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Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]11<!--
12
13If Passes.html is up to date, the following "one-liner" should print
14an empty diff.
15
16egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \
17 -e '^ <a name=".*">.*</a>$' < Passes.html >html; \
18perl >help <<'EOT' && diff -u help html; rm -f help html
19open HTML, "<Passes.html" or die "open: Passes.html: $!\n";
20while (<HTML>) {
21 m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next;
22 $order{$1} = sprintf("%03d", 1 + int %order);
23}
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]24open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n";
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]25while (<HELP>) {
26 m:^ -([^ ]+) +- (.*)$: or next;
27 my $o = $order{$1};
28 $o = "000" unless defined $o;
29 push @x, "$o<tr><td><a href=\"#$1\">-$1</a></td><td>$2</td></tr>\n";
30 push @y, "$o <a name=\"$1\">$2</a>\n";
31}
32@x = map { s/^\d\d\d//; $_ } sort @x;
33@y = map { s/^\d\d\d//; $_ } sort @y;
34print @x, @y;
35EOT
36
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]37This (real) one-liner can also be helpful when converting comments to HTML:
38
39perl -e '$/ = undef; for (split(/\n/, <>)) { s:^ *///? ?::; print " <p>\n" if !$on && $_ =~ /\S/; print " </p>\n" if $on && $_ =~ /^\s*$/; print " $_\n"; $on = ($_ =~ /\S/); } print " </p>\n" if $on'
40
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]41 -->
42
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]43<div class="doc_title">LLVM's Analysis and Transform Passes</div>
44
45<ol>
46 <li><a href="#intro">Introduction</a></li>
47 <li><a href="#analyses">Analysis Passes</a>
48 <li><a href="#transforms">Transform Passes</a></li>
49 <li><a href="#utilities">Utility Passes</a></li>
50</ol>
51
52<div class="doc_author">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]53 <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
54 and Gordon Henriksen</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]55</div>
56
57<!-- ======================================================================= -->
58<div class="doc_section"> <a name="intro">Introduction</a> </div>
59<div class="doc_text">
60 <p>This document serves as a high level summary of the optimization features
61 that LLVM provides. Optimizations are implemented as Passes that traverse some
62 portion of a program to either collect information or transform the program.
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]63 The table below divides the passes that LLVM provides into three categories.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]64 Analysis passes compute information that other passes can use or for debugging
65 or program visualization purposes. Transform passes can use (or invalidate)
66 the analysis passes. Transform passes all mutate the program in some way.
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]67 Utility passes provides some utility but don't otherwise fit categorization.
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]68 For example passes to extract functions to bitcode or write a module to
69 bitcode are neither analysis nor transform passes.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]70 <p>The table below provides a quick summary of each pass and links to the more
71 complete pass description later in the document.</p>
72</div>
73<div class="doc_text" >
74<table>
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]75<tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr>
76<tr><th>Option</th><th>Name</th></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]77<tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr>
78<tr><td><a href="#anders-aa">-anders-aa</a></td><td>Andersen's Interprocedural Alias Analysis</td></tr>
79<tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (default AA impl)</td></tr>
80<tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr>
81<tr><td><a href="#basicvn">-basicvn</a></td><td>Basic Value Numbering (default GVN impl)</td></tr>
82<tr><td><a href="#callgraph">-callgraph</a></td><td>Print a call graph</td></tr>
83<tr><td><a href="#callscc">-callscc</a></td><td>Print SCCs of the Call Graph</td></tr>
84<tr><td><a href="#cfgscc">-cfgscc</a></td><td>Print SCCs of each function CFG</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]85<tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]86<tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
87<tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
88<tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]89<tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]90<tr><td><a href="#externalfnconstants">-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
91<tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]92<tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr>
93<tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr>
94<tr><td><a href="#load-vn">-load-vn</a></td><td>Load Value Numbering</td></tr>
95<tr><td><a href="#loops">-loops</a></td><td>Natural Loop Construction</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]96<tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]97<tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
98<tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
99<tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]100<tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]101<tr><td><a href="#print">-print</a></td><td>Print function to stderr</td></tr>
102<tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
103<tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
104<tr><td><a href="#print-cfg">-print-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
105<tr><td><a href="#print-cfg-only">-print-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr>
106<tr><td><a href="#printm">-printm</a></td><td>Print module to stderr</td></tr>
107<tr><td><a href="#printusedtypes">-printusedtypes</a></td><td>Find Used Types</td></tr>
108<tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr>
109<tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr>
110<tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
111
112
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]113<tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr>
114<tr><th>Option</th><th>Name</th></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]115<tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr>
116<tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr>
117<tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]118<tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]119<tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Prepare a function for code generation </td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]120<tr><td><a href="#condprop">-condprop</a></td><td>Conditional Propagation</td></tr>
121<tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr>
122<tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr>
123<tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr>
124<tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr>
125<tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr>
126<tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr>
127<tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr>
128<tr><td><a href="#gcse">-gcse</a></td><td>Global Common Subexpression Elimination</td></tr>
129<tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr>
130<tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr>
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]131<tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]132<tr><td><a href="#gvnpre">-gvnpre</a></td><td>Global Value Numbering/Partial Redundancy Elimination</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]133<tr><td><a href="#indmemrem">-indmemrem</a></td><td>Indirect Malloc and Free Removal</td></tr>
134<tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr>
135<tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr>
136<tr><td><a href="#insert-block-profiling">-insert-block-profiling</a></td><td>Insert instrumentation for block profiling</td></tr>
137<tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr>
138<tr><td><a href="#insert-function-profiling">-insert-function-profiling</a></td><td>Insert instrumentation for function profiling</td></tr>
139<tr><td><a href="#insert-null-profiling-rs">-insert-null-profiling-rs</a></td><td>Measure profiling framework overhead</td></tr>
140<tr><td><a href="#insert-rs-profiling-framework">-insert-rs-profiling-framework</a></td><td>Insert random sampling instrumentation framework</td></tr>
141<tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr>
142<tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr>
143<tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr>
144<tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]145<tr><td><a href="#jump-threading">-jump-threading</a></td><td>Thread control through conditional blocks </td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]146<tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr>
147<tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]148<tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Dead Loop Deletion Pass </td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]149<tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr>
150<tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]151<tr><td><a href="#loop-index-split">-loop-index-split</a></td><td>Index Split Loops</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]152<tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]153<tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr>
154<tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr>
155<tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr>
156<tr><td><a href="#loopsimplify">-loopsimplify</a></td><td>Canonicalize natural loops</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]157<tr><td><a href="#lowerallocs">-lowerallocs</a></td><td>Lower allocations from instructions to calls</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]158<tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]159<tr><td><a href="#lowersetjmp">-lowersetjmp</a></td><td>Lower Set Jump</td></tr>
160<tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr>
161<tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]162<tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>Optimize use of memcpy and friends</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]163<tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]164<tr><td><a href="#predsimplify">-predsimplify</a></td><td>Predicate Simplifier</td></tr>
165<tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr>
166<tr><td><a href="#raiseallocs">-raiseallocs</a></td><td>Raise allocations from calls to instructions</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]167<tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr>
168<tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]169<tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates</td></tr>
170<tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr>
171<tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr>
172<tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr>
173<tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]174<tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Remove unused function declarations</td></tr>
175<tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]176<tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr>
177<tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]178
179
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]180<tr><th colspan="2"><b>UTILITY PASSES</b></th></tr>
181<tr><th>Option</th><th>Name</th></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]182<tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr>
183<tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr>
Gordon Henriksen90a52142007-11-05 02:05:35 +0000[diff] [blame]184<tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]185<tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]186<tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr>
187<tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]188</table>
189</div>
190
191<!-- ======================================================================= -->
192<div class="doc_section"> <a name="example">Analysis Passes</a></div>
193<div class="doc_text">
194 <p>This section describes the LLVM Analysis Passes.</p>
195</div>
196
197<!-------------------------------------------------------------------------- -->
198<div class="doc_subsection">
199 <a name="aa-eval">Exhaustive Alias Analysis Precision Evaluator</a>
200</div>
201<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]202 <p>This is a simple N^2 alias analysis accuracy evaluator.
