1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <title>LLVM's Analysis and Transform Passes</title> 6 <link rel="stylesheet" href="llvm.css" type="text/css"> 7 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 8 </head> 9 <body> 10 11 <!-- 12 13 If Passes.html is up to date, the following "one-liner" should print 14 an empty diff. 15 16 egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \ 17 -e '^ <a name=".*">.*</a>$' < Passes.html >html; \ 18 perl >help <<'EOT' && diff -u help html; rm -f help html 19 open HTML, "<Passes.html" or die "open: Passes.html: $!\n"; 20 while (<HTML>) { 21 m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next; 22 $order{$1} = sprintf("%03d", 1 + int %order); 23 } 24 open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n"; 25 while (<HELP>) { 26 m:^ -([^ ]+) +- (.*)$: 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\">-$1: $2</a>\n"; 31 } 32 @x = map { s/^\d\d\d//; $_ } sort @x; 33 @y = map { s/^\d\d\d//; $_ } sort @y; 34 print @x, @y; 35 EOT 36 37 This (real) one-liner can also be helpful when converting comments to HTML: 38 39 perl -e '$/ = 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 41 --> 42 43 <h1>LLVM's Analysis and Transform Passes</h1> 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"> 53 <p>Written by <a href="mailto:rspencer (a] x10sys.com">Reid Spencer</a> 54 and Gordon Henriksen</p> 55 </div> 56 57 <!-- ======================================================================= --> 58 <h2><a name="intro">Introduction</a></h2> 59 <div> 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. 63 The table below divides the passes that LLVM provides into three categories. 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. 67 Utility passes provides some utility but don't otherwise fit categorization. 68 For example passes to extract functions to bitcode or write a module to 69 bitcode are neither analysis nor transform passes. 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 73 <table> 74 <tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr> 75 <tr><th>Option</th><th>Name</th></tr> 76 <tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr> 77 <tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (stateless AA impl)</td></tr> 78 <tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr> 79 <tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr> 80 <tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr> 81 <tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr> 82 <tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr> 83 <tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr> 84 <tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr> 85 <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> 86 <tr><td><a href="#dot-dom">-dot-dom</a></td><td>Print dominance tree of function to 'dot' file</td></tr> 87 <tr><td><a href="#dot-dom-only">-dot-dom-only</a></td><td>Print dominance tree of function to 'dot' file (with no function bodies)</td></tr> 88 <tr><td><a href="#dot-postdom">-dot-postdom</a></td><td>Print postdominance tree of function to 'dot' file</td></tr> 89 <tr><td><a href="#dot-postdom-only">-dot-postdom-only</a></td><td>Print postdominance tree of function to 'dot' file (with no function bodies)</td></tr> 90 <tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr> 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="#iv-users">-iv-users</a></td><td>Induction Variable Users</td></tr> 94 <tr><td><a href="#lazy-value-info">-lazy-value-info</a></td><td>Lazy Value Information Analysis</td></tr> 95 <tr><td><a href="#lda">-lda</a></td><td>Loop Dependence Analysis</td></tr> 96 <tr><td><a href="#libcall-aa">-libcall-aa</a></td><td>LibCall Alias Analysis</td></tr> 97 <tr><td><a href="#lint">-lint</a></td><td>Statically lint-checks LLVM IR</td></tr> 98 <tr><td><a href="#loops">-loops</a></td><td>Natural Loop Information</td></tr> 99 <tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr> 100 <tr><td><a href="#module-debuginfo">-module-debuginfo</a></td><td>Decodes module-level debug info</td></tr> 101 <tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr> 102 <tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr> 103 <tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr> 104 <tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr> 105 <tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr> 106 <tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr> 107 <tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr> 108 <tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr> 109 <tr><td><a href="#print-dbginfo">-print-dbginfo</a></td><td>Print debug info in human readable form</td></tr> 110 <tr><td><a href="#print-dom-info">-print-dom-info</a></td><td>Dominator Info Printer</td></tr> 111 <tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr> 112 <tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr> 113 <tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr> 114 <tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr> 115 <tr><td><a href="#profile-estimator">-profile-estimator</a></td><td>Estimate profiling information</td></tr> 116 <tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr> 117 <tr><td><a href="#profile-verifier">-profile-verifier</a></td><td>Verify profiling information</td></tr> 118 <tr><td><a href="#regions">-regions</a></td><td>Detect single entry single exit regions</td></tr> 119 <tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr> 120 <tr><td><a href="#scev-aa">-scev-aa</a></td><td>ScalarEvolution-based Alias Analysis</td></tr> 121 <tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr> 122 123 124 <tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr> 125 <tr><th>Option</th><th>Name</th></tr> 126 <tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr> 127 <tr><td><a href="#always-inline">-always-inline</a></td><td>Inliner for always_inline functions</td></tr> 128 <tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr> 129 <tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr> 130 <tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr> 131 <tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr> 132 <tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr> 133 <tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr> 134 <tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr> 135 <tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr> 136 <tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr> 137 <tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr> 138 <tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr> 139 <tr><td><a href="#functionattrs">-functionattrs</a></td><td>Deduce function attributes</td></tr> 140 <tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr> 141 <tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr> 142 <tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr> 143 <tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr> 144 <tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr> 145 <tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr> 146 <tr><td><a href="#insert-optimal-edge-profiling">-insert-optimal-edge-profiling</a></td><td>Insert optimal instrumentation for edge profiling</td></tr> 147 <tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr> 148 <tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr> 149 <tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr> 150 <tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr> 151 <tr><td><a href="#jump-threading">-jump-threading</a></td><td>Jump Threading</td></tr> 152 <tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr> 153 <tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr> 154 <tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Delete dead loops</td></tr> 155 <tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr> 156 <tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr> 157 <tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr> 158 <tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr> 159 <tr><td><a href="#loop-simplify">-loop-simplify</a></td><td>Canonicalize natural loops</td></tr> 160 <tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr> 161 <tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr> 162 <tr><td><a href="#loweratomic">-loweratomic</a></td><td>Lower atomic intrinsics to non-atomic form</td></tr> 163 <tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr> 164 <tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr> 165 <tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr> 166 <tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>MemCpy Optimization</td></tr> 167 <tr><td><a href="#mergefunc">-mergefunc</a></td><td>Merge Functions</td></tr> 168 <tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr> 169 <tr><td><a href="#partial-inliner">-partial-inliner</a></td><td>Partial Inliner</td></tr> 170 <tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr> 171 <tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr> 172 <tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr> 173 <tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates (DT)</td></tr> 174 <tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr> 175 <tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr> 176 <tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr> 177 <tr><td><a href="#sink">-sink</a></td><td>Code sinking</td></tr> 178 <tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments to multiple ret values</td></tr> 179 <tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr> 180 <tr><td><a href="#strip-dead-debug-info">-strip-dead-debug-info</a></td><td>Strip debug info for