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10
11<div class="doc_title">
12 Writing an LLVM Pass
13</div>
14
15<ol>
16 <li><a href="#introduction">Introduction - What is a pass?</a></li>
17 <li><a href="#quickstart">Quick Start - Writing hello world</a>
18 <ul>
19 <li><a href="#makefile">Setting up the build environment</a></li>
20 <li><a href="#basiccode">Basic code required</a></li>
21 <li><a href="#running">Running a pass with <tt>opt</tt></a></li>
22 </ul></li>
23 <li><a href="#passtype">Pass classes and requirements</a>
24 <ul>
25 <li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li>
26 <li><a href="#ModulePass">The <tt>ModulePass</tt> class</a>
27 <ul>
28 <li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li>
29 </ul></li>
30 <li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
31 <ul>
32 <li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph
33 &amp;)</tt> method</a></li>
34 <li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li>
35 <li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph
36 &amp;)</tt> method</a></li>
37 </ul></li>
38 <li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
39 <ul>
40 <li><a href="#doInitialization_mod">The <tt>doInitialization(Module
41 &amp;)</tt> method</a></li>
42 <li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li>
43 <li><a href="#doFinalization_mod">The <tt>doFinalization(Module
44 &amp;)</tt> method</a></li>
45 </ul></li>
46 <li><a href="#LoopPass">The <tt>LoopPass</tt> class</a>
47 <ul>
48 <li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *,
49 LPPassManager &amp;)</tt> method</a></li>
50 <li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li>
51 <li><a href="#doFinalization_loop">The <tt>doFinalization()
52 </tt> method</a></li>
53 </ul></li>
54 <li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
55 <ul>
56 <li><a href="#doInitialization_fn">The <tt>doInitialization(Function
57 &amp;)</tt> method</a></li>
58 <li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt>
59 method</a></li>
60 <li><a href="#doFinalization_fn">The <tt>doFinalization(Function
61 &amp;)</tt> method</a></li>
62 </ul></li>
63 <li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt>
64 class</a>
65 <ul>
66 <li><a href="#runOnMachineFunction">The
67 <tt>runOnMachineFunction(MachineFunction &amp;)</tt> method</a></li>
68 </ul></li>
69 </ul>
70 <li><a href="#registration">Pass Registration</a>
71 <ul>
72 <li><a href="#print">The <tt>print</tt> method</a></li>
73 </ul></li>
74 <li><a href="#interaction">Specifying interactions between passes</a>
75 <ul>
76 <li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt>
77 method</a></li>
78 <li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired&lt;&gt;</tt> and <tt>AnalysisUsage::addRequiredTransitive&lt;&gt;</tt> methods</a></li>
79 <li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved&lt;&gt;</tt> method</a></li>
80 <li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
81 <li><a href="#getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> and <tt>getAnalysisToUpdate&lt;&gt;</tt> methods</a></li>
82 </ul></li>
83 <li><a href="#analysisgroup">Implementing Analysis Groups</a>
84 <ul>
85 <li><a href="#agconcepts">Analysis Group Concepts</a></li>
86 <li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li>
87 </ul></li>
88 <li><a href="#passStatistics">Pass Statistics</a>
89 <li><a href="#passmanager">What PassManager does</a>
90 <ul>
91 <li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li>
92 </ul></li>
93 <li><a href="#registering">Registering dynamically loaded passes</a>
94 <ul>
95 <li><a href="#registering_existing">Using existing registries</a></li>
96 <li><a href="#registering_new">Creating new registries</a></li>
97 </ul></li>
98 <li><a href="#debughints">Using GDB with dynamically loaded passes</a>
99 <ul>
100 <li><a href="#breakpoint">Setting a breakpoint in your pass</a></li>
101 <li><a href="#debugmisc">Miscellaneous Problems</a></li>
102 </ul></li>
103 <li><a href="#future">Future extensions planned</a>
104 <ul>
105 <li><a href="#SMP">Multithreaded LLVM</a></li>
106 </ul></li>
107</ol>
108
109<div class="doc_author">
110 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
111 <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
112</div>
113
114<!-- *********************************************************************** -->
115<div class="doc_section">
116 <a name="introduction">Introduction - What is a pass?</a>
117</div>
118<!-- *********************************************************************** -->
119
120<div class="doc_text">
121
122<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM
123passes are where most of the interesting parts of the compiler exist. Passes
124perform the transformations and optimizations that make up the compiler, they
125build the analysis results that are used by these transformations, and they are,
126above all, a structuring technique for compiler code.</p>
127
128<p>All LLVM passes are subclasses of the <tt><a
129href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>
130class, which implement functionality by overriding virtual methods inherited
131from <tt>Pass</tt>. Depending on how your pass works, you should inherit from
132the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a
133href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a
134href="#FunctionPass">FunctionPass</a></tt>, or <tt><a
135href="#LoopPass">LoopPass</a></tt>, or <tt><a
136href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system
137more information about what your pass does, and how it can be combined with
138other passes. One of the main features of the LLVM Pass Framework is that it
139schedules passes to run in an efficient way based on the constraints that your
140pass meets (which are indicated by which class they derive from).</p>
141
142<p>We start by showing you how to construct a pass, everything from setting up
143the code, to compiling, loading, and executing it. After the basics are down,
144more advanced features are discussed.</p>
145
146</div>
147
148<!-- *********************************************************************** -->
149<div class="doc_section">
150 <a name="quickstart">Quick Start - Writing hello world</a>
151</div>
152<!-- *********************************************************************** -->
153
154<div class="doc_text">
155
156<p>Here we describe how to write the "hello world" of passes. The "Hello" pass
157is designed to simply print out the name of non-external functions that exist in
158the program being compiled. It does not modify the program at all, it just
159inspects it. The source code and files for this pass are available in the LLVM
160source tree in the <tt>lib/Transforms/Hello</tt> directory.</p>
161
162</div>
163
164<!-- ======================================================================= -->
165<div class="doc_subsection">
166 <a name="makefile">Setting up the build environment</a>
167</div>
168
169<div class="doc_text">
170
171 <p>First, you need to create a new directory somewhere in the LLVM source
172 base. For this example, we'll assume that you made
173 <tt>lib/Transforms/Hello</tt>. Next, you must set up a build script
174 (Makefile) that will compile the source code for the new pass. To do this,
175 copy the following into <tt>Makefile</tt>:</p>
176 <hr/>
177
178<div class="doc_code"><pre>
179# Makefile for hello pass
180
181# Path to top level of LLVM heirarchy
182LEVEL = ../../..
183
184# Name of the library to build
185LIBRARYNAME = Hello
186
187# Make the shared library become a loadable module so the tools can
188# dlopen/dlsym on the resulting library.
189LOADABLE_MODULE = 1
190
191# Tell the build system which LLVM libraries your pass needs. You'll probably
192# need at least LLVMSystem.a, LLVMSupport.a, LLVMCore.a but possibly several
193# others too.
194LLVMLIBS = LLVMCore.a LLVMSupport.a LLVMSystem.a
195
196# Include the makefile implementation stuff
197include $(LEVEL)/Makefile.common
198</pre></div>
199
200<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current
201directory are to be compiled and linked together into a
202<tt>Debug/lib/Hello.so</tt> shared object that can be dynamically loaded by
203the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options.
204If your operating system uses a suffix other than .so (such as windows or
205Mac OS/X), the appropriate extension will be used.</p>
206
207<p>Now that we have the build scripts set up, we just need to write the code for
208the pass itself.</p>
209
210</div>
211
212<!-- ======================================================================= -->
213<div class="doc_subsection">
214 <a name="basiccode">Basic code required</a>
215</div>
216
217<div class="doc_text">
218
219<p>Now that we have a way to compile our new pass, we just have to write it.
