this mode, steps 3 and 4 are
				738	omitted.
				739
				740	<p>
				741	These flags combine, so that <code>--instrument=no</code> together with
				742	<code>--optimise=no</code> means only steps 1, 5 and 6 are used.
				743	<code>--single-step=yes</code> causes each x86 instruction to be
				744	treated as a single basic block. The translations are terrible but
				745	this is sometimes instructive.
				746
				747	<p>
				748	The <code>--stop-after=N</code> flag switches back to the real CPU
				749	after <code>N</code> basic blocks. It also re-JITs the final basic
				750	block executed and prints the debugging info resulting, so this
				751	gives you a way to get a quick snapshot of how a basic block looks as
				752	it passes through the six stages mentioned above. If you want to
				753	see full information for every block translated (probably not, but
				754	still ...) find, in <code>VG_(translate)</code>, the lines
				755	<br><code> dis = True;</code>
				756	<br><code> dis = debugging_translation;</code>
				757	<br>
				758	and comment out the second line. This will spew out debugging
				759	junk faster than you can possibly imagine.
				760
				761
				762
				763	<h3>UCode operand tags: type <code>Tag</code></h3>
				764
				765	UCode is, more or less, a simple two-address RISC-like code. In
				766	keeping with the x86 AT&T assembly syntax, generally speaking the
				767	first operand is the source operand, and the second is the destination
				768	operand, which is modified when the uinstr is notionally executed.
				769
				770	<p>
				771	UCode instructions have up to three operand fields, each of which has
				772	a corresponding <code>Tag</code> describing it. Possible values for
				773	the tag are:
				774
				775	<ul>
				776	<li><code>NoValue</code>: indicates that the field is not in use.
				777	<p>
				778	<li><code>Lit16</code>: the field contains a 16-bit literal.
				779	<p>
				780	<li><code>Literal</code>: the field denotes a 32-bit literal, whose
				781	value is stored in the <code>lit32</code> field of the uinstr
				782	itself. Since there is only one <code>lit32</code> for the whole
				783	uinstr, only one operand field may contain this tag.
				784	<p>
				785	<li><code>SpillNo</code>: the field contains a spill slot number, in
				786	the range 0 to 23 inclusive, denoting one of the spill slots
				787	contained inside <code>VG_(baseBlock)</code>. Such tags only
				788	exist after register allocation.
				789	<p>
				790	<li><code>RealReg</code>: the field contains a number in the range 0
				791	to 7 denoting an integer x86 ("real") register on the host. The
				792	number is the Intel encoding for integer registers. Such tags
				793	only exist after register allocation.
				794	<p>
				795	<li><code>ArchReg</code>: the field contains a number in the range 0
				796	to 7 denoting an integer x86 register on the simulated CPU. In
				797	reality this means a reference to one of the first 8 words of
				798	<code>VG_(baseBlock)</code>. Such tags can exist at any point in
				799	the translation process.
				800	<p>
				801	<li>Last, but not least, <code>TempReg</code>. The field contains the
				802	number of one of an infinite set of virtual (integer)
				803	registers. <code>TempReg</code>s are used everywhere throughout
				804	the translation process; you can have as many as you want. The
				805	register allocator maps as many as it can into
				806	<code>RealReg</code>s and turns the rest into
				807	<code>SpillNo</code>s, so <code>TempReg</code>s should not exist
				808	after the register allocation phase.
				809	<p>
				810	<code>TempReg</code>s are always 32 bits long, even if the data
				811	they hold is logically shorter. In that case the upper unused
				812	bits are required, and, I think, generally assumed, to be zero.
				813	<code>TempReg</code>s holding V bits for quantities shorter than
				814	32 bits are expected to have ones in the unused places, since a
				815	one denotes "undefined".
				816	</ul>
				817
				818
				819	<h3>UCode instructions: type <code>UInstr</code></h3>
				820
				821	<p>
				822	UCode was carefully designed to make it possible to do register
				823	allocation on UCode and then translate the result into x86 code
				824	without needing any extra registers ... well, that was the original
				825	plan, anyway. Things have gotten a little more complicated since
				826	then. In what follows, UCode instructions are referred to as uinstrs,
				827	to distinguish them from x86 instructions. Uinstrs of course have
				828	uopcodes which are (naturally) different from x86 opcodes.
				829
				830	<p>
				831	A uinstr (type <code>UInstr</code>) contains
				832	various fields, not all of which are used by any one uopcode:
				833	<ul>
				834	<li>Three 16-bit operand fields, <code>val1</code>, <code>val2</code>
				835	and <code>val3</code>.
				836	<p>
				837	<li>Three tag fields, <code>tag1</code>, <code>tag2</code>
				838	and <code>tag3</code>. Each of these has a value of type
				839	<code>Tag</code>,
				840	and they describe what the <code>val1</code>, <code>val2</code>
				841	and <code>val3</code> fields contain.
				842	<p>
				843	<li>A 32-bit literal field.
				844	<p>
				845	<li>Two <code>FlagSet</code>s, specifying which x86 condition codes are
				846	read and written by the uinstr.
				847	<p>
				848	<li>An opcode byte, containing a value of type <code>Opcode</code>.
				849	<p>
				850	<li>A size field, indicating the data transfer size (1/2/4/8/10) in
				851	cases where this makes sense, or zero otherwise.
				852	<p>
				853	<li>A condition-code field, which, for jumps, holds a
				854	value of type <code>Condcode</code>, indicating the condition
				855	which applies. The encoding is as it is in the x86 insn stream,
				856	except we add a 17th value <code>CondAlways</code> to indicate
				857	an unconditional transfer.
				858	<p>
				859	<li>Various 1-bit flags, indicating whether this insn pertains to an
				860	x86 CALL or RET instruction, whether a widening is signed or not,
				861	etc.
				862	</ul>
				863
				864	<p>
				865	UOpcodes (type <code>Opcode</code>) are divided into two groups: those
				866	necessary merely to express the functionality of the x86 code, and
				867	extra uopcodes needed to express the instrumentation. The former
				868	group contains:
				869	<ul>
				870	<li><code>GET</code> and <code>PUT</code>, which move values from the
				871	simulated CPU's integer registers (<code>ArchReg</code>s) into
				872	<code>TempReg</code>s, and back. <code>GETF</code> and
				873	<code>PUTF</code> do the corresponding thing for the simulated
				874	<code>%EFLAGS</code>. There are no corresponding insns for the
				875	FPU register stack, since we don't explicitly simulate its
				876	registers.
				877	<p>
				878	<li><code>LOAD</code> and <code>STORE</code>, which, in RISC-like
				879	fashion, are the only uinstrs able to interact with memory.
				880	<p>
				881	<li><code>MOV</code> and <code>CMOV</code> allow unconditional and
				882	conditional moves of values between <code>TempReg</code>s.
				883	<p>
				884	<li>ALU operations. Again in RISC-like fashion, these only operate on
				885	<code>TempReg</code>s (before reg-alloc) or <code>RealReg</code>s
				886	(after reg-alloc). These are: <code>ADD</code>, <code>ADC</code>,
				887	<code>AND</code>, <code>OR</code>, <code>XOR</code>,
				888	<code>SUB</code>, <code>SBB</code>, <code>SHL</code>,
				889	<code>SHR</code>, <code>SAR</code>, <code>ROL</code>,
				890	<code>ROR</code>, <code>RCL</code>, <code>RCR</code>,
				891	<code>NOT</code>, <code>NEG</code>, <code>INC</code>,
				892	<code>DEC</code>, <code>BSWAP</code>, <code>CC2VAL</code> and
				893	<code>WIDEN</code>. <code>WIDEN</code> does signed or unsigned
				894	value widening. <code>CC2VAL</code> is used to convert condition
				895	codes into a value, zero or one. The rest are obvious.
				896	<p>
				897	To allow for more efficient code generation, we bend slightly the
				898	restriction at the start of the previous para: for
				899	<code>ADD</code>, <code>ADC</code>, <code>XOR</code>,
				900	<code>SUB</code> and <code>SBB</code>, we allow the first (source)
				901	operand to also be an <code>ArchReg</code>, that is, one of the
				902	simulated machine's registers. Also, many of these ALU ops allow
				903	the source operand to be a literal. See
				904	<code>VG_(saneUInstr)</code> for the final word on the allowable
				905	forms of uinstrs.