unused symbols</td></tr> 181 <tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Strip Unused Function Prototypes</td></tr> 182 <tr><td><a href="#strip-debug-declare">-strip-debug-declare</a></td><td>Strip all llvm.dbg.declare intrinsics</td></tr> 183 <tr><td><a href="#strip-nondebug">-strip-nondebug</a></td><td>Strip all symbols, except dbg symbols, from a module</td></tr> 184 <tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr> 185 <tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr> 186 187 188 <tr><th colspan="2"><b>UTILITY PASSES</b></th></tr> 189 <tr><th>Option</th><th>Name</th></tr> 190 <tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr> 191 <tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr> 192 <tr><td><a href="#instnamer">-instnamer</a></td><td>Assign names to anonymous instructions</td></tr> 193 <tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr> 194 <tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr> 195 <tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr> 196 <tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr> 197 <tr><td><a href="#view-dom">-view-dom</a></td><td>View dominance tree of function</td></tr> 198 <tr><td><a href="#view-dom-only">-view-dom-only</a></td><td>View dominance tree of function (with no function bodies)</td></tr> 199 <tr><td><a href="#view-postdom">-view-postdom</a></td><td>View postdominance tree of function</td></tr> 200 <tr><td><a href="#view-postdom-only">-view-postdom-only</a></td><td>View postdominance tree of function (with no function bodies)</td></tr> 201 </table> 202 203 </div> 204 205 <!-- ======================================================================= --> 206 <h2><a name="analyses">Analysis Passes</a></h2> 207 <div> 208 <p>This section describes the LLVM Analysis Passes.</p> 209 210 <!-------------------------------------------------------------------------- --> 211 <h3> 212 <a name="aa-eval">-aa-eval: Exhaustive Alias Analysis Precision Evaluator</a> 213 </h3> 214 <div> 215 <p>This is a simple N^2 alias analysis accuracy evaluator. 216 Basically, for each function in the program, it simply queries to see how the 217 alias analysis implementation answers alias queries between each pair of 218 pointers in the function.</p> 219 220 <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco 221 Spadini, and Wojciech Stryjewski.</p> 222 </div> 223 224 <!-------------------------------------------------------------------------- --> 225 <h3> 226 <a name="basicaa">-basicaa: Basic Alias Analysis (stateless AA impl)</a> 227 </h3> 228 <div> 229 <p> 230 This is the default implementation of the Alias Analysis interface 231 that simply implements a few identities (two different globals cannot alias, 232 etc), but otherwise does no analysis. 233 </p> 234 </div> 235 236 <!-------------------------------------------------------------------------- --> 237 <h3> 238 <a name="basiccg">-basiccg: Basic CallGraph Construction</a> 239 </h3> 240 <div> 241 <p>Yet to be written.</p> 242 </div> 243 244 <!-------------------------------------------------------------------------- --> 245 <h3> 246 <a name="count-aa">-count-aa: Count Alias Analysis Query Responses</a> 247 </h3> 248 <div> 249 <p> 250 A pass which can be used to count how many alias queries 251 are being made and how the alias analysis implementation being used responds. 252 </p> 253 </div> 254 255 <!-------------------------------------------------------------------------- --> 256 <h3> 257 <a name="debug-aa">-debug-aa: AA use debugger</a> 258 </h3> 259 <div> 260 <p> 261 This simple pass checks alias analysis users to ensure that if they 262 create a new value, they do not query AA without informing it of the value. 263 It acts as a shim over any other AA pass you want. 264 </p> 265 266 <p> 267 Yes keeping track of every value in the program is expensive, but this is 268 a debugging pass. 269 </p> 270 </div> 271 272 <!-------------------------------------------------------------------------- --> 273 <h3> 274 <a name="domfrontier">-domfrontier: Dominance Frontier Construction</a> 275 </h3> 276 <div> 277 <p> 278 This pass is a simple dominator construction algorithm for finding forward 279 dominator frontiers. 280 </p> 281 </div> 282 283 <!-------------------------------------------------------------------------- --> 284 <h3> 285 <a name="domtree">-domtree: Dominator Tree Construction</a> 286 </h3> 287 <div> 288 <p> 289 This pass is a simple dominator construction algorithm for finding forward 290 dominators. 291 </p> 292 </div> 293 294 <!-------------------------------------------------------------------------- --> 295 <h3> 296 <a name="dot-callgraph">-dot-callgraph: Print Call Graph to 'dot' file</a> 297 </h3> 298 <div> 299 <p> 300 This pass, only available in <code>opt</code>, prints the call graph into a 301 <code>.dot</code> graph. This graph can then be processed with the "dot" tool 302 to convert it to postscript or some other suitable format. 303 </p> 304 </div> 305 306 <!-------------------------------------------------------------------------- --> 307 <h3> 308 <a name="dot-cfg">-dot-cfg: Print CFG of function to 'dot' file</a> 309 </h3> 310 <div> 311 <p> 312 This pass, only available in <code>opt</code>, prints the control flow graph 313 into a <code>.dot</code> graph. This graph can then be processed with the 314 "dot" tool to convert it to postscript or some other suitable format. 315 </p> 316 </div> 317 318 <!-------------------------------------------------------------------------- --> 319 <h3> 320 <a name="dot-cfg-only">-dot-cfg-only: Print CFG of function to 'dot' file (with no function bodies)</a> 321 </h3> 322 <div> 323 <p> 324 This pass, only available in <code>opt</code>, prints the control flow graph 325 into a <code>.dot</code> graph, omitting the function bodies. This graph can 326 then be processed with the "dot" tool to convert it to postscript or some 327 other suitable format. 328 </p> 329 </div> 330 331 <!-------------------------------------------------------------------------- --> 332 <h3> 333 <a name="dot-dom">-dot-dom: Print dominance tree of function to 'dot' file</a> 334 </h3> 335 <div> 336 <p> 337 This pass, only available in <code>opt</code>, prints the dominator tree 338 into a <code>.dot</code> graph. This graph can then be processed with the 339 "dot" tool to convert it to postscript or some other suitable format. 340 </p> 341 </div> 342 343 <!-------------------------------------------------------------------------- --> 344 <h3> 345 <a name="dot-dom-only">-dot-dom-only: Print dominance tree of function to 'dot' file (with no function bodies)</a> 346 </h3> 347 <div> 348 <p> 349 This pass, only available in <code>opt</code>, prints the dominator tree 350 into a <code>.dot</code> graph, omitting the function bodies. This graph can 351 then be processed with the "dot" tool to convert it to postscript or some 352 other suitable format. 353 </p> 354 </div> 355 356 <!-------------------------------------------------------------------------- --> 357 <h3> 358 <a name="dot-postdom">-dot-postdom: Print postdominance tree of function to 'dot' file</a> 359 </h3> 360 <div> 361 <p> 362 This pass, only available in <code>opt</code>, prints the post dominator tree 363 into a <code>.dot</code> graph. This graph can then be processed with the 364 "dot" tool to convert it to postscript or some other suitable format. 365 </p> 366 </div> 367 368 <!-------------------------------------------------------------------------- --> 369 <h3> 370 <a name="dot-postdom-only">-dot-postdom-only: Print postdominance tree of function to 'dot' file (with no function bodies)</a> 371 </h3> 372 <div> 373 <p> 374 This pass, only available in <code>opt</code>, prints the post dominator tree 375 into a <code>.dot</code> graph, omitting the function bodies. This graph can 376 then be processed with the "dot" tool to convert it to postscript or some 377 other suitable format. 378 </p> 379 </div> 380 381 <!-------------------------------------------------------------------------- --> 382 <h3> 383 <a name="globalsmodref-aa">-globalsmodref-aa: Simple mod/ref analysis for globals</a> 384 </h3> 385 <div> 386 <p> 387 This simple pass provides alias and mod/ref information for global values 388 that do not have their address taken, and keeps track of whether functions 389 read or write memory (are "pure"). For this simple (but very common) case, 390 we can provide pretty accurate and useful information. 391 </p> 392 </div> 393 394 <!-------------------------------------------------------------------------- --> 395 <h3> 396 <a name="instcount">-instcount: Counts the various types of Instructions</a> 397 </h3> 398 <div> 399 <p> 400 This pass collects the count of all instructions and reports them 401 </p> 402 </div> 403 404 <!-------------------------------------------------------------------------- --> 405 <h3> 406 <a name="intervals">-intervals: Interval Partition Construction</a> 407 </h3> 408 <div> 409 <p> 410 This analysis calculates and represents the interval partition of a function, 411 or a preexisting interval partition. 412 </p> 413 414 <p> 415 In this way, the interval partition may be used to reduce a flow graph down 416 to its degenerate single node interval partition (unless it is irreducible). 417 </p> 418 </div> 419 420 <!-------------------------------------------------------------------------- --> 421 <h3> 422 <a name="iv-users">-iv-users: Induction Variable Users</a> 423 </h3> 424 <div> 425 <p>Bookkeeping for "interesting" users of expressions computed from 426 induction variables.</p> 427 </div> 428 429 <!