220Start out with:</p>
221
222<div class="doc_code"><pre>
223<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
224<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
225</pre></div>
226
227<p>Which are needed because we are writing a <tt><a
228href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, and
229we are operating on <tt><a
230href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s.</p>
231
232<p>Next we have:</p>
233<div class="doc_code"><pre>
234<b>using namespace llvm;</b>
235</pre></div>
236<p>... which is required because the functions from the include files
237live in the llvm namespace.
238</p>
239
240<p>Next we have:</p>
241
242<div class="doc_code"><pre>
243<b>namespace</b> {
244</pre></div>
245
246<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++
247what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
248things declared inside of the anonymous namespace only visible to the current
249file. If you're not familiar with them, consult a decent C++ book for more
250information.</p>
251
252<p>Next, we declare our pass itself:</p>
253
254<div class="doc_code"><pre>
255 <b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
256</pre></div><p>
257
258<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
259href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
260The different builtin pass subclasses are described in detail <a
261href="#passtype">later</a>, but for now, know that <a
262href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate a function at a
263time.</p>
264
265<div class="doc_code"><pre>
266 static char ID;
267 Hello() : FunctionPass((intptr_t)&amp;ID) {}
268</pre></div><p>
269
270<p> This declares pass identifier used by LLVM to identify pass. This allows LLVM to
271avoid using expensive C++ runtime information.</p>
272
273<div class="doc_code"><pre>
274 <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &amp;F) {
275 llvm::cerr &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
276 <b>return false</b>;
277 }
278 }; <i>// end of struct Hello</i>
279</pre></div>
280
281<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method,
282which overloads an abstract virtual method inherited from <a
283href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed
284to do our thing, so we just print out our message with the name of each
285function.</p>
286
287<div class="doc_code"><pre>
288 char Hello::ID = 0;
289</pre></div>
290
291<p> We initialize pass ID here. LLVM uses ID's address to identify pass so
292initialization value is not important.</p>
293
294<div class="doc_code"><pre>
295 RegisterPass&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>");
296} <i>// end of anonymous namespace</i>
297</pre></div>
298
299<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
300giving it a command line
301argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>".</p>
302
303<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
304
305<div class="doc_code"><pre>
306<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
307<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
308
309<b>using namespace llvm;</b>
310
311<b>namespace</b> {
312 <b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
313
314 static char ID;
315 Hello() : FunctionPass((intptr_t)&amp;ID) {}
316
317 <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &amp;F) {
318 llvm::cerr &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
319 <b>return false</b>;
320 }
321 };
322
Devang Patel8e46f052007-07-25 21:05:39 +0000[diff] [blame]323 char Hello::ID = 0;
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]324 RegisterPass&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>");
325}
326</pre></div>
327
328<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
329command in the local directory and you should get a new
330"<tt>Debug/lib/Hello.so</tt> file. Note that everything in this file is
331contained in an anonymous namespace: this reflects the fact that passes are self
332contained units that do not need external interfaces (although they can have
333them) to be useful.</p>
334
335</div>
336
337<!-- ======================================================================= -->
338<div class="doc_subsection">
339 <a name="running">Running a pass with <tt>opt</tt></a>
340</div>
341
342<div class="doc_text">
343
344<p>Now that you have a brand new shiny shared object file, we can use the
345<tt>opt</tt> command to run an LLVM program through your pass. Because you
346registered your pass with the <tt>RegisterPass</tt> template, you will be able to
347use the <tt>opt</tt> tool to access it, once loaded.</p>
348
349<p>To test it, follow the example at the end of the <a
350href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
351LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program
352through our transformation like this (or course, any bitcode file will
353work):</p>
354
355<div class="doc_code"><pre>
356$ opt -load ../../../Debug/lib/Hello.so -hello &lt; hello.bc &gt; /dev/null
357Hello: __main
358Hello: puts
359Hello: main
360</pre></div>
361
362<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your
363pass as a shared object, which makes '<tt>-hello</tt>' a valid command line
364argument (which is one reason you need to <a href="#registration">register your
365pass</a>). Because the hello pass does not modify the program in any
366interesting way, we just throw away the result of <tt>opt</tt> (sending it to
367<tt>/dev/null</tt>).</p>
368
369<p>To see what happened to the other string you registered, try running
370<tt>opt</tt> with the <tt>--help</tt> option:</p>
371
372<div class="doc_code"><pre>
373$ opt -load ../../../Debug/lib/Hello.so --help
374OVERVIEW: llvm .bc -&gt; .bc modular optimizer
375
376USAGE: opt [options] &lt;input bitcode&gt;
377
378OPTIONS:
379 Optimizations available:
380...
381 -funcresolve - Resolve Functions
382 -gcse - Global Common Subexpression Elimination
383 -globaldce - Dead Global Elimination
384 <b>-hello - Hello World Pass</b>
385 -indvars - Canonicalize Induction Variables
386 -inline - Function Integration/Inlining
387 -instcombine - Combine redundant instructions
388...
389</pre></div>
390
391<p>The pass name get added as the information string for your pass, giving some
392documentation to users of <tt>opt</tt>. Now that you have a working pass, you
393would go ahead and make it do the cool transformations you want. Once you get
394it all working and tested, it may become useful to find out how fast your pass
395is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
396line option (<tt>--time-passes</tt>) that allows you to get information about
397the execution time of your pass along with the other passes you queue up. For
398example:</p>
399
400<div class="doc_code"><pre>
401$ opt -load ../../../Debug/lib/Hello.so -hello -time-passes &lt; hello.bc &gt; /dev/null
402Hello: __main
403Hello: puts
404Hello: main
405===============================================================================
406 ... Pass execution timing report ...
407===============================================================================
408 Total Execution Time: 0.02 seconds (0.0479059 wall clock)
409
410 ---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
411 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer
412 0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
413 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
414 <b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
415 0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
416</pre></div>
417
418<p>As you can see, our implementation above is pretty fast :). The additional
419passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
420that the LLVM emitted by your pass is still valid and well formed LLVM, which
421hasn't been broken somehow.</p>
422
423<p>Now that you have seen the basics of the mechanics behind passes, we can talk
424about some more details of how they work and how to use them.</p>
425
426</div>
427
428<!-- *********************************************************************** -->
429<div class="doc_section">
430 <a name="passtype">Pass classes and requirements</a>
431</div>
432<!-- *********************************************************************** -->
433
434<div class="doc_text">
435
436<p>One of the first things that you should do when designing a new pass is to
437decide what class you should subclass for your pass. The <a
438href="#basiccode">Hello World</a> example uses the <tt><a
439href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we
440did not discuss why or when this should occur. Here we talk about the classes
441available, from the most general to the most specific.</p>
442
443<p>When choosing a superclass for your Pass, you should choose the <b>most
444specific</b> class possible, while still being able to meet the requirements
445listed. This gives the LLVM Pass Infrastructure information necessary to
446optimize how passes are run, so that the resultant compiler isn't unneccesarily
447slow.</p>
448
449</div>
450
451<!-- ======================================================================= -->
452<div class="doc_subsection">
453 <a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a>
454</div>
455
456<div class="doc_text">
457
458<p>The most plain and boring type of pass is the "<tt><a
459href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
460class. This pass type is used for passes that do not have to be run, do not
461change state, and never need to be updated. This is not a normal type of
462transformation or analysis, but can provide information about the current
463compiler configuration.</p>
464
465<p>Although this pass class is very infrequently used, it is important for
466providing information about the current target machine being compiled for, and
467other static information that can affect the various transformations.</p>
468
469<p><tt>ImmutablePass</tt>es never invalidate other transformations, are never
470invalidated, and are never "run".</p>
471
472</div>
473
474<!-- ======================================================================= -->
475<div class="doc_subsection">
476 <a name="ModulePass">The <tt>ModulePass</tt> class</a>
477</div>
478
479<div class="doc_text">
480
481<p>The "<tt><a
482href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>"
483class is the most general of all superclasses that you can use. Deriving from
484<tt>ModulePass</tt> indicates that your pass uses the entire program as a unit,
485refering to function bodies in no predictable order, or adding and removing
486functions. Because nothing is known about the behavior of <tt>ModulePass</tt>
487subclasses, no optimization can be done for their execution. A module pass
488can use function level passes (e.g. dominators) using getAnalysis interface
489<tt> getAnalysis&lt;DominatorTree&gt;(Function)</tt>. </p>
490
491<p>To write a correct <tt>ModulePass</tt> subclass, derive from
492<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the
493following signature:</p>
494
495</div>
496
497<!-- _______________________________________________________________________ -->
498<div class="doc_subsubsection">
499 <a name="runOnModule">The <tt>runOnModule</tt> method</a>
500</div>
501
502<div class="doc_text">
503
504<div class="doc_code"><pre>
505 <b>virtual bool</b> runOnModule(Module &amp;M) = 0;
506</pre></div>
507
508<p>The <tt>runOnModule</tt> method performs the interesting work of the pass.
