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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>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]82<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]83<tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
84<tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
85<tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]86<tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]87<tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
88<tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
89<tr><td><a href="#dot-cfg-only">-dot-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]90<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]91<tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr>
92<tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr>
93<tr><td><a href="#load-vn">-load-vn</a></td><td>Load Value Numbering</td></tr>
94<tr><td><a href="#loops">-loops</a></td><td>Natural Loop Construction</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]95<tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]96<tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
97<tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
98<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]99<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]100<tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]101<tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr>
102<tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr>
103<tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr>
104<tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
105<tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr>
106<tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr>
107<tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]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>
Chris Lattnerb80e1ab2009-08-28 00:45:47 +0000[diff] [blame]277
278 <p>
279 Please keep in mind that the current andersen's pass has many known
280 problems and bugs. It should be considered "research quality".
281 </p>
282
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]283</div>
284
285<!-------------------------------------------------------------------------- -->
286<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]287 <a name="basicaa">Basic Alias Analysis (default AA impl)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]288</div>
289<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]290 <p>
291 This is the default implementation of the Alias Analysis interface
292 that simply implements a few identities (two different globals cannot alias,
293 etc), but otherwise does no analysis.
294 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]295</div>
296
297<!-------------------------------------------------------------------------- -->
298<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]299 <a name="basiccg">Basic CallGraph Construction</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]300</div>
301<div class="doc_text">
302 <p>Yet to be written.</p>
303</div>
304
305<!-------------------------------------------------------------------------- -->
306<div class="doc_subsection">
Matthijs Kooijman845f5242008-06-05 07:55:49 +0000[diff] [blame]307 <a name="basicvn">Basic Value Numbering (default Value Numbering impl)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]308</div>
309<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]310 <p>
311 This is the default implementation of the <code>ValueNumbering</code>
312 interface. It walks the SSA def-use chains to trivially identify
313 lexically identical expressions. This does not require any ahead of time
314 analysis, so it is a very fast default implementation.
315 </p>
Matthijs Kooijman845f5242008-06-05 07:55:49 +0000[diff] [blame]316 <p>
317 The ValueNumbering analysis passes are mostly deprecated. They are only used
318 by the <a href="#gcse">Global Common Subexpression Elimination pass</a>, which
319 is deprecated by the <a href="#gvn">Global Value Numbering pass</a> (which
320 does its value numbering on its own).
321 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]322</div>
323
324<!-------------------------------------------------------------------------- -->
325<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]326 <a name="codegenprepare">Optimize for code generation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]327</div>
328<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]329 <p>
330 This pass munges the code in the input function to better prepare it for
331 SelectionDAG-based code generation. This works around limitations in it's
332 basic-block-at-a-time approach. It should eventually be removed.
333 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]334</div>
335
336<!-------------------------------------------------------------------------- -->
337<div class="doc_subsection">
338 <a name="count-aa">Count Alias Analysis Query Responses</a>
339</div>
340<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]341 <p>
342 A pass which can be used to count how many alias queries
343 are being made and how the alias analysis implementation being used responds.
344 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]345</div>
346
347<!-------------------------------------------------------------------------- -->
348<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]349 <a name="debug-aa">AA use debugger</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]350</div>
351<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]352 <p>
353 This simple pass checks alias analysis users to ensure that if they
354 create a new value, they do not query AA without informing it of the value.
355 It acts as a shim over any other AA pass you want.
356 </p>
357
358 <p>
359 Yes keeping track of every value in the program is expensive, but this is
360 a debugging pass.
361 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]362</div>
363
364<!-------------------------------------------------------------------------- -->
365<div class="doc_subsection">
366 <a name="domfrontier">Dominance Frontier Construction</a>
367</div>
368<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]369 <p>
370 This pass is a simple dominator construction algorithm for finding forward
371 dominator frontiers.
372 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]373</div>
374
375<!-------------------------------------------------------------------------- -->
376<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]377 <a name="domtree">Dominator Tree Construction</a>
378</div>
379<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]380 <p>
381 This pass is a simple dominator construction algorithm for finding forward
382 dominators.
383 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]384</div>
385
386<!-------------------------------------------------------------------------- -->
387<div class="doc_subsection">
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]388 <a name="dot-callgraph">Print Call Graph to 'dot' file</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]389</div>
390<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]391 <p>
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]392 This pass, only available in <code>opt</code>, prints the call graph into a
393 <code>.dot</code> graph. This graph can then be processed with the "dot" tool
394 to convert it to postscript or some other suitable format.
395 </p>
396</div>
397
398<!-------------------------------------------------------------------------- -->
399<div class="doc_subsection">
400 <a name="dot-cfg">Print CFG of function to 'dot' file</a>
401</div>
402<div class="doc_text">
403 <p>
404 This pass, only available in <code>opt</code>, prints the control flow graph
405 into a <code>.dot</code> graph. This graph can then be processed with the
406 "dot" tool to convert it to postscript or some other suitable format.
407 </p>
408</div>
409
410<!-------------------------------------------------------------------------- -->
411<div class="doc_subsection">
412 <a name="dot-cfg-only">Print CFG of function to 'dot' file (with no function bodies)</a>
413</div>
414<div class="doc_text">
415 <p>
416 This pass, only available in <code>opt</code>, prints the control flow graph
417 into a <code>.dot</code> graph, omitting the function bodies. This graph can
418 then be processed with the "dot" tool to convert it to postscript or some
419 other suitable format.
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]420 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]421</div>
422
423<!-------------------------------------------------------------------------- -->
424<div class="doc_subsection">
425 <a name="globalsmodref-aa">Simple mod/ref analysis for globals</a>
426</div>
427<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]428 <p>
429 This simple pass provides alias and mod/ref information for global values
430 that do not have their address taken, and keeps track of whether functions
431 read or write memory (are "pure"). For this simple (but very common) case,
432 we can provide pretty accurate and useful information.
433 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]434</div>
435
436<!-------------------------------------------------------------------------- -->
437<div class="doc_subsection">
438 <a name="instcount">Counts the various types of Instructions</a>
439</div>
440<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]441 <p>
442 This pass collects the count of all instructions and reports them
443 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]444</div>
445
446<!-------------------------------------------------------------------------- -->
447<div class="doc_subsection">
448 <a name="intervals">Interval Partition Construction</a>
449</div>
450<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]451 <p>
452 This analysis calculates and represents the interval partition of a function,
453 or a preexisting interval partition.
454 </p>
455
456 <p>
457 In this way, the interval partition may be used to reduce a flow graph down
458 to its degenerate single node interval partition (unless it is irreducible).
459 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]460</div>
461
462<!-------------------------------------------------------------------------- -->
463<div class="doc_subsection">
464 <a name="load-vn">Load Value Numbering</a>
465</div>
466<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]467 <p>
468 This pass value numbers load and call instructions. To do this, it finds
469 lexically identical load instructions, and uses alias analysis to determine
470 which loads are guaranteed to produce the same value. To value number call
471 instructions, it looks for calls to functions that do not write to memory
472 which do not have intervening instructions that clobber the memory that is
473 read from.
474 </p>
475
476 <p>
477 This pass builds off of another value numbering pass to implement value
478 numbering for non-load and non-call instructions. It uses Alias Analysis so
479 that it can disambiguate the load instructions. The more powerful these base
480 analyses are, the more powerful the resultant value numbering will be.