-------------------------------------------------------------------------- --> 430 <h3> 431 <a name="lazy-value-info">-lazy-value-info: Lazy Value Information Analysis</a> 432 </h3> 433 <div> 434 <p>Interface for lazy computation of value constraint information.</p> 435 </div> 436 437 <!-------------------------------------------------------------------------- --> 438 <h3> 439 <a name="lda">-lda: Loop Dependence Analysis</a> 440 </h3> 441 <div> 442 <p>Loop dependence analysis framework, which is used to detect dependences in 443 memory accesses in loops.</p> 444 </div> 445 446 <!-------------------------------------------------------------------------- --> 447 <h3> 448 <a name="libcall-aa">-libcall-aa: LibCall Alias Analysis</a> 449 </h3> 450 <div> 451 <p>LibCall Alias Analysis.</p> 452 </div> 453 454 <!-------------------------------------------------------------------------- --> 455 <h3> 456 <a name="lint">-lint: Statically lint-checks LLVM IR</a> 457 </h3> 458 <div> 459 <p>This pass statically checks for common and easily-identified constructs 460 which produce undefined or likely unintended behavior in LLVM IR.</p> 461 462 <p>It is not a guarantee of correctness, in two ways. First, it isn't 463 comprehensive. There are checks which could be done statically which are 464 not yet implemented. Some of these are indicated by TODO comments, but 465 those aren't comprehensive either. Second, many conditions cannot be 466 checked statically. This pass does no dynamic instrumentation, so it 467 can't check for all possible problems.</p> 468 469 <p>Another limitation is that it assumes all code will be executed. A store 470 through a null pointer in a basic block which is never reached is harmless, 471 but this pass will warn about it anyway.</p> 472 473 <p>Optimization passes may make conditions that this pass checks for more or 474 less obvious. If an optimization pass appears to be introducing a warning, 475 it may be that the optimization pass is merely exposing an existing 476 condition in the code.</p> 477 478 <p>This code may be run before instcombine. In many cases, instcombine checks 479 for the same kinds of things and turns instructions with undefined behavior 480 into unreachable (or equivalent). Because of this, this pass makes some 481 effort to look through bitcasts and so on. 482 </p> 483 </div> 484 485 <!-------------------------------------------------------------------------- --> 486 <h3> 487 <a name="loops">-loops: Natural Loop Information</a> 488 </h3> 489 <div> 490 <p> 491 This analysis is used to identify natural loops and determine the loop depth 492 of various nodes of the CFG. Note that the loops identified may actually be 493 several natural loops that share the same header node... not just a single 494 natural loop. 495 </p> 496 </div> 497 498 <!-------------------------------------------------------------------------- --> 499 <h3> 500 <a name="memdep">-memdep: Memory Dependence Analysis</a> 501 </h3> 502 <div> 503 <p> 504 An analysis that determines, for a given memory operation, what preceding 505 memory operations it depends on. It builds on alias analysis information, and 506 tries to provide a lazy, caching interface to a common kind of alias 507 information query. 508 </p> 509 </div> 510 511 <!-------------------------------------------------------------------------- --> 512 <h3> 513 <a name="module-debuginfo">-module-debuginfo: Decodes module-level debug info</a> 514 </h3> 515 <div> 516 <p>This pass decodes the debug info metadata in a module and prints in a 517 (sufficiently-prepared-) human-readable form. 518 519 For example, run this pass from opt along with the -analyze option, and 520 it'll print to standard output. 521 </p> 522 </div> 523 524 <!-------------------------------------------------------------------------- --> 525 <h3> 526 <a name="no-aa">-no-aa: No Alias Analysis (always returns 'may' alias)</a> 527 </h3> 528 <div> 529 <p> 530 Always returns "I don't know" for alias queries. NoAA is unlike other alias 531 analysis implementations, in that it does not chain to a previous analysis. As 532 such it doesn't follow many of the rules that other alias analyses must. 533 </p> 534 </div> 535 536 <!-------------------------------------------------------------------------- --> 537 <h3> 538 <a name="no-profile">-no-profile: No Profile Information</a> 539 </h3> 540 <div> 541 <p> 542 The default "no profile" implementation of the abstract 543 <code>ProfileInfo</code> interface. 544 </p> 545 </div> 546 547 <!-------------------------------------------------------------------------- --> 548 <h3> 549 <a name="postdomfrontier">-postdomfrontier: Post-Dominance Frontier Construction</a> 550 </h3> 551 <div> 552 <p> 553 This pass is a simple post-dominator construction algorithm for finding 554 post-dominator frontiers. 555 </p> 556 </div> 557 558 <!-------------------------------------------------------------------------- --> 559 <h3> 560 <a name="postdomtree">-postdomtree: Post-Dominator Tree Construction</a> 561 </h3> 562 <div> 563 <p> 564 This pass is a simple post-dominator construction algorithm for finding 565 post-dominators. 566 </p> 567 </div> 568 569 <!-------------------------------------------------------------------------- --> 570 <h3> 571 <a name="print-alias-sets">-print-alias-sets: Alias Set Printer</a> 572 </h3> 573 <div> 574 <p>Yet to be written.</p> 575 </div> 576 577 <!-------------------------------------------------------------------------- --> 578 <h3> 579 <a name="print-callgraph">-print-callgraph: Print a call graph</a> 580 </h3> 581 <div> 582 <p> 583 This pass, only available in <code>opt</code>, prints the call graph to 584 standard error in a human-readable form. 585 </p> 586 </div> 587 588 <!-------------------------------------------------------------------------- --> 589 <h3> 590 <a name="print-callgraph-sccs">-print-callgraph-sccs: Print SCCs of the Call Graph</a> 591 </h3> 592 <div> 593 <p> 594 This pass, only available in <code>opt</code>, prints the SCCs of the call 595 graph to standard error in a human-readable form. 596 </p> 597 </div> 598 599 <!-------------------------------------------------------------------------- --> 600 <h3> 601 <a name="print-cfg-sccs">-print-cfg-sccs: Print SCCs of each function CFG</a> 602 </h3> 603 <div> 604 <p> 605 This pass, only available in <code>opt</code>, prints the SCCs of each 606 function CFG to standard error in a human-readable form. 607 </p> 608 </div> 609 610 <!-------------------------------------------------------------------------- --> 611 <h3> 612 <a name="print-dbginfo">-print-dbginfo: Print debug info in human readable form</a> 613 </h3> 614 <div> 615 <p>Pass that prints instructions, and associated debug info:</p> 616 <ul> 617 618 <li>source/line/col information</li> 619 <li>original variable name</li> 620 <li>original type name</li> 621 </ul> 622 </div> 623 624 <!-------------------------------------------------------------------------- --> 625 <h3> 626 <a name="print-dom-info">-print-dom-info: Dominator Info Printer</a> 627 </h3> 628 <div> 629 <p>Dominator Info Printer.</p> 630 </div> 631 632 <!-------------------------------------------------------------------------- --> 633 <h3> 634 <a name="print-externalfnconstants">-print-externalfnconstants: Print external fn callsites passed constants</a> 635 </h3> 636 <div> 637 <p> 638 This pass, only available in <code>opt</code>, prints out call sites to 639 external functions that are called with constant arguments. This can be 640 useful when looking for standard library functions we should constant fold 641 or handle in alias analyses. 642 </p> 643 </div> 644 645 <!-------------------------------------------------------------------------- --> 646 <h3> 647 <a name="print-function">-print-function: Print function to stderr</a> 648 </h3> 649 <div> 650 <p> 651 The <code>PrintFunctionPass</code> class is designed to be pipelined with 652 other <code>FunctionPass</code>es, and prints out the functions of the module 653 as they are processed. 654 </p> 655 </div> 656 657 <!-------------------------------------------------------------------------- --> 658 <h3> 659 <a name="print-module">-print-module: Print module to stderr</a> 660 </h3> 661 <div> 662 <p> 663 This pass simply prints out the entire module when it is executed. 664 </p> 665 </div> 666 667 <!-------------------------------------------------------------------------- --> 668 <h3> 669 <a name="print-used-types">-print-used-types: Find Used Types</a> 670 </h3> 671 <div> 672 <p> 673 This pass is used to seek out all of the types in use by the program. Note 674 that this analysis explicitly does not include types only used by the symbol 675 table. 676 </div> 677 678 <!-------------------------------------------------------------------------- --> 679 <h3> 680 <a name="profile-estimator">-profile-estimator: Estimate profiling information</a> 681 </h3> 682 <div> 683 <p>Profiling information that estimates the profiling information 684 in a very crude and unimaginative way. 685 </p> 686 </div> 687 688 <!-------------------------------------------------------------------------- --> 689 <h3> 690 <a name="profile-loader">-profile-loader: Load profile information from llvmprof.out</a> 691 </h3> 692 <div> 693 <p> 694 A concrete implementation of profiling information that loads the information 695 from a profile dump file. 696 </p> 697 </div> 698 699 <!-------------------------------------------------------------------------- --> 700 <h3> 701 <a name="profile-verifier">-profile-verifier: Verify profiling information</a> 702 </h3> 703 <div> 704 <p>Pass that checks profiling information for plausibility.