203 Basically, for each function in the program, it simply queries to see how the
204 alias analysis implementation answers alias queries between each pair of
205 pointers in the function.</p>
206
207 <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco
208 Spadini, and Wojciech Stryjewski.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]209</div>
210
211<!-------------------------------------------------------------------------- -->
212<div class="doc_subsection">
213 <a name="anders-aa">Andersen's Interprocedural Alias Analysis</a>
214</div>
215<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]216 <p>
217 This is an implementation of Andersen's interprocedural alias
218 analysis
219 </p>
220
221 <p>
222 In pointer analysis terms, this is a subset-based, flow-insensitive,
223 field-sensitive, and context-insensitive algorithm pointer algorithm.
224 </p>
225
226 <p>
227 This algorithm is implemented as three stages:
228 </p>
229
230 <ol>
231 <li>Object identification.</li>
232 <li>Inclusion constraint identification.</li>
233 <li>Offline constraint graph optimization.</li>
234 <li>Inclusion constraint solving.</li>
235 </ol>
236
237 <p>
238 The object identification stage identifies all of the memory objects in the
239 program, which includes globals, heap allocated objects, and stack allocated
240 objects.
241 </p>
242
243 <p>
244 The inclusion constraint identification stage finds all inclusion constraints
245 in the program by scanning the program, looking for pointer assignments and
246 other statements that effect the points-to graph. For a statement like
247 <code><var>A</var> = <var>B</var></code>, this statement is processed to
248 indicate that <var>A</var> can point to anything that <var>B</var> can point
249 to. Constraints can handle copies, loads, and stores, and address taking.
250 </p>
251
252 <p>
253 The offline constraint graph optimization portion includes offline variable
254 substitution algorithms intended to computer pointer and location
255 equivalences. Pointer equivalences are those pointers that will have the
256 same points-to sets, and location equivalences are those variables that
257 always appear together in points-to sets.
258 </p>
259
260 <p>
261 The inclusion constraint solving phase iteratively propagates the inclusion
262 constraints until a fixed point is reached. This is an O(<var>n</var>³)
263 algorithm.
264 </p>
265
266 <p>
267 Function constraints are handled as if they were structs with <var>X</var>
268 fields. Thus, an access to argument <var>X</var> of function <var>Y</var> is
269 an access to node index <code>getNode(<var>Y</var>) + <var>X</var></code>.
270 This representation allows handling of indirect calls without any issues. To
271 wit, an indirect call <code><var>Y</var>(<var>a</var>,<var>b</var>)</code> is
272 equivalent to <code>*(<var>Y</var> + 1) = <var>a</var>, *(<var>Y</var> + 2) =
273 <var>b</var></code>. The return node for a function <var>F</var> is always
274 located at <code>getNode(<var>F</var>) + CallReturnPos</code>. The arguments
275 start at <code>getNode(<var>F</var>) + CallArgPos</code>.
276 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]277</div>
278
279<!-------------------------------------------------------------------------- -->
280<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]281 <a name="basicaa">Basic Alias Analysis (default AA impl)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]282</div>
283<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]284 <p>
285 This is the default implementation of the Alias Analysis interface
286 that simply implements a few identities (two different globals cannot alias,
287 etc), but otherwise does no analysis.
288 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]289</div>
290
291<!-------------------------------------------------------------------------- -->
292<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]293 <a name="basiccg">Basic CallGraph Construction</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]294</div>
295<div class="doc_text">
296 <p>Yet to be written.</p>
297</div>
298
299<!-------------------------------------------------------------------------- -->
300<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]301 <a name="basicvn">Basic Value Numbering (default GVN impl)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]302</div>
303<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]304 <p>
305 This is the default implementation of the <code>ValueNumbering</code>
306 interface. It walks the SSA def-use chains to trivially identify
307 lexically identical expressions. This does not require any ahead of time
308 analysis, so it is a very fast default implementation.
309 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]310</div>
311
312<!-------------------------------------------------------------------------- -->
313<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]314 <a name="callgraph">Print a call graph</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]315</div>
316<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]317 <p>
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]318 This pass, only available in <code>opt</code>, prints the call graph to
319 standard output in a human-readable form.
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]320 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]321</div>
322
323<!-------------------------------------------------------------------------- -->
324<div class="doc_subsection">
325 <a name="callscc">Print SCCs of the Call Graph</a>
326</div>
327<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]328 <p>
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]329 This pass, only available in <code>opt</code>, prints the SCCs of the call
330 graph to standard output in a human-readable form.
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]331 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]332</div>
333
334<!-------------------------------------------------------------------------- -->
335<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]336 <a name="cfgscc">Print SCCs of each function CFG</a>
337</div>
338<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]339 <p>
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]340 This pass, only available in <code>opt</code>, prints the SCCs of each
341 function CFG to standard output in a human-readable form.
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]342 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]343</div>
344
345<!-------------------------------------------------------------------------- -->
346<div class="doc_subsection">
347 <a name="codegenprepare">Optimize for code generation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]348</div>
349<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]350 <p>
351 This pass munges the code in the input function to better prepare it for
352 SelectionDAG-based code generation. This works around limitations in it's
353 basic-block-at-a-time approach. It should eventually be removed.
354 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]355</div>
356
357<!-------------------------------------------------------------------------- -->
358<div class="doc_subsection">
359 <a name="count-aa">Count Alias Analysis Query Responses</a>
360</div>
361<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]362 <p>
363 A pass which can be used to count how many alias queries
364 are being made and how the alias analysis implementation being used responds.
365 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]366</div>
367
368<!-------------------------------------------------------------------------- -->
369<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]370 <a name="debug-aa">AA use debugger</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]371</div>
372<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]373 <p>
374 This simple pass checks alias analysis users to ensure that if they
375 create a new value, they do not query AA without informing it of the value.
376 It acts as a shim over any other AA pass you want.
377 </p>
378
379 <p>
380 Yes keeping track of every value in the program is expensive, but this is
381 a debugging pass.
382 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]383</div>
384
385<!-------------------------------------------------------------------------- -->
386<div class="doc_subsection">
387 <a name="domfrontier">Dominance Frontier Construction</a>
388</div>
389<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]390 <p>
391 This pass is a simple dominator construction algorithm for finding forward
392 dominator frontiers.
393 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]394</div>
395
396<!-------------------------------------------------------------------------- -->
397<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]398 <a name="domtree">Dominator Tree Construction</a>
399</div>
400<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]401 <p>
402 This pass is a simple dominator construction algorithm for finding forward
403 dominators.
404 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]405</div>
406
407<!-------------------------------------------------------------------------- -->
408<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]409 <a name="externalfnconstants">Print external fn callsites passed constants</a>
410</div>
411<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]412 <p>
413 This pass, only available in <code>opt</code>, prints out call sites to
414 external functions that are called with constant arguments. This can be
415 useful when looking for standard library functions we should constant fold
416 or handle in alias analyses.
417 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]418</div>
419
420<!-------------------------------------------------------------------------- -->
421<div class="doc_subsection">
422 <a name="globalsmodref-aa">Simple mod/ref analysis for globals</a>
423</div>
424<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]425 <p>
426 This simple pass provides alias and mod/ref information for global values
427 that do not have their address taken, and keeps track of whether functions
428 read or write memory (are "pure"). For this simple (but very common) case,
429 we can provide pretty accurate and useful information.
430 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]431</div>
432
433<!-------------------------------------------------------------------------- -->
434<div class="doc_subsection">
435 <a name="instcount">Counts the various types of Instructions</a>
436</div>
437<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]438 <p>
439 This pass collects the count of all instructions and reports them
440 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]441</div>
442
443<!-------------------------------------------------------------------------- -->
444<div class="doc_subsection">
445 <a name="intervals">Interval Partition Construction</a>
446</div>
447<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]448 <p>
449 This analysis calculates and represents the interval partition of a function,
450 or a preexisting interval partition.
451 </p>
452
453 <p>
454 In this way, the interval partition may be used to reduce a flow graph down
455 to its degenerate single node interval partition (unless it is irreducible).
456 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]457</div>
458
459<!-------------------------------------------------------------------------- -->
460<div class="doc_subsection">
461 <a name="load-vn">Load Value Numbering</a>
462</div>
463<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]464 <p>
465 This pass value numbers load and call instructions. To do this, it finds
466 lexically identical load instructions, and uses alias analysis to determine
467 which loads are guaranteed to produce the same value. To value number call
468 instructions, it looks for calls to functions that do not write to memory
469 which do not have intervening instructions that clobber the memory that is
470 read from.