509It should return true if the module was modified by the transformation and
510false otherwise.</p>
511
512</div>
513
514<!-- ======================================================================= -->
515<div class="doc_subsection">
516 <a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
517</div>
518
519<div class="doc_text">
520
521<p>The "<tt><a
522href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>"
523is used by passes that need to traverse the program bottom-up on the call graph
524(callees before callers). Deriving from CallGraphSCCPass provides some
525mechanics for building and traversing the CallGraph, but also allows the system
526to optimize execution of CallGraphSCCPass's. If your pass meets the
527requirements outlined below, and doesn't meet the requirements of a <tt><a
528href="#FunctionPass">FunctionPass</a></tt> or <tt><a
529href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from
530<tt>CallGraphSCCPass</tt>.</p>
531
532<p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p>
533
534<p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p>
535
536<ol>
537
538<li>... <em>not allowed</em> to modify any <tt>Function</tt>s that are not in
539the current SCC.</li>
540
541<li>... <em>not allowed</em> to inspect any Function's other than those in the
542current SCC and the direct callees of the SCC.</li>
543
544<li>... <em>required</em> to preserve the current CallGraph object, updating it
545to reflect any changes made to the program.</li>
546
547<li>... <em>not allowed</em> to add or remove SCC's from the current Module,
548though they may change the contents of an SCC.</li>
549
550<li>... <em>allowed</em> to add or remove global variables from the current
551Module.</li>
552
553<li>... <em>allowed</em> to maintain state across invocations of
554 <a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li>
555</ol>
556
557<p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases
558because it has to handle SCCs with more than one node in it. All of the virtual
559methods described below should return true if they modified the program, or
560false if they didn't.</p>
561
562</div>
563
564<!-- _______________________________________________________________________ -->
565<div class="doc_subsubsection">
566 <a name="doInitialization_scc">The <tt>doInitialization(CallGraph &amp;)</tt>
567 method</a>
568</div>
569
570<div class="doc_text">
571
572<div class="doc_code"><pre>
573 <b>virtual bool</b> doInitialization(CallGraph &amp;CG);
574</pre></div>
575
576<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
577<tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove
578functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
579is designed to do simple initialization type of stuff that does not depend on
580the SCCs being processed. The <tt>doInitialization</tt> method call is not
581scheduled to overlap with any other pass executions (thus it should be very
582fast).</p>
583
584</div>
585
586<!-- _______________________________________________________________________ -->
587<div class="doc_subsubsection">
588 <a name="runOnSCC">The <tt>runOnSCC</tt> method</a>
589</div>
590
591<div class="doc_text">
592
593<div class="doc_code"><pre>
594 <b>virtual bool</b> runOnSCC(const std::vector&lt;CallGraphNode *&gt; &amp;SCCM) = 0;
595</pre></div>
596
597<p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and
598should return true if the module was modified by the transformation, false
599otherwise.</p>
600
601</div>
602
603<!-- _______________________________________________________________________ -->
604<div class="doc_subsubsection">
605 <a name="doFinalization_scc">The <tt>doFinalization(CallGraph
606 &amp;)</tt> method</a>
607</div>
608
609<div class="doc_text">
610
611<div class="doc_code"><pre>
612 <b>virtual bool</b> doFinalization(CallGraph &amp;CG);
613</pre></div>
614
615<p>The <tt>doFinalization</tt> method is an infrequently used method that is
616called when the pass framework has finished calling <a
617href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
618program being compiled.</p>
619
620</div>
621
622<!-- ======================================================================= -->
623<div class="doc_subsection">
624 <a name="FunctionPass">The <tt>FunctionPass</tt> class</a>
625</div>
626
627<div class="doc_text">
628
629<p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a
630href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
631subclasses do have a predictable, local behavior that can be expected by the
632system. All <tt>FunctionPass</tt> execute on each function in the program
633independent of all of the other functions in the program.
634<tt>FunctionPass</tt>'s do not require that they are executed in a particular
635order, and <tt>FunctionPass</tt>'s do not modify external functions.</p>
636
637<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p>
638
639<ol>
640<li>Modify a Function other than the one currently being processed.</li>
641<li>Add or remove Function's from the current Module.</li>
642<li>Add or remove global variables from the current Module.</li>
643<li>Maintain state across invocations of
644 <a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li>
645</ol>
646
647<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
648href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s
649may overload three virtual methods to do their work. All of these methods
650should return true if they modified the program, or false if they didn't.</p>
651
652</div>
653
654<!-- _______________________________________________________________________ -->
655<div class="doc_subsubsection">
656 <a name="doInitialization_mod">The <tt>doInitialization(Module &amp;)</tt>
657 method</a>
658</div>
659
660<div class="doc_text">
661
662<div class="doc_code"><pre>
663 <b>virtual bool</b> doInitialization(Module &amp;M);
664</pre></div>
665
666<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
667<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove
668functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
669is designed to do simple initialization type of stuff that does not depend on
670the functions being processed. The <tt>doInitialization</tt> method call is not
671scheduled to overlap with any other pass executions (thus it should be very
672fast).</p>
673
674<p>A good example of how this method should be used is the <a
675href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
676pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
677platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It
678uses the <tt>doInitialization</tt> method to get a reference to the malloc and
679free functions that it needs, adding prototypes to the module if necessary.</p>
680
681</div>
682
683<!-- _______________________________________________________________________ -->
684<div class="doc_subsubsection">
685 <a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
686</div>
687
688<div class="doc_text">
689
690<div class="doc_code"><pre>
691 <b>virtual bool</b> runOnFunction(Function &amp;F) = 0;
692</pre></div><p>
693
694<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do
695the transformation or analysis work of your pass. As usual, a true value should
696be returned if the function is modified.</p>
697
698</div>
699
700<!-- _______________________________________________________________________ -->
701<div class="doc_subsubsection">
702 <a name="doFinalization_mod">The <tt>doFinalization(Module
703 &amp;)</tt> method</a>
704</div>
705
706<div class="doc_text">
707
708<div class="doc_code"><pre>
709 <b>virtual bool</b> doFinalization(Module &amp;M);
710</pre></div>
711
712<p>The <tt>doFinalization</tt> method is an infrequently used method that is
713called when the pass framework has finished calling <a
714href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
715program being compiled.</p>
716
717</div>
718
719<!-- ======================================================================= -->
720<div class="doc_subsection">
721 <a name="LoopPass">The <tt>LoopPass</tt> class </a>
722</div>
723
724<div class="doc_text">
725
726<p> All <tt>LoopPass</tt> execute on each loop in the function independent of
727all of the other loops in the function. <tt>LoopPass</tt> processes loops in
728loop nest order such that outer most loop is processed last. </p>
729
730<p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using
731<tt>LPPassManager</tt> interface. Implementing a loop pass is usually
732straightforward. <tt>Looppass</tt>'s may overload three virtual methods to
733do their work. All these methods should return true if they modified the
734program, or false if they didn't. </p>
735</div>
736
737<!-- _______________________________________________________________________ -->
738<div class="doc_subsubsection">
739 <a name="doInitialization_loop">The <tt>doInitialization(Loop *,