</p> 705 </div> 706 <h3> 707 <a name="regions">-regions: Detect single entry single exit regions</a> 708 </h3> 709 <div> 710 <p> 711 The <code>RegionInfo</code> pass detects single entry single exit regions in a 712 function, where a region is defined as any subgraph that is connected to the 713 remaining graph at only two spots. Furthermore, an hierarchical region tree is 714 built. 715 </p> 716 </div> 717 718 <!-------------------------------------------------------------------------- --> 719 <h3> 720 <a name="scalar-evolution">-scalar-evolution: Scalar Evolution Analysis</a> 721 </h3> 722 <div> 723 <p> 724 The <code>ScalarEvolution</code> analysis can be used to analyze and 725 catagorize scalar expressions in loops. It specializes in recognizing general 726 induction variables, representing them with the abstract and opaque 727 <code>SCEV</code> class. Given this analysis, trip counts of loops and other 728 important properties can be obtained. 729 </p> 730 731 <p> 732 This analysis is primarily useful for induction variable substitution and 733 strength reduction. 734 </p> 735 </div> 736 737 <!-------------------------------------------------------------------------- --> 738 <h3> 739 <a name="scev-aa">-scev-aa: ScalarEvolution-based Alias Analysis</a> 740 </h3> 741 <div> 742 <p>Simple alias analysis implemented in terms of ScalarEvolution queries. 743 744 This differs from traditional loop dependence analysis in that it tests 745 for dependencies within a single iteration of a loop, rather than 746 dependencies between different iterations. 747 748 ScalarEvolution has a more complete understanding of pointer arithmetic 749 than BasicAliasAnalysis' collection of ad-hoc analyses. 750 </p> 751 </div> 752 753 <!-------------------------------------------------------------------------- --> 754 <h3> 755 <a name="targetdata">-targetdata: Target Data Layout</a> 756 </h3> 757 <div> 758 <p>Provides other passes access to information on how the size and alignment 759 required by the the target ABI for various data types.</p> 760 </div> 761 762 </div> 763 764 <!-- ======================================================================= --> 765 <h2><a name="transforms">Transform Passes</a></h2> 766 <div> 767 <p>This section describes the LLVM Transform Passes.</p> 768 769 <!-------------------------------------------------------------------------- --> 770 <h3> 771 <a name="adce">-adce: Aggressive Dead Code Elimination</a> 772 </h3> 773 <div> 774 <p>ADCE aggressively tries to eliminate code. This pass is similar to 775 <a href="#dce">DCE</a> but it assumes that values are dead until proven 776 otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to 777 the liveness of values.</p> 778 </div> 779 780 <!-------------------------------------------------------------------------- --> 781 <h3> 782 <a name="always-inline">-always-inline: Inliner for always_inline functions</a> 783 </h3> 784 <div> 785 <p>A custom inliner that handles only functions that are marked as 786 "always inline".</p> 787 </div> 788 789 <!-------------------------------------------------------------------------- --> 790 <h3> 791 <a name="argpromotion">-argpromotion: Promote 'by reference' arguments to scalars</a> 792 </h3> 793 <div> 794 <p> 795 This pass promotes "by reference" arguments to be "by value" arguments. In 796 practice, this means looking for internal functions that have pointer 797 arguments. If it can prove, through the use of alias analysis, that an 798 argument is *only* loaded, then it can pass the value into the function 799 instead of the address of the value. This can cause recursive simplification 800 of code and lead to the elimination of allocas (especially in C++ template 801 code like the STL). 802 </p> 803 804 <p> 805 This pass also handles aggregate arguments that are passed into a function, 806 scalarizing them if the elements of the aggregate are only loaded. Note that 807 it refuses to scalarize aggregates which would require passing in more than 808 three operands to the function, because passing thousands of operands for a 809 large array or structure is unprofitable! 810 </p> 811 812 <p> 813 Note that this transformation could also be done for arguments that are only 814 stored to (returning the value instead), but does not currently. This case 815 would be best handled when and if LLVM starts supporting multiple return 816 values from functions. 817 </p> 818 </div> 819 820 <!-------------------------------------------------------------------------- --> 821 <h3> 822 <a name="block-placement">-block-placement: Profile Guided Basic Block Placement</a> 823 </h3> 824 <div> 825 <p>This pass is a very simple profile guided basic block placement algorithm. 826 The idea is to put frequently executed blocks together at the start of the 827 function and hopefully increase the number of fall-through conditional 828 branches. If there is no profile information for a particular function, this 829 pass basically orders blocks in depth-first order.</p> 830 </div> 831 832 <!-------------------------------------------------------------------------- --> 833 <h3> 834 <a name="break-crit-edges">-break-crit-edges: Break critical edges in CFG</a> 835 </h3> 836 <div> 837 <p> 838 Break all of the critical edges in the CFG by inserting a dummy basic block. 839 It may be "required" by passes that cannot deal with critical edges. This 840 transformation obviously invalidates the CFG, but can update forward dominator 841 (set, immediate dominators, tree, and frontier) information. 842 </p> 843 </div> 844 845 <!-------------------------------------------------------------------------- --> 846 <h3> 847 <a name="codegenprepare">-codegenprepare: Optimize for code generation</a> 848 </h3> 849 <div> 850 This pass munges the code in the input function to better prepare it for 851 SelectionDAG-based code generation. This works around limitations in it's 852 basic-block-at-a-time approach. It should eventually be removed. 853 </div> 854 855 <!-------------------------------------------------------------------------- --> 856 <h3> 857 <a name="constmerge">-constmerge: Merge Duplicate Global Constants</a> 858 </h3> 859 <div> 860 <p> 861 Merges duplicate global constants together into a single constant that is 862 shared. This is useful because some passes (ie TraceValues) insert a lot of 863 string constants into the program, regardless of whether or not an existing 864 string is available. 865 </p> 866 </div> 867 868 <!-------------------------------------------------------------------------- --> 869 <h3> 870 <a name="constprop">-constprop: Simple constant propagation</a> 871 </h3> 872 <div> 873 <p>This file implements constant propagation and merging. It looks for 874 instructions involving only constant operands and replaces them with a 875 constant value instead of an instruction. For example:</p> 876 <blockquote><pre>add i32 1, 2</pre></blockquote> 877 <p>becomes</p> 878 <blockquote><pre>i32 3</pre></blockquote> 879 <p>NOTE: this pass has a habit of making definitions be dead. It is a good 880 idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass 881 sometime after running this pass.</p> 882 </div> 883 884 <!-------------------------------------------------------------------------- --> 885 <h3> 886 <a name="dce">-dce: Dead Code Elimination</a> 887 </h3> 888 <div> 889 <p> 890 Dead code elimination is similar to <a href="#die">dead instruction 891 elimination</a>, but it rechecks instructions that were used by removed 892 instructions to see if they are newly dead. 893 </p> 894 </div> 895 896 <!-------------------------------------------------------------------------- --> 897 <h3> 898 <a name="deadargelim">-deadargelim: Dead Argument Elimination</a> 899 </h3> 900 <div> 901 <p> 902 This pass deletes dead arguments from internal functions. Dead argument 903 elimination removes arguments which are directly dead, as well as arguments 904 only passed into function calls as dead arguments of other functions. This 905 pass also deletes dead arguments in a similar way. 906 </p> 907 908 <p> 909 This pass is often useful as a cleanup pass to run after aggressive 910 interprocedural passes, which add possibly-dead arguments. 911 </p> 912 </div> 913 914 <!-------------------------------------------------------------------------- --> 915 <h3> 916 <a name="deadtypeelim">-deadtypeelim: Dead Type Elimination</a> 917 </h3> 918 <div> 919 <p> 920 This pass is used to cleanup the output of GCC. It eliminate names for types 921 that are unused in the entire translation unit, using the <a 922 href="#findusedtypes">find used types</a> pass. 923 </p> 924 </div> 925 926 <!-------------------------------------------------------------------------- --> 927 <h3> 928 <a name="die">-die: Dead Instruction Elimination</a> 929 </h3> 930 <div> 931 <p> 932 Dead instruction elimination performs a single pass over the function, 933 removing instructions that are obviously dead. 934 </p> 935 </div> 936 937 <!-------------------------------------------------------------------------- --> 938 <h3> 939 <a name="dse">-dse: Dead Store Elimination</a> 940 </h3> 941 <div> 942 <p> 943 A trivial dead store elimination that only considers basic-block local 944 redundant stores. 945 </p> 946 </div> 947 948 <!-------------------------------------------------------------------------- --> 949 <h3> 950 <a name="functionattrs">-functionattrs: Deduce function attributes</a> 951 </h3> 952 <div> 953 <p>A simple interprocedural pass which walks the call-graph, looking for 954 functions which do not access or only read non-local memory, and marking them 955 readnone/readonly. In addition, it marks function arguments (of pointer type) 956 'nocapture' if a call to the function does not create any copies of the pointer 957 value that outlive the call. This more or less means that the pointer is only 958 dereferenced, and not returned from the function or stored in a global. 