471 </p>
472
473 <p>
474 This pass builds off of another value numbering pass to implement value
475 numbering for non-load and non-call instructions. It uses Alias Analysis so
476 that it can disambiguate the load instructions. The more powerful these base
477 analyses are, the more powerful the resultant value numbering will be.
478 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]479</div>
480
481<!-------------------------------------------------------------------------- -->
482<div class="doc_subsection">
483 <a name="loops">Natural Loop Construction</a>
484</div>
485<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]486 <p>
487 This analysis is used to identify natural loops and determine the loop depth
488 of various nodes of the CFG. Note that the loops identified may actually be
489 several natural loops that share the same header node... not just a single
490 natural loop.
491 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]492</div>
493
494<!-------------------------------------------------------------------------- -->
495<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]496 <a name="memdep">Memory Dependence Analysis</a>
497</div>
498<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]499 <p>
500 An analysis that determines, for a given memory operation, what preceding
501 memory operations it depends on. It builds on alias analysis information, and
502 tries to provide a lazy, caching interface to a common kind of alias
503 information query.
504 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]505</div>
506
507<!-------------------------------------------------------------------------- -->
508<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]509 <a name="no-aa">No Alias Analysis (always returns 'may' alias)</a>
510</div>
511<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]512 <p>
513 Always returns "I don't know" for alias queries. NoAA is unlike other alias
514 analysis implementations, in that it does not chain to a previous analysis. As
515 such it doesn't follow many of the rules that other alias analyses must.
516 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]517</div>
518
519<!-------------------------------------------------------------------------- -->
520<div class="doc_subsection">
521 <a name="no-profile">No Profile Information</a>
522</div>
523<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]524 <p>
525 The default "no profile" implementation of the abstract
526 <code>ProfileInfo</code> interface.
527 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]528</div>
529
530<!-------------------------------------------------------------------------- -->
531<div class="doc_subsection">
532 <a name="postdomfrontier">Post-Dominance Frontier Construction</a>
533</div>
534<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]535 <p>
536 This pass is a simple post-dominator construction algorithm for finding
537 post-dominator frontiers.
538 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]539</div>
540
541<!-------------------------------------------------------------------------- -->
542<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]543 <a name="postdomtree">Post-Dominator Tree Construction</a>
544</div>
545<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]546 <p>
547 This pass is a simple post-dominator construction algorithm for finding
548 post-dominators.
549 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]550</div>
551
552<!-------------------------------------------------------------------------- -->
553<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]554 <a name="print">Print function to stderr</a>
555</div>
556<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]557 <p>
558 The <code>PrintFunctionPass</code> class is designed to be pipelined with
559 other <code>FunctionPass</code>es, and prints out the functions of the module
560 as they are processed.
561 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]562</div>
563
564<!-------------------------------------------------------------------------- -->
565<div class="doc_subsection">
566 <a name="print-alias-sets">Alias Set Printer</a>
567</div>
568<div class="doc_text">
569 <p>Yet to be written.</p>
570</div>
571
572<!-------------------------------------------------------------------------- -->
573<div class="doc_subsection">
574 <a name="print-callgraph">Print Call Graph to 'dot' file</a>
575</div>
576<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]577 <p>
578 This pass, only available in <code>opt</code>, prints the call graph into a
579 <code>.dot</code> graph. This graph can then be processed with the "dot" tool
580 to convert it to postscript or some other suitable format.
581 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]582</div>
583
584<!-------------------------------------------------------------------------- -->
585<div class="doc_subsection">
586 <a name="print-cfg">Print CFG of function to 'dot' file</a>
587</div>
588<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]589 <p>
590 This pass, only available in <code>opt</code>, prints the control flow graph
591 into a <code>.dot</code> graph. This graph can then be processed with the
592 "dot" tool to convert it to postscript or some other suitable format.
593 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]594</div>
595
596<!-------------------------------------------------------------------------- -->
597<div class="doc_subsection">
598 <a name="print-cfg-only">Print CFG of function to 'dot' file (with no function bodies)</a>
599</div>
600<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]601 <p>
602 This pass, only available in <code>opt</code>, prints the control flow graph
603 into a <code>.dot</code> graph, omitting the function bodies. This graph can
604 then be processed with the "dot" tool to convert it to postscript or some
605 other suitable format.
606 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]607</div>
608
609<!-------------------------------------------------------------------------- -->
610<div class="doc_subsection">
611 <a name="printm">Print module to stderr</a>
612</div>
613<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]614 <p>
615 This pass simply prints out the entire module when it is executed.
616 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]617</div>
618
619<!-------------------------------------------------------------------------- -->
620<div class="doc_subsection">
621 <a name="printusedtypes">Find Used Types</a>
622</div>
623<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]624 <p>
625 This pass is used to seek out all of the types in use by the program. Note
626 that this analysis explicitly does not include types only used by the symbol
627 table.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]628</div>
629
630<!-------------------------------------------------------------------------- -->
631<div class="doc_subsection">
632 <a name="profile-loader">Load profile information from llvmprof.out</a>
633</div>
634<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]635 <p>
636 A concrete implementation of profiling information that loads the information
637 from a profile dump file.
638 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]639</div>
640
641<!-------------------------------------------------------------------------- -->
642<div class="doc_subsection">
643 <a name="scalar-evolution">Scalar Evolution Analysis</a>
644</div>
645<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]646 <p>
647 The <code>ScalarEvolution</code> analysis can be used to analyze and
648 catagorize scalar expressions in loops. It specializes in recognizing general
649 induction variables, representing them with the abstract and opaque
650 <code>SCEV</code> class. Given this analysis, trip counts of loops and other
651 important properties can be obtained.
652 </p>
653
654 <p>
655 This analysis is primarily useful for induction variable substitution and
656 strength reduction.
657 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]658</div>
659
660<!-------------------------------------------------------------------------- -->
661<div class="doc_subsection">
662 <a name="targetdata">Target Data Layout</a>
663</div>
664<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]665 <p>Provides other passes access to information on how the size and alignment
666 required by the the target ABI for various data types.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]667</div>
668
669<!-- ======================================================================= -->
670<div class="doc_section"> <a name="transform">Transform Passes</a></div>
671<div class="doc_text">
672 <p>This section describes the LLVM Transform Passes.</p>
673</div>
674
675<!-------------------------------------------------------------------------- -->
676<div class="doc_subsection">
677 <a name="adce">Aggressive Dead Code Elimination</a>
678</div>
679<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]680 <p>ADCE aggressively tries to eliminate code. This pass is similar to
681 <a href="#dce">DCE</a> but it assumes that values are dead until proven
682 otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to
683 the liveness of values.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]684</div>
685
686<!-------------------------------------------------------------------------- -->
687<div class="doc_subsection">
688 <a name="argpromotion">Promote 'by reference' arguments to scalars</a>
689</div>
690<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]691 <p>
692 This pass promotes "by reference" arguments to be "by value" arguments. In
693 practice, this means looking for internal functions that have pointer
694 arguments. If it can prove, through the use of alias analysis, that an
695 argument is *only* loaded, then it can pass the value into the function
696 instead of the address of the value. This can cause recursive simplification
697 of code and lead to the elimination of allocas (especially in C++ template
698 code like the STL).
699 </p>
700
701 <p>
702 This pass also handles aggregate arguments that are passed into a function,
703 scalarizing them if the elements of the aggregate are only loaded. Note that
704 it refuses to scalarize aggregates which would require passing in more than
705 three operands to the function, because passing thousands of operands for a
706 large array or structure is unprofitable!
707 </p>
708
709 <p>
710 Note that this transformation could also be done for arguments that are only
711 stored to (returning the value instead), but does not currently. This case
712 would be best handled when and if LLVM starts supporting multiple return
713 values from functions.
714 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]715</div>
716
717<!-------------------------------------------------------------------------- -->
718<div class="doc_subsection">
719 <a name="block-placement">Profile Guided Basic Block Placement</a>
720</div>
721<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]722 <p>This pass is a very simple profile guided basic block placement algorithm.
723 The idea is to put frequently executed blocks together at the start of the
724 function and hopefully increase the number of fall-through conditional
725 branches. If there is no profile information for a particular function, this
726 pass basically orders blocks in depth-first order.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]727</div>
728
729<!-------------------------------------------------------------------------- -->
730<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]731 <a name="break-crit-edges">Break critical edges in CFG</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]732</div>
733<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]734 <p>
735 Break all of the critical edges in the CFG by inserting a dummy basic block.
736 It may be "required" by passes that cannot deal with critical edges. This
737 transformation obviously invalidates the CFG, but can update forward dominator
738 (set, immediate dominators, tree, and frontier) information.