				906	<p>
				907	<li><code>LEA1</code> and <code>LEA2</code> are not strictly
				908	necessary, but allow faciliate better translations. They
				909	record the fancy x86 addressing modes in a direct way, which
				910	allows those amodes to be emitted back into the final
				911	instruction stream more or less verbatim.
				912	<p>
				913	<li><code>CALLM</code> calls a machine-code helper, one of the methods
				914	whose address is stored at some <code>VG_(baseBlock)</code>
				915	offset. <code>PUSH</code> and <code>POP</code> move values
				916	to/from <code>TempReg</code> to the real (Valgrind's) stack, and
				917	<code>CLEAR</code> removes values from the stack.
				918	<code>CALLM_S</code> and <code>CALLM_E</code> delimit the
				919	boundaries of call setups and clearings, for the benefit of the
				920	instrumentation passes. Getting this right is critical, and so
				921	<code>VG_(saneUCodeBlock)</code> makes various checks on the use
				922	of these uopcodes.
				923	<p>
				924	It is important to understand that these uopcodes have nothing to
				925	do with the x86 <code>call</code>, <code>return,</code>
				926	<code>push</code> or <code>pop</code> instructions, and are not
				927	used to implement them. Those guys turn into combinations of
				928	<code>GET</code>, <code>PUT</code>, <code>LOAD</code>,
				929	<code>STORE</code>, <code>ADD</code>, <code>SUB</code>, and
				930	<code>JMP</code>. What these uopcodes support is calling of
				931	helper functions such as <code>VG_(helper_imul_32_64)</code>,
				932	which do stuff which is too difficult or tedious to emit inline.
				933	<p>
				934	<li><code>FPU</code>, <code>FPU_R</code> and <code>FPU_W</code>.
				935	Valgrind doesn't attempt to simulate the internal state of the
				936	FPU at all. Consequently it only needs to be able to distinguish
				937	FPU ops which read and write memory from those that don't, and
				938	for those which do, it needs to know the effective address and
				939	data transfer size. This is made easier because the x86 FP
				940	instruction encoding is very regular, basically consisting of
				941	16 bits for a non-memory FPU insn and 11 (IIRC) bits + an address mode
				942	for a memory FPU insn. So our <code>FPU</code> uinstr carries
				943	the 16 bits in its <code>val1</code> field. And
				944	<code>FPU_R</code> and <code>FPU_W</code> carry 11 bits in that
				945	field, together with the identity of a <code>TempReg</code> or
				946	(later) <code>RealReg</code> which contains the address.
				947	<p>
				948	<li><code>JIFZ</code> is unique, in that it allows a control-flow
				949	transfer which is not deemed to end a basic block. It causes a
				950	jump to a literal (original) address if the specified argument
				951	is zero.
				952	<p>
				953	<li>Finally, <code>INCEIP</code> advances the simulated
				954	<code>%EIP</code> by the specified literal amount. This supports
				955	lazy <code>%EIP</code> updating, as described below.
				956	</ul>
				957
				958	<p>
				959	Stages 1 and 2 of the 6-stage translation process mentioned above
				960	deal purely with these uopcodes, and no others. They are
				961	sufficient to express pretty much all the x86 32-bit protected-mode
				962	instruction set, at
				963	least everything understood by a pre-MMX original Pentium (P54C).
				964
				965	<p>
				966	Stages 3, 4, 5 and 6 also deal with the following extra
				967	"instrumentation" uopcodes. They are used to express all the
				968	definedness-tracking and -checking machinery which valgrind does. In
				969	later sections we show how to create checking code for each of the
				970	uopcodes above. Note that these instrumentation uopcodes, although
				971	some appearing complicated, have been carefully chosen so that
				972	efficient x86 code can be generated for them. GNU superopt v2.5 did a
				973	great job helping out here. Anyways, the uopcodes are as follows:
				974
				975	<ul>
				976	<li><code>GETV</code> and <code>PUTV</code> are analogues to
				977	<code>GET</code> and <code>PUT</code> above. They are identical
				978	except that they move the V bits for the specified values back and
				979	forth to <code>TempRegs</code>, rather than moving the values
				980	themselves.
				981	<p>
				982	<li>Similarly, <code>LOADV</code> and <code>STOREV</code> read and
				983	write V bits from the synthesised shadow memory that Valgrind
				984	maintains. In fact they do more than that, since they also do
				985	address-validity checks, and emit complaints if the read/written
				986	addresses are unaddressible.
				987	<p>
				988	<li><code>TESTV</code>, whose parameters are a <code>TempReg</code>
				989	and a size, tests the V bits in the <code>TempReg</code>, at the
				990	specified operation size (0/1/2/4 byte) and emits an error if any
				991	of them indicate undefinedness. This is the only uopcode capable
				992	of doing such tests.
				993	<p>
				994	<li><code>SETV</code>, whose parameters are also <code>TempReg</code>
				995	and a size, makes the V bits in the <code>TempReg</code> indicated
				996	definedness, at the specified operation size. This is usually
				997	used to generate the correct V bits for a literal value, which is
				998	of course fully defined.
				999	<p>
				1000	<li><code>GETVF</code> and <code>PUTVF</code> are analogues to
				1001	<code>GETF</code> and <code>PUTF</code>. They move the single V
				1002	bit used to model definedness of <code>%EFLAGS</code> between its
				1003	home in <code>VG_(baseBlock)</code> and the specified
				1004	<code>TempReg</code>.
				1005	<p>
				1006	<li><code>TAG1</code> denotes one of a family of unary operations on
				1007	<code>TempReg</code>s containing V bits. Similarly,
				1008	<code>TAG2</code> denotes one in a family of binary operations on
				1009	V bits.
				1010	</ul>
				1011
				1012	<p>
				1013	These 10 uopcodes are sufficient to express Valgrind's entire
				1014	definedness-checking semantics. In fact most of the interesting magic
				1015	is done by the <code>TAG1</code> and <code>TAG2</code>
				1016	suboperations.
				1017
				1018	<p>
				1019	First, however, I need to explain about V-vector operation sizes.
				1020	There are 4 sizes: 1, 2 and 4, which operate on groups of 8, 16 and 32
				1021	V bits at a time, supporting the usual 1, 2 and 4 byte x86 operations.
				1022	However there is also the mysterious size 0, which really means a
				1023	single V bit. Single V bits are used in various circumstances; in
				1024	particular, the definedness of <code>%EFLAGS</code> is modelled with a
				1025	single V bit. Now might be a good time to also point out that for
				1026	V bits, 1 means "undefined" and 0 means "defined". Similarly, for A
				1027	bits, 1 means "invalid address" and 0 means "valid address". This
				1028	seems counterintuitive (and so it is), but testing against zero on
				1029	x86s saves instructions compared to testing against all 1s, because
				1030	many ALU operations set the Z flag for free, so to speak.
				1031
				1032	<p>
				1033	With that in mind, the tag ops are:
				1034
				1035	<ul>
				1036	<li><b>(UNARY) Pessimising casts</b>: <code>VgT_PCast40</code>,
				1037	<code>VgT_PCast20</code>, <code>VgT_PCast10</code>,
				1038	<code>VgT_PCast01</code>, <code>VgT_PCast02</code> and
				1039	<code>VgT_PCast04</code>. A "pessimising cast" takes a V-bit
				1040	vector at one size, and creates a new one at another size,
				1041	pessimised in the sense that if any of the bits in the source
				1042	vector indicate undefinedness, then all the bits in the result
				1043	indicate undefinedness. In this case the casts are all to or from
				1044	a single V bit, so for example <code>VgT_PCast40</code> is a
				1045	pessimising cast from 32 bits to 1, whereas
				1046	<code>VgT_PCast04</code> simply copies the single source V bit
				1047	into all 32 bit positions in the result. Surprisingly, these ops
				1048	can all be implemented very efficiently.
				1049	<p>
				1050	There are also the pessimising casts <code>VgT_PCast14</code>,
				1051	from 8 bits to 32, <code>VgT_PCast12</code>, from 8 bits to 16,
				1052	and <code>VgT_PCast11</code>, from 8 bits to 8. This last one
				1053	seems nonsensical, but in fact it isn't a no-op because, as
				1054	mentioned above, any undefined (1) bits in the source infect the
				1055	entire result.