481 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]482</div>
483
484<!-------------------------------------------------------------------------- -->
485<div class="doc_subsection">
486 <a name="loops">Natural Loop Construction</a>
487</div>
488<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]489 <p>
490 This analysis is used to identify natural loops and determine the loop depth
491 of various nodes of the CFG. Note that the loops identified may actually be
492 several natural loops that share the same header node... not just a single
493 natural loop.
494 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]495</div>
496
497<!-------------------------------------------------------------------------- -->
498<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]499 <a name="memdep">Memory Dependence Analysis</a>
500</div>
501<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]502 <p>
503 An analysis that determines, for a given memory operation, what preceding
504 memory operations it depends on. It builds on alias analysis information, and
505 tries to provide a lazy, caching interface to a common kind of alias
506 information query.
507 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]508</div>
509
510<!-------------------------------------------------------------------------- -->
511<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]512 <a name="no-aa">No Alias Analysis (always returns 'may' alias)</a>
513</div>
514<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]515 <p>
516 Always returns "I don't know" for alias queries. NoAA is unlike other alias
517 analysis implementations, in that it does not chain to a previous analysis. As
518 such it doesn't follow many of the rules that other alias analyses must.
519 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]520</div>
521
522<!-------------------------------------------------------------------------- -->
523<div class="doc_subsection">
524 <a name="no-profile">No Profile Information</a>
525</div>
526<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]527 <p>
528 The default "no profile" implementation of the abstract
529 <code>ProfileInfo</code> interface.
530 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]531</div>
532
533<!-------------------------------------------------------------------------- -->
534<div class="doc_subsection">
535 <a name="postdomfrontier">Post-Dominance Frontier Construction</a>
536</div>
537<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]538 <p>
539 This pass is a simple post-dominator construction algorithm for finding
540 post-dominator frontiers.
541 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]542</div>
543
544<!-------------------------------------------------------------------------- -->
545<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]546 <a name="postdomtree">Post-Dominator Tree Construction</a>
547</div>
548<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]549 <p>
550 This pass is a simple post-dominator construction algorithm for finding
551 post-dominators.
552 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]553</div>
554
555<!-------------------------------------------------------------------------- -->
556<div class="doc_subsection">
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]557 <a name="print-alias-sets">Alias Set Printer</a>
558</div>
559<div class="doc_text">
560 <p>Yet to be written.</p>
561</div>
562
563<!-------------------------------------------------------------------------- -->
564<div class="doc_subsection">
565 <a name="print-callgraph">Print a call graph</a>
566</div>
567<div class="doc_text">
568 <p>
569 This pass, only available in <code>opt</code>, prints the call graph to
570 standard output in a human-readable form.
571 </p>
572</div>
573
574<!-------------------------------------------------------------------------- -->
575<div class="doc_subsection">
576 <a name="print-callgraph-sccs">Print SCCs of the Call Graph</a>
577</div>
578<div class="doc_text">
579 <p>
580 This pass, only available in <code>opt</code>, prints the SCCs of the call
581 graph to standard output in a human-readable form.
582 </p>
583</div>
584
585<!-------------------------------------------------------------------------- -->
586<div class="doc_subsection">
587 <a name="print-cfg-sccs">Print SCCs of each function CFG</a>
588</div>
589<div class="doc_text">
590 <p>
591 This pass, only available in <code>opt</code>, prints the SCCs of each
592 function CFG to standard output in a human-readable form.
593 </p>
594</div>
595
596<!-------------------------------------------------------------------------- -->
597<div class="doc_subsection">
598 <a name="print-externalfnconstants">Print external fn callsites passed constants</a>
599</div>
600<div class="doc_text">
601 <p>
602 This pass, only available in <code>opt</code>, prints out call sites to
603 external functions that are called with constant arguments. This can be
604 useful when looking for standard library functions we should constant fold
605 or handle in alias analyses.
606 </p>
607</div>
608
609<!-------------------------------------------------------------------------- -->
610<div class="doc_subsection">
611 <a name="print-function">Print function to stderr</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]612</div>
613<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]614 <p>
615 The <code>PrintFunctionPass</code> class is designed to be pipelined with
616 other <code>FunctionPass</code>es, and prints out the functions of the module
617 as they are processed.
618 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]619</div>
620
621<!-------------------------------------------------------------------------- -->
622<div class="doc_subsection">
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]623 <a name="print-module">Print module to stderr</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]624</div>
625<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]626 <p>
627 This pass simply prints out the entire module when it is executed.
628 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]629</div>
630
631<!-------------------------------------------------------------------------- -->
632<div class="doc_subsection">
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]633 <a name="print-used-types">Find Used Types</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]634</div>
635<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]636 <p>
637 This pass is used to seek out all of the types in use by the program. Note
638 that this analysis explicitly does not include types only used by the symbol
639 table.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]640</div>
641
642<!-------------------------------------------------------------------------- -->
643<div class="doc_subsection">
644 <a name="profile-loader">Load profile information from llvmprof.out</a>
645</div>
646<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]647 <p>
648 A concrete implementation of profiling information that loads the information
649 from a profile dump file.
650 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]651</div>
652
653<!-------------------------------------------------------------------------- -->
654<div class="doc_subsection">
655 <a name="scalar-evolution">Scalar Evolution Analysis</a>
656</div>
657<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]658 <p>
659 The <code>ScalarEvolution</code> analysis can be used to analyze and
660 catagorize scalar expressions in loops. It specializes in recognizing general
661 induction variables, representing them with the abstract and opaque
662 <code>SCEV</code> class. Given this analysis, trip counts of loops and other
663 important properties can be obtained.
664 </p>
665
666 <p>
667 This analysis is primarily useful for induction variable substitution and
668 strength reduction.
669 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]670</div>
671
672<!-------------------------------------------------------------------------- -->
673<div class="doc_subsection">
674 <a name="targetdata">Target Data Layout</a>
675</div>
676<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]677 <p>Provides other passes access to information on how the size and alignment
678 required by the the target ABI for various data types.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]679</div>
680
681<!-- ======================================================================= -->
682<div class="doc_section"> <a name="transform">Transform Passes</a></div>
683<div class="doc_text">
684 <p>This section describes the LLVM Transform Passes.</p>
685</div>
686
687<!-------------------------------------------------------------------------- -->
688<div class="doc_subsection">
689 <a name="adce">Aggressive Dead Code Elimination</a>
690</div>
691<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]692 <p>ADCE aggressively tries to eliminate code. This pass is similar to
693 <a href="#dce">DCE</a> but it assumes that values are dead until proven
694 otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to
695 the liveness of values.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]696</div>
697
698<!-------------------------------------------------------------------------- -->
699<div class="doc_subsection">
700 <a name="argpromotion">Promote 'by reference' arguments to scalars</a>
701</div>
702<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]703 <p>
704 This pass promotes "by reference" arguments to be "by value" arguments. In
705 practice, this means looking for internal functions that have pointer
706 arguments. If it can prove, through the use of alias analysis, that an
707 argument is *only* loaded, then it can pass the value into the function
708 instead of the address of the value. This can cause recursive simplification
709 of code and lead to the elimination of allocas (especially in C++ template
710 code like the STL).