740 LPPassManager &amp;)</tt>
741 method</a>
742</div>
743
744<div class="doc_text">
745
746<div class="doc_code"><pre>
747 <b>virtual bool</b> doInitialization(Loop *, LPPassManager &amp;LPM);
748</pre></div>
749
750<p>The <tt>doInitialization</tt> method is designed to do simple initialization
751type of stuff that does not depend on the functions being processed. The
752<tt>doInitialization</tt> method call is not scheduled to overlap with any
753other pass executions (thus it should be very fast). LPPassManager
754interface should be used to access Function or Module level analysis
755information.</p>
756
757</div>
758
759
760<!-- _______________________________________________________________________ -->
761<div class="doc_subsubsection">
762 <a name="runOnLoop">The <tt>runOnLoop</tt> method</a>
763</div>
764
765<div class="doc_text">
766
767<div class="doc_code"><pre>
768 <b>virtual bool</b> runOnLoop(Loop *, LPPassManager &amp;LPM) = 0;
769</pre></div><p>
770
771<p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do
772the transformation or analysis work of your pass. As usual, a true value should
773be returned if the function is modified. <tt>LPPassManager</tt> interface
774should be used to update loop nest.</p>
775
776</div>
777
778<!-- _______________________________________________________________________ -->
779<div class="doc_subsubsection">
780 <a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a>
781</div>
782
783<div class="doc_text">
784
785<div class="doc_code"><pre>
786 <b>virtual bool</b> doFinalization();
787</pre></div>
788
789<p>The <tt>doFinalization</tt> method is an infrequently used method that is
790called when the pass framework has finished calling <a
791href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the
792program being compiled. </p>
793
794</div>
795
796
797
798<!-- ======================================================================= -->
799<div class="doc_subsection">
800 <a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
801</div>
802
803<div class="doc_text">
804
805<p><tt>BasicBlockPass</tt>'s are just like <a
806href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
807their scope of inspection and modification to a single basic block at a time.
808As such, they are <b>not</b> allowed to do any of the following:</p>
809
810<ol>
811<li>Modify or inspect any basic blocks outside of the current one</li>
812<li>Maintain state across invocations of
813 <a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li>
814<li>Modify the control flow graph (by altering terminator instructions)</li>
815<li>Any of the things forbidden for
816 <a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li>
817</ol>
818
819<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole"
820optimizations. They may override the same <a
821href="#doInitialization_mod"><tt>doInitialization(Module &amp;)</tt></a> and <a
822href="#doFinalization_mod"><tt>doFinalization(Module &amp;)</tt></a> methods that <a
823href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p>
824
825</div>
826
827<!-- _______________________________________________________________________ -->
828<div class="doc_subsubsection">
829 <a name="doInitialization_fn">The <tt>doInitialization(Function
830 &amp;)</tt> method</a>
831</div>
832
833<div class="doc_text">
834
835<div class="doc_code"><pre>
836 <b>virtual bool</b> doInitialization(Function &amp;F);
837</pre></div>
838
839<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
840<tt>BasicBlockPass</tt>'s are not allowed to do, but that
841<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed
842to do simple initialization that does not depend on the
843BasicBlocks being processed. The <tt>doInitialization</tt> method call is not
844scheduled to overlap with any other pass executions (thus it should be very
845fast).</p>
846
847</div>
848
849<!-- _______________________________________________________________________ -->
850<div class="doc_subsubsection">
851 <a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
852</div>
853
854<div class="doc_text">
855
856<div class="doc_code"><pre>
857 <b>virtual bool</b> runOnBasicBlock(BasicBlock &amp;BB) = 0;
858</pre></div>
859
860<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This
861function is not allowed to inspect or modify basic blocks other than the
862parameter, and are not allowed to modify the CFG. A true value must be returned
863if the basic block is modified.</p>
864
865</div>
866
867<!-- _______________________________________________________________________ -->
868<div class="doc_subsubsection">
869 <a name="doFinalization_fn">The <tt>doFinalization(Function &amp;)</tt>
870 method</a>
871</div>
872
873<div class="doc_text">
874
875<div class="doc_code"><pre>
876 <b>virtual bool</b> doFinalization(Function &amp;F);
877</pre></div>
878
879<p>The <tt>doFinalization</tt> method is an infrequently used method that is
880called when the pass framework has finished calling <a
881href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
882program being compiled. This can be used to perform per-function
883finalization.</p>
884
885</div>
886
887<!-- ======================================================================= -->
888<div class="doc_subsection">
889 <a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a>
890</div>
891
892<div class="doc_text">
893
894<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
895executes on the machine-dependent representation of each LLVM function in the
896program. A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
897the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
898<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
899<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>
900
901<ol>
902<li>Modify any LLVM Instructions, BasicBlocks or Functions.</li>
903<li>Modify a MachineFunction other than the one currently being processed.</li>
904<li>Add or remove MachineFunctions from the current Module.</li>
905<li>Add or remove global variables from the current Module.</li>
906<li>Maintain state across invocations of <a
907href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global
908data)</li>
909</ol>
910
911</div>
912
913<!-- _______________________________________________________________________ -->
914<div class="doc_subsubsection">
915 <a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
916 &amp;MF)</tt> method</a>
917</div>
918
919<div class="doc_text">
920
921<div class="doc_code"><pre>
922 <b>virtual bool</b> runOnMachineFunction(MachineFunction &amp;MF) = 0;
923</pre></div>
924
925<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a
926<tt>MachineFunctionPass</tt>; that is, you should override this method to do the
927work of your <tt>MachineFunctionPass</tt>.</p>
928
929<p>The <tt>runOnMachineFunction</tt> method is called on every
930<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the
931<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent
932representation of the function. If you want to get at the LLVM <tt>Function</tt>
933for the <tt>MachineFunction</tt> you're working on, use
934<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but
935remember, you may not modify the LLVM <tt>Function</tt> or its contents from a
936<tt>MachineFunctionPass</tt>.</p>
937
938</div>
939
940<!-- *********************************************************************** -->
941<div class="doc_section">
942 <a name="registration">Pass registration</a>
943</div>
944<!-- *********************************************************************** -->
945
946<div class="doc_text">
947
948<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how
949pass registration works, and discussed some of the reasons that it is used and
950what it does. Here we discuss how and why passes are registered.</p>
951
952<p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b>
953template, which requires you to pass at least two
954parameters. The first parameter is the name of the pass that is to be used on
955the command line to specify that the pass should be added to a program (for
956example, with <tt>opt</tt> or <tt>bugpoint</tt>). The second argument is the
957name of the pass, which is to be used for the <tt>--help</tt> output of
958programs, as
959well as for debug output generated by the <tt>--debug-pass</tt> option.</p>
960
961<p>If you want your pass to be easily dumpable, you should
962implement the virtual <tt>print</tt> method:</p>
963
964</div>
965
966<!-- _______________________________________________________________________ -->
967<div class="doc_subsubsection">
968 <a name="print">The <tt>print</tt> method</a>
969</div>
970
971<div class="doc_text">
972
973<div class="doc_code"><pre>
974 <b>virtual void</b> print(llvm::OStream &amp;O, <b>const</b> Module *M) <b>const</b>;
975</pre></div>
976
977<p>The <tt>print</tt> method must be implemented by "analyses" in order to print