				1056	<p>
				1057	<li><b>(UNARY) Propagating undefinedness upwards in a word</b>:
				1058	<code>VgT_Left4</code>, <code>VgT_Left2</code> and
				1059	<code>VgT_Left1</code>. These are used to simulate the worst-case
				1060	effects of carry propagation in adds and subtracts. They return a
				1061	V vector identical to the original, except that if the original
				1062	contained any undefined bits, then it and all bits above it are
				1063	marked as undefined too. Hence the Left bit in the names.
				1064	<p>
				1065	<li><b>(UNARY) Signed and unsigned value widening</b>:
				1066	<code>VgT_SWiden14</code>, <code>VgT_SWiden24</code>,
				1067	<code>VgT_SWiden12</code>, <code>VgT_ZWiden14</code>,
				1068	<code>VgT_ZWiden24</code> and <code>VgT_ZWiden12</code>. These
				1069	mimic the definedness effects of standard signed and unsigned
				1070	integer widening. Unsigned widening creates zero bits in the new
				1071	positions, so <code>VgT_ZWiden*</code> accordingly park mark
				1072	those parts of their argument as defined. Signed widening copies
				1073	the sign bit into the new positions, so <code>VgT_SWiden*</code>
				1074	copies the definedness of the sign bit into the new positions.
				1075	Because 1 means undefined and 0 means defined, these operations
				1076	can (fascinatingly) be done by the same operations which they
				1077	mimic. Go figure.
				1078	<p>
				1079	<li><b>(BINARY) Undefined-if-either-Undefined,
				1080	Defined-if-either-Defined</b>: <code>VgT_UifU4</code>,
				1081	<code>VgT_UifU2</code>, <code>VgT_UifU1</code>,
				1082	<code>VgT_UifU0</code>, <code>VgT_DifD4</code>,
				1083	<code>VgT_DifD2</code>, <code>VgT_DifD1</code>. These do simple
				1084	bitwise operations on pairs of V-bit vectors, with
				1085	<code>UifU</code> giving undefined if either arg bit is
				1086	undefined, and <code>DifD</code> giving defined if either arg bit
				1087	is defined. Abstract interpretation junkies, if any make it this
				1088	far, may like to think of them as meets and joins (or is it joins
				1089	and meets) in the definedness lattices.
				1090	<p>
				1091	<li><b>(BINARY; one value, one V bits) Generate argument improvement
				1092	terms for AND and OR</b>: <code>VgT_ImproveAND4_TQ</code>,
				1093	<code>VgT_ImproveAND2_TQ</code>, <code>VgT_ImproveAND1_TQ</code>,
				1094	<code>VgT_ImproveOR4_TQ</code>, <code>VgT_ImproveOR2_TQ</code>,
				1095	<code>VgT_ImproveOR1_TQ</code>. These help out with AND and OR
				1096	operations. AND and OR have the inconvenient property that the
				1097	definedness of the result depends on the actual values of the
				1098	arguments as well as their definedness. At the bit level:
				1099	<br><code>1 AND undefined = undefined</code>, but
				1100	<br><code>0 AND undefined = 0</code>, and similarly
				1101	<br><code>0 OR undefined = undefined</code>, but
				1102	<br><code>1 OR undefined = 1</code>.
				1103	<br>
				1104	<p>
				1105	It turns out that gcc (quite legitimately) generates code which
				1106	relies on this fact, so we have to model it properly in order to
				1107	avoid flooding users with spurious value errors. The ultimate
				1108	definedness result of AND and OR is calculated using
				1109	<code>UifU</code> on the definedness of the arguments, but we
				1110	also <code>DifD</code> in some "improvement" terms which
				1111	take into account the above phenomena.
				1112	<p>
				1113	<code>ImproveAND</code> takes as its first argument the actual
				1114	value of an argument to AND (the T) and the definedness of that
				1115	argument (the Q), and returns a V-bit vector which is defined (0)
				1116	for bits which have value 0 and are defined; this, when
				1117	<code>DifD</code> into the final result causes those bits to be
				1118	defined even if the corresponding bit in the other argument is undefined.
				1119	<p>
				1120	The <code>ImproveOR</code> ops do the dual thing for OR
				1121	arguments. Note that XOR does not have this property that one
				1122	argument can make the other irrelevant, so there is no need for
				1123	such complexity for XOR.
				1124	</ul>
				1125
				1126	<p>
				1127	That's all the tag ops. If you stare at this long enough, and then
				1128	run Valgrind and stare at the pre- and post-instrumented ucode, it
				1129	should be fairly obvious how the instrumentation machinery hangs
				1130	together.
				1131
				1132	<p>
				1133	One point, if you do this: in order to make it easy to differentiate
				1134	<code>TempReg</code>s carrying values from <code>TempReg</code>s
				1135	carrying V bit vectors, Valgrind prints the former as (for example)
				1136	<code>t28</code> and the latter as <code>q28</code>; the fact that
				1137	they carry the same number serves to indicate their relationship.
				1138	This is purely for the convenience of the human reader; the register
				1139	allocator and code generator don't regard them as different.
				1140
				1141
				1142	<h3>Translation into UCode</h3>
				1143
				1144	<code>VG_(disBB)</code> allocates a new <code>UCodeBlock</code> and
				1145	then uses <code>disInstr</code> to translate x86 instructions one at a
				1146	time into UCode, dumping the result in the <code>UCodeBlock</code>.
				1147	This goes on until a control-flow transfer instruction is encountered.
				1148
				1149	<p>
				1150	Despite the large size of <code>vg_to_ucode.c</code>, this translation
				1151	is really very simple. Each x86 instruction is translated entirely
				1152	independently of its neighbours, merrily allocating new
				1153	<code>TempReg</code>s as it goes. The idea is to have a simple
				1154	translator -- in reality, no more than a macro-expander -- and the --
				1155	resulting bad UCode translation is cleaned up by the UCode
				1156	optimisation phase which follows. To give you an idea of some x86
				1157	instructions and their translations (this is a complete basic block,
				1158	as Valgrind sees it):
				1159	<pre>
				1160	0x40435A50: incl %edx
				1161
				1162	0: GETL %EDX, t0
				1163	1: INCL t0 (-wOSZAP)
				1164	2: PUTL t0, %EDX
				1165
				1166	0x40435A51: movsbl (%edx),%eax
				1167
				1168	3: GETL %EDX, t2
				1169	4: LDB (t2), t2
				1170	5: WIDENL_Bs t2
				1171	6: PUTL t2, %EAX
				1172
				1173	0x40435A54: testb $0x20, 1(%ecx,%eax,2)
				1174
				1175	7: GETL %EAX, t6
				1176	8: GETL %ECX, t8
				1177	9: LEA2L 1(t8,t6,2), t4
				1178	10: LDB (t4), t10
				1179	11: MOVB $0x20, t12
				1180	12: ANDB t12, t10 (-wOSZACP)
				1181	13: INCEIPo $9
				1182
				1183	0x40435A59: jnz-8 0x40435A50
				1184
				1185	14: Jnzo $0x40435A50 (-rOSZACP)
				1186	15: JMPo $0x40435A5B
				1187	</pre>
				1188
				1189	<p>
				1190	Notice how the block always ends with an unconditional jump to the
				1191	next block. This is a bit unnecessary, but makes many things simpler.
				1192
				1193	<p>
				1194	Most x86 instructions turn into sequences of <code>GET</code>,
				1195	</code>PUT</code>, <code>LEA1</code>, <code>LEA2</code>,
				1196	<code>LOAD</code> and <code>STORE</code>. Some complicated ones
				1197	however rely on calling helper bits of code in
				1198	<code>vg_helpers.S</code>. The ucode instructions <code>PUSH</code>,
				1199	<code>POP</code>, <code>CALL</code>, <code>CALLM_S</code> and
				1200	<code>CALLM_E</code> support this. The calling convention is somewhat
				1201	ad-hoc and is not the C calling convention. The helper routines must
				1202	save all integer registers, and the flags, that they use. Args are
				1203	passed on the stack underneath the return address, as usual, and if
				1204	result(s) are to be returned, it (they) are either placed in dummy arg
				1205	slots created by the ucode <code>PUSH</code> sequence, or just
				1206	overwrite the incoming args.
				1207
				1208	<p>
				1209	In order that the instrumentation mechanism can handle calls to these
				1210	helpers, <code>VG_(saneUCodeBlock)</code> enforces the following
				1211	restrictions on calls to helpers:
				1212
				1213	<ul>
				1214	<li>Each <code>CALL</code> uinstr must be bracketed by a preceding
				1215	<code>CALLM_S</code> marker (dummy uinstr) and a trailing
				1216	<code>CALLM_E</code> marker. These markers are used by the
				1217	instrumentation mechanism later to establish the boundaries of the
				1218	<code>PUSH</code>, <code>POP</code> and <code>CLEAR</code>
				1219	sequences for the call.