711 </p>
712
713 <p>
714 This pass also handles aggregate arguments that are passed into a function,
715 scalarizing them if the elements of the aggregate are only loaded. Note that
716 it refuses to scalarize aggregates which would require passing in more than
717 three operands to the function, because passing thousands of operands for a
718 large array or structure is unprofitable!
719 </p>
720
721 <p>
722 Note that this transformation could also be done for arguments that are only
723 stored to (returning the value instead), but does not currently. This case
724 would be best handled when and if LLVM starts supporting multiple return
725 values from functions.
726 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]727</div>
728
729<!-------------------------------------------------------------------------- -->
730<div class="doc_subsection">
731 <a name="block-placement">Profile Guided Basic Block Placement</a>
732</div>
733<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]734 <p>This pass is a very simple profile guided basic block placement algorithm.
735 The idea is to put frequently executed blocks together at the start of the
736 function and hopefully increase the number of fall-through conditional
737 branches. If there is no profile information for a particular function, this
738 pass basically orders blocks in depth-first order.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]739</div>
740
741<!-------------------------------------------------------------------------- -->
742<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]743 <a name="break-crit-edges">Break critical edges in CFG</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]744</div>
745<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]746 <p>
747 Break all of the critical edges in the CFG by inserting a dummy basic block.
748 It may be "required" by passes that cannot deal with critical edges. This
749 transformation obviously invalidates the CFG, but can update forward dominator
750 (set, immediate dominators, tree, and frontier) information.
751 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]752</div>
753
754<!-------------------------------------------------------------------------- -->
755<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]756 <a name="codegenprepare">Prepare a function for code generation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]757</div>
758<div class="doc_text">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]759 This pass munges the code in the input function to better prepare it for
760 SelectionDAG-based code generation. This works around limitations in it's
761 basic-block-at-a-time approach. It should eventually be removed.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]762</div>
763
764<!-------------------------------------------------------------------------- -->
765<div class="doc_subsection">
766 <a name="condprop">Conditional Propagation</a>
767</div>
768<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]769 <p>This pass propagates information about conditional expressions through the
770 program, allowing it to eliminate conditional branches in some cases.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]771</div>
772
773<!-------------------------------------------------------------------------- -->
774<div class="doc_subsection">
775 <a name="constmerge">Merge Duplicate Global Constants</a>
776</div>
777<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]778 <p>
779 Merges duplicate global constants together into a single constant that is
780 shared. This is useful because some passes (ie TraceValues) insert a lot of
781 string constants into the program, regardless of whether or not an existing
782 string is available.
783 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]784</div>
785
786<!-------------------------------------------------------------------------- -->
787<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]788 <a name="constprop">Simple constant propagation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]789</div>
790<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]791 <p>This file implements constant propagation and merging. It looks for
792 instructions involving only constant operands and replaces them with a
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]793 constant value instead of an instruction. For example:</p>
794 <blockquote><pre>add i32 1, 2</pre></blockquote>
795 <p>becomes</p>
796 <blockquote><pre>i32 3</pre></blockquote>
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]797 <p>NOTE: this pass has a habit of making definitions be dead. It is a good
798 idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass
799 sometime after running this pass.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]800</div>
801
802<!-------------------------------------------------------------------------- -->
803<div class="doc_subsection">
804 <a name="dce">Dead Code Elimination</a>
805</div>
806<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]807 <p>
808 Dead code elimination is similar to <a href="#die">dead instruction
809 elimination</a>, but it rechecks instructions that were used by removed
810 instructions to see if they are newly dead.
811 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]812</div>
813
814<!-------------------------------------------------------------------------- -->
815<div class="doc_subsection">
816 <a name="deadargelim">Dead Argument Elimination</a>
817</div>
818<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]819 <p>
820 This pass deletes dead arguments from internal functions. Dead argument
821 elimination removes arguments which are directly dead, as well as arguments
822 only passed into function calls as dead arguments of other functions. This
823 pass also deletes dead arguments in a similar way.
824 </p>
825
826 <p>
827 This pass is often useful as a cleanup pass to run after aggressive
828 interprocedural passes, which add possibly-dead arguments.
829 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]830</div>
831
832<!-------------------------------------------------------------------------- -->
833<div class="doc_subsection">
834 <a name="deadtypeelim">Dead Type Elimination</a>
835</div>
836<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]837 <p>
838 This pass is used to cleanup the output of GCC. It eliminate names for types
839 that are unused in the entire translation unit, using the <a
840 href="#findusedtypes">find used types</a> pass.
841 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]842</div>
843
844<!-------------------------------------------------------------------------- -->
845<div class="doc_subsection">
846 <a name="die">Dead Instruction Elimination</a>
847</div>
848<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]849 <p>
850 Dead instruction elimination performs a single pass over the function,
851 removing instructions that are obviously dead.
852 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]853</div>
854
855<!-------------------------------------------------------------------------- -->
856<div class="doc_subsection">
857 <a name="dse">Dead Store Elimination</a>
858</div>
859<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]860 <p>
861 A trivial dead store elimination that only considers basic-block local
862 redundant stores.
863 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]864</div>
865
866<!-------------------------------------------------------------------------- -->
867<div class="doc_subsection">
868 <a name="gcse">Global Common Subexpression Elimination</a>
869</div>
870<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]871 <p>
872 This pass is designed to be a very quick global transformation that
873 eliminates global common subexpressions from a function. It does this by
Matthijs Kooijman845f5242008-06-05 07:55:49 +0000[diff] [blame]874 using an existing value numbering analysis pass to identify the common
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]875 subexpressions, eliminating them when possible.
876 </p>
Matthijs Kooijman845f5242008-06-05 07:55:49 +0000[diff] [blame]877 <p>
878 This pass is deprecated by the <a href="#gvn">Global Value Numbering pass</a>
879 (which does a better job with its own value numbering).
880 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]881</div>
882
883<!-------------------------------------------------------------------------- -->
884<div class="doc_subsection">
885 <a name="globaldce">Dead Global Elimination</a>
886</div>
887<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]888 <p>
889 This transform is designed to eliminate unreachable internal globals from the
890 program. It uses an aggressive algorithm, searching out globals that are
891 known to be alive. After it finds all of the globals which are needed, it
892 deletes whatever is left over. This allows it to delete recursive chunks of
893 the program which are unreachable.
894 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]895</div>
896
897<!-------------------------------------------------------------------------- -->
898<div class="doc_subsection">
899 <a name="globalopt">Global Variable Optimizer</a>
900</div>
901<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]902 <p>
903 This pass transforms simple global variables that never have their address
904 taken. If obviously true, it marks read/write globals as constant, deletes
905 variables only stored to, etc.
906 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]907</div>
908
909<!-------------------------------------------------------------------------- -->
910<div class="doc_subsection">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]911 <a name="gvn">Global Value Numbering</a>
912</div>
913<div class="doc_text">
914 <p>
915 This pass performs global value numbering to eliminate fully redundant
916 instructions. It also performs simple dead load elimination.
917 </p>
Matthijs Kooijman845f5242008-06-05 07:55:49 +0000[diff] [blame]918 <p>
919 Note that this pass does the value numbering itself, it does not use the
920 ValueNumbering analysis passes.
921 </p>
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]922</div>
923
924<!-------------------------------------------------------------------------- -->
925<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]926 <a name="gvnpre">Global Value Numbering/Partial Redundancy Elimination</a>
927</div>
928<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]929 <p>
930 This pass performs a hybrid of global value numbering and partial redundancy
931 elimination, known as GVN-PRE. It performs partial redundancy elimination on
932 values, rather than lexical expressions, allowing a more comprehensive view
933 the optimization. It replaces redundant values with uses of earlier
934 occurences of the same value. While this is beneficial in that it eliminates
935 unneeded computation, it also increases register pressure by creating large
936 live ranges, and should be used with caution on platforms that are very
937 sensitive to register pressure.