959 This pass is implemented as a bottom-up traversal of the call-graph. 960 </p> 961 </div> 962 963 <!-------------------------------------------------------------------------- --> 964 <h3> 965 <a name="globaldce">-globaldce: Dead Global Elimination</a> 966 </h3> 967 <div> 968 <p> 969 This transform is designed to eliminate unreachable internal globals from the 970 program. It uses an aggressive algorithm, searching out globals that are 971 known to be alive. After it finds all of the globals which are needed, it 972 deletes whatever is left over. This allows it to delete recursive chunks of 973 the program which are unreachable. 974 </p> 975 </div> 976 977 <!-------------------------------------------------------------------------- --> 978 <h3> 979 <a name="globalopt">-globalopt: Global Variable Optimizer</a> 980 </h3> 981 <div> 982 <p> 983 This pass transforms simple global variables that never have their address 984 taken. If obviously true, it marks read/write globals as constant, deletes 985 variables only stored to, etc. 986 </p> 987 </div> 988 989 <!-------------------------------------------------------------------------- --> 990 <h3> 991 <a name="gvn">-gvn: Global Value Numbering</a> 992 </h3> 993 <div> 994 <p> 995 This pass performs global value numbering to eliminate fully and partially 996 redundant instructions. It also performs redundant load elimination. 997 </p> 998 </div> 999 1000 <!-------------------------------------------------------------------------- --> 1001 <h3> 1002 <a name="indvars">-indvars: Canonicalize Induction Variables</a> 1003 </h3> 1004 <div> 1005 <p> 1006 This transformation analyzes and transforms the induction variables (and 1007 computations derived from them) into simpler forms suitable for subsequent 1008 analysis and transformation. 1009 </p> 1010 1011 <p> 1012 This transformation makes the following changes to each loop with an 1013 identifiable induction variable: 1014 </p> 1015 1016 <ol> 1017 <li>All loops are transformed to have a <em>single</em> canonical 1018 induction variable which starts at zero and steps by one.</li> 1019 <li>The canonical induction variable is guaranteed to be the first PHI node 1020 in the loop header block.</li> 1021 <li>Any pointer arithmetic recurrences are raised to use array 1022 subscripts.</li> 1023 </ol> 1024 1025 <p> 1026 If the trip count of a loop is computable, this pass also makes the following 1027 changes: 1028 </p> 1029 1030 <ol> 1031 <li>The exit condition for the loop is canonicalized to compare the 1032 induction value against the exit value. This turns loops like: 1033 <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote> 1034 into 1035 <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li> 1036 <li>Any use outside of the loop of an expression derived from the indvar 1037 is changed to compute the derived value outside of the loop, eliminating 1038 the dependence on the exit value of the induction variable. If the only 1039 purpose of the loop is to compute the exit value of some derived 1040 expression, this transformation will make the loop dead.</li> 1041 </ol> 1042 1043 <p> 1044 This transformation should be followed by strength reduction after all of the 1045 desired loop transformations have been performed. Additionally, on targets 1046 where it is profitable, the loop could be transformed to count down to zero 1047 (the "do loop" optimization). 1048 </p> 1049 </div> 1050 1051 <!-------------------------------------------------------------------------- --> 1052 <h3> 1053 <a name="inline">-inline: Function Integration/Inlining</a> 1054 </h3> 1055 <div> 1056 <p> 1057 Bottom-up inlining of functions into callees. 1058 </p> 1059 </div> 1060 1061 <!-------------------------------------------------------------------------- --> 1062 <h3> 1063 <a name="insert-edge-profiling">-insert-edge-profiling: Insert instrumentation for edge profiling</a> 1064 </h3> 1065 <div> 1066 <p> 1067 This pass instruments the specified program with counters for edge profiling. 1068 Edge profiling can give a reasonable approximation of the hot paths through a 1069 program, and is used for a wide variety of program transformations. 1070 </p> 1071 1072 <p> 1073 Note that this implementation is very nave. It inserts a counter for 1074 <em>every</em> edge in the program, instead of using control flow information 1075 to prune the number of counters inserted. 1076 </p> 1077 </div> 1078 1079 <!-------------------------------------------------------------------------- --> 1080 <h3> 1081 <a name="insert-optimal-edge-profiling">-insert-optimal-edge-profiling: Insert optimal instrumentation for edge profiling</a> 1082 </h3> 1083 <div> 1084 <p>This pass instruments the specified program with counters for edge profiling. 1085 Edge profiling can give a reasonable approximation of the hot paths through a 1086 program, and is used for a wide variety of program transformations. 1087 </p> 1088 </div> 1089 1090 <!-------------------------------------------------------------------------- --> 1091 <h3> 1092 <a name="instcombine">-instcombine: Combine redundant instructions</a> 1093 </h3> 1094 <div> 1095 <p> 1096 Combine instructions to form fewer, simple 1097 instructions. This pass does not modify the CFG This pass is where algebraic 1098 simplification happens. 1099 </p> 1100 1101 <p> 1102 This pass combines things like: 1103 </p> 1104 1105 <blockquote><pre 1106 >%Y = add i32 %X, 1 1107 %Z = add i32 %Y, 1</pre></blockquote> 1108 1109 <p> 1110 into: 1111 </p> 1112 1113 <blockquote><pre 1114 >%Z = add i32 %X, 2</pre></blockquote> 1115 1116 <p> 1117 This is a simple worklist driven algorithm. 1118 </p> 1119 1120 <p> 1121 This pass guarantees that the following canonicalizations are performed on 1122 the program: 1123 </p> 1124 1125 <ul> 1126 <li>If a binary operator has a constant operand, it is moved to the right- 1127 hand side.</li> 1128 <li>Bitwise operators with constant operands are always grouped so that 1129 shifts are performed first, then <code>or</code>s, then 1130 <code>and</code>s, then <code>xor</code>s.</li> 1131 <li>Compare instructions are converted from <code><</code>, 1132 <code>></code>, <code></code>, or <code></code> to 1133 <code>=</code> or <code></code> if possible.</li> 1134 <li>All <code>cmp</code> instructions on boolean values are replaced with 1135 logical operations.</li> 1136 <li><code>add <var>X</var>, <var>X</var></code> is represented as 1137 <code>mul <var>X</var>, 2</code> <code>shl <var>X</var>, 1</code></li> 1138 <li>Multiplies with a constant power-of-two argument are transformed into 1139 shifts.</li> 1140 <li> etc.</li> 1141 </ul> 1142 </div> 1143 1144 <!-------------------------------------------------------------------------- --> 1145 <h3> 1146 <a name="internalize">-internalize: Internalize Global Symbols</a> 1147 </h3> 1148 <div> 1149 <p> 1150 This pass loops over all of the functions in the input module, looking for a 1151 main function. If a main function is found, all other functions and all 1152 global variables with initializers are marked as internal. 1153 </p> 1154 </div> 1155 1156 <!-------------------------------------------------------------------------- --> 1157 <h3> 1158 <a name="ipconstprop">-ipconstprop: Interprocedural constant propagation</a> 1159 </h3> 1160 <div> 1161 <p> 1162 This pass implements an <em>extremely</em> simple interprocedural constant 1163 propagation pass. It could certainly be improved in many different ways, 1164 like using a worklist. This pass makes arguments dead, but does not remove 1165 them. The existing dead argument elimination pass should be run after this 1166 to clean up the mess. 1167 </p> 1168 </div> 1169 1170 <!-------------------------------------------------------------------------- --> 1171 <h3> 1172 <a name="ipsccp">-ipsccp: Interprocedural Sparse Conditional Constant Propagation</a> 1173 </h3> 1174 <div> 1175 <p> 1176 An interprocedural variant of <a href="#sccp">Sparse Conditional Constant 1177 Propagation</a>. 1178 </p> 1179 </div> 1180 1181 <!-------------------------------------------------------------------------- --> 1182 <h3> 1183 <a name="jump-threading">-jump-threading: Jump Threading</a> 1184 </h3> 1185 <div> 1186 <p> 1187 Jump threading tries to find distinct threads of control flow running through 1188 a basic block. This pass looks at blocks that have multiple predecessors and 1189 multiple successors. If one or more of the predecessors of the block can be 1190 proven to always cause a jump to one of the successors, we forward the edge 1191 from the predecessor to the successor by duplicating the contents of this 1192 block. 1193 </p> 1194 <p> 1195 An example of when this can occur is code like this: 1196 </p> 1197 1198 <pre 1199 >if () { ... 1200 X = 4; 1201 } 1202 if (X < 3) {</pre> 1203 1204 <p> 1205 In this case, the unconditional branch at the end of the first if can be 1206 revectored to the false side of the second if. 1207 </p> 1208 </div> 1209 1210 <!-------------------------------------------------------------------------- --> 1211 <h3> 1212 <a name="lcssa">-lcssa: Loop-Closed SSA Form Pass</a> 1213 </h3> 1214 <div> 1215 <p> 1216 This pass transforms loops by placing phi nodes at the end of the loops for 1217 all values that are live across the loop boundary. For example, it turns 1218 the left into the right code: 1219 </p> 1220 1221 <pre 1222 >for (...) for (...) 