				1220	<p>
				1221	<li><code>PUSH</code>, <code>POP</code> and <code>CLEAR</code>
				1222	may only appear inside sections bracketed by <code>CALLM_S</code>
				1223	and <code>CALLM_E</code>, and nowhere else.
				1224	<p>
				1225	<li>In any such bracketed section, no two <code>PUSH</code> insns may
				1226	push the same <code>TempReg</code>. Dually, no two two
				1227	<code>POP</code>s may pop the same <code>TempReg</code>.
				1228	<p>
				1229	<li>Finally, although this is not checked, args should be removed from
				1230	the stack with <code>CLEAR</code>, rather than <code>POP</code>s
				1231	into a <code>TempReg</code> which is not subsequently used. This
				1232	is because the instrumentation mechanism assumes that all values
				1233	<code>POP</code>ped from the stack are actually used.
				1234	</ul>
				1235
				1236	Some of the translations may appear to have redundant
				1237	<code>TempReg</code>-to-<code>TempReg</code> moves. This helps the
				1238	next phase, UCode optimisation, to generate better code.
				1239
				1240
				1241
				1242	<h3>UCode optimisation</h3>
				1243
				1244	UCode is then subjected to an improvement pass
				1245	(<code>vg_improve()</code>), which blurs the boundaries between the
				1246	translations of the original x86 instructions. It's pretty
				1247	straightforward. Three transformations are done:
				1248
				1249	<ul>
				1250	<li>Redundant <code>GET</code> elimination. Actually, more general
				1251	than that -- eliminates redundant fetches of ArchRegs. In our
				1252	running example, uinstr 3 <code>GET</code>s <code>%EDX</code> into
				1253	<code>t2</code> despite the fact that, by looking at the previous
				1254	uinstr, it is already in <code>t0</code>. The <code>GET</code> is
				1255	therefore removed, and <code>t2</code> renamed to <code>t0</code>.
				1256	Assuming <code>t0</code> is allocated to a host register, it means
				1257	the simulated <code>%EDX</code> will exist in a host CPU register
				1258	for more than one simulated x86 instruction, which seems to me to
				1259	be a highly desirable property.
				1260	<p>
				1261	There is some mucking around to do with subregisters;
				1262	<code>%AL</code> vs <code>%AH</code> <code>%AX</code> vs
				1263	<code>%EAX</code> etc. I can't remember how it works, but in
				1264	general we are very conservative, and these tend to invalidate the
				1265	caching.
				1266	<p>
				1267	<li>Redundant <code>PUT</code> elimination. This annuls
				1268	<code>PUT</code>s of values back to simulated CPU registers if a
				1269	later <code>PUT</code> would overwrite the earlier
				1270	<code>PUT</code> value, and there is no intervening reads of the
				1271	simulated register (<code>ArchReg</code>).
				1272	<p>
				1273	As before, we are paranoid when faced with subregister references.
				1274	Also, <code>PUT</code>s of <code>%ESP</code> are never annulled,
				1275	because it is vital the instrumenter always has an up-to-date
				1276	<code>%ESP</code> value available, <code>%ESP</code> changes
				1277	affect addressibility of the memory around the simulated stack
				1278	pointer.
				1279	<p>
				1280	The implication of the above paragraph is that the simulated
				1281	machine's registers are only lazily updated once the above two
				1282	optimisation phases have run, with the exception of
				1283	<code>%ESP</code>. <code>TempReg</code>s go dead at the end of
				1284	every basic block, from which is is inferrable that any
				1285	<code>TempReg</code> caching a simulated CPU reg is flushed (back
				1286	into the relevant <code>VG_(baseBlock)</code> slot) at the end of
				1287	every basic block. The further implication is that the simulated
				1288	registers are only up-to-date at in between basic blocks, and not
				1289	at arbitrary points inside basic blocks. And the consequence of
				1290	that is that we can only deliver signals to the client in between
				1291	basic blocks. None of this seems any problem in practice.
				1292	<p>
				1293	<li>Finally there is a simple def-use thing for condition codes. If
				1294	an earlier uinstr writes the condition codes, and the next uinsn
				1295	along which actually cares about the condition codes writes the
				1296	same or larger set of them, but does not read any, the earlier
				1297	uinsn is marked as not writing any condition codes. This saves
				1298	a lot of redundant cond-code saving and restoring.
				1299	</ul>
				1300
				1301	The effect of these transformations on our short block is rather
				1302	unexciting, and shown below. On longer basic blocks they can
				1303	dramatically improve code quality.
				1304
				1305	<pre>
				1306	at 3: delete GET, rename t2 to t0 in (4 .. 6)
				1307	at 7: delete GET, rename t6 to t0 in (8 .. 9)
				1308	at 1: annul flag write OSZAP due to later OSZACP
				1309
				1310	Improved code:
				1311	0: GETL %EDX, t0
				1312	1: INCL t0
				1313	2: PUTL t0, %EDX
				1314	4: LDB (t0), t0
				1315	5: WIDENL_Bs t0
				1316	6: PUTL t0, %EAX
				1317	8: GETL %ECX, t8
				1318	9: LEA2L 1(t8,t0,2), t4
				1319	10: LDB (t4), t10
				1320	11: MOVB $0x20, t12
				1321	12: ANDB t12, t10 (-wOSZACP)
				1322	13: INCEIPo $9
				1323	14: Jnzo $0x40435A50 (-rOSZACP)
				1324	15: JMPo $0x40435A5B
				1325	</pre>
				1326
				1327	<h3>UCode instrumentation</h3>
				1328
				1329	Once you understand the meaning of the instrumentation uinstrs,
				1330	discussed in detail above, the instrumentation scheme is fairly
				1331	straighforward. Each uinstr is instrumented in isolation, and the
				1332	instrumentation uinstrs are placed before the original uinstr.
				1333	Our running example continues below. I have placed a blank line
				1334	after every original ucode, to make it easier to see which
				1335	instrumentation uinstrs correspond to which originals.
				1336
				1337	<p>
				1338	As mentioned somewhere above, <code>TempReg</code>s carrying values
				1339	have names like <code>t28</code>, and each one has a shadow carrying
				1340	its V bits, with names like <code>q28</code>. This pairing aids in
				1341	reading instrumented ucode.
				1342
				1343	<p>
				1344	One decision about all this is where to have "observation points",
				1345	that is, where to check that V bits are valid. I use a minimalistic
				1346	scheme, only checking where a failure of validity could cause the
				1347	original program to (seg)fault. So the use of values as memory
				1348	addresses causes a check, as do conditional jumps (these cause a check
				1349	on the definedness of the condition codes). And arguments
				1350	<code>PUSH</code>ed for helper calls are checked, hence the wierd
				1351	restrictions on help call preambles described above.
				1352
				1353	<p>
				1354	Another decision is that once a value is tested, it is thereafter
				1355	regarded as defined, so that we do not emit multiple undefined-value
				1356	errors for the same undefined value. That means that
				1357	<code>TESTV</code> uinstrs are always followed by <code>SETV</code>
				1358	on the same (shadow) <code>TempReg</code>s. Most of these
				1359	<code>SETV</code>s are redundant and are removed by the
				1360	post-instrumentation cleanup phase.
				1361
				1362	<p>
				1363	The instrumentation for calling helper functions deserves further
				1364	comment. The definedness of results from a helper is modelled using
				1365	just one V bit. So, in short, we do pessimising casts of the
				1366	definedness of all the args, down to a single bit, and then
				1367	<code>UifU</code> these bits together. So this single V bit will say
				1368	"undefined" if any part of any arg is undefined. This V bit is then
				1369	pessimally cast back up to the result(s) sizes, as needed. If, by
				1370	seeing that all the args are got rid of with <code>CLEAR</code> and
				1371	none with <code>POP</code>, Valgrind sees that the result of the call
				1372	is not actually used, it immediately examines the result V bit with a
				1373	<code>TESTV</code> -- <code>SETV</code> pair. If it did not do this,
				1374	there would be no observation point to detect that the some of the
				1375	args to the helper were undefined. Of course, if the helper's results
				1376	are indeed used, we don't do this, since the result usage will
				1377	presumably cause the result definedness to be checked at some suitable
				1378	future point.