938 </p>
Matthijs Kooijman845f5242008-06-05 07:55:49 +0000[diff] [blame]939 <p>
940 Note that this pass does the value numbering itself, it does not use the
941 ValueNumbering analysis passes.
942 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]943</div>
944
945<!-------------------------------------------------------------------------- -->
946<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]947 <a name="indmemrem">Indirect Malloc and Free Removal</a>
948</div>
949<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]950 <p>
951 This pass finds places where memory allocation functions may escape into
952 indirect land. Some transforms are much easier (aka possible) only if free
953 or malloc are not called indirectly.
954 </p>
955
956 <p>
957 Thus find places where the address of memory functions are taken and construct
958 bounce functions with direct calls of those functions.
959 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]960</div>
961
962<!-------------------------------------------------------------------------- -->
963<div class="doc_subsection">
964 <a name="indvars">Canonicalize Induction Variables</a>
965</div>
966<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]967 <p>
968 This transformation analyzes and transforms the induction variables (and
969 computations derived from them) into simpler forms suitable for subsequent
970 analysis and transformation.
971 </p>
972
973 <p>
974 This transformation makes the following changes to each loop with an
975 identifiable induction variable:
976 </p>
977
978 <ol>
979 <li>All loops are transformed to have a <em>single</em> canonical
980 induction variable which starts at zero and steps by one.</li>
981 <li>The canonical induction variable is guaranteed to be the first PHI node
982 in the loop header block.</li>
983 <li>Any pointer arithmetic recurrences are raised to use array
984 subscripts.</li>
985 </ol>
986
987 <p>
988 If the trip count of a loop is computable, this pass also makes the following
989 changes:
990 </p>
991
992 <ol>
993 <li>The exit condition for the loop is canonicalized to compare the
994 induction value against the exit value. This turns loops like:
995 <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote>
996 into
997 <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li>
998 <li>Any use outside of the loop of an expression derived from the indvar
999 is changed to compute the derived value outside of the loop, eliminating
1000 the dependence on the exit value of the induction variable. If the only
1001 purpose of the loop is to compute the exit value of some derived
1002 expression, this transformation will make the loop dead.</li>
Gordon Henriksene626bbe2007-11-04 16:17:00 +0000[diff] [blame]1003 </ol>
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1004
1005 <p>
1006 This transformation should be followed by strength reduction after all of the
1007 desired loop transformations have been performed. Additionally, on targets
1008 where it is profitable, the loop could be transformed to count down to zero
1009 (the "do loop" optimization).
1010 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1011</div>
1012
1013<!-------------------------------------------------------------------------- -->
1014<div class="doc_subsection">
1015 <a name="inline">Function Integration/Inlining</a>
1016</div>
1017<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1018 <p>
1019 Bottom-up inlining of functions into callees.
1020 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1021</div>
1022
1023<!-------------------------------------------------------------------------- -->
1024<div class="doc_subsection">
1025 <a name="insert-block-profiling">Insert instrumentation for block profiling</a>
1026</div>
1027<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1028 <p>
1029 This pass instruments the specified program with counters for basic block
1030 profiling, which counts the number of times each basic block executes. This
1031 is the most basic form of profiling, which can tell which blocks are hot, but
1032 cannot reliably detect hot paths through the CFG.
1033 </p>
1034
1035 <p>
1036 Note that this implementation is very naïve. Control equivalent regions of
1037 the CFG should not require duplicate counters, but it does put duplicate
1038 counters in.
1039 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1040</div>
1041
1042<!-------------------------------------------------------------------------- -->
1043<div class="doc_subsection">
1044 <a name="insert-edge-profiling">Insert instrumentation for edge profiling</a>
1045</div>
1046<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1047 <p>
1048 This pass instruments the specified program with counters for edge profiling.
1049 Edge profiling can give a reasonable approximation of the hot paths through a
1050 program, and is used for a wide variety of program transformations.
1051 </p>
1052
1053 <p>
1054 Note that this implementation is very naïve. It inserts a counter for
1055 <em>every</em> edge in the program, instead of using control flow information
1056 to prune the number of counters inserted.
1057 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1058</div>
1059
1060<!-------------------------------------------------------------------------- -->
1061<div class="doc_subsection">
1062 <a name="insert-function-profiling">Insert instrumentation for function profiling</a>
1063</div>
1064<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1065 <p>
1066 This pass instruments the specified program with counters for function
1067 profiling, which counts the number of times each function is called.
1068 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1069</div>
1070
1071<!-------------------------------------------------------------------------- -->
1072<div class="doc_subsection">
1073 <a name="insert-null-profiling-rs">Measure profiling framework overhead</a>
1074</div>
1075<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1076 <p>
1077 The basic profiler that does nothing. It is the default profiler and thus
1078 terminates <code>RSProfiler</code> chains. It is useful for measuring
1079 framework overhead.
1080 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1081</div>
1082
1083<!-------------------------------------------------------------------------- -->
1084<div class="doc_subsection">
1085 <a name="insert-rs-profiling-framework">Insert random sampling instrumentation framework</a>
1086</div>
1087<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1088 <p>
1089 The second stage of the random-sampling instrumentation framework, duplicates
1090 all instructions in a function, ignoring the profiling code, then connects the
1091 two versions together at the entry and at backedges. At each connection point
1092 a choice is made as to whether to jump to the profiled code (take a sample) or
1093 execute the unprofiled code.
1094 </p>
1095
1096 <p>
1097 After this pass, it is highly recommended to run<a href="#mem2reg">mem2reg</a>
1098 and <a href="#adce">adce</a>. <a href="#instcombine">instcombine</a>,
1099 <a href="#load-vn">load-vn</a>, <a href="#gdce">gdce</a>, and
1100 <a href="#dse">dse</a> also are good to run afterwards.
1101 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1102</div>
1103
1104<!-------------------------------------------------------------------------- -->
1105<div class="doc_subsection">
1106 <a name="instcombine">Combine redundant instructions</a>
1107</div>
1108<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]1109 <p>
1110 Combine instructions to form fewer, simple
1111 instructions. This pass does not modify the CFG This pass is where algebraic
1112 simplification happens.