739 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]740</div>
741
742<!-------------------------------------------------------------------------- -->
743<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]744 <a name="codegenprepare">Prepare a function for code generation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]745</div>
746<div class="doc_text">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]747 This pass munges the code in the input function to better prepare it for
748 SelectionDAG-based code generation. This works around limitations in it's
749 basic-block-at-a-time approach. It should eventually be removed.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]750</div>
751
752<!-------------------------------------------------------------------------- -->
753<div class="doc_subsection">
754 <a name="condprop">Conditional Propagation</a>
755</div>
756<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]757 <p>This pass propagates information about conditional expressions through the
758 program, allowing it to eliminate conditional branches in some cases.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]759</div>
760
761<!-------------------------------------------------------------------------- -->
762<div class="doc_subsection">
763 <a name="constmerge">Merge Duplicate Global Constants</a>
764</div>
765<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]766 <p>
767 Merges duplicate global constants together into a single constant that is
768 shared. This is useful because some passes (ie TraceValues) insert a lot of
769 string constants into the program, regardless of whether or not an existing
770 string is available.
771 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]772</div>
773
774<!-------------------------------------------------------------------------- -->
775<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]776 <a name="constprop">Simple constant propagation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]777</div>
778<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]779 <p>This file implements constant propagation and merging. It looks for
780 instructions involving only constant operands and replaces them with a
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]781 constant value instead of an instruction. For example:</p>
782 <blockquote><pre>add i32 1, 2</pre></blockquote>
783 <p>becomes</p>
784 <blockquote><pre>i32 3</pre></blockquote>
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]785 <p>NOTE: this pass has a habit of making definitions be dead. It is a good
786 idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass
787 sometime after running this pass.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]788</div>
789
790<!-------------------------------------------------------------------------- -->
791<div class="doc_subsection">
792 <a name="dce">Dead Code Elimination</a>
793</div>
794<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]795 <p>
796 Dead code elimination is similar to <a href="#die">dead instruction
797 elimination</a>, but it rechecks instructions that were used by removed
798 instructions to see if they are newly dead.
799 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]800</div>
801
802<!-------------------------------------------------------------------------- -->
803<div class="doc_subsection">
804 <a name="deadargelim">Dead Argument Elimination</a>
805</div>
806<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]807 <p>
808 This pass deletes dead arguments from internal functions. Dead argument
809 elimination removes arguments which are directly dead, as well as arguments
810 only passed into function calls as dead arguments of other functions. This
811 pass also deletes dead arguments in a similar way.
812 </p>
813
814 <p>
815 This pass is often useful as a cleanup pass to run after aggressive
816 interprocedural passes, which add possibly-dead arguments.
817 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]818</div>
819
820<!-------------------------------------------------------------------------- -->
821<div class="doc_subsection">
822 <a name="deadtypeelim">Dead Type Elimination</a>
823</div>
824<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]825 <p>
826 This pass is used to cleanup the output of GCC. It eliminate names for types
827 that are unused in the entire translation unit, using the <a
828 href="#findusedtypes">find used types</a> pass.
829 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]830</div>
831
832<!-------------------------------------------------------------------------- -->
833<div class="doc_subsection">
834 <a name="die">Dead Instruction Elimination</a>
835</div>
836<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]837 <p>
838 Dead instruction elimination performs a single pass over the function,
839 removing instructions that are obviously dead.
840 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]841</div>
842
843<!-------------------------------------------------------------------------- -->
844<div class="doc_subsection">
845 <a name="dse">Dead Store Elimination</a>
846</div>
847<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]848 <p>
849 A trivial dead store elimination that only considers basic-block local
850 redundant stores.
851 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]852</div>
853
854<!-------------------------------------------------------------------------- -->
855<div class="doc_subsection">
856 <a name="gcse">Global Common Subexpression Elimination</a>
857</div>
858<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]859 <p>
860 This pass is designed to be a very quick global transformation that
861 eliminates global common subexpressions from a function. It does this by
862 using an existing value numbering implementation to identify the common
863 subexpressions, eliminating them when possible.
864 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]865</div>
866
867<!-------------------------------------------------------------------------- -->
868<div class="doc_subsection">
869 <a name="globaldce">Dead Global Elimination</a>
870</div>
871<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]872 <p>
873 This transform is designed to eliminate unreachable internal globals from the
874 program. It uses an aggressive algorithm, searching out globals that are
875 known to be alive. After it finds all of the globals which are needed, it
876 deletes whatever is left over. This allows it to delete recursive chunks of
877 the program which are unreachable.
878 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]879</div>
880
881<!-------------------------------------------------------------------------- -->
882<div class="doc_subsection">
883 <a name="globalopt">Global Variable Optimizer</a>
884</div>
885<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]886 <p>
887 This pass transforms simple global variables that never have their address
888 taken. If obviously true, it marks read/write globals as constant, deletes
889 variables only stored to, etc.
890 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]891</div>
892
893<!-------------------------------------------------------------------------- -->
894<div class="doc_subsection">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]895 <a name="gvn">Global Value Numbering</a>
896</div>
897<div class="doc_text">
898 <p>
899 This pass performs global value numbering to eliminate fully redundant
900 instructions. It also performs simple dead load elimination.
901 </p>
902</div>
903
904<!-------------------------------------------------------------------------- -->
905<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]906 <a name="gvnpre">Global Value Numbering/Partial Redundancy Elimination</a>
907</div>
908<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]909 <p>
910 This pass performs a hybrid of global value numbering and partial redundancy
911 elimination, known as GVN-PRE. It performs partial redundancy elimination on
912 values, rather than lexical expressions, allowing a more comprehensive view
913 the optimization. It replaces redundant values with uses of earlier
914 occurences of the same value. While this is beneficial in that it eliminates
915 unneeded computation, it also increases register pressure by creating large
916 live ranges, and should be used with caution on platforms that are very
917 sensitive to register pressure.
918 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]919</div>
920
921<!-------------------------------------------------------------------------- -->
922<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]923 <a name="indmemrem">Indirect Malloc and Free Removal</a>
924</div>
925<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]926 <p>
927 This pass finds places where memory allocation functions may escape into
928 indirect land. Some transforms are much easier (aka possible) only if free
929 or malloc are not called indirectly.
930 </p>
931
932 <p>
933 Thus find places where the address of memory functions are taken and construct
934 bounce functions with direct calls of those functions.
935 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]936</div>
937
938<!-------------------------------------------------------------------------- -->
939<div class="doc_subsection">
940 <a name="indvars">Canonicalize Induction Variables</a>
941</div>
942<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]943 <p>
944 This transformation analyzes and transforms the induction variables (and
945 computations derived from them) into simpler forms suitable for subsequent
946 analysis and transformation.
947 </p>
948
949 <p>
950 This transformation makes the following changes to each loop with an
951 identifiable induction variable:
952 </p>
953
954 <ol>
955 <li>All loops are transformed to have a <em>single</em> canonical
956 induction variable which starts at zero and steps by one.</li>
957 <li>The canonical induction variable is guaranteed to be the first PHI node
958 in the loop header block.</li>
959 <li>Any pointer arithmetic recurrences are raised to use array
960 subscripts.</li>
961 </ol>
962
963 <p>
964 If the trip count of a loop is computable, this pass also makes the following
965 changes:
966 </p>
967
968 <ol>
969 <li>The exit condition for the loop is canonicalized to compare the
970 induction value against the exit value. This turns loops like:
971 <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote>
972 into
973 <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li>
974 <li>Any use outside of the loop of an expression derived from the indvar
975 is changed to compute the derived value outside of the loop, eliminating
976 the dependence on the exit value of the induction variable. If the only
977 purpose of the loop is to compute the exit value of some derived
978 expression, this transformation will make the loop dead.</li>
Gordon Henriksene626bbe2007-11-04 16:17:00 +0000[diff] [blame]979 </ol>
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]980
981 <p>
982 This transformation should be followed by strength reduction after all of the
983 desired loop transformations have been performed. Additionally, on targets
984 where it is profitable, the loop could be transformed to count down to zero
985 (the "do loop" optimization).