				1379
				1380	<p>
				1381	In general Valgrind tries to track definedness on a bit-for-bit basis,
				1382	but as the above para shows, for calls to helpers we throw in the
				1383	towel and approximate down to a single bit. This is because it's too
				1384	complex and difficult to track bit-level definedness through complex
				1385	ops such as integer multiply and divide, and in any case there is no
				1386	reasonable code fragments which attempt to (eg) multiply two
				1387	partially-defined values and end up with something meaningful, so
				1388	there seems little point in modelling multiplies, divides, etc, in
				1389	that level of detail.
				1390
				1391	<p>
				1392	Integer loads and stores are instrumented with firstly a test of the
				1393	definedness of the address, followed by a <code>LOADV</code> or
				1394	<code>STOREV</code> respectively. These turn into calls to
				1395	(for example) <code>VG_(helperc_LOADV4)</code>. These helpers do two
				1396	things: they perform an address-valid check, and they load or store V
				1397	bits from/to the relevant address in the (simulated V-bit) memory.
				1398
				1399	<p>
				1400	FPU loads and stores are different. As above the definedness of the
				1401	address is first tested. However, the helper routine for FPU loads
				1402	(<code>VGM_(fpu_read_check)</code>) emits an error if either the
				1403	address is invalid or the referenced area contains undefined values.
				1404	It has to do this because we do not simulate the FPU at all, and so
				1405	cannot track definedness of values loaded into it from memory, so we
				1406	have to check them as soon as they are loaded into the FPU, ie, at
				1407	this point. We notionally assume that everything in the FPU is
				1408	defined.
				1409
				1410	<p>
				1411	It follows therefore that FPU writes first check the definedness of
				1412	the address, then the validity of the address, and finally mark the
				1413	written bytes as well-defined.
				1414
				1415	<p>
				1416	If anyone is inspired to extend Valgrind to MMX/SSE insns, I suggest
				1417	you use the same trick. It works provided that the FPU/MMX unit is
				1418	not used to merely as a conduit to copy partially undefined data from
				1419	one place in memory to another. Unfortunately the integer CPU is used
				1420	like that (when copying C structs with holes, for example) and this is
				1421	the cause of much of the elaborateness of the instrumentation here
				1422	described.
				1423
				1424	<p>
				1425	<code>vg_instrument()</code> in <code>vg_translate.c</code> actually
				1426	does the instrumentation. There are comments explaining how each
				1427	uinstr is handled, so we do not repeat that here. As explained
				1428	already, it is bit-accurate, except for calls to helper functions.
				1429	Unfortunately the x86 insns <code>bt/bts/btc/btr</code> are done by
				1430	helper fns, so bit-level accuracy is lost there. This should be fixed
				1431	by doing them inline; it will probably require adding a couple new
				1432	uinstrs. Also, left and right rotates through the carry flag (x86
				1433	<code>rcl</code> and <code>rcr</code>) are approximated via a single
				1434	V bit; so far this has not caused anyone to complain. The
				1435	non-carry rotates, <code>rol</code> and <code>ror</code>, are much
				1436	more common and are done exactly. Re-visiting the instrumentation for
				1437	AND and OR, they seem rather verbose, and I wonder if it could be done
				1438	more concisely now.
				1439
				1440	<p>
				1441	The lowercase <code>o</code> on many of the uopcodes in the running
				1442	example indicates that the size field is zero, usually meaning a
				1443	single-bit operation.
				1444
				1445	<p>
				1446	Anyroads, the post-instrumented version of our running example looks
				1447	like this:
				1448
				1449	<pre>
				1450	Instrumented code:
				1451	0: GETVL %EDX, q0
				1452	1: GETL %EDX, t0
				1453
				1454	2: TAG1o q0 = Left4 ( q0 )
				1455	3: INCL t0
				1456
				1457	4: PUTVL q0, %EDX
				1458	5: PUTL t0, %EDX
				1459
				1460	6: TESTVL q0
				1461	7: SETVL q0
				1462	8: LOADVB (t0), q0
				1463	9: LDB (t0), t0
				1464
				1465	10: TAG1o q0 = SWiden14 ( q0 )
				1466	11: WIDENL_Bs t0
				1467
				1468	12: PUTVL q0, %EAX
				1469	13: PUTL t0, %EAX
				1470
				1471	14: GETVL %ECX, q8
				1472	15: GETL %ECX, t8
				1473
				1474	16: MOVL q0, q4
				1475	17: SHLL $0x1, q4
				1476	18: TAG2o q4 = UifU4 ( q8, q4 )
				1477	19: TAG1o q4 = Left4 ( q4 )
				1478	20: LEA2L 1(t8,t0,2), t4
				1479
				1480	21: TESTVL q4
				1481	22: SETVL q4
				1482	23: LOADVB (t4), q10
				1483	24: LDB (t4), t10
				1484
				1485	25: SETVB q12
				1486	26: MOVB $0x20, t12
				1487
				1488	27: MOVL q10, q14
				1489	28: TAG2o q14 = ImproveAND1_TQ ( t10, q14 )
				1490	29: TAG2o q10 = UifU1 ( q12, q10 )
				1491	30: TAG2o q10 = DifD1 ( q14, q10 )
				1492	31: MOVL q12, q14
				1493	32: TAG2o q14 = ImproveAND1_TQ ( t12, q14 )
				1494	33: TAG2o q10 = DifD1 ( q14, q10 )
				1495	34: MOVL q10, q16
				1496	35: TAG1o q16 = PCast10 ( q16 )
				1497	36: PUTVFo q16
				1498	37: ANDB t12, t10 (-wOSZACP)
				1499
				1500	38: INCEIPo $9
				1501
				1502	39: GETVFo q18
				1503	40: TESTVo q18
				1504	41: SETVo q18
				1505	42: Jnzo $0x40435A50 (-rOSZACP)
				1506
				1507	43: JMPo $0x40435A5B
				1508	</pre>
				1509
				1510
				1511	<h3>UCode post-instrumentation cleanup</h3>
				1512
				1513	<p>
				1514	This pass, coordinated by <code>vg_cleanup()</code>, removes redundant
				1515	definedness computation created by the simplistic instrumentation
				1516	pass. It consists of two passes,
				1517	<code>vg_propagate_definedness()</code> followed by
				1518	<code>vg_delete_redundant_SETVs</code>.
				1519
				1520	<p>
				1521	<code>vg_propagate_definedness()</code> is a simple
				1522	constant-propagation and constant-folding pass. It tries to determine
				1523	which <code>TempReg</code>s containing V bits will always indicate
				1524	"fully defined", and it propagates this information as far as it can,
				1525	and folds out as many operations as possible. For example, the
				1526	instrumentation for an ADD of a literal to a variable quantity will be
				1527	reduced down so that the definedness of the result is simply the
				1528	definedness of the variable quantity, since the literal is by
				1529	definition fully defined.
				1530
				1531	<p>
				1532	<code>vg_delete_redundant_SETVs</code> removes <code>SETV</code>s on
				1533	shadow <code>TempReg</code>s for which the next action is a write.
				1534	I don't think there's anything else worth saying about this; it is
				1535	simple. Read the sources for details.
				1536
				1537	<p>
				1538	So the cleaned-up running example looks like this. As above, I have
				1539	inserted line breaks after every original (non-instrumentation) uinstr
				1540	to aid readability. As with straightforward ucode optimisation, the
				1541	results in this block are undramatic because it is so short; longer
				1542	blocks benefit more because they have more redundancy which gets
				1543	eliminated.