1113 </p>
1114
1115 <p>
1116 This pass combines things like:
1117 </p>
1118
1119<blockquote><pre
1120>%Y = add i32 %X, 1
1121%Z = add i32 %Y, 1</pre></blockquote>
1122
1123 <p>
1124 into:
1125 </p>
1126
1127<blockquote><pre
1128>%Z = add i32 %X, 2</pre></blockquote>
1129
1130 <p>
1131 This is a simple worklist driven algorithm.
1132 </p>
1133
1134 <p>
1135 This pass guarantees that the following canonicalizations are performed on
1136 the program:
1137 </p>
1138
1139 <ul>
1140 <li>If a binary operator has a constant operand, it is moved to the right-
1141 hand side.</li>
1142 <li>Bitwise operators with constant operands are always grouped so that
1143 shifts are performed first, then <code>or</code>s, then
1144 <code>and</code>s, then <code>xor</code>s.</li>
1145 <li>Compare instructions are converted from <code>&lt;</code>,
1146 <code>&gt;</code>, <code>≤</code>, or <code>≥</code> to
1147 <code>=</code> or <code>≠</code> if possible.</li>
1148 <li>All <code>cmp</code> instructions on boolean values are replaced with
1149 logical operations.</li>
1150 <li><code>add <var>X</var>, <var>X</var></code> is represented as
1151 <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li>
1152 <li>Multiplies with a constant power-of-two argument are transformed into
1153 shifts.</li>
1154 <li>… etc.</li>
1155 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1156</div>
1157
1158<!-------------------------------------------------------------------------- -->
1159<div class="doc_subsection">
1160 <a name="internalize">Internalize Global Symbols</a>
1161</div>
1162<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1163 <p>
1164 This pass loops over all of the functions in the input module, looking for a
1165 main function. If a main function is found, all other functions and all
1166 global variables with initializers are marked as internal.
1167 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1168</div>
1169
1170<!-------------------------------------------------------------------------- -->
1171<div class="doc_subsection">
1172 <a name="ipconstprop">Interprocedural constant propagation</a>
1173</div>
1174<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1175 <p>
1176 This pass implements an <em>extremely</em> simple interprocedural constant
1177 propagation pass. It could certainly be improved in many different ways,
1178 like using a worklist. This pass makes arguments dead, but does not remove
1179 them. The existing dead argument elimination pass should be run after this
1180 to clean up the mess.
1181 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1182</div>
1183
1184<!-------------------------------------------------------------------------- -->
1185<div class="doc_subsection">
1186 <a name="ipsccp">Interprocedural Sparse Conditional Constant Propagation</a>
1187</div>
1188<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1189 <p>
1190 An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
1191 Propagation</a>.
1192 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1193</div>
1194
1195<!-------------------------------------------------------------------------- -->
1196<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]1197 <a name="jump-threading">Thread control through conditional blocks</a>
1198</div>
1199<div class="doc_text">
1200 <p>
1201 Jump threading tries to find distinct threads of control flow running through
1202 a basic block. This pass looks at blocks that have multiple predecessors and
1203 multiple successors. If one or more of the predecessors of the block can be
1204 proven to always cause a jump to one of the successors, we forward the edge
1205 from the predecessor to the successor by duplicating the contents of this
1206 block.
1207 </p>
1208 <p>
1209 An example of when this can occur is code like this:
1210 </p>
1211
1212 <pre
1213>if () { ...
1214 X = 4;
1215}
1216if (X &lt; 3) {</pre>
1217
1218 <p>
1219 In this case, the unconditional branch at the end of the first if can be
1220 revectored to the false side of the second if.
1221 </p>
1222</div>
1223
1224<!-------------------------------------------------------------------------- -->
1225<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1226 <a name="lcssa">Loop-Closed SSA Form Pass</a>
1227</div>
1228<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1229 <p>
1230 This pass transforms loops by placing phi nodes at the end of the loops for
1231 all values that are live across the loop boundary. For example, it turns
1232 the left into the right code:
1233 </p>
1234
1235 <pre
1236>for (...) for (...)
1237 if (c) if (c)
1238 X1 = ... X1 = ...
1239 else else
1240 X2 = ... X2 = ...
1241 X3 = phi(X1, X2) X3 = phi(X1, X2)
1242... = X3 + 4 X4 = phi(X3)
1243 ... = X4 + 4</pre>
1244
1245 <p>
1246 This is still valid LLVM; the extra phi nodes are purely redundant, and will
1247 be trivially eliminated by <code>InstCombine</code>. The major benefit of
1248 this transformation is that it makes many other loop optimizations, such as
1249 LoopUnswitching, simpler.
1250 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1251</div>
1252
1253<!-------------------------------------------------------------------------- -->
1254<div class="doc_subsection">
1255 <a name="licm">Loop Invariant Code Motion</a>
1256</div>
1257<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1258 <p>
1259 This pass performs loop invariant code motion, attempting to remove as much
1260 code from the body of a loop as possible. It does this by either hoisting
1261 code into the preheader block, or by sinking code to the exit blocks if it is
1262 safe. This pass also promotes must-aliased memory locations in the loop to
1263 live in registers, thus hoisting and sinking "invariant" loads and stores.
1264 </p>
1265
1266 <p>
1267 This pass uses alias analysis for two purposes:
1268 </p>
1269
1270 <ul>
1271 <li>Moving loop invariant loads and calls out of loops. If we can determine
1272 that a load or call inside of a loop never aliases anything stored to,
1273 we can hoist it or sink it like any other instruction.</li>
1274 <li>Scalar Promotion of Memory - If there is a store instruction inside of
1275 the loop, we try to move the store to happen AFTER the loop instead of
1276 inside of the loop. This can only happen if a few conditions are true:
1277 <ul>
1278 <li>The pointer stored through is loop invariant.</li>
1279 <li>There are no stores or loads in the loop which <em>may</em> alias
1280 the pointer. There are no calls in the loop which mod/ref the
1281 pointer.</li>
1282 </ul>
1283 If these conditions are true, we can promote the loads and stores in the
1284 loop of the pointer to use a temporary alloca'd variable. We then use
1285 the mem2reg functionality to construct the appropriate SSA form for the
1286 variable.</li>
1287 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1288</div>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]1289<!-------------------------------------------------------------------------- -->
1290<div class="doc_subsection">
1291 <a name="loop-deletion">Dead Loop Deletion Pass</a>
1292</div>
1293<div class="doc_text">
1294 <p>
1295 This file implements the Dead Loop Deletion Pass. This pass is responsible
1296 for eliminating loops with non-infinite computable trip counts that have no
1297 side effects or volatile instructions, and do not contribute to the
1298 computation of the function's return value.
1299 </p>
1300</div>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1301
1302<!-------------------------------------------------------------------------- -->
1303<div class="doc_subsection">
1304 <a name="loop-extract">Extract loops into new functions</a>
1305</div>
1306<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1307 <p>
1308 A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to
1309 extract each top-level loop into its own new function. If the loop is the
1310 <em>only</em> loop in a given function, it is not touched. This is a pass most
1311 useful for debugging via bugpoint.
1312 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1313</div>
1314
1315<!-------------------------------------------------------------------------- -->
1316<div class="doc_subsection">
1317 <a name="loop-extract-single">Extract at most one loop into a new function</a>
1318</div>
1319<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1320 <p>
1321 Similar to <a href="#loop-extract">Extract loops into new functions</a>,
1322 this pass extracts one natural loop from the program into a function if it
1323 can. This is used by bugpoint.
1324 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1325</div>
1326
1327<!-------------------------------------------------------------------------- -->
1328<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1329 <a name="loop-index-split">Index Split Loops</a>
1330</div>
1331<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1332 <p>
1333 This pass divides loop's iteration range by spliting loop such that each
1334 individual loop is executed efficiently.
1335 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1336</div>
1337
1338<!-------------------------------------------------------------------------- -->
1339<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1340 <a name="loop-reduce">Loop Strength Reduction</a>
1341</div>
1342<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1343 <p>
1344 This pass performs a strength reduction on array references inside loops that
1345 have as one or more of their components the loop induction variable. This is
1346 accomplished by creating a new value to hold the initial value of the array
1347 access for the first iteration, and then creating a new GEP instruction in
1348 the loop to increment the value by the appropriate amount.