1223 if (c) if (c) 1224 X1 = ... X1 = ... 1225 else else 1226 X2 = ... X2 = ... 1227 X3 = phi(X1, X2) X3 = phi(X1, X2) 1228 ... = X3 + 4 X4 = phi(X3) 1229 ... = X4 + 4</pre> 1230 1231 <p> 1232 This is still valid LLVM; the extra phi nodes are purely redundant, and will 1233 be trivially eliminated by <code>InstCombine</code>. The major benefit of 1234 this transformation is that it makes many other loop optimizations, such as 1235 LoopUnswitching, simpler. 1236 </p> 1237 </div> 1238 1239 <!-------------------------------------------------------------------------- --> 1240 <h3> 1241 <a name="licm">-licm: Loop Invariant Code Motion</a> 1242 </h3> 1243 <div> 1244 <p> 1245 This pass performs loop invariant code motion, attempting to remove as much 1246 code from the body of a loop as possible. It does this by either hoisting 1247 code into the preheader block, or by sinking code to the exit blocks if it is 1248 safe. This pass also promotes must-aliased memory locations in the loop to 1249 live in registers, thus hoisting and sinking "invariant" loads and stores. 1250 </p> 1251 1252 <p> 1253 This pass uses alias analysis for two purposes: 1254 </p> 1255 1256 <ul> 1257 <li>Moving loop invariant loads and calls out of loops. If we can determine 1258 that a load or call inside of a loop never aliases anything stored to, 1259 we can hoist it or sink it like any other instruction.</li> 1260 <li>Scalar Promotion of Memory - If there is a store instruction inside of 1261 the loop, we try to move the store to happen AFTER the loop instead of 1262 inside of the loop. This can only happen if a few conditions are true: 1263 <ul> 1264 <li>The pointer stored through is loop invariant.</li> 1265 <li>There are no stores or loads in the loop which <em>may</em> alias 1266 the pointer. There are no calls in the loop which mod/ref the 1267 pointer.</li> 1268 </ul> 1269 If these conditions are true, we can promote the loads and stores in the 1270 loop of the pointer to use a temporary alloca'd variable. We then use 1271 the mem2reg functionality to construct the appropriate SSA form for the 1272 variable.</li> 1273 </ul> 1274 </div> 1275 1276 <!-------------------------------------------------------------------------- --> 1277 <h3> 1278 <a name="loop-deletion">-loop-deletion: Delete dead loops</a> 1279 </h3> 1280 <div> 1281 <p> 1282 This file implements the Dead Loop Deletion Pass. This pass is responsible 1283 for eliminating loops with non-infinite computable trip counts that have no 1284 side effects or volatile instructions, and do not contribute to the 1285 computation of the function's return value. 1286 </p> 1287 </div> 1288 1289 <!-------------------------------------------------------------------------- --> 1290 <h3> 1291 <a name="loop-extract">-loop-extract: Extract loops into new functions</a> 1292 </h3> 1293 <div> 1294 <p> 1295 A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to 1296 extract each top-level loop into its own new function. If the loop is the 1297 <em>only</em> loop in a given function, it is not touched. This is a pass most 1298 useful for debugging via bugpoint. 1299 </p> 1300 </div> 1301 1302 <!-------------------------------------------------------------------------- --> 1303 <h3> 1304 <a name="loop-extract-single">-loop-extract-single: Extract at most one loop into a new function</a> 1305 </h3> 1306 <div> 1307 <p> 1308 Similar to <a href="#loop-extract">Extract loops into new functions</a>, 1309 this pass extracts one natural loop from the program into a function if it 1310 can. This is used by bugpoint. 1311 </p> 1312 </div> 1313 1314 <!-------------------------------------------------------------------------- --> 1315 <h3> 1316 <a name="loop-reduce">-loop-reduce: Loop Strength Reduction</a> 1317 </h3> 1318 <div> 1319 <p> 1320 This pass performs a strength reduction on array references inside loops that 1321 have as one or more of their components the loop induction variable. This is 1322 accomplished by creating a new value to hold the initial value of the array 1323 access for the first iteration, and then creating a new GEP instruction in 1324 the loop to increment the value by the appropriate amount. 1325 </p> 1326 </div> 1327 1328 <!-------------------------------------------------------------------------- --> 1329 <h3> 1330 <a name="loop-rotate">-loop-rotate: Rotate Loops</a> 1331 </h3> 1332 <div> 1333 <p>A simple loop rotation transformation.</p> 1334 </div> 1335 1336 <!-------------------------------------------------------------------------- --> 1337 <h3> 1338 <a name="loop-simplify">-loop-simplify: Canonicalize natural loops</a> 1339 </h3> 1340 <div> 1341 <p> 1342 This pass performs several transformations to transform natural loops into a 1343 simpler form, which makes subsequent analyses and transformations simpler and 1344 more effective. 1345 </p> 1346 1347 <p> 1348 Loop pre-header insertion guarantees that there is a single, non-critical 1349 entry edge from outside of the loop to the loop header. This simplifies a 1350 number of analyses and transformations, such as LICM. 1351 </p> 1352 1353 <p> 1354 Loop exit-block insertion guarantees that all exit blocks from the loop 1355 (blocks which are outside of the loop that have predecessors inside of the 1356 loop) only have predecessors from inside of the loop (and are thus dominated 1357 by the loop header). This simplifies transformations such as store-sinking 1358 that are built into LICM. 1359 </p> 1360 1361 <p> 1362 This pass also guarantees that loops will have exactly one backedge. 1363 </p> 1364 1365 <p> 1366 Note that the simplifycfg pass will clean up blocks which are split out but 1367 end up being unnecessary, so usage of this pass should not pessimize 1368 generated code. 1369 </p> 1370 1371 <p> 1372 This pass obviously modifies the CFG, but updates loop information and 1373 dominator information. 1374 </p> 1375 </div> 1376 1377 <!-------------------------------------------------------------------------- --> 1378 <h3> 1379 <a name="loop-unroll">-loop-unroll: Unroll loops</a> 1380 </h3> 1381 <div> 1382 <p> 1383 This pass implements a simple loop unroller. It works best when loops have 1384 been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass, 1385 allowing it to determine the trip counts of loops easily. 1386 </p> 1387 </div> 1388 1389 <!-------------------------------------------------------------------------- --> 1390 <h3> 1391 <a name="loop-unswitch">-loop-unswitch: Unswitch loops</a> 1392 </h3> 1393 <div> 1394 <p> 1395 This pass transforms loops that contain branches on loop-invariant conditions 1396 to have multiple loops. For example, it turns the left into the right code: 1397 </p> 1398 1399 <pre 1400 >for (...) if (lic) 1401 A for (...) 1402 if (lic) A; B; C 1403 B else 1404 C for (...) 1405 A; C</pre> 1406 1407 <p> 1408 This can increase the size of the code exponentially (doubling it every time 1409 a loop is unswitched) so we only unswitch if the resultant code will be 1410 smaller than a threshold. 1411 </p> 1412 1413 <p> 1414 This pass expects LICM to be run before it to hoist invariant conditions out 1415 of the loop, to make the unswitching opportunity obvious. 1416 </p> 1417 </div> 1418 1419 <!-------------------------------------------------------------------------- --> 1420 <h3> 1421 <a name="loweratomic">-loweratomic: Lower atomic intrinsics to non-atomic form</a> 1422 </h3> 1423 <div> 1424 <p> 1425 This pass lowers atomic intrinsics to non-atomic form for use in a known 1426 non-preemptible environment. 1427 </p> 1428 1429 <p> 1430 The pass does not verify that the environment is non-preemptible (in 1431 general this would require knowledge of the entire call graph of the 1432 program including any libraries which may not be available in bitcode form); 1433 it simply lowers every atomic intrinsic. 1434 </p> 1435 </div> 1436 1437 <!-------------------------------------------------------------------------- --> 1438 <h3> 1439 <a name="lowerinvoke">-lowerinvoke: Lower invoke and unwind, for unwindless code generators</a> 1440 </h3> 1441 <div> 1442 <p> 1443 This transformation is designed for use by code generators which do not yet 1444 support stack unwinding. This pass supports two models of exception handling 1445 lowering, the 'cheap' support and the 'expensive' support. 1446 </p> 1447 1448 <p> 1449 'Cheap' exception handling support gives the program the ability to execute 1450 any program which does not "throw an exception", by turning 'invoke' 1451 instructions into calls and by turning 'unwind' instructions into calls to 1452 abort(). If the program does dynamically use the unwind instruction, the 1453 program will print a message then abort. 1454 </p> 1455 1456 <p> 1457 'Expensive' exception handling support gives the full exception handling 1458 support to the program at the cost of making the 'invoke' instruction 1459 really expensive. It basically inserts setjmp/longjmp calls to emulate the 1460 exception handling as necessary. 1461 </p> 1462 1463 <p> 1464 Because the 'expensive' support slows down programs a lot, and EH is only 1465 used for a subset of the programs, it must be specifically enabled by the 1466 <tt>-enable-correct-eh-support</tt> option. 1467 </p> 1468 1469 <p> 1470 Note that after this pass runs the CFG is not entirely accurate (exceptional 1471 control flow edges are not correct anymore) so only very simple things should 1472 be done after the lowerinvoke pass has run (like generation of native code). 1473 This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't 1474 support the invoke instruction yet" lowering pass. 1475 </p> 1476 </div> 1477 1478 <!