				1544
				1545
				1546	<pre>
				1547	at 29: delete UifU1 due to defd arg1
				1548	at 32: change ImproveAND1_TQ to MOV due to defd arg2
				1549	at 41: delete SETV
				1550	at 31: delete MOV
				1551	at 25: delete SETV
				1552	at 22: delete SETV
				1553	at 7: delete SETV
				1554
				1555	0: GETVL %EDX, q0
				1556	1: GETL %EDX, t0
				1557
				1558	2: TAG1o q0 = Left4 ( q0 )
				1559	3: INCL t0
				1560
				1561	4: PUTVL q0, %EDX
				1562	5: PUTL t0, %EDX
				1563
				1564	6: TESTVL q0
				1565	8: LOADVB (t0), q0
				1566	9: LDB (t0), t0
				1567
				1568	10: TAG1o q0 = SWiden14 ( q0 )
				1569	11: WIDENL_Bs t0
				1570
				1571	12: PUTVL q0, %EAX
				1572	13: PUTL t0, %EAX
				1573
				1574	14: GETVL %ECX, q8
				1575	15: GETL %ECX, t8
				1576
				1577	16: MOVL q0, q4
				1578	17: SHLL $0x1, q4
				1579	18: TAG2o q4 = UifU4 ( q8, q4 )
				1580	19: TAG1o q4 = Left4 ( q4 )
				1581	20: LEA2L 1(t8,t0,2), t4
				1582
				1583	21: TESTVL q4
				1584	23: LOADVB (t4), q10
				1585	24: LDB (t4), t10
				1586
				1587	26: MOVB $0x20, t12
				1588
				1589	27: MOVL q10, q14
				1590	28: TAG2o q14 = ImproveAND1_TQ ( t10, q14 )
				1591	30: TAG2o q10 = DifD1 ( q14, q10 )
				1592	32: MOVL t12, q14
				1593	33: TAG2o q10 = DifD1 ( q14, q10 )
				1594	34: MOVL q10, q16
				1595	35: TAG1o q16 = PCast10 ( q16 )
				1596	36: PUTVFo q16
				1597	37: ANDB t12, t10 (-wOSZACP)
				1598
				1599	38: INCEIPo $9
				1600	39: GETVFo q18
				1601	40: TESTVo q18
				1602	42: Jnzo $0x40435A50 (-rOSZACP)
				1603
				1604	43: JMPo $0x40435A5B
				1605	</pre>
				1606
				1607
				1608	<h3>Translation from UCode</h3>
				1609
				1610	This is all very simple, even though <code>vg_from_ucode.c</code>
				1611	is a big file. Position-independent x86 code is generated into
				1612	a dynamically allocated array <code>emitted_code</code>; this is
				1613	doubled in size when it overflows. Eventually the array is handed
				1614	back to the caller of <code>VG_(translate)</code>, who must copy
				1615	the result into TC and TT, and free the array.
				1616
				1617	<p>
				1618	This file is structured into four layers of abstraction, which,
				1619	thankfully, are glued back together with extensive
				1620	<code>__inline__</code> directives. From the bottom upwards:
				1621
				1622	<ul>
				1623	<li>Address-mode emitters, <code>emit_amode_regmem_reg</code> et al.
				1624	<p>
				1625	<li>Emitters for specific x86 instructions. There are quite a lot of
				1626	these, with names such as <code>emit_movv_offregmem_reg</code>.
				1627	The <code>v</code> suffix is Intel parlance for a 16/32 bit insn;
				1628	there are also <code>b</code> suffixes for 8 bit insns.
				1629	<p>
				1630	<li>The next level up are the <code>synth_*</code> functions, which
				1631	synthesise possibly a sequence of raw x86 instructions to do some
				1632	simple task. Some of these are quite complex because they have to
				1633	work around Intel's silly restrictions on subregister naming. See
				1634	<code>synth_nonshiftop_reg_reg</code> for example.
				1635	<p>
				1636	<li>Finally, at the top of the heap, we have
				1637	<code>emitUInstr()</code>,
				1638	which emits code for a single uinstr.
				1639	</ul>
				1640
				1641	<p>
				1642	Some comments:
				1643	<ul>
				1644	<li>The hack for FPU instructions becomes apparent here. To do a
				1645	<code>FPU</code> ucode instruction, we load the simulated FPU's
				1646	state into from its <code>VG_(baseBlock)</code> into the real FPU
				1647	using an x86 <code>frstor</code> insn, do the ucode
				1648	<code>FPU</code> insn on the real CPU, and write the updated FPU
				1649	state back into <code>VG_(baseBlock)</code> using an
				1650	<code>fnsave</code> instruction. This is pretty brutal, but is
				1651	simple and it works, and even seems tolerably efficient. There is
				1652	no attempt to cache the simulated FPU state in the real FPU over
				1653	multiple back-to-back ucode FPU instructions.
				1654	<p>
				1655	<code>FPU_R</code> and <code>FPU_W</code> are also done this way,
				1656	with the minor complication that we need to patch in some
				1657	addressing mode bits so the resulting insn knows the effective
				1658	address to use. This is easy because of the regularity of the x86
				1659	FPU instruction encodings.
				1660	<p>
				1661	<li>An analogous trick is done with ucode insns which claim, in their
				1662	<code>flags_r</code> and <code>flags_w</code> fields, that they
				1663	read or write the simulated <code>%EFLAGS</code>. For such cases
				1664	we first copy the simulated <code>%EFLAGS</code> into the real
				1665	<code>%eflags</code>, then do the insn, then, if the insn says it
				1666	writes the flags, copy back to <code>%EFLAGS</code>. This is a
				1667	bit expensive, which is why the ucode optimisation pass goes to
				1668	some effort to remove redundant flag-update annotations.
				1669	</ul>
				1670
				1671	<p>
				1672	And so ... that's the end of the documentation for the instrumentating
				1673	translator! It's really not that complex, because it's composed as a
				1674	sequence of simple(ish) self-contained transformations on
				1675	straight-line blocks of code.
				1676
				1677
				1678	<h3>Top-level dispatch loop</h3>
				1679
				1680	Urk. In <code>VG_(toploop)</code>. This is basically boring and
				1681	unsurprising, not to mention fiddly and fragile. It needs to be
				1682	cleaned up.
				1683
				1684	<p>
				1685	The only perhaps surprise is that the whole thing is run
				1686	on top of a <code>setjmp</code>-installed exception handler, because,
				1687	supposing a translation got a segfault, we have to bail out of the
				1688	Valgrind-supplied exception handler <code>VG_(oursignalhandler)</code>
				1689	and immediately start running the client's segfault handler, if it has
				1690	one. In particular we can't finish the current basic block and then
				1691	deliver the signal at some convenient future point, because signals
				1692	like SIGILL, SIGSEGV and SIGBUS mean that the faulting insn should not
				1693	simply be re-tried. (I'm sure there is a clearer way to explain this).
				1694
				1695
				1696	<h3>Exceptions, creating new translations</h3>
				1697	<h3>Self-modifying code</h3>
				1698
				1699	<h3>Lazy updates of the simulated program counter</h3>
				1700
				1701	Simulated <code>%EIP</code> is not updated after every simulated x86
				1702	insn as this was regarded as too expensive. Instead ucode
				1703	<code>INCEIP</code> insns move it along as and when necessary.
				1704	Currently we don't allow it to fall more than 4 bytes behind reality
				1705	(see <code>VG_(disBB)</code> for the way this works).
				1706	<p>
				1707	Note that <code>%EIP</code> is always brought up to date by the inner
				1708	dispatch loop in <code>VG_(dispatch)</code>, so that if the client
				1709	takes a fault we know at least which basic block this happened in.
				1710
				1711
				1712	<h3>The translation cache and translation table</h3>
				1713
				1714	<h3>Signals</h3>
				1715
				1716	Horrible, horrible. <code>vg_signals.c</code>.
				1717	Basically, since we have to intercept all system
				1718	calls anyway, we can see when the client tries to install a signal
				1719	handler. If it does so, we make a note of what the client asked to
				1720	happen, and ask the kernel to route the signal to our own signal
				1721	handler, <code>VG_(oursignalhandler)</code>. This simply notes the
				1722	delivery of signals, and returns.
				1723
				1724	<p>
				1725	Every 1000 basic blocks, we see if more signals have arrived. If so,
				1726	<code>VG_(deliver_signals)</code> builds signal delivery frames on the
				1727	client's stack, and allows their handlers to be run. Valgrind places
				1728	in these signal delivery frames a bogus return address,
				1729	</code>VG_(signalreturn_bogusRA)</code>, and checks all jumps to see
				1730	if any jump to it. If so, this is a sign that a signal handler is
				1731	returning, and if so Valgrind removes the relevant signal frame from
				1732	the client's stack, restores the from the signal frame the simulated
				1733	state before the signal was delivered, and allows the client to run
				1734	onwards. We have to do it this way because some signal handlers never
				1735	return, they just <code>longjmp()</code>, which nukes the signal
				1736	delivery frame.
				1737
				1738	<p>
				1739	The Linux kernel has a different but equally horrible hack for
				1740	detecting signal handler returns. Discovering it is left as an
				1741	exercise for the reader.