1349 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1350</div>
1351
1352<!-------------------------------------------------------------------------- -->
1353<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1354 <a name="loop-rotate">Rotate Loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1355</div>
1356<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1357 <p>A simple loop rotation transformation.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1358</div>
1359
1360<!-------------------------------------------------------------------------- -->
1361<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1362 <a name="loop-unroll">Unroll loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1363</div>
1364<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1365 <p>
1366 This pass implements a simple loop unroller. It works best when loops have
1367 been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass,
1368 allowing it to determine the trip counts of loops easily.
1369 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1370</div>
1371
1372<!-------------------------------------------------------------------------- -->
1373<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1374 <a name="loop-unswitch">Unswitch loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1375</div>
1376<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1377 <p>
1378 This pass transforms loops that contain branches on loop-invariant conditions
1379 to have multiple loops. For example, it turns the left into the right code:
1380 </p>
1381
1382 <pre
1383>for (...) if (lic)
1384 A for (...)
1385 if (lic) A; B; C
1386 B else
1387 C for (...)
1388 A; C</pre>
1389
1390 <p>
1391 This can increase the size of the code exponentially (doubling it every time
1392 a loop is unswitched) so we only unswitch if the resultant code will be
1393 smaller than a threshold.
1394 </p>
1395
1396 <p>
1397 This pass expects LICM to be run before it to hoist invariant conditions out
1398 of the loop, to make the unswitching opportunity obvious.
1399 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1400</div>
1401
1402<!-------------------------------------------------------------------------- -->
1403<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1404 <a name="loopsimplify">Canonicalize natural loops</a>
1405</div>
1406<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1407 <p>
1408 This pass performs several transformations to transform natural loops into a
1409 simpler form, which makes subsequent analyses and transformations simpler and
1410 more effective.
1411 </p>
1412
1413 <p>
1414 Loop pre-header insertion guarantees that there is a single, non-critical
1415 entry edge from outside of the loop to the loop header. This simplifies a
1416 number of analyses and transformations, such as LICM.
1417 </p>
1418
1419 <p>
1420 Loop exit-block insertion guarantees that all exit blocks from the loop
1421 (blocks which are outside of the loop that have predecessors inside of the
1422 loop) only have predecessors from inside of the loop (and are thus dominated
1423 by the loop header). This simplifies transformations such as store-sinking
1424 that are built into LICM.
1425 </p>
1426
1427 <p>
1428 This pass also guarantees that loops will have exactly one backedge.
1429 </p>
1430
1431 <p>
1432 Note that the simplifycfg pass will clean up blocks which are split out but
1433 end up being unnecessary, so usage of this pass should not pessimize
1434 generated code.
1435 </p>
1436
1437 <p>
1438 This pass obviously modifies the CFG, but updates loop information and
1439 dominator information.
1440 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1441</div>
1442
1443<!-------------------------------------------------------------------------- -->
1444<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1445 <a name="lowerallocs">Lower allocations from instructions to calls</a>
1446</div>
1447<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1448 <p>
1449 Turn <tt>malloc</tt> and <tt>free</tt> instructions into <tt>@malloc</tt> and
1450 <tt>@free</tt> calls.
1451 </p>
1452
1453 <p>
1454 This is a target-dependent tranformation because it depends on the size of
1455 data types and alignment constraints.
1456 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1457</div>
1458
1459<!-------------------------------------------------------------------------- -->
1460<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1461 <a name="lowerinvoke">Lower invoke and unwind, for unwindless code generators</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1462</div>
1463<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1464 <p>
1465 This transformation is designed for use by code generators which do not yet
1466 support stack unwinding. This pass supports two models of exception handling
1467 lowering, the 'cheap' support and the 'expensive' support.
1468 </p>
1469
1470 <p>
1471 'Cheap' exception handling support gives the program the ability to execute
1472 any program which does not "throw an exception", by turning 'invoke'
1473 instructions into calls and by turning 'unwind' instructions into calls to
1474 abort(). If the program does dynamically use the unwind instruction, the
1475 program will print a message then abort.
1476 </p>
1477
1478 <p>
1479 'Expensive' exception handling support gives the full exception handling
1480 support to the program at the cost of making the 'invoke' instruction
1481 really expensive. It basically inserts setjmp/longjmp calls to emulate the
1482 exception handling as necessary.
1483 </p>
1484
1485 <p>
1486 Because the 'expensive' support slows down programs a lot, and EH is only
1487 used for a subset of the programs, it must be specifically enabled by the
1488 <tt>-enable-correct-eh-support</tt> option.
1489 </p>
1490
1491 <p>
1492 Note that after this pass runs the CFG is not entirely accurate (exceptional
1493 control flow edges are not correct anymore) so only very simple things should
1494 be done after the lowerinvoke pass has run (like generation of native code).
1495 This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't
1496 support the invoke instruction yet" lowering pass.
1497 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1498</div>
1499
1500<!-------------------------------------------------------------------------- -->
1501<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1502 <a name="lowersetjmp">Lower Set Jump</a>
1503</div>
1504<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1505 <p>
1506 Lowers <tt>setjmp</tt> and <tt>longjmp</tt> to use the LLVM invoke and unwind
1507 instructions as necessary.
1508 </p>
1509
1510 <p>
1511 Lowering of <tt>longjmp</tt> is fairly trivial. We replace the call with a
1512 call to the LLVM library function <tt>__llvm_sjljeh_throw_longjmp()</tt>.
1513 This unwinds the stack for us calling all of the destructors for
1514 objects allocated on the stack.
1515 </p>
1516
1517 <p>
1518 At a <tt>setjmp</tt> call, the basic block is split and the <tt>setjmp</tt>
1519 removed. The calls in a function that have a <tt>setjmp</tt> are converted to
1520 invoke where the except part checks to see if it's a <tt>longjmp</tt>
1521 exception and, if so, if it's handled in the function. If it is, then it gets
1522 the value returned by the <tt>longjmp</tt> and goes to where the basic block
1523 was split. <tt>invoke</tt> instructions are handled in a similar fashion with
1524 the original except block being executed if it isn't a <tt>longjmp</tt>
1525 except that is handled by that function.
1526 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1527</div>
1528
1529<!-------------------------------------------------------------------------- -->
1530<div class="doc_subsection">
1531 <a name="lowerswitch">Lower SwitchInst's to branches</a>
1532</div>
1533<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1534 <p>
1535 Rewrites <tt>switch</tt> instructions with a sequence of branches, which
1536 allows targets to get away with not implementing the switch instruction until
1537 it is convenient.
1538 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1539</div>
1540
1541<!-------------------------------------------------------------------------- -->
1542<div class="doc_subsection">
1543 <a name="mem2reg">Promote Memory to Register</a>
1544</div>
1545<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1546 <p>
1547 This file promotes memory references to be register references. It promotes
1548 <tt>alloca</tt> instructions which only have <tt>load</tt>s and
1549 <tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator
1550 frontiers to place <tt>phi</tt> nodes, then traversing the function in
1551 depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as
1552 appropriate. This is just the standard SSA construction algorithm to construct
1553 "pruned" SSA form.