978a human readable version of the analysis results. This is useful for debugging
979an analysis itself, as well as for other people to figure out how an analysis
980works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p>
981
982<p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on,
983and the <tt>Module</tt> parameter gives a pointer to the top level module of the
984program that has been analyzed. Note however that this pointer may be null in
985certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
986debugger), so it should only be used to enhance debug output, it should not be
987depended on.</p>
988
989</div>
990
991<!-- *********************************************************************** -->
992<div class="doc_section">
993 <a name="interaction">Specifying interactions between passes</a>
994</div>
995<!-- *********************************************************************** -->
996
997<div class="doc_text">
998
999<p>One of the main responsibilities of the <tt>PassManager</tt> is the make sure
1000that passes interact with each other correctly. Because <tt>PassManager</tt>
1001tries to <a href="#passmanager">optimize the execution of passes</a> it must
1002know how the passes interact with each other and what dependencies exist between
1003the various passes. To track this, each pass can declare the set of passes that
1004are required to be executed before the current pass, and the passes which are
1005invalidated by the current pass.</p>
1006
1007<p>Typically this functionality is used to require that analysis results are
1008computed before your pass is run. Running arbitrary transformation passes can
1009invalidate the computed analysis results, which is what the invalidation set
1010specifies. If a pass does not implement the <tt><a
1011href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
1012having any prerequisite passes, and invalidating <b>all</b> other passes.</p>
1013
1014</div>
1015
1016<!-- _______________________________________________________________________ -->
1017<div class="doc_subsubsection">
1018 <a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
1019</div>
1020
1021<div class="doc_text">
1022
1023<div class="doc_code"><pre>
1024 <b>virtual void</b> getAnalysisUsage(AnalysisUsage &amp;Info) <b>const</b>;
1025</pre></div>
1026
1027<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and
1028invalidated sets may be specified for your transformation. The implementation
1029should fill in the <tt><a
1030href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
1031object with information about which passes are required and not invalidated. To
1032do this, a pass may call any of the following methods on the AnalysisUsage
1033object:</p>
1034</div>
1035
1036<!-- _______________________________________________________________________ -->
1037<div class="doc_subsubsection">
1038 <a name="AU::addRequired">The <tt>AnalysisUsage::addRequired&lt;&gt;</tt> and <tt>AnalysisUsage::addRequiredTransitive&lt;&gt;</tt> methods</a>
1039</div>
1040
1041<div class="doc_text">
1042<p>
1043If your pass requires a previous pass to be executed (an analysis for example),
1044it can use one of these methods to arrange for it to be run before your pass.
1045LLVM has many different types of analyses and passes that can be required,
1046spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
1047Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
1048be no critical edges in the CFG when your pass has been run.
1049</p>
1050
1051<p>
1052Some analyses chain to other analyses to do their job. For example, an <a
1053href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
1054href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In
1055cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
1056used instead of the <tt>addRequired</tt> method. This informs the PassManager
1057that the transitively required pass should be alive as long as the requiring
1058pass is.
1059</p>
1060</div>
1061
1062<!-- _______________________________________________________________________ -->
1063<div class="doc_subsubsection">
1064 <a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved&lt;&gt;</tt> method</a>
1065</div>
1066
1067<div class="doc_text">
1068<p>
1069One of the jobs of the PassManager is to optimize how and when analyses are run.
1070In particular, it attempts to avoid recomputing data unless it needs to. For
1071this reason, passes are allowed to declare that they preserve (i.e., they don't
1072invalidate) an existing analysis if it's available. For example, a simple
1073constant folding pass would not modify the CFG, so it can't possibly affect the
1074results of dominator analysis. By default, all passes are assumed to invalidate
1075all others.
1076</p>
1077
1078<p>
1079The <tt>AnalysisUsage</tt> class provides several methods which are useful in
1080certain circumstances that are related to <tt>addPreserved</tt>. In particular,
1081the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
1082not modify the LLVM program at all (which is true for analyses), and the
1083<tt>setPreservesCFG</tt> method can be used by transformations that change
1084instructions in the program but do not modify the CFG or terminator instructions
1085(note that this property is implicitly set for <a
1086href="#BasicBlockPass">BasicBlockPass</a>'s).
1087</p>
1088
1089<p>
1090<tt>addPreserved</tt> is particularly useful for transformations like
1091<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop
1092and dominator related analyses if they exist, so it can preserve them, despite
1093the fact that it hacks on the CFG.
1094</p>
1095</div>
1096
1097<!-- _______________________________________________________________________ -->
1098<div class="doc_subsubsection">
1099 <a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
1100</div>
1101
1102<div class="doc_text">
1103
1104<div class="doc_code"><pre>
1105 <i>// This is an example implementation from an analysis, which does not modify
1106 // the program at all, yet has a prerequisite.</i>
1107 <b>void</b> <a href="http://llvm.org/doxygen/classllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
1108 AU.setPreservesAll();
1109 AU.addRequired&lt;<a href="http://llvm.org/doxygen/classllvm_1_1PostDominatorTree.html">PostDominatorTree</a>&gt;();
1110 }
1111</pre></div>
1112
1113<p>and:</p>
1114
1115<div class="doc_code"><pre>
1116 <i>// This example modifies the program, but does not modify the CFG</i>
1117 <b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
1118 AU.setPreservesCFG();
1119 AU.addRequired&lt;<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>&gt;();
1120 }
1121</pre></div>
1122
1123</div>
1124
1125<!-- _______________________________________________________________________ -->
1126<div class="doc_subsubsection">
1127 <a name="getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> and <tt>getAnalysisToUpdate&lt;&gt;</tt> methods</a>
1128</div>
1129
1130<div class="doc_text">
1131
1132<p>The <tt>Pass::getAnalysis&lt;&gt;</tt> method is automatically inherited by
1133your class, providing you with access to the passes that you declared that you
1134required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
1135method. It takes a single template argument that specifies which pass class you
1136want, and returns a reference to that pass. For example:</p>
1137
1138<div class="doc_code"><pre>
1139 bool LICM::runOnFunction(Function &amp;F) {
1140 LoopInfo &amp;LI = getAnalysis&lt;LoopInfo&gt;();
1141 ...
1142 }
1143</pre></div>
1144
1145<p>This method call returns a reference to the pass desired. You may get a
1146runtime assertion failure if you attempt to get an analysis that you did not
1147declare as required in your <a
1148href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This
1149method can be called by your <tt>run*</tt> method implementation, or by any
1150other local method invoked by your <tt>run*</tt> method.
1151
1152A module level pass can use function level analysis info using this interface.
1153For example:</p>
1154
1155<div class="doc_code"><pre>
1156 bool ModuleLevelPass::runOnModule(Module &amp;M) {
1157 ...
1158 DominatorTree &amp;DT = getAnalysis&lt;DominatorTree&gt;(Func);
1159 ...
1160 }
1161</pre></div>
1162
1163<p>In above example, runOnFunction for DominatorTree is called by pass manager
1164before returning a reference to the desired pass.</p>
1165
1166<p>
1167If your pass is capable of updating analyses if they exist (e.g.,
1168<tt>BreakCriticalEdges</tt>, as described above), you can use the
1169<tt>getAnalysisToUpdate</tt> method, which returns a pointer to the analysis if
1170it is active. For example:</p>
1171
1172<div class="doc_code"><pre>
1173 ...
1174 if (DominatorSet *DS = getAnalysisToUpdate&lt;DominatorSet&gt;()) {
1175 <i>// A DominatorSet is active. This code will update it.</i>
1176 }
1177 ...