986 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]987</div>
988
989<!-------------------------------------------------------------------------- -->
990<div class="doc_subsection">
991 <a name="inline">Function Integration/Inlining</a>
992</div>
993<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]994 <p>
995 Bottom-up inlining of functions into callees.
996 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]997</div>
998
999<!-------------------------------------------------------------------------- -->
1000<div class="doc_subsection">
1001 <a name="insert-block-profiling">Insert instrumentation for block profiling</a>
1002</div>
1003<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1004 <p>
1005 This pass instruments the specified program with counters for basic block
1006 profiling, which counts the number of times each basic block executes. This
1007 is the most basic form of profiling, which can tell which blocks are hot, but
1008 cannot reliably detect hot paths through the CFG.
1009 </p>
1010
1011 <p>
1012 Note that this implementation is very naïve. Control equivalent regions of
1013 the CFG should not require duplicate counters, but it does put duplicate
1014 counters in.
1015 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1016</div>
1017
1018<!-------------------------------------------------------------------------- -->
1019<div class="doc_subsection">
1020 <a name="insert-edge-profiling">Insert instrumentation for edge profiling</a>
1021</div>
1022<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1023 <p>
1024 This pass instruments the specified program with counters for edge profiling.
1025 Edge profiling can give a reasonable approximation of the hot paths through a
1026 program, and is used for a wide variety of program transformations.
1027 </p>
1028
1029 <p>
1030 Note that this implementation is very naïve. It inserts a counter for
1031 <em>every</em> edge in the program, instead of using control flow information
1032 to prune the number of counters inserted.
1033 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1034</div>
1035
1036<!-------------------------------------------------------------------------- -->
1037<div class="doc_subsection">
1038 <a name="insert-function-profiling">Insert instrumentation for function profiling</a>
1039</div>
1040<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1041 <p>
1042 This pass instruments the specified program with counters for function
1043 profiling, which counts the number of times each function is called.
1044 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1045</div>
1046
1047<!-------------------------------------------------------------------------- -->
1048<div class="doc_subsection">
1049 <a name="insert-null-profiling-rs">Measure profiling framework overhead</a>
1050</div>
1051<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1052 <p>
1053 The basic profiler that does nothing. It is the default profiler and thus
1054 terminates <code>RSProfiler</code> chains. It is useful for measuring
1055 framework overhead.
1056 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1057</div>
1058
1059<!-------------------------------------------------------------------------- -->
1060<div class="doc_subsection">
1061 <a name="insert-rs-profiling-framework">Insert random sampling instrumentation framework</a>
1062</div>
1063<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1064 <p>
1065 The second stage of the random-sampling instrumentation framework, duplicates
1066 all instructions in a function, ignoring the profiling code, then connects the
1067 two versions together at the entry and at backedges. At each connection point
1068 a choice is made as to whether to jump to the profiled code (take a sample) or
1069 execute the unprofiled code.
1070 </p>
1071
1072 <p>
1073 After this pass, it is highly recommended to run<a href="#mem2reg">mem2reg</a>
1074 and <a href="#adce">adce</a>. <a href="#instcombine">instcombine</a>,
1075 <a href="#load-vn">load-vn</a>, <a href="#gdce">gdce</a>, and
1076 <a href="#dse">dse</a> also are good to run afterwards.
1077 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1078</div>
1079
1080<!-------------------------------------------------------------------------- -->
1081<div class="doc_subsection">
1082 <a name="instcombine">Combine redundant instructions</a>
1083</div>
1084<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1085 <p>
1086 Combine instructions to form fewer, simple
1087 instructions. This pass does not modify the CFG This pass is where algebraic
1088 simplification happens.
1089 </p>
1090
1091 <p>
1092 This pass combines things like:
1093 </p>
1094
1095<blockquote><pre
1096>%Y = add i32 %X, 1
1097%Z = add i32 %Y, 1</pre></blockquote>
1098
1099 <p>
1100 into:
1101 </p>
1102
1103<blockquote><pre
1104>%Z = add i32 %X, 2</pre></blockquote>
1105
1106 <p>
1107 This is a simple worklist driven algorithm.
1108 </p>
1109
1110 <p>
1111 This pass guarantees that the following canonicalizations are performed on
1112 the program:
1113 </p>
1114
1115 <ul>
1116 <li>If a binary operator has a constant operand, it is moved to the right-
1117 hand side.</li>
1118 <li>Bitwise operators with constant operands are always grouped so that
1119 shifts are performed first, then <code>or</code>s, then
1120 <code>and</code>s, then <code>xor</code>s.</li>
1121 <li>Compare instructions are converted from <code>&lt;</code>,
1122 <code>&gt;</code>, <code>≤</code>, or <code>≥</code> to
1123 <code>=</code> or <code>≠</code> if possible.</li>
1124 <li>All <code>cmp</code> instructions on boolean values are replaced with
1125 logical operations.</li>
1126 <li><code>add <var>X</var>, <var>X</var></code> is represented as
1127 <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li>
1128 <li>Multiplies with a constant power-of-two argument are transformed into
1129 shifts.</li>
1130 <li>… etc.</li>
1131 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1132</div>
1133
1134<!-------------------------------------------------------------------------- -->
1135<div class="doc_subsection">
1136 <a name="internalize">Internalize Global Symbols</a>
1137</div>
1138<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1139 <p>
1140 This pass loops over all of the functions in the input module, looking for a
1141 main function. If a main function is found, all other functions and all
1142 global variables with initializers are marked as internal.
1143 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1144</div>
1145
1146<!-------------------------------------------------------------------------- -->
1147<div class="doc_subsection">
1148 <a name="ipconstprop">Interprocedural constant propagation</a>
1149</div>
1150<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1151 <p>
1152 This pass implements an <em>extremely</em> simple interprocedural constant
1153 propagation pass. It could certainly be improved in many different ways,
1154 like using a worklist. This pass makes arguments dead, but does not remove
1155 them. The existing dead argument elimination pass should be run after this
1156 to clean up the mess.
1157 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1158</div>
1159
1160<!-------------------------------------------------------------------------- -->
1161<div class="doc_subsection">
1162 <a name="ipsccp">Interprocedural Sparse Conditional Constant Propagation</a>
1163</div>
1164<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1165 <p>
1166 An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
1167 Propagation</a>.
1168 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1169</div>
1170
1171<!-------------------------------------------------------------------------- -->
1172<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]1173 <a name="jump-threading">Thread control through conditional blocks</a>
1174</div>
1175<div class="doc_text">
1176 <p>
1177 Jump threading tries to find distinct threads of control flow running through
1178 a basic block. This pass looks at blocks that have multiple predecessors and
1179 multiple successors. If one or more of the predecessors of the block can be
1180 proven to always cause a jump to one of the successors, we forward the edge
1181 from the predecessor to the successor by duplicating the contents of this
1182 block.
1183 </p>
1184 <p>
1185 An example of when this can occur is code like this:
1186 </p>
1187
1188 <pre
1189>if () { ...
1190 X = 4;
1191}
1192if (X &lt; 3) {</pre>
1193
1194 <p>
1195 In this case, the unconditional branch at the end of the first if can be
1196 revectored to the false side of the second if.
1197 </p>
1198</div>
1199
1200<!-------------------------------------------------------------------------- -->
1201<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1202 <a name="lcssa">Loop-Closed SSA Form Pass</a>
1203</div>
1204<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1205 <p>
1206 This pass transforms loops by placing phi nodes at the end of the loops for
1207 all values that are live across the loop boundary. For example, it turns
1208 the left into the right code:
1209 </p>
1210
1211 <pre
1212>for (...) for (...)
1213 if (c) if (c)
1214 X1 = ... X1 = ...
1215 else else
1216 X2 = ... X2 = ...
1217 X3 = phi(X1, X2) X3 = phi(X1, X2)
1218... = X3 + 4 X4 = phi(X3)
1219 ... = X4 + 4</pre>
1220
1221 <p>
1222 This is still valid LLVM; the extra phi nodes are purely redundant, and will
1223 be trivially eliminated by <code>InstCombine</code>. The major benefit of
1224 this transformation is that it makes many other loop optimizations, such as
1225 LoopUnswitching, simpler.
1226 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1227</div>
1228
1229<!-------------------------------------------------------------------------- -->
1230<div class="doc_subsection">
1231 <a name="licm">Loop Invariant Code Motion</a>
1232</div>
1233<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1234 <p>
1235 This pass performs loop invariant code motion, attempting to remove as much
1236 code from the body of a loop as possible. It does this by either hoisting
1237 code into the preheader block, or by sinking code to the exit blocks if it is
1238 safe. This pass also promotes must-aliased memory locations in the loop to
1239 live in registers, thus hoisting and sinking "invariant" loads and stores.