1554 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1555</div>
1556
1557<!-------------------------------------------------------------------------- -->
1558<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]1559 <a name="memcpyopt">Optimize use of memcpy and friend</a>
1560</div>
1561<div class="doc_text">
1562 <p>
1563 This pass performs various transformations related to eliminating memcpy
1564 calls, or transforming sets of stores into memset's.
1565 </p>
1566</div>
1567
1568<!-------------------------------------------------------------------------- -->
1569<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1570 <a name="mergereturn">Unify function exit nodes</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1571</div>
1572<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1573 <p>
1574 Ensure that functions have at most one <tt>ret</tt> instruction in them.
1575 Additionally, it keeps track of which node is the new exit node of the CFG.
1576 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1577</div>
1578
1579<!-------------------------------------------------------------------------- -->
1580<div class="doc_subsection">
1581 <a name="predsimplify">Predicate Simplifier</a>
1582</div>
1583<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1584 <p>
1585 Path-sensitive optimizer. In a branch where <tt>x == y</tt>, replace uses of
1586 <tt>x</tt> with <tt>y</tt>. Permits further optimization, such as the
1587 elimination of the unreachable call:
1588 </p>
1589
1590<blockquote><pre
1591>void test(int *p, int *q)
1592{
1593 if (p != q)
1594 return;
1595
1596 if (*p != *q)
1597 foo(); // unreachable
1598}</pre></blockquote>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1599</div>
1600
1601<!-------------------------------------------------------------------------- -->
1602<div class="doc_subsection">
1603 <a name="prune-eh">Remove unused exception handling info</a>
1604</div>
1605<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1606 <p>
1607 This file implements a simple interprocedural pass which walks the call-graph,
1608 turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and
1609 only if the callee cannot throw an exception. It implements this as a
1610 bottom-up traversal of the call-graph.
1611 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1612</div>
1613
1614<!-------------------------------------------------------------------------- -->
1615<div class="doc_subsection">
1616 <a name="raiseallocs">Raise allocations from calls to instructions</a>
1617</div>
1618<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1619 <p>
1620 Converts <tt>@malloc</tt> and <tt>@free</tt> calls to <tt>malloc</tt> and
1621 <tt>free</tt> instructions.
1622 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1623</div>
1624
1625<!-------------------------------------------------------------------------- -->
1626<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1627 <a name="reassociate">Reassociate expressions</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1628</div>
1629<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1630 <p>
1631 This pass reassociates commutative expressions in an order that is designed
1632 to promote better constant propagation, GCSE, LICM, PRE, etc.
1633 </p>
1634
1635 <p>
1636 For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5)
1637 </p>
1638
1639 <p>
1640 In the implementation of this algorithm, constants are assigned rank = 0,
1641 function arguments are rank = 1, and other values are assigned ranks
1642 corresponding to the reverse post order traversal of current function
1643 (starting at 2), which effectively gives values in deep loops higher rank
1644 than values not in loops.
1645 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1646</div>
1647
1648<!-------------------------------------------------------------------------- -->
1649<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1650 <a name="reg2mem">Demote all values to stack slots</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1651</div>
1652<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1653 <p>
1654 This file demotes all registers to memory references. It is intented to be
1655 the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to
1656 <tt>load</tt> instructions, the only values live accross basic blocks are
1657 <tt>alloca</tt> instructions and <tt>load</tt> instructions before
1658 <tt>phi</tt> nodes. It is intended that this should make CFG hacking much
1659 easier. To make later hacking easier, the entry block is split into two, such
1660 that all introduced <tt>alloca</tt> instructions (and nothing else) are in the
1661 entry block.
1662 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1663</div>
1664
1665<!-------------------------------------------------------------------------- -->
1666<div class="doc_subsection">
1667 <a name="scalarrepl">Scalar Replacement of Aggregates</a>
1668</div>
1669<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1670 <p>
1671 The well-known scalar replacement of aggregates transformation. This
1672 transform breaks up <tt>alloca</tt> instructions of aggregate type (structure
1673 or array) into individual <tt>alloca</tt> instructions for each member if
1674 possible. Then, if possible, it transforms the individual <tt>alloca</tt>
1675 instructions into nice clean scalar SSA form.
1676 </p>
1677
1678 <p>
1679 This combines a simple scalar replacement of aggregates algorithm with the <a
1680 href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact,
1681 especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>,
1682 then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to
1683 promote works well.
1684 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1685</div>
1686
1687<!-------------------------------------------------------------------------- -->
1688<div class="doc_subsection">
1689 <a name="sccp">Sparse Conditional Constant Propagation</a>
1690</div>
1691<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1692 <p>
1693 Sparse conditional constant propagation and merging, which can be summarized
1694 as:
1695 </p>
1696
1697 <ol>
1698 <li>Assumes values are constant unless proven otherwise</li>
1699 <li>Assumes BasicBlocks are dead unless proven otherwise</li>
1700 <li>Proves values to be constant, and replaces them with constants</li>
1701 <li>Proves conditional branches to be unconditional</li>
1702 </ol>
1703
1704 <p>
1705 Note that this pass has a habit of making definitions be dead. It is a good
1706 idea to to run a DCE pass sometime after running this pass.
1707 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1708</div>
1709
1710<!-------------------------------------------------------------------------- -->
1711<div class="doc_subsection">
1712 <a name="simplify-libcalls">Simplify well-known library calls</a>
1713</div>
1714<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1715 <p>
1716 Applies a variety of small optimizations for calls to specific well-known
1717 function calls (e.g. runtime library functions). For example, a call
1718 <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be
1719 transformed into simply <tt>return 3</tt>.
1720 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1721</div>
1722
1723<!-------------------------------------------------------------------------- -->
1724<div class="doc_subsection">
1725 <a name="simplifycfg">Simplify the CFG</a>
1726</div>
1727<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1728 <p>
1729 Performs dead code elimination and basic block merging. Specifically:
1730 </p>
1731
1732 <ol>
1733 <li>Removes basic blocks with no predecessors.</li>
1734 <li>Merges a basic block into its predecessor if there is only one and the
1735 predecessor only has one successor.</li>
1736 <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li>
1737 <li>Eliminates a basic block that only contains an unconditional
1738 branch.</li>
1739 </ol>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1740</div>
1741
1742<!-------------------------------------------------------------------------- -->
1743<div class="doc_subsection">
1744 <a name="strip">Strip all symbols from a module</a>
1745</div>
1746<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1747 <p>
1748 Performs code stripping. This transformation can delete:
1749 </p>
1750
1751 <ol>
1752 <li>names for virtual registers</li>
1753 <li>symbols for internal globals and functions</li>
1754 <li>debug information</li>
1755 </ol>
1756
1757 <p>
1758 Note that this transformation makes code much less readable, so it should
1759 only be used in situations where the <tt>strip</tt> utility would be used,
1760 such as reducing code size or making it harder to reverse engineer code.
1761 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1762</div>
1763
1764<!-------------------------------------------------------------------------- -->
1765<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]1766 <a name="strip-dead-prototypes">Remove unused function declarations</a>
1767</div>
1768<div class="doc_text">
1769 <p>
1770 This pass loops over all of the functions in the input module, looking for
1771 dead declarations and removes them. Dead declarations are declarations of
1772 functions for which no implementation is available (i.e., declarations for
1773 unused library functions).