1178</pre></div>
1179
1180</div>
1181
1182<!-- *********************************************************************** -->
1183<div class="doc_section">
1184 <a name="analysisgroup">Implementing Analysis Groups</a>
1185</div>
1186<!-- *********************************************************************** -->
1187
1188<div class="doc_text">
1189
Chris Lattner942c3952007-11-16 05:32:05 +0000[diff] [blame]1190<p>Now that we understand the basics of how passes are defined, how they are
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]1191used, and how they are required from other passes, it's time to get a little bit
1192fancier. All of the pass relationships that we have seen so far are very
1193simple: one pass depends on one other specific pass to be run before it can run.
1194For many applications, this is great, for others, more flexibility is
1195required.</p>
1196
1197<p>In particular, some analyses are defined such that there is a single simple
1198interface to the analysis results, but multiple ways of calculating them.
1199Consider alias analysis for example. The most trivial alias analysis returns
1200"may alias" for any alias query. The most sophisticated analysis a
1201flow-sensitive, context-sensitive interprocedural analysis that can take a
1202significant amount of time to execute (and obviously, there is a lot of room
1203between these two extremes for other implementations). To cleanly support
1204situations like this, the LLVM Pass Infrastructure supports the notion of
1205Analysis Groups.</p>
1206
1207</div>
1208
1209<!-- _______________________________________________________________________ -->
1210<div class="doc_subsubsection">
1211 <a name="agconcepts">Analysis Group Concepts</a>
1212</div>
1213
1214<div class="doc_text">
1215
1216<p>An Analysis Group is a single simple interface that may be implemented by
1217multiple different passes. Analysis Groups can be given human readable names
1218just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
1219class. An analysis group may have one or more implementations, one of which is
1220the "default" implementation.</p>
1221
1222<p>Analysis groups are used by client passes just like other passes are: the
1223<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
1224In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
1225scans the available passes to see if any implementations of the analysis group
1226are available. If none is available, the default implementation is created for
1227the pass to use. All standard rules for <A href="#interaction">interaction
1228between passes</a> still apply.</p>
1229
1230<p>Although <a href="#registration">Pass Registration</a> is optional for normal
1231passes, all analysis group implementations must be registered, and must use the
1232<A href="#registerag"><tt>RegisterAnalysisGroup</tt></a> template to join the
1233implementation pool. Also, a default implementation of the interface
1234<b>must</b> be registered with <A
1235href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>
1236
1237<p>As a concrete example of an Analysis Group in action, consider the <a
1238href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
1239analysis group. The default implementation of the alias analysis interface (the
1240<tt><a
1241href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
1242pass) just does a few simple checks that don't require significant analysis to
1243compute (such as: two different globals can never alias each other, etc).
1244Passes that use the <tt><a
1245href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
1246interface (for example the <tt><a
1247href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do
1248not care which implementation of alias analysis is actually provided, they just
1249use the designated interface.</p>
1250
1251<p>From the user's perspective, commands work just like normal. Issuing the
1252command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
1253instantiated and added to the pass sequence. Issuing the command '<tt>opt
1254-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
1255<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
1256hypothetical example) instead.</p>
1257
1258</div>
1259
1260<!-- _______________________________________________________________________ -->
1261<div class="doc_subsubsection">
1262 <a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
1263</div>
1264
1265<div class="doc_text">
1266
1267<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
1268group itself as well as add pass implementations to the analysis group. First,
1269an analysis should be registered, with a human readable name provided for it.
1270Unlike registration of passes, there is no command line argument to be specified
1271for the Analysis Group Interface itself, because it is "abstract":</p>
1272
1273<div class="doc_code"><pre>
1274 <b>static</b> RegisterAnalysisGroup&lt;<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>&gt; A("<i>Alias Analysis</i>");
1275</pre></div>
1276
1277<p>Once the analysis is registered, passes can declare that they are valid
1278implementations of the interface by using the following code:</p>
1279
1280<div class="doc_code"><pre>
1281<b>namespace</b> {
1282 //<i> Analysis Group implementations <b>must</b> be registered normally...</i>
1283 RegisterPass&lt;FancyAA&gt;
1284 B("<i>somefancyaa</i>", "<i>A more complex alias analysis implementation</i>");
1285
1286 //<i> Declare that we implement the AliasAnalysis interface</i>
1287 RegisterAnalysisGroup&lt;<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>&gt; C(B);
1288}
1289</pre></div>
1290
1291<p>This just shows a class <tt>FancyAA</tt> that is registered normally, then
1292uses the <tt>RegisterAnalysisGroup</tt> template to "join" the <tt><a
1293href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
1294analysis group. Every implementation of an analysis group should join using
1295this template. A single pass may join multiple different analysis groups with
1296no problem.</p>
1297
1298<div class="doc_code"><pre>
1299<b>namespace</b> {
1300 //<i> Analysis Group implementations <b>must</b> be registered normally...</i>
1301 RegisterPass&lt;<a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>&gt;
1302 D("<i>basicaa</i>", "<i>Basic Alias Analysis (default AA impl)</i>");
1303
1304 //<i> Declare that we implement the AliasAnalysis interface</i>
1305 RegisterAnalysisGroup&lt;<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <b>true</b>&gt; E(D);
1306}
1307</pre></div>
1308
1309<p>Here we show how the default implementation is specified (using the extra
1310argument to the <tt>RegisterAnalysisGroup</tt> template). There must be exactly
1311one default implementation available at all times for an Analysis Group to be
1312used. Only default implementation can derive from <tt>ImmutablePass</tt>.
1313Here we declare that the
1314 <tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
1315pass is the default implementation for the interface.</p>
1316
1317</div>
1318
1319<!-- *********************************************************************** -->
1320<div class="doc_section">
1321 <a name="passStatistics">Pass Statistics</a>
1322</div>
1323<!-- *********************************************************************** -->
1324
1325<div class="doc_text">
1326<p>The <a
1327href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a>
1328class is designed to be an easy way to expose various success
1329metrics from passes. These statistics are printed at the end of a
1330run, when the -stats command line option is enabled on the command
1331line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details.
1332
1333</div>
1334
1335
1336<!-- *********************************************************************** -->
1337<div class="doc_section">
1338 <a name="passmanager">What PassManager does</a>
1339</div>
1340<!-- *********************************************************************** -->
1341
1342<div class="doc_text">
1343
1344<p>The <a
1345href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
1346<a
1347href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a>
1348takes a list of passes, ensures their <a href="#interaction">prerequisites</a>
1349are set up correctly, and then schedules passes to run efficiently. All of the
1350LLVM tools that run passes use the <tt>PassManager</tt> for execution of these
1351passes.</p>
1352
1353<p>The <tt>PassManager</tt> does two main things to try to reduce the execution
1354time of a series of passes:</p>
1355
1356<ol>
1357<li><b>Share analysis results</b> - The PassManager attempts to avoid
1358recomputing analysis results as much as possible. This means keeping track of
1359which analyses are available already, which analyses get invalidated, and which
1360analyses are needed to be run for a pass. An important part of work is that the
1361<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
1362it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
1363results as soon as they are no longer needed.</li>
1364
1365<li><b>Pipeline the execution of passes on the program</b> - The
1366<tt>PassManager</tt> attempts to get better cache and memory usage behavior out
1367of a series of passes by pipelining the passes together. This means that, given
1368a series of consequtive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
1369will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
1370the first function, then all of the <a
1371href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function,
1372etc... until the entire program has been run through the passes.