				1742
				1743
				1744
				1745	<h3>Errors, error contexts, error reporting, suppressions</h3>
				1746	<h3>Client malloc/free</h3>
				1747	<h3>Low-level memory management</h3>
				1748	<h3>A and V bitmaps</h3>
				1749	<h3>Symbol table management</h3>
				1750	<h3>Dealing with system calls</h3>
				1751	<h3>Namespace management</h3>
				1752	<h3>GDB attaching</h3>
				1753	<h3>Non-dependence on glibc or anything else</h3>
				1754	<h3>The leak detector</h3>
				1755	<h3>Performance problems</h3>
				1756	<h3>Continuous sanity checking</h3>
				1757	<h3>Tracing, or not tracing, child processes</h3>
				1758	<h3>Assembly glue for syscalls</h3>
				1759
				1760
				1761	<hr width="100%">
				1762
				1763	<h2>Extensions</h2>
				1764
				1765	Some comments about Stuff To Do.
				1766
				1767	<h3>Bugs</h3>
				1768
				1769	Stephan Kulow and Marc Mutz report problems with kmail in KDE 3 CVS
				1770	(RC2 ish) when run on Valgrind. Stephan has it deadlocking; Marc has
				1771	it looping at startup. I can't repro either behaviour. Needs
				1772	repro-ing and fixing.
				1773
				1774
				1775	<h3>Threads</h3>
				1776
				1777	Doing a good job of thread support strikes me as almost a
				1778	research-level problem. The central issues are how to do fast cheap
				1779	locking of the <code>VG_(primary_map)</code> structure, whether or not
				1780	accesses to the individual secondary maps need locking, what
				1781	race-condition issues result, and whether the already-nasty mess that
				1782	is the signal simulator needs further hackery.
				1783
				1784	<p>
				1785	I realise that threads are the most-frequently-requested feature, and
				1786	I am thinking about it all. If you have guru-level understanding of
				1787	fast mutual exclusion mechanisms and race conditions, I would be
				1788	interested in hearing from you.
				1789
				1790
				1791	<h3>Verification suite</h3>
				1792
				1793	Directory <code>tests/</code> contains various ad-hoc tests for
				1794	Valgrind. However, there is no systematic verification or regression
				1795	suite, that, for example, exercises all the stuff in
				1796	<code>vg_memory.c</code>, to ensure that illegal memory accesses and
				1797	undefined value uses are detected as they should be. It would be good
				1798	to have such a suite.
				1799
				1800
				1801	<h3>Porting to other platforms</h3>
				1802
				1803	It would be great if Valgrind was ported to FreeBSD and x86 NetBSD,
				1804	and to x86 OpenBSD, if it's possible (doesn't OpenBSD use a.out-style
				1805	executables, not ELF ?)
				1806
				1807	<p>
				1808	The main difficulties, for an x86-ELF platform, seem to be:
				1809
				1810	<ul>
				1811	<li>You'd need to rewrite the <code>/proc/self/maps</code> parser
				1812	(<code>vg_procselfmaps.c</code>).
				1813	Easy.
				1814	<p>
				1815	<li>You'd need to rewrite <code>vg_syscall_mem.c</code>, or, more
				1816	specifically, provide one for your OS. This is tedious, but you
				1817	can implement syscalls on demand, and the Linux kernel interface
				1818	is, for the most part, going to look very similar to the *BSD
				1819	interfaces, so it's really a copy-paste-and-modify-on-demand job.
				1820	As part of this, you'd need to supply a new
				1821	<code>vg_kerneliface.h</code> file.
				1822	<p>
				1823	<li>You'd also need to change the syscall wrappers for Valgrind's
				1824	internal use, in <code>vg_mylibc.c</code>.
				1825	</ul>
				1826
				1827	All in all, I think a port to x86-ELF *BSDs is not really very
				1828	difficult, and in some ways I would like to see it happen, because
				1829	that would force a more clear factoring of Valgrind into platform
				1830	dependent and independent pieces. Not to mention, *BSD folks also
				1831	deserve to use Valgrind just as much as the Linux crew do.
				1832
				1833
				1834	<p>
				1835	<hr width="100%">
				1836
				1837	<h2>Easy stuff which ought to be done</h2>
				1838
				1839	<h3>MMX instructions</h3>
				1840
				1841	MMX insns should be supported, using the same trick as for FPU insns.
				1842	If the MMX registers are not used to copy uninitialised junk from one
				1843	place to another in memory, this means we don't have to actually
				1844	simulate the internal MMX unit state, so the FPU hack applies. This
				1845	should be fairly easy.
				1846
				1847
				1848
				1849	<h3>Fix stabs-info reader</h3>
				1850
				1851	The machinery in <code>vg_symtab2.c</code> which reads "stabs" style
				1852	debugging info is pretty weak. It usually correctly translates
				1853	simulated program counter values into line numbers and procedure
				1854	names, but the file name is often completely wrong. I think the
				1855	logic used to parse "stabs" entries is weak. It should be fixed.
				1856	The simplest solution, IMO, is to copy either the logic or simply the
				1857	code out of GNU binutils which does this; since GDB can clearly get it
				1858	right, binutils (or GDB?) must have code to do this somewhere.
				1859
				1860
				1861
				1862
				1863
				1864	<h3>BT/BTC/BTS/BTR</h3>
				1865
				1866	These are x86 instructions which test, complement, set, or reset, a
				1867	single bit in a word. At the moment they are both incorrectly
				1868	implemented and incorrectly instrumented.
				1869
				1870	<p>
				1871	The incorrect instrumentation is due to use of helper functions. This
				1872	means we lose bit-level definedness tracking, which could wind up
				1873	giving spurious uninitialised-value use errors. The Right Thing to do
				1874	is to invent a couple of new UOpcodes, I think <code>GET_BIT</code>
				1875	and <code>SET_BIT</code>, which can be used to implement all 4 x86
				1876	insns, get rid of the helpers, and give bit-accurate instrumentation
				1877	rules for the two new UOpcodes.
				1878
				1879	<p>
				1880	I realised the other day that they are mis-implemented too. The x86
				1881	insns take a bit-index and a register or memory location to access.
				1882	For registers the bit index clearly can only be in the range zero to
				1883	register-width minus 1, and I assumed the same applied to memory
				1884	locations too. But evidently not; for memory locations the index can
				1885	be arbitrary, and the processor will index arbitrarily into memory as
				1886	a result. This too should be fixed. Sigh. Presumably indexing
				1887	outside the immediate word is not actually used by any programs yet
				1888	tested on Valgrind, for otherwise they (presumably) would simply not
				1889	work at all. If you plan to hack on this, first check the Intel docs
				1890	to make sure my understanding is really correct.
				1891
				1892
				1893
				1894	<h3>Using PREFETCH instructions</h3>
				1895
				1896	Here's a small but potentially interesting project for performance
				1897	junkies. Experiments with valgrind's code generator and optimiser(s)
				1898	suggest that reducing the number of instructions executed in the
				1899	translations and mem-check helpers gives disappointingly small
				1900	performance improvements. Perhaps this is because performance of
				1901	Valgrindified code is limited by cache misses. After all, each read
				1902	in the original program now gives rise to at least three reads, one
				1903	for the <code>VG_(primary_map)</code>, one of the resulting
				1904	secondary, and the original. Not to mention, the instrumented
				1905	translations are 13 to 14 times larger than the originals. All in all
				1906	one would expect the memory system to be hammered to hell and then
				1907	some.
				1908
				1909	<p>
				1910	So here's an idea. An x86 insn involving a read from memory, after
				1911	instrumentation, will turn into ucode of the following form:
				1912	<pre>
				1913	... calculate effective addr, into ta and qa ...
				1914	TESTVL qa -- is the addr defined?
				1915	LOADV (ta), qloaded -- fetch V bits for the addr
				1916	LOAD (ta), tloaded -- do the original load
				1917	</pre>
				1918	At the point where the <code>LOADV</code> is done, we know the actual
				1919	address (<code>ta</code>) from which the real <code>LOAD</code> will
				1920	be done. We also know that the <code>LOADV</code> will take around
				1921	20 x86 insns to do. So it seems plausible that doing a prefetch of
				1922	<code>ta</code> just before the <code>LOADV</code> might just avoid a
				1923	miss at the <code>LOAD</code> point, and that might be a significant
				1924	performance win.