1774 </p>
1775</div>
1776
1777<!-------------------------------------------------------------------------- -->
1778<div class="doc_subsection">
1779 <a name="sretpromotion">Promote sret arguments</a>
1780</div>
1781<div class="doc_text">
1782 <p>
1783 This pass finds functions that return a struct (using a pointer to the struct
1784 as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and
1785 replaces them with a new function that simply returns each of the elements of
1786 that struct (using multiple return values).
1787 </p>
1788
1789 <p>
1790 This pass works under a number of conditions:
1791 </p>
1792
1793 <ul>
1794 <li>The returned struct must not contain other structs</li>
1795 <li>The returned struct must only be used to load values from</li>
1796 <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li>
1797 </ul>
1798</div>
1799
1800<!-------------------------------------------------------------------------- -->
1801<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1802 <a name="tailcallelim">Tail Call Elimination</a>
1803</div>
1804<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1805 <p>
1806 This file transforms calls of the current function (self recursion) followed
1807 by a return instruction with a branch to the entry of the function, creating
1808 a loop. This pass also implements the following extensions to the basic
1809 algorithm:
1810 </p>
1811
1812 <ul>
1813 <li>Trivial instructions between the call and return do not prevent the
1814 transformation from taking place, though currently the analysis cannot
1815 support moving any really useful instructions (only dead ones).
1816 <li>This pass transforms functions that are prevented from being tail
1817 recursive by an associative expression to use an accumulator variable,
1818 thus compiling the typical naive factorial or <tt>fib</tt> implementation
1819 into efficient code.
1820 <li>TRE is performed if the function returns void, if the return
1821 returns the result returned by the call, or if the function returns a
1822 run-time constant on all exits from the function. It is possible, though
1823 unlikely, that the return returns something else (like constant 0), and
1824 can still be TRE'd. It can be TRE'd if <em>all other</em> return
1825 instructions in the function return the exact same value.
1826 <li>If it can prove that callees do not access theier caller stack frame,
1827 they are marked as eligible for tail call elimination (by the code
1828 generator).
1829 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1830</div>
1831
1832<!-------------------------------------------------------------------------- -->
1833<div class="doc_subsection">
1834 <a name="tailduplicate">Tail Duplication</a>
1835</div>
1836<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1837 <p>
1838 This pass performs a limited form of tail duplication, intended to simplify
1839 CFGs by removing some unconditional branches. This pass is necessary to
1840 straighten out loops created by the C front-end, but also is capable of
1841 making other code nicer. After this pass is run, the CFG simplify pass
1842 should be run to clean up the mess.
1843 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1844</div>
1845
1846<!-- ======================================================================= -->
1847<div class="doc_section"> <a name="transform">Utility Passes</a></div>
1848<div class="doc_text">
1849 <p>This section describes the LLVM Utility Passes.</p>
1850</div>
1851
1852<!-------------------------------------------------------------------------- -->
1853<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1854 <a name="deadarghaX0r">Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1855</div>
1856<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1857 <p>
1858 Same as dead argument elimination, but deletes arguments to functions which
1859 are external. This is only for use by <a
1860 href="Bugpoint.html">bugpoint</a>.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1861</div>
1862
1863<!-------------------------------------------------------------------------- -->
1864<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1865 <a name="extract-blocks">Extract Basic Blocks From Module (for bugpoint use)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1866</div>
1867<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1868 <p>
1869 This pass is used by bugpoint to extract all blocks from the module into their
1870 own functions.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1871</div>
1872
1873<!-------------------------------------------------------------------------- -->
1874<div class="doc_subsection">
Gordon Henriksen90a52142007-11-05 02:05:35 +0000[diff] [blame]1875 <a name="preverify">Preliminary module verification</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1876</div>
1877<div class="doc_text">
Gordon Henriksen90a52142007-11-05 02:05:35 +0000[diff] [blame]1878 <p>
1879 Ensures that the module is in the form required by the <a
1880 href="#verifier">Module Verifier</a> pass.
1881 </p>
1882
1883 <p>
1884 Running the verifier runs this pass automatically, so there should be no need
1885 to use it directly.
1886 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1887</div>
1888
1889<!-------------------------------------------------------------------------- -->
1890<div class="doc_subsection">
1891 <a name="verify">Module Verifier</a>
1892</div>
1893<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1894 <p>
1895 Verifies an LLVM IR code. This is useful to run after an optimization which is
1896 undergoing testing. Note that <tt>llvm-as</tt> verifies its input before
1897 emitting bitcode, and also that malformed bitcode is likely to make LLVM
1898 crash. All language front-ends are therefore encouraged to verify their output
1899 before performing optimizing transformations.
1900 </p>
1901
Gordon Henriksen23a8ce52007-11-04 18:14:08 +0000[diff] [blame]1902 <ul>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1903 <li>Both of a binary operator's parameters are of the same type.</li>
1904 <li>Verify that the indices of mem access instructions match other
1905 operands.</li>
1906 <li>Verify that arithmetic and other things are only performed on
1907 first-class types. Verify that shifts and logicals only happen on
1908 integrals f.e.</li>
1909 <li>All of the constants in a switch statement are of the correct type.</li>
1910 <li>The code is in valid SSA form.</li>
1911 <li>It should be illegal to put a label into any other type (like a
1912 structure) or to return one. [except constant arrays!]</li>
Nick Lewycky0c78ac12008-03-28 06:46:51 +0000[diff] [blame]1913 <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]1914 invalid.</li>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1915 <li>PHI nodes must have an entry for each predecessor, with no extras.</li>
1916 <li>PHI nodes must be the first thing in a basic block, all grouped
1917 together.</li>
1918 <li>PHI nodes must have at least one entry.</li>
1919 <li>All basic blocks should only end with terminator insts, not contain
1920 them.</li>
1921 <li>The entry node to a function must not have predecessors.</li>
1922 <li>All Instructions must be embedded into a basic block.</li>
1923 <li>Functions cannot take a void-typed parameter.</li>
1924 <li>Verify that a function's argument list agrees with its declared
1925 type.</li>
1926 <li>It is illegal to specify a name for a void value.</li>
1927 <li>It is illegal to have a internal global value with no initializer.</li>
1928 <li>It is illegal to have a ret instruction that returns a value that does
1929 not agree with the function return value type.</li>
1930 <li>Function call argument types match the function prototype.</li>
1931 <li>All other things that are tested by asserts spread about the code.</li>
Gordon Henriksen23a8ce52007-11-04 18:14:08 +0000[diff] [blame]1932 </ul>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1933
1934 <p>
1935 Note that this does not provide full security verification (like Java), but
1936 instead just tries to ensure that code is well-formed.
1937 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1938</div>
1939
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1940<!-------------------------------------------------------------------------- -->
1941<div class="doc_subsection">
1942 <a name="view-cfg">View CFG of function</a>
1943</div>
1944<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1945 <p>
1946 Displays the control flow graph using the GraphViz tool.
1947 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1948</div>
1949
1950<!-------------------------------------------------------------------------- -->
1951<div class="doc_subsection">
1952 <a name="view-cfg-only">View CFG of function (with no function bodies)</a>
1953</div>
1954<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1955 <p>
1956 Displays the control flow graph using the GraphViz tool, but omitting function
1957 bodies.
1958 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1959</div>
1960
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1961<!-- *********************************************************************** -->
1962
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Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1969
1970 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
1971 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
1972 Last modified: $Date$
1973</address>
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