-------------------------------------------------------------------------- --> 1479 <h3> 1480 <a name="lowerswitch">-lowerswitch: Lower SwitchInst's to branches</a> 1481 </h3> 1482 <div> 1483 <p> 1484 Rewrites <tt>switch</tt> instructions with a sequence of branches, which 1485 allows targets to get away with not implementing the switch instruction until 1486 it is convenient. 1487 </p> 1488 </div> 1489 1490 <!-------------------------------------------------------------------------- --> 1491 <h3> 1492 <a name="mem2reg">-mem2reg: Promote Memory to Register</a> 1493 </h3> 1494 <div> 1495 <p> 1496 This file promotes memory references to be register references. It promotes 1497 <tt>alloca</tt> instructions which only have <tt>load</tt>s and 1498 <tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator 1499 frontiers to place <tt>phi</tt> nodes, then traversing the function in 1500 depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as 1501 appropriate. This is just the standard SSA construction algorithm to construct 1502 "pruned" SSA form. 1503 </p> 1504 </div> 1505 1506 <!-------------------------------------------------------------------------- --> 1507 <h3> 1508 <a name="memcpyopt">-memcpyopt: MemCpy Optimization</a> 1509 </h3> 1510 <div> 1511 <p> 1512 This pass performs various transformations related to eliminating memcpy 1513 calls, or transforming sets of stores into memset's. 1514 </p> 1515 </div> 1516 1517 <!-------------------------------------------------------------------------- --> 1518 <h3> 1519 <a name="mergefunc">-mergefunc: Merge Functions</a> 1520 </h3> 1521 <div> 1522 <p>This pass looks for equivalent functions that are mergable and folds them. 1523 1524 A hash is computed from the function, based on its type and number of 1525 basic blocks. 1526 1527 Once all hashes are computed, we perform an expensive equality comparison 1528 on each function pair. This takes n^2/2 comparisons per bucket, so it's 1529 important that the hash function be high quality. The equality comparison 1530 iterates through each instruction in each basic block. 1531 1532 When a match is found the functions are folded. If both functions are 1533 overridable, we move the functionality into a new internal function and 1534 leave two overridable thunks to it. 1535 </p> 1536 </div> 1537 1538 <!-------------------------------------------------------------------------- --> 1539 <h3> 1540 <a name="mergereturn">-mergereturn: Unify function exit nodes</a> 1541 </h3> 1542 <div> 1543 <p> 1544 Ensure that functions have at most one <tt>ret</tt> instruction in them. 1545 Additionally, it keeps track of which node is the new exit node of the CFG. 1546 </p> 1547 </div> 1548 1549 <!-------------------------------------------------------------------------- --> 1550 <h3> 1551 <a name="partial-inliner">-partial-inliner: Partial Inliner</a> 1552 </h3> 1553 <div> 1554 <p>This pass performs partial inlining, typically by inlining an if 1555 statement that surrounds the body of the function. 1556 </p> 1557 </div> 1558 1559 <!-------------------------------------------------------------------------- --> 1560 <h3> 1561 <a name="prune-eh">-prune-eh: Remove unused exception handling info</a> 1562 </h3> 1563 <div> 1564 <p> 1565 This file implements a simple interprocedural pass which walks the call-graph, 1566 turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and 1567 only if the callee cannot throw an exception. It implements this as a 1568 bottom-up traversal of the call-graph. 1569 </p> 1570 </div> 1571 1572 <!-------------------------------------------------------------------------- --> 1573 <h3> 1574 <a name="reassociate">-reassociate: Reassociate expressions</a> 1575 </h3> 1576 <div> 1577 <p> 1578 This pass reassociates commutative expressions in an order that is designed 1579 to promote better constant propagation, GCSE, LICM, PRE, etc. 1580 </p> 1581 1582 <p> 1583 For example: 4 + (<var>x</var> + 5) <var>x</var> + (4 + 5) 1584 </p> 1585 1586 <p> 1587 In the implementation of this algorithm, constants are assigned rank = 0, 1588 function arguments are rank = 1, and other values are assigned ranks 1589 corresponding to the reverse post order traversal of current function 1590 (starting at 2), which effectively gives values in deep loops higher rank 1591 than values not in loops. 1592 </p> 1593 </div> 1594 1595 <!-------------------------------------------------------------------------- --> 1596 <h3> 1597 <a name="reg2mem">-reg2mem: Demote all values to stack slots</a> 1598 </h3> 1599 <div> 1600 <p> 1601 This file demotes all registers to memory references. It is intented to be 1602 the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to 1603 <tt>load</tt> instructions, the only values live across basic blocks are 1604 <tt>alloca</tt> instructions and <tt>load</tt> instructions before 1605 <tt>phi</tt> nodes. It is intended that this should make CFG hacking much 1606 easier. To make later hacking easier, the entry block is split into two, such 1607 that all introduced <tt>alloca</tt> instructions (and nothing else) are in the 1608 entry block. 1609 </p> 1610 </div> 1611 1612 <!-------------------------------------------------------------------------- --> 1613 <h3> 1614 <a name="scalarrepl">-scalarrepl: Scalar Replacement of Aggregates (DT)</a> 1615 </h3> 1616 <div> 1617 <p> 1618 The well-known scalar replacement of aggregates transformation. This 1619 transform breaks up <tt>alloca</tt> instructions of aggregate type (structure 1620 or array) into individual <tt>alloca</tt> instructions for each member if 1621 possible. Then, if possible, it transforms the individual <tt>alloca</tt> 1622 instructions into nice clean scalar SSA form. 1623 </p> 1624 1625 <p> 1626 This combines a simple scalar replacement of aggregates algorithm with the <a 1627 href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact, 1628 especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>, 1629 then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to 1630 promote works well. 1631 </p> 1632 </div> 1633 1634 <!-------------------------------------------------------------------------- --> 1635 <h3> 1636 <a name="sccp">-sccp: Sparse Conditional Constant Propagation</a> 1637 </h3> 1638 <div> 1639 <p> 1640 Sparse conditional constant propagation and merging, which can be summarized 1641 as: 1642 </p> 1643 1644 <ol> 1645 <li>Assumes values are constant unless proven otherwise</li> 1646 <li>Assumes BasicBlocks are dead unless proven otherwise</li> 1647 <li>Proves values to be constant, and replaces them with constants</li> 1648 <li>Proves conditional branches to be unconditional</li> 1649 </ol> 1650 1651 <p> 1652 Note that this pass has a habit of making definitions be dead. It is a good 1653 idea to to run a DCE pass sometime after running this pass. 1654 </p> 1655 </div> 1656 1657 <!-------------------------------------------------------------------------- --> 1658 <h3> 1659 <a name="simplify-libcalls">-simplify-libcalls: Simplify well-known library calls</a> 1660 </h3> 1661 <div> 1662 <p> 1663 Applies a variety of small optimizations for calls to specific well-known 1664 function calls (e.g. runtime library functions). For example, a call 1665 <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be 1666 transformed into simply <tt>return 3</tt>. 1667 </p> 1668 </div> 1669 1670 <!-------------------------------------------------------------------------- --> 1671 <h3> 1672 <a name="simplifycfg">-simplifycfg: Simplify the CFG</a> 1673 </h3> 1674 <div> 1675 <p> 1676 Performs dead code elimination and basic block merging. Specifically: 1677 </p> 1678 1679 <ol> 1680 <li>Removes basic blocks with no predecessors.</li> 1681 <li>Merges a basic block into its predecessor if there is only one and the 1682 predecessor only has one successor.</li> 1683 <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li> 1684 <li>Eliminates a basic block that only contains an unconditional 1685 branch.</li> 1686 </ol> 1687 </div> 1688 1689 <!-------------------------------------------------------------------------- --> 1690 <h3> 1691 <a name="sink">-sink: Code sinking</a> 1692 </h3> 1693 <div> 1694 <p>This pass moves instructions into successor blocks, when possible, so that 1695 they aren't executed on paths where their results aren't needed. 1696 </p> 1697 </div> 1698 1699 <!-------------------------------------------------------------------------- --> 1700 <h3> 1701 <a name="sretpromotion">-sretpromotion: Promote sret arguments to multiple ret values</a> 1702 </h3> 1703 <div> 1704 <p> 1705 This pass finds functions that return a struct (using a pointer to the struct 1706 as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and 1707 replaces them with a new function that simply returns each of the elements of 1708 that struct (using multiple return values). 1709 </p> 1710 1711 <p> 1712 This pass works under a number of conditions: 1713 </p> 1714 1715 <ul> 1716 <li>The returned struct must not contain other structs</li> 1717 <li>The returned struct must only be used to load values from</li> 1718 <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li> 1719 </ul> 1720 </div> 1721 1722 <!-------------------------------------------------------------------------- --> 1723 <h3> 1724 <a name="strip">-strip: Strip all symbols from a module</a> 1725 </h3> 1726 <div> 1727 <p> 1728 performs code stripping. this transformation can delete: 1729 </p> 1730 1731 <ol> 1732 <li>names for virtual registers</li> 1733 <li>symbols for internal globals and functions</li> 1734 <li>debug information</li> 1735 </ol> 1736 1737 <p> 1738 note that this transformation makes code much less readable, so it should 1739 only be used in situations where the <tt>strip</tt> utility would be used, 1740 such as reducing code size or making it harder to reverse engineer code. 1741 </p> 1742 </div> 1743 1744 <!