1240 </p>
1241
1242 <p>
1243 This pass uses alias analysis for two purposes:
1244 </p>
1245
1246 <ul>
1247 <li>Moving loop invariant loads and calls out of loops. If we can determine
1248 that a load or call inside of a loop never aliases anything stored to,
1249 we can hoist it or sink it like any other instruction.</li>
1250 <li>Scalar Promotion of Memory - If there is a store instruction inside of
1251 the loop, we try to move the store to happen AFTER the loop instead of
1252 inside of the loop. This can only happen if a few conditions are true:
1253 <ul>
1254 <li>The pointer stored through is loop invariant.</li>
1255 <li>There are no stores or loads in the loop which <em>may</em> alias
1256 the pointer. There are no calls in the loop which mod/ref the
1257 pointer.</li>
1258 </ul>
1259 If these conditions are true, we can promote the loads and stores in the
1260 loop of the pointer to use a temporary alloca'd variable. We then use
1261 the mem2reg functionality to construct the appropriate SSA form for the
1262 variable.</li>
1263 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1264</div>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]1265<!-------------------------------------------------------------------------- -->
1266<div class="doc_subsection">
1267 <a name="loop-deletion">Dead Loop Deletion Pass</a>
1268</div>
1269<div class="doc_text">
1270 <p>
1271 This file implements the Dead Loop Deletion Pass. This pass is responsible
1272 for eliminating loops with non-infinite computable trip counts that have no
1273 side effects or volatile instructions, and do not contribute to the
1274 computation of the function's return value.
1275 </p>
1276</div>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1277
1278<!-------------------------------------------------------------------------- -->
1279<div class="doc_subsection">
1280 <a name="loop-extract">Extract loops into new functions</a>
1281</div>
1282<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1283 <p>
1284 A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to
1285 extract each top-level loop into its own new function. If the loop is the
1286 <em>only</em> loop in a given function, it is not touched. This is a pass most
1287 useful for debugging via bugpoint.
1288 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1289</div>
1290
1291<!-------------------------------------------------------------------------- -->
1292<div class="doc_subsection">
1293 <a name="loop-extract-single">Extract at most one loop into a new function</a>
1294</div>
1295<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1296 <p>
1297 Similar to <a href="#loop-extract">Extract loops into new functions</a>,
1298 this pass extracts one natural loop from the program into a function if it
1299 can. This is used by bugpoint.
1300 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1301</div>
1302
1303<!-------------------------------------------------------------------------- -->
1304<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1305 <a name="loop-index-split">Index Split Loops</a>
1306</div>
1307<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1308 <p>
1309 This pass divides loop's iteration range by spliting loop such that each
1310 individual loop is executed efficiently.
1311 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1312</div>
1313
1314<!-------------------------------------------------------------------------- -->
1315<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1316 <a name="loop-reduce">Loop Strength Reduction</a>
1317</div>
1318<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1319 <p>
1320 This pass performs a strength reduction on array references inside loops that
1321 have as one or more of their components the loop induction variable. This is
1322 accomplished by creating a new value to hold the initial value of the array
1323 access for the first iteration, and then creating a new GEP instruction in
1324 the loop to increment the value by the appropriate amount.
1325 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1326</div>
1327
1328<!-------------------------------------------------------------------------- -->
1329<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1330 <a name="loop-rotate">Rotate Loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1331</div>
1332<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1333 <p>A simple loop rotation transformation.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1334</div>
1335
1336<!-------------------------------------------------------------------------- -->
1337<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1338 <a name="loop-unroll">Unroll loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1339</div>
1340<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1341 <p>
1342 This pass implements a simple loop unroller. It works best when loops have
1343 been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass,
1344 allowing it to determine the trip counts of loops easily.
1345 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1346</div>
1347
1348<!-------------------------------------------------------------------------- -->
1349<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1350 <a name="loop-unswitch">Unswitch loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1351</div>
1352<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1353 <p>
1354 This pass transforms loops that contain branches on loop-invariant conditions
1355 to have multiple loops. For example, it turns the left into the right code:
1356 </p>
1357
1358 <pre
1359>for (...) if (lic)
1360 A for (...)
1361 if (lic) A; B; C
1362 B else
1363 C for (...)
1364 A; C</pre>
1365
1366 <p>
1367 This can increase the size of the code exponentially (doubling it every time
1368 a loop is unswitched) so we only unswitch if the resultant code will be
1369 smaller than a threshold.
1370 </p>
1371
1372 <p>
1373 This pass expects LICM to be run before it to hoist invariant conditions out
1374 of the loop, to make the unswitching opportunity obvious.
1375 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1376</div>
1377
1378<!-------------------------------------------------------------------------- -->
1379<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1380 <a name="loopsimplify">Canonicalize natural loops</a>
1381</div>
1382<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1383 <p>
1384 This pass performs several transformations to transform natural loops into a
1385 simpler form, which makes subsequent analyses and transformations simpler and
1386 more effective.
1387 </p>
1388
1389 <p>
1390 Loop pre-header insertion guarantees that there is a single, non-critical
1391 entry edge from outside of the loop to the loop header. This simplifies a
1392 number of analyses and transformations, such as LICM.
1393 </p>
1394
1395 <p>
1396 Loop exit-block insertion guarantees that all exit blocks from the loop
1397 (blocks which are outside of the loop that have predecessors inside of the
1398 loop) only have predecessors from inside of the loop (and are thus dominated
1399 by the loop header). This simplifies transformations such as store-sinking
1400 that are built into LICM.
1401 </p>
1402
1403 <p>
1404 This pass also guarantees that loops will have exactly one backedge.
1405 </p>
1406
1407 <p>
1408 Note that the simplifycfg pass will clean up blocks which are split out but
1409 end up being unnecessary, so usage of this pass should not pessimize
1410 generated code.
1411 </p>
1412
1413 <p>
1414 This pass obviously modifies the CFG, but updates loop information and
1415 dominator information.
1416 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1417</div>
1418
1419<!-------------------------------------------------------------------------- -->
1420<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1421 <a name="lowerallocs">Lower allocations from instructions to calls</a>
1422</div>
1423<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1424 <p>
1425 Turn <tt>malloc</tt> and <tt>free</tt> instructions into <tt>@malloc</tt> and
1426 <tt>@free</tt> calls.
1427 </p>
1428
1429 <p>
1430 This is a target-dependent tranformation because it depends on the size of
1431 data types and alignment constraints.
1432 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1433</div>
1434
1435<!-------------------------------------------------------------------------- -->
1436<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1437 <a name="lowerinvoke">Lower invoke and unwind, for unwindless code generators</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1438</div>
1439<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1440 <p>
1441 This transformation is designed for use by code generators which do not yet
1442 support stack unwinding. This pass supports two models of exception handling
1443 lowering, the 'cheap' support and the 'expensive' support.
1444 </p>
1445
1446 <p>
1447 'Cheap' exception handling support gives the program the ability to execute
1448 any program which does not "throw an exception", by turning 'invoke'
1449 instructions into calls and by turning 'unwind' instructions into calls to
1450 abort(). If the program does dynamically use the unwind instruction, the
1451 program will print a message then abort.
1452 </p>
1453
1454 <p>
1455 'Expensive' exception handling support gives the full exception handling
1456 support to the program at the cost of making the 'invoke' instruction
1457 really expensive. It basically inserts setjmp/longjmp calls to emulate the
1458 exception handling as necessary.
1459 </p>
1460
1461 <p>
1462 Because the 'expensive' support slows down programs a lot, and EH is only
1463 used for a subset of the programs, it must be specifically enabled by the
1464 <tt>-enable-correct-eh-support</tt> option.
1465 </p>
1466
1467 <p>
1468 Note that after this pass runs the CFG is not entirely accurate (exceptional
1469 control flow edges are not correct anymore) so only very simple things should
1470 be done after the lowerinvoke pass has run (like generation of native code).
1471 This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't
1472 support the invoke instruction yet" lowering pass.
1473 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1474</div>
1475
1476<!-------------------------------------------------------------------------- -->
1477<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1478 <a name="lowersetjmp">Lower Set Jump</a>
1479</div>
1480<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1481 <p>
1482 Lowers <tt>setjmp</tt> and <tt>longjmp</tt> to use the LLVM invoke and unwind
1483 instructions as necessary.