1373
1374<p>This improves the cache behavior of the compiler, because it is only touching
1375the LLVM program representation for a single function at a time, instead of
1376traversing the entire program. It reduces the memory consumption of compiler,
1377because, for example, only one <a
1378href="http://llvm.org/doxygen/classllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a>
1379needs to be calculated at a time. This also makes it possible some <a
1380href="#SMP">interesting enhancements</a> in the future.</p></li>
1381
1382</ol>
1383
1384<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how
1385much information it has about the behaviors of the passes it is scheduling. For
1386example, the "preserved" set is intentionally conservative in the face of an
1387unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
1388Not implementing when it should be implemented will have the effect of not
1389allowing any analysis results to live across the execution of your pass.</p>
1390
1391<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
1392options that is useful for debugging pass execution, seeing how things work, and
1393diagnosing when you should be preserving more analyses than you currently are
1394(To get information about all of the variants of the <tt>--debug-pass</tt>
1395option, just type '<tt>opt --help-hidden</tt>').</p>
1396
1397<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see
1398how our <a href="#basiccode">Hello World</a> pass interacts with other passes.
1399Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p>
1400
1401<div class="doc_code"><pre>
1402$ opt -load ../../../Debug/lib/Hello.so -gcse -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
1403Module Pass Manager
1404 Function Pass Manager
1405 Dominator Set Construction
1406 Immediate Dominators Construction
1407 Global Common Subexpression Elimination
1408-- Immediate Dominators Construction
1409-- Global Common Subexpression Elimination
1410 Natural Loop Construction
1411 Loop Invariant Code Motion
1412-- Natural Loop Construction
1413-- Loop Invariant Code Motion
1414 Module Verifier
1415-- Dominator Set Construction
1416-- Module Verifier
1417 Bitcode Writer
1418--Bitcode Writer
1419</pre></div>
1420
1421<p>This output shows us when passes are constructed and when the analysis
1422results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that
1423GCSE uses dominator and immediate dominator information to do its job. The LICM
1424pass uses natural loop information, which uses dominator sets, but not immediate
1425dominators. Because immediate dominators are no longer useful after the GCSE
1426pass, it is immediately destroyed. The dominator sets are then reused to
1427compute natural loop information, which is then used by the LICM pass.</p>
1428
1429<p>After the LICM pass, the module verifier runs (which is automatically added
1430by the '<tt>opt</tt>' tool), which uses the dominator set to check that the
1431resultant LLVM code is well formed. After it finishes, the dominator set
1432information is destroyed, after being computed once, and shared by three
1433passes.</p>
1434
1435<p>Lets see how this changes when we run the <a href="#basiccode">Hello
1436World</a> pass in between the two passes:</p>
1437
1438<div class="doc_code"><pre>
1439$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
1440Module Pass Manager
1441 Function Pass Manager
1442 Dominator Set Construction
1443 Immediate Dominators Construction
1444 Global Common Subexpression Elimination
1445<b>-- Dominator Set Construction</b>
1446-- Immediate Dominators Construction
1447-- Global Common Subexpression Elimination
1448<b> Hello World Pass
1449-- Hello World Pass
1450 Dominator Set Construction</b>
1451 Natural Loop Construction
1452 Loop Invariant Code Motion
1453-- Natural Loop Construction
1454-- Loop Invariant Code Motion
1455 Module Verifier
1456-- Dominator Set Construction
1457-- Module Verifier
1458 Bitcode Writer
1459--Bitcode Writer
1460Hello: __main
1461Hello: puts
1462Hello: main
1463</pre></div>
1464
1465<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
1466Dominator Set pass, even though it doesn't modify the code at all! To fix this,
1467we need to add the following <a
1468href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p>
1469
1470<div class="doc_code"><pre>
1471 <i>// We don't modify the program, so we preserve all analyses</i>
1472 <b>virtual void</b> getAnalysisUsage(AnalysisUsage &amp;AU) <b>const</b> {
1473 AU.setPreservesAll();
1474 }
1475</pre></div>
1476
1477<p>Now when we run our pass, we get this output:</p>
1478
1479<div class="doc_code"><pre>
1480$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure &lt; hello.bc &gt; /dev/null
1481Pass Arguments: -gcse -hello -licm
1482Module Pass Manager
1483 Function Pass Manager
1484 Dominator Set Construction
1485 Immediate Dominators Construction
1486 Global Common Subexpression Elimination
1487-- Immediate Dominators Construction
1488-- Global Common Subexpression Elimination
1489 Hello World Pass
1490-- Hello World Pass
1491 Natural Loop Construction
1492 Loop Invariant Code Motion
1493-- Loop Invariant Code Motion
1494-- Natural Loop Construction
1495 Module Verifier
1496-- Dominator Set Construction
1497-- Module Verifier
1498 Bitcode Writer
1499--Bitcode Writer
1500Hello: __main
1501Hello: puts
1502Hello: main
1503</pre></div>
1504
1505<p>Which shows that we don't accidentally invalidate dominator information
1506anymore, and therefore do not have to compute it twice.</p>
1507
1508</div>
1509
1510<!-- _______________________________________________________________________ -->
1511<div class="doc_subsubsection">
1512 <a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
1513</div>
1514
1515<div class="doc_text">
1516
1517<div class="doc_code"><pre>
1518 <b>virtual void</b> releaseMemory();
1519</pre></div>
1520
1521<p>The <tt>PassManager</tt> automatically determines when to compute analysis
1522results, and how long to keep them around for. Because the lifetime of the pass
1523object itself is effectively the entire duration of the compilation process, we
1524need some way to free analysis results when they are no longer useful. The
1525<tt>releaseMemory</tt> virtual method is the way to do this.</p>
1526
1527<p>If you are writing an analysis or any other pass that retains a significant
1528amount of state (for use by another pass which "requires" your pass and uses the
1529<a href="#getAnalysis">getAnalysis</a> method) you should implement
1530<tt>releaseMEmory</tt> to, well, release the memory allocated to maintain this
1531internal state. This method is called after the <tt>run*</tt> method for the
1532class, before the next call of <tt>run*</tt> in your pass.</p>
1533
1534</div>
1535
1536<!-- *********************************************************************** -->
1537<div class="doc_section">
1538 <a name="registering">Registering dynamically loaded passes</a>
1539</div>
1540<!-- *********************************************************************** -->
1541
1542<div class="doc_text">
1543
1544<p><i>Size matters</i> when constructing production quality tools using llvm,
1545both for the purposes of distribution, and for regulating the resident code size
1546when running on the target system. Therefore, it becomes desirable to
1547selectively use some passes, while omitting others and maintain the flexibility
1548to change configurations later on. You want to be able to do all this, and,
1549provide feedback to the user. This is where pass registration comes into
1550play.</p>
1551
1552<p>The fundamental mechanisms for pass registration are the
1553<tt>MachinePassRegistry</tt> class and subclasses of
1554<tt>MachinePassRegistryNode</tt>.</p>
1555
1556<p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of
1557<tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and
1558communicates additions and deletions to the command line interface.</p>
1559
1560<p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain
1561information provided about a particular pass. This information includes the
1562command line name, the command help string and the address of the function used
1563to create an instance of the pass. A global static constructor of one of these
1564instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>,
1565the static destructor <i>unregisters</i>. Thus a pass that is statically linked
1566in the tool will be registered at start up. A dynamically loaded pass will
1567register on load and unregister at unload.</p>
1568
1569</div>
1570
1571<!-- _______________________________________________________________________ -->
1572<div class="doc_subsection">
1573 <a name="registering_existing">Using existing registries</a>
1574</div>
1575
1576<div class="doc_text">
1577
1578<p>There are predefined registries to track instruction scheduling
1579(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>)
1580machine passes. Here we will describe how to <i>register</i> a register
1581allocator machine pass.</p>
1582
1583<p>Implement your register allocator machine pass. In your register allocator
1584.cpp file add the following include;</p>
1585
1586<div class="doc_code"><pre>
1587 #include "llvm/CodeGen/RegAllocRegistry.h"
1588</pre></div>
1589
1590<p>Also in your register allocator .cpp file, define a creator function in the
1591form; </p>
1592
1593<div class="doc_code"><pre>
1594 FunctionPass *createMyRegisterAllocator() {
1595 return new MyRegisterAllocator();
1596 }
1597</pre></div>
1598
1599<p>Note that the signature of this function should match the type of
1600<tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the
1601"installing" declaration, in the form;</p>
1602
1603<div class="doc_code"><pre>
1604 static RegisterRegAlloc myRegAlloc("myregalloc",
1605 " my register allocator help string",
1606 createMyRegisterAllocator);
1607</pre></div>
1608
1609<p>Note the two spaces prior to the help string produces a tidy result on the
1610--help query.</p>
1611
1612<div class="doc_code"><pre>
1613$ llc --help
1614 ...