-------------------------------------------------------------------------- --> 1745 <h3> 1746 <a name="strip-dead-debug-info">-strip-dead-debug-info: Strip debug info for unused symbols</a> 1747 </h3> 1748 <div> 1749 <p> 1750 performs code stripping. this transformation can delete: 1751 </p> 1752 1753 <ol> 1754 <li>names for virtual registers</li> 1755 <li>symbols for internal globals and functions</li> 1756 <li>debug information</li> 1757 </ol> 1758 1759 <p> 1760 note that this transformation makes code much less readable, so it should 1761 only be used in situations where the <tt>strip</tt> utility would be used, 1762 such as reducing code size or making it harder to reverse engineer code. 1763 </p> 1764 </div> 1765 1766 <!-------------------------------------------------------------------------- --> 1767 <h3> 1768 <a name="strip-dead-prototypes">-strip-dead-prototypes: Strip Unused Function Prototypes</a> 1769 </h3> 1770 <div> 1771 <p> 1772 This pass loops over all of the functions in the input module, looking for 1773 dead declarations and removes them. Dead declarations are declarations of 1774 functions for which no implementation is available (i.e., declarations for 1775 unused library functions). 1776 </p> 1777 </div> 1778 1779 <!-------------------------------------------------------------------------- --> 1780 <h3> 1781 <a name="strip-debug-declare">-strip-debug-declare: Strip all llvm.dbg.declare intrinsics</a> 1782 </h3> 1783 <div> 1784 <p>This pass implements code stripping. Specifically, it can delete:</p> 1785 <ul> 1786 <li>names for virtual registers</li> 1787 <li>symbols for internal globals and functions</li> 1788 <li>debug information</li> 1789 </ul> 1790 <p> 1791 Note that this transformation makes code much less readable, so it should 1792 only be used in situations where the 'strip' utility would be used, such as 1793 reducing code size or making it harder to reverse engineer code. 1794 </p> 1795 </div> 1796 1797 <!-------------------------------------------------------------------------- --> 1798 <h3> 1799 <a name="strip-nondebug">-strip-nondebug: Strip all symbols, except dbg symbols, from a module</a> 1800 </h3> 1801 <div> 1802 <p>This pass implements code stripping. Specifically, it can delete:</p> 1803 <ul> 1804 <li>names for virtual registers</li> 1805 <li>symbols for internal globals and functions</li> 1806 <li>debug information</li> 1807 </ul> 1808 <p> 1809 Note that this transformation makes code much less readable, so it should 1810 only be used in situations where the 'strip' utility would be used, such as 1811 reducing code size or making it harder to reverse engineer code. 1812 </p> 1813 </div> 1814 1815 <!-------------------------------------------------------------------------- --> 1816 <h3> 1817 <a name="tailcallelim">-tailcallelim: Tail Call Elimination</a> 1818 </h3> 1819 <div> 1820 <p> 1821 This file transforms calls of the current function (self recursion) followed 1822 by a return instruction with a branch to the entry of the function, creating 1823 a loop. This pass also implements the following extensions to the basic 1824 algorithm: 1825 </p> 1826 1827 <ul> 1828 <li>Trivial instructions between the call and return do not prevent the 1829 transformation from taking place, though currently the analysis cannot 1830 support moving any really useful instructions (only dead ones). 1831 <li>This pass transforms functions that are prevented from being tail 1832 recursive by an associative expression to use an accumulator variable, 1833 thus compiling the typical naive factorial or <tt>fib</tt> implementation 1834 into efficient code. 1835 <li>TRE is performed if the function returns void, if the return 1836 returns the result returned by the call, or if the function returns a 1837 run-time constant on all exits from the function. It is possible, though 1838 unlikely, that the return returns something else (like constant 0), and 1839 can still be TRE'd. It can be TRE'd if <em>all other</em> return 1840 instructions in the function return the exact same value. 1841 <li>If it can prove that callees do not access theier caller stack frame, 1842 they are marked as eligible for tail call elimination (by the code 1843 generator). 1844 </ul> 1845 </div> 1846 1847 <!-------------------------------------------------------------------------- --> 1848 <h3> 1849 <a name="tailduplicate">-tailduplicate: Tail Duplication</a> 1850 </h3> 1851 <div> 1852 <p> 1853 This pass performs a limited form of tail duplication, intended to simplify 1854 CFGs by removing some unconditional branches. This pass is necessary to 1855 straighten out loops created by the C front-end, but also is capable of 1856 making other code nicer. After this pass is run, the CFG simplify pass 1857 should be run to clean up the mess. 1858 </p> 1859 </div> 1860 1861 </div> 1862 1863 <!-- ======================================================================= --> 1864 <h2><a name="utilities">Utility Passes</a></h2> 1865 <div> 1866 <p>This section describes the LLVM Utility Passes.</p> 1867 1868 <!-------------------------------------------------------------------------- --> 1869 <h3> 1870 <a name="deadarghaX0r">-deadarghaX0r: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a> 1871 </h3> 1872 <div> 1873 <p> 1874 Same as dead argument elimination, but deletes arguments to functions which 1875 are external. This is only for use by <a 1876 href="Bugpoint.html">bugpoint</a>.</p> 1877 </div> 1878 1879 <!-------------------------------------------------------------------------- --> 1880 <h3> 1881 <a name="extract-blocks">-extract-blocks: Extract Basic Blocks From Module (for bugpoint use)</a> 1882 </h3> 1883 <div> 1884 <p> 1885 This pass is used by bugpoint to extract all blocks from the module into their 1886 own functions.</p> 1887 </div> 1888 1889 <!-------------------------------------------------------------------------- --> 1890 <h3> 1891 <a name="instnamer">-instnamer: Assign names to anonymous instructions</a> 1892 </h3> 1893 <div> 1894 <p>This is a little utility pass that gives instructions names, this is mostly 1895 useful when diffing the effect of an optimization because deleting an 1896 unnamed instruction can change all other instruction numbering, making the 1897 diff very noisy. 1898 </p> 1899 </div> 1900 1901 <!-------------------------------------------------------------------------- --> 1902 <h3> 1903 <a name="preverify">-preverify: Preliminary module verification</a> 1904 </h3> 1905 <div> 1906 <p> 1907 Ensures that the module is in the form required by the <a 1908 href="#verifier">Module Verifier</a> pass. 1909 </p> 1910 1911 <p> 1912 Running the verifier runs this pass automatically, so there should be no need 1913 to use it directly. 1914 </p> 1915 </div> 1916 1917 <!-------------------------------------------------------------------------- --> 1918 <h3> 1919 <a name="verify">-verify: Module Verifier</a> 1920 </h3> 1921 <div> 1922 <p> 1923 Verifies an LLVM IR code. This is useful to run after an optimization which is 1924 undergoing testing. Note that <tt>llvm-as</tt> verifies its input before 1925 emitting bitcode, and also that malformed bitcode is likely to make LLVM 1926 crash. All language front-ends are therefore encouraged to verify their output 1927 before performing optimizing transformations. 1928 </p> 1929 1930 <ul> 1931 <li>Both of a binary operator's parameters are of the same type.</li> 1932 <li>Verify that the indices of mem access instructions match other 1933 operands.</li> 1934 <li>Verify that arithmetic and other things are only performed on 1935 first-class types. Verify that shifts and logicals only happen on 1936 integrals f.e.</li> 1937 <li>All of the constants in a switch statement are of the correct type.</li> 1938 <li>The code is in valid SSA form.</li> 1939 <li>It is illegal to put a label into any other type (like a structure) or 1940 to return one.</li> 1941 <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is 1942 invalid.</li> 1943 <li>PHI nodes must have an entry for each predecessor, with no extras.</li> 1944 <li>PHI nodes must be the first thing in a basic block, all grouped 1945 together.</li> 1946 <li>PHI nodes must have at least one entry.</li> 1947 <li>All basic blocks should only end with terminator insts, not contain 1948 them.</li> 1949 <li>The entry node to a function must not have predecessors.</li> 1950 <li>All Instructions must be embedded into a basic block.</li> 1951 <li>Functions cannot take a void-typed parameter.</li> 1952 <li>Verify that a function's argument list agrees with its declared 1953 type.</li> 1954 <li>It is illegal to specify a name for a void value.</li> 1955 <li>It is illegal to have a internal global value with no initializer.</li> 1956 <li>It is illegal to have a ret instruction that returns a value that does 1957 not agree with the function return value type.</li> 1958 <li>Function call argument types match the function prototype.</li> 1959 <li>All other things that are tested by asserts spread about the code.</li> 1960 </ul> 1961 1962 <p> 1963 Note that this does not provide full security verification (like Java), but 1964 instead just tries to ensure that code is well-formed. 1965 </p> 1966 </div> 1967 1968 <!-------------------------------------------------------------------------- --> 1969 <h3> 1