1484 </p>
1485
1486 <p>
1487 Lowering of <tt>longjmp</tt> is fairly trivial. We replace the call with a
1488 call to the LLVM library function <tt>__llvm_sjljeh_throw_longjmp()</tt>.
1489 This unwinds the stack for us calling all of the destructors for
1490 objects allocated on the stack.
1491 </p>
1492
1493 <p>
1494 At a <tt>setjmp</tt> call, the basic block is split and the <tt>setjmp</tt>
1495 removed. The calls in a function that have a <tt>setjmp</tt> are converted to
1496 invoke where the except part checks to see if it's a <tt>longjmp</tt>
1497 exception and, if so, if it's handled in the function. If it is, then it gets
1498 the value returned by the <tt>longjmp</tt> and goes to where the basic block
1499 was split. <tt>invoke</tt> instructions are handled in a similar fashion with
1500 the original except block being executed if it isn't a <tt>longjmp</tt>
1501 except that is handled by that function.
1502 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1503</div>
1504
1505<!-------------------------------------------------------------------------- -->
1506<div class="doc_subsection">
1507 <a name="lowerswitch">Lower SwitchInst's to branches</a>
1508</div>
1509<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1510 <p>
1511 Rewrites <tt>switch</tt> instructions with a sequence of branches, which
1512 allows targets to get away with not implementing the switch instruction until
1513 it is convenient.
1514 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1515</div>
1516
1517<!-------------------------------------------------------------------------- -->
1518<div class="doc_subsection">
1519 <a name="mem2reg">Promote Memory to Register</a>
1520</div>
1521<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1522 <p>
1523 This file promotes memory references to be register references. It promotes
1524 <tt>alloca</tt> instructions which only have <tt>load</tt>s and
1525 <tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator
1526 frontiers to place <tt>phi</tt> nodes, then traversing the function in
1527 depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as
1528 appropriate. This is just the standard SSA construction algorithm to construct
1529 "pruned" SSA form.
1530 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1531</div>
1532
1533<!-------------------------------------------------------------------------- -->
1534<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]1535 <a name="memcpyopt">Optimize use of memcpy and friend</a>
1536</div>
1537<div class="doc_text">
1538 <p>
1539 This pass performs various transformations related to eliminating memcpy
1540 calls, or transforming sets of stores into memset's.
1541 </p>
1542</div>
1543
1544<!-------------------------------------------------------------------------- -->
1545<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1546 <a name="mergereturn">Unify function exit nodes</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1547</div>
1548<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1549 <p>
1550 Ensure that functions have at most one <tt>ret</tt> instruction in them.
1551 Additionally, it keeps track of which node is the new exit node of the CFG.
1552 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1553</div>
1554
1555<!-------------------------------------------------------------------------- -->
1556<div class="doc_subsection">
1557 <a name="predsimplify">Predicate Simplifier</a>
1558</div>
1559<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1560 <p>
1561 Path-sensitive optimizer. In a branch where <tt>x == y</tt>, replace uses of
1562 <tt>x</tt> with <tt>y</tt>. Permits further optimization, such as the
1563 elimination of the unreachable call:
1564 </p>
1565
1566<blockquote><pre
1567>void test(int *p, int *q)
1568{
1569 if (p != q)
1570 return;
1571
1572 if (*p != *q)
1573 foo(); // unreachable
1574}</pre></blockquote>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1575</div>
1576
1577<!-------------------------------------------------------------------------- -->
1578<div class="doc_subsection">
1579 <a name="prune-eh">Remove unused exception handling info</a>
1580</div>
1581<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1582 <p>
1583 This file implements a simple interprocedural pass which walks the call-graph,
1584 turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and
1585 only if the callee cannot throw an exception. It implements this as a
1586 bottom-up traversal of the call-graph.
1587 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1588</div>
1589
1590<!-------------------------------------------------------------------------- -->
1591<div class="doc_subsection">
1592 <a name="raiseallocs">Raise allocations from calls to instructions</a>
1593</div>
1594<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1595 <p>
1596 Converts <tt>@malloc</tt> and <tt>@free</tt> calls to <tt>malloc</tt> and
1597 <tt>free</tt> instructions.
1598 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1599</div>
1600
1601<!-------------------------------------------------------------------------- -->
1602<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1603 <a name="reassociate">Reassociate expressions</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1604</div>
1605<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1606 <p>
1607 This pass reassociates commutative expressions in an order that is designed
1608 to promote better constant propagation, GCSE, LICM, PRE, etc.
1609 </p>
1610
1611 <p>
1612 For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5)
1613 </p>
1614
1615 <p>
1616 In the implementation of this algorithm, constants are assigned rank = 0,
1617 function arguments are rank = 1, and other values are assigned ranks
1618 corresponding to the reverse post order traversal of current function
1619 (starting at 2), which effectively gives values in deep loops higher rank
1620 than values not in loops.
1621 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1622</div>
1623
1624<!-------------------------------------------------------------------------- -->
1625<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1626 <a name="reg2mem">Demote all values to stack slots</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1627</div>
1628<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1629 <p>
1630 This file demotes all registers to memory references. It is intented to be
1631 the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to
1632 <tt>load</tt> instructions, the only values live accross basic blocks are
1633 <tt>alloca</tt> instructions and <tt>load</tt> instructions before
1634 <tt>phi</tt> nodes. It is intended that this should make CFG hacking much
1635 easier. To make later hacking easier, the entry block is split into two, such
1636 that all introduced <tt>alloca</tt> instructions (and nothing else) are in the
1637 entry block.
1638 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1639</div>
1640
1641<!-------------------------------------------------------------------------- -->
1642<div class="doc_subsection">
1643 <a name="scalarrepl">Scalar Replacement of Aggregates</a>
1644</div>
1645<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1646 <p>
1647 The well-known scalar replacement of aggregates transformation. This
1648 transform breaks up <tt>alloca</tt> instructions of aggregate type (structure
1649 or array) into individual <tt>alloca</tt> instructions for each member if
1650 possible. Then, if possible, it transforms the individual <tt>alloca</tt>
1651 instructions into nice clean scalar SSA form.
1652 </p>
1653
1654 <p>
1655 This combines a simple scalar replacement of aggregates algorithm with the <a
1656 href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact,
1657 especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>,
1658 then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to
1659 promote works well.
1660 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1661</div>
1662
1663<!-------------------------------------------------------------------------- -->
1664<div class="doc_subsection">
1665 <a name="sccp">Sparse Conditional Constant Propagation</a>
1666</div>
1667<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1668 <p>
1669 Sparse conditional constant propagation and merging, which can be summarized
1670 as:
1671 </p>
1672
1673 <ol>
1674 <li>Assumes values are constant unless proven otherwise</li>
1675 <li>Assumes BasicBlocks are dead unless proven otherwise</li>
1676 <li>Proves values to be constant, and replaces them with constants</li>
1677 <li>Proves conditional branches to be unconditional</li>
1678 </ol>
1679
1680 <p>
1681 Note that this pass has a habit of making definitions be dead. It is a good
1682 idea to to run a DCE pass sometime after running this pass.
1683 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1684</div>
1685
1686<!-------------------------------------------------------------------------- -->
1687<div class="doc_subsection">
1688 <a name="simplify-libcalls">Simplify well-known library calls</a>
1689</div>
1690<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1691 <p>
1692 Applies a variety of small optimizations for calls to specific well-known
1693 function calls (e.g. runtime library functions). For example, a call
1694 <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be
1695 transformed into simply <tt>return 3</tt>.
1696 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1697</div>
1698
1699<!-------------------------------------------------------------------------- -->
1700<div class="doc_subsection">
1701 <a name="simplifycfg">Simplify the CFG</a>
1702</div>
1703<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1704 <p>
1705 Performs dead code elimination and basic block merging. Specifically:
1706 </p>
1707
1708 <ol>
1709 <li>Removes basic blocks with no predecessors.</li>
1710 <li>Merges a basic block into its predecessor if there is only one and the
1711 predecessor only has one successor.</li>
1712 <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li>
1713 <li>Eliminates a basic block that only contains an unconditional
1714 branch.</li>
1715 </ol>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1716</div>
1717
1718<!-------------------------------------------------------------------------- -->
1719<div class="doc_subsection">
1720 <a name="strip">Strip all symbols from a module</a>
1721</div>
1722<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1723 <p>
1724 Performs code stripping. This transformation can delete:
1725 </p>
1726
1727 <ol>
1728 <li>names for virtual registers</li>
1729 <li>symbols for internal globals and functions</li>
1730 <li>debug information</li>
1731 </ol>
1732
1733 <p>
1734 Note that this transformation makes code much less readable, so it should
1735 only be used in situations where the <tt>strip</tt> utility would be used,
1736 such as reducing code size or making it harder to reverse engineer code.