1615 -regalloc - Register allocator to use: (default = linearscan)
1616 =linearscan - linear scan register allocator
1617 =local - local register allocator
1618 =simple - simple register allocator
1619 =myregalloc - my register allocator help string
1620 ...
1621</pre></div>
1622
1623<p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as
1624an option. Registering instruction schedulers is similar except use the
1625<tt>RegisterScheduler</tt> class. Note that the
1626<tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from
1627<tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p>
1628
1629<p>To force the load/linking of your register allocator into the llc/lli tools,
1630add your creator function's global declaration to "Passes.h" and add a "pseudo"
1631call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p>
1632
1633</div>
1634
1635
1636<!-- _______________________________________________________________________ -->
1637<div class="doc_subsection">
1638 <a name="registering_new">Creating new registries</a>
1639</div>
1640
1641<div class="doc_text">
1642
1643<p>The easiest way to get started is to clone one of the existing registries; we
1644recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify
1645are the class name and the <tt>FunctionPassCtor</tt> type.</p>
1646
1647<p>Then you need to declare the registry. Example: if your pass registry is
1648<tt>RegisterMyPasses</tt> then define;</p>
1649
1650<div class="doc_code"><pre>
1651MachinePassRegistry RegisterMyPasses::Registry;
1652</pre></div>
1653
1654<p>And finally, declare the command line option for your passes. Example:</p>
1655
1656<div class="doc_code"><pre>
1657 cl::opt&lt;RegisterMyPasses::FunctionPassCtor, false,
1658 RegisterPassParser&lt;RegisterMyPasses&gt &gt
1659 MyPassOpt("mypass",
1660 cl::init(&amp;createDefaultMyPass),
1661 cl::desc("my pass option help"));
1662</pre></div>
1663
1664<p>Here the command option is "mypass", with createDefaultMyPass as the default
1665creator.</p>
1666
1667</div>
1668
1669<!-- *********************************************************************** -->
1670<div class="doc_section">
1671 <a name="debughints">Using GDB with dynamically loaded passes</a>
1672</div>
1673<!-- *********************************************************************** -->
1674
1675<div class="doc_text">
1676
1677<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it
1678should be. First of all, you can't set a breakpoint in a shared object that has
1679not been loaded yet, and second of all there are problems with inlined functions
1680in shared objects. Here are some suggestions to debugging your pass with
1681GDB.</p>
1682
1683<p>For sake of discussion, I'm going to assume that you are debugging a
1684transformation invoked by <tt>opt</tt>, although nothing described here depends
1685on that.</p>
1686
1687</div>
1688
1689<!-- _______________________________________________________________________ -->
1690<div class="doc_subsubsection">
1691 <a name="breakpoint">Setting a breakpoint in your pass</a>
1692</div>
1693
1694<div class="doc_text">
1695
1696<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
1697
1698<div class="doc_code"><pre>
1699$ <b>gdb opt</b>
1700GNU gdb 5.0
1701Copyright 2000 Free Software Foundation, Inc.
1702GDB is free software, covered by the GNU General Public License, and you are
1703welcome to change it and/or distribute copies of it under certain conditions.
1704Type "show copying" to see the conditions.
1705There is absolutely no warranty for GDB. Type "show warranty" for details.
1706This GDB was configured as "sparc-sun-solaris2.6"...
1707(gdb)
1708</pre></div>
1709
1710<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
1711time to load. Be patient. Since we cannot set a breakpoint in our pass yet
1712(the shared object isn't loaded until runtime), we must execute the process, and
1713have it stop before it invokes our pass, but after it has loaded the shared
1714object. The most foolproof way of doing this is to set a breakpoint in
1715<tt>PassManager::run</tt> and then run the process with the arguments you
1716want:</p>
1717
1718<div class="doc_code"><pre>
1719(gdb) <b>break llvm::PassManager::run</b>
1720Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
1721(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]</b>
1722Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]
1723Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
172470 bool PassManager::run(Module &amp;M) { return PM-&gt;run(M); }
1725(gdb)
1726</pre></div>
1727
1728<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are
1729now free to set breakpoints in your pass so that you can trace through execution
1730or do other standard debugging stuff.</p>
1731
1732</div>
1733
1734<!-- _______________________________________________________________________ -->
1735<div class="doc_subsubsection">
1736 <a name="debugmisc">Miscellaneous Problems</a>
1737</div>
1738
1739<div class="doc_text">
1740
1741<p>Once you have the basics down, there are a couple of problems that GDB has,
1742some with solutions, some without.</p>
1743
1744<ul>
1745<li>Inline functions have bogus stack information. In general, GDB does a
1746pretty good job getting stack traces and stepping through inline functions.
1747When a pass is dynamically loaded however, it somehow completely loses this
1748capability. The only solution I know of is to de-inline a function (move it
1749from the body of a class to a .cpp file).</li>
1750
1751<li>Restarting the program breaks breakpoints. After following the information
1752above, you have succeeded in getting some breakpoints planted in your pass. Nex
1753thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
1754and you start getting errors about breakpoints being unsettable. The only way I
1755have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
1756already set in your pass, run the program, and re-set the breakpoints once
1757execution stops in <tt>PassManager::run</tt>.</li>
1758
1759</ul>
1760
1761<p>Hopefully these tips will help with common case debugging situations. If
1762you'd like to contribute some tips of your own, just contact <a
1763href="mailto:sabre@nondot.org">Chris</a>.</p>
1764
1765</div>
1766
1767<!-- *********************************************************************** -->
1768<div class="doc_section">
1769 <a name="future">Future extensions planned</a>
1770</div>
1771<!-- *********************************************************************** -->
1772
1773<div class="doc_text">
1774
1775<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does
1776some nifty stuff, there are things we'd like to add in the future. Here is
1777where we are going:</p>
1778
1779</div>
1780
1781<!-- _______________________________________________________________________ -->
1782<div class="doc_subsubsection">
1783 <a name="SMP">Multithreaded LLVM</a>
1784</div>
1785
1786<div class="doc_text">
1787
1788<p>Multiple CPU machines are becoming more common and compilation can never be
1789fast enough: obviously we should allow for a multithreaded compiler. Because of
1790the semantics defined for passes above (specifically they cannot maintain state
1791across invocations of their <tt>run*</tt> methods), a nice clean way to
1792implement a multithreaded compiler would be for the <tt>PassManager</tt> class
1793to create multiple instances of each pass object, and allow the separate
1794instances to be hacking on different parts of the program at the same time.</p>
1795
1796<p>This implementation would prevent each of the passes from having to implement
1797multithreaded constructs, requiring only the LLVM core to have locking in a few
1798places (for global resources). Although this is a simple extension, we simply
1799haven't had time (or multiprocessor machines, thus a reason) to implement this.
1800Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>
1801
1802</div>
1803
1804<!-- *********************************************************************** -->
1805<hr>
1806<address>
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1811
1812 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
1813 <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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