				1925
				1926	<p>
				1927	Prefetch insns are notoriously tempermental, more often than not
				1928	making things worse rather than better, so this would require
				1929	considerable fiddling around. It's complicated because Intels and
				1930	AMDs have different prefetch insns with different semantics, so that
				1931	too needs to be taken into account. As a general rule, even placing
				1932	the prefetches before the <code>LOADV</code> insn is too near the
				1933	<code>LOAD</code>; the ideal distance is apparently circa 200 CPU
				1934	cycles. So it might be worth having another analysis/transformation
				1935	pass which pushes prefetches as far back as possible, hopefully
				1936	immediately after the effective address becomes available.
				1937
				1938	<p>
				1939	Doing too many prefetches is also bad because they soak up bus
				1940	bandwidth / cpu resources, so some cleverness in deciding which loads
				1941	to prefetch and which to not might be helpful. One can imagine not
				1942	prefetching client-stack-relative (<code>%EBP</code> or
				1943	<code>%ESP</code>) accesses, since the stack in general tends to show
				1944	good locality anyway.
				1945
				1946	<p>
				1947	There's quite a lot of experimentation to do here, but I think it
				1948	might make an interesting week's work for someone.
				1949
				1950	<p>
				1951	As of 15-ish March 2002, I've started to experiment with this, using
				1952	the AMD <code>prefetch/prefetchw</code> insns.
				1953
				1954
				1955
				1956	<h3>User-defined permission ranges</h3>
				1957
				1958	This is quite a large project -- perhaps a month's hacking for a
				1959	capable hacker to do a good job -- but it's potentially very
				1960	interesting. The outcome would be that Valgrind could detect a
				1961	whole class of bugs which it currently cannot.
				1962
				1963	<p>
				1964	The presentation falls into two pieces.
				1965
				1966	<p>
				1967	<b>Part 1: user-defined address-range permission setting</b>
				1968	<p>
				1969
				1970	Valgrind intercepts the client's <code>malloc</code>,
				1971	<code>free</code>, etc calls, watches system calls, and watches the
				1972	stack pointer move. This is currently the only way it knows about
				1973	which addresses are valid and which not. Sometimes the client program
				1974	knows extra information about its memory areas. For example, the
				1975	client could at some point know that all elements of an array are
				1976	out-of-date. We would like to be able to convey to Valgrind this
				1977	information that the array is now addressable-but-uninitialised, so
				1978	that Valgrind can then warn if elements are used before they get new
				1979	values.
				1980
				1981	<p>
				1982	What I would like are some macros like this:
				1983	<pre>
				1984	VALGRIND_MAKE_NOACCESS(addr, len)
				1985	VALGRIND_MAKE_WRITABLE(addr, len)
				1986	VALGRIND_MAKE_READABLE(addr, len)
				1987	</pre>
				1988	and also, to check that memory is addressible/initialised,
				1989	<pre>
				1990	VALGRIND_CHECK_ADDRESSIBLE(addr, len)
				1991	VALGRIND_CHECK_INITIALISED(addr, len)
				1992	</pre>
				1993
				1994	<p>
				1995	I then include in my sources a header defining these macros, rebuild
				1996	my app, run under Valgrind, and get user-defined checks.
				1997
				1998	<p>
				1999	Now here's a neat trick. It's a nuisance to have to re-link the app
				2000	with some new library which implements the above macros. So the idea
				2001	is to define the macros so that the resulting executable is still
				2002	completely stand-alone, and can be run without Valgrind, in which case
				2003	the macros do nothing, but when run on Valgrind, the Right Thing
				2004	happens. How to do this? The idea is for these macros to turn into a
				2005	piece of inline assembly code, which (1) has no effect when run on the
				2006	real CPU, (2) is easily spotted by Valgrind's JITter, and (3) no sane
				2007	person would ever write, which is important for avoiding false matches
				2008	in (2). So here's a suggestion:
				2009	<pre>
				2010	VALGRIND_MAKE_NOACCESS(addr, len)
				2011	</pre>
				2012	becomes (roughly speaking)
				2013	<pre>
				2014	movl addr, %eax
				2015	movl len, %ebx
				2016	movl $1, %ecx -- 1 describes the action; MAKE_WRITABLE might be
				2017	-- 2, etc
				2018	rorl $13, %ecx
				2019	rorl $19, %ecx
				2020	rorl $11, %eax
				2021	rorl $21, %eax
				2022	</pre>
				2023	The rotate sequences have no effect, and it's unlikely they would
				2024	appear for any other reason, but they define a unique byte-sequence
				2025	which the JITter can easily spot. Using the operand constraints
				2026	section at the end of a gcc inline-assembly statement, we can tell gcc
				2027	that the assembly fragment kills <code>%eax</code>, <code>%ebx</code>,
				2028	<code>%ecx</code> and the condition codes, so this fragment is made
				2029	harmless when not running on Valgrind, runs quickly when not on
				2030	Valgrind, and does not require any other library support.
				2031
				2032
				2033	<p>
				2034	<b>Part 2: using it to detect interference between stack variables</b>
				2035	<p>
				2036
				2037	Currently Valgrind cannot detect errors of the following form:
				2038	<pre>
				2039	void fooble ( void )
				2040	{
				2041	int a[10];
				2042	int b[10];
				2043	a[10] = 99;
				2044	}
				2045	</pre>
				2046	Now imagine rewriting this as
				2047	<pre>
				2048	void fooble ( void )
				2049	{
				2050	int spacer0;
				2051	int a[10];
				2052	int spacer1;
				2053	int b[10];
				2054	int spacer2;
				2055	VALGRIND_MAKE_NOACCESS(&spacer0, sizeof(int));
				2056	VALGRIND_MAKE_NOACCESS(&spacer1, sizeof(int));
				2057	VALGRIND_MAKE_NOACCESS(&spacer2, sizeof(int));
				2058	a[10] = 99;
				2059	}
				2060	</pre>
				2061	Now the invalid write is certain to hit <code>spacer0</code> or
				2062	<code>spacer1</code>, so Valgrind will spot the error.
				2063
				2064	<p>
				2065	There are two complications.
				2066
				2067	<p>
				2068	The first is that we don't want to annotate sources by hand, so the
				2069	Right Thing to do is to write a C/C++ parser, annotator, prettyprinter
				2070	which does this automatically, and run it on post-CPP'd C/C++ source.
				2071	See http://www.cacheprof.org for an example of a system which
				2072	transparently inserts another phase into the gcc/g++ compilation
				2073	route. The parser/prettyprinter is probably not as hard as it sounds;
				2074	I would write it in Haskell, a powerful functional language well
				2075	suited to doing symbolic computation, with which I am intimately
				2076	familar. There is already a C parser written in Haskell by someone in
				2077	the Haskell community, and that would probably be a good starting
				2078	point.
				2079
				2080	<p>
				2081	The second complication is how to get rid of these
				2082	<code>NOACCESS</code> records inside Valgrind when the instrumented
				2083	function exits; after all, these refer to stack addresses and will
				2084	make no sense whatever when some other function happens to re-use the
				2085	same stack address range, probably shortly afterwards. I think I
				2086	would be inclined to define a special stack-specific macro
				2087	<pre>
				2088	VALGRIND_MAKE_NOACCESS_STACK(addr, len)
				2089	</pre>
				2090	which causes Valgrind to record the client's <code>%ESP</code> at the
				2091	time it is executed. Valgrind will then watch for changes in
				2092	<code>%ESP</code> and discard such records as soon as the protected
				2093	area is uncovered by an increase in <code>%ESP</code>. I hesitate
				2094	with this scheme only because it is potentially expensive, if there
				2095	are hundreds of such records, and considering that changes in
				2096	<code>%ESP</code> already require expensive messing with stack access
				2097	permissions.
				2098
				2099	<p>
				2100	This is probably easier and more robust than for the instrumenter
				2101	program to try and spot all exit points for the procedure and place
				2102	suitable deallocation annotations there. Plus C++ procedures can
				2103	bomb out at any point if they get an exception, so spotting return
				2104	points at the source level just won't work at all.
				2105
				2106	<p>
				2107	Although some work, it's all eminently doable, and it would make
				2108	Valgrind into an even-more-useful tool.
				2109
				2110	<p>
				2111	Update: as of 17 March 2002, this (these hooks) are done.
				2112
				2113
				2114	<p>
				2115	</body>
				2116	</html>