1737 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1738</div>
1739
1740<!-------------------------------------------------------------------------- -->
1741<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame^]1742 <a name="strip-dead-prototypes">Remove unused function declarations</a>
1743</div>
1744<div class="doc_text">
1745 <p>
1746 This pass loops over all of the functions in the input module, looking for
1747 dead declarations and removes them. Dead declarations are declarations of
1748 functions for which no implementation is available (i.e., declarations for
1749 unused library functions).
1750 </p>
1751</div>
1752
1753<!-------------------------------------------------------------------------- -->
1754<div class="doc_subsection">
1755 <a name="sretpromotion">Promote sret arguments</a>
1756</div>
1757<div class="doc_text">
1758 <p>
1759 This pass finds functions that return a struct (using a pointer to the struct
1760 as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and
1761 replaces them with a new function that simply returns each of the elements of
1762 that struct (using multiple return values).
1763 </p>
1764
1765 <p>
1766 This pass works under a number of conditions:
1767 </p>
1768
1769 <ul>
1770 <li>The returned struct must not contain other structs</li>
1771 <li>The returned struct must only be used to load values from</li>
1772 <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li>
1773 </ul>
1774</div>
1775
1776<!-------------------------------------------------------------------------- -->
1777<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1778 <a name="tailcallelim">Tail Call Elimination</a>
1779</div>
1780<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1781 <p>
1782 This file transforms calls of the current function (self recursion) followed
1783 by a return instruction with a branch to the entry of the function, creating
1784 a loop. This pass also implements the following extensions to the basic
1785 algorithm:
1786 </p>
1787
1788 <ul>
1789 <li>Trivial instructions between the call and return do not prevent the
1790 transformation from taking place, though currently the analysis cannot
1791 support moving any really useful instructions (only dead ones).
1792 <li>This pass transforms functions that are prevented from being tail
1793 recursive by an associative expression to use an accumulator variable,
1794 thus compiling the typical naive factorial or <tt>fib</tt> implementation
1795 into efficient code.
1796 <li>TRE is performed if the function returns void, if the return
1797 returns the result returned by the call, or if the function returns a
1798 run-time constant on all exits from the function. It is possible, though
1799 unlikely, that the return returns something else (like constant 0), and
1800 can still be TRE'd. It can be TRE'd if <em>all other</em> return
1801 instructions in the function return the exact same value.
1802 <li>If it can prove that callees do not access theier caller stack frame,
1803 they are marked as eligible for tail call elimination (by the code
1804 generator).
1805 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1806</div>
1807
1808<!-------------------------------------------------------------------------- -->
1809<div class="doc_subsection">
1810 <a name="tailduplicate">Tail Duplication</a>
1811</div>
1812<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1813 <p>
1814 This pass performs a limited form of tail duplication, intended to simplify
1815 CFGs by removing some unconditional branches. This pass is necessary to
1816 straighten out loops created by the C front-end, but also is capable of
1817 making other code nicer. After this pass is run, the CFG simplify pass
1818 should be run to clean up the mess.
1819 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1820</div>
1821
1822<!-- ======================================================================= -->
1823<div class="doc_section"> <a name="transform">Utility Passes</a></div>
1824<div class="doc_text">
1825 <p>This section describes the LLVM Utility Passes.</p>
1826</div>
1827
1828<!-------------------------------------------------------------------------- -->
1829<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1830 <a name="deadarghaX0r">Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1831</div>
1832<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1833 <p>
1834 Same as dead argument elimination, but deletes arguments to functions which
1835 are external. This is only for use by <a
1836 href="Bugpoint.html">bugpoint</a>.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1837</div>
1838
1839<!-------------------------------------------------------------------------- -->
1840<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1841 <a name="extract-blocks">Extract Basic Blocks From Module (for bugpoint use)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1842</div>
1843<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1844 <p>
1845 This pass is used by bugpoint to extract all blocks from the module into their
1846 own functions.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1847</div>
1848
1849<!-------------------------------------------------------------------------- -->
1850<div class="doc_subsection">
Gordon Henriksen90a52142007-11-05 02:05:35 +0000[diff] [blame]1851 <a name="preverify">Preliminary module verification</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1852</div>
1853<div class="doc_text">
Gordon Henriksen90a52142007-11-05 02:05:35 +0000[diff] [blame]1854 <p>
1855 Ensures that the module is in the form required by the <a
1856 href="#verifier">Module Verifier</a> pass.
1857 </p>
1858
1859 <p>
1860 Running the verifier runs this pass automatically, so there should be no need
1861 to use it directly.
1862 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1863</div>
1864
1865<!-------------------------------------------------------------------------- -->
1866<div class="doc_subsection">
1867 <a name="verify">Module Verifier</a>
1868</div>
1869<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1870 <p>
1871 Verifies an LLVM IR code. This is useful to run after an optimization which is
1872 undergoing testing. Note that <tt>llvm-as</tt> verifies its input before
1873 emitting bitcode, and also that malformed bitcode is likely to make LLVM
1874 crash. All language front-ends are therefore encouraged to verify their output
1875 before performing optimizing transformations.
1876 </p>
1877
Gordon Henriksen23a8ce52007-11-04 18:14:08 +0000[diff] [blame]1878 <ul>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1879 <li>Both of a binary operator's parameters are of the same type.</li>
1880 <li>Verify that the indices of mem access instructions match other
1881 operands.</li>
1882 <li>Verify that arithmetic and other things are only performed on
1883 first-class types. Verify that shifts and logicals only happen on
1884 integrals f.e.</li>
1885 <li>All of the constants in a switch statement are of the correct type.</li>
1886 <li>The code is in valid SSA form.</li>
1887 <li>It should be illegal to put a label into any other type (like a
1888 structure) or to return one. [except constant arrays!]</li>
Nick Lewycky0c78ac12008-03-28 06:46:51 +0000[diff] [blame]1889 <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is
Gordon Henriksen873390e2007-11-04 18:17:58 +0000[diff] [blame]1890 invalid.</li>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1891 <li>PHI nodes must have an entry for each predecessor, with no extras.</li>
1892 <li>PHI nodes must be the first thing in a basic block, all grouped
1893 together.</li>
1894 <li>PHI nodes must have at least one entry.</li>
1895 <li>All basic blocks should only end with terminator insts, not contain
1896 them.</li>
1897 <li>The entry node to a function must not have predecessors.</li>
1898 <li>All Instructions must be embedded into a basic block.</li>
1899 <li>Functions cannot take a void-typed parameter.</li>
1900 <li>Verify that a function's argument list agrees with its declared
1901 type.</li>
1902 <li>It is illegal to specify a name for a void value.</li>
1903 <li>It is illegal to have a internal global value with no initializer.</li>
1904 <li>It is illegal to have a ret instruction that returns a value that does
1905 not agree with the function return value type.</li>
1906 <li>Function call argument types match the function prototype.</li>
1907 <li>All other things that are tested by asserts spread about the code.</li>
Gordon Henriksen23a8ce52007-11-04 18:14:08 +0000[diff] [blame]1908 </ul>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1909
1910 <p>
1911 Note that this does not provide full security verification (like Java), but
1912 instead just tries to ensure that code is well-formed.
1913 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1914</div>
1915
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1916<!-------------------------------------------------------------------------- -->
1917<div class="doc_subsection">
1918 <a name="view-cfg">View CFG of function</a>
1919</div>
1920<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1921 <p>
1922 Displays the control flow graph using the GraphViz tool.
1923 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1924</div>
1925
1926<!-------------------------------------------------------------------------- -->
1927<div class="doc_subsection">
1928 <a name="view-cfg-only">View CFG of function (with no function bodies)</a>
1929</div>
1930<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1931 <p>
1932 Displays the control flow graph using the GraphViz tool, but omitting function
1933 bodies.
1934 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1935</div>
1936
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1937<!-- *********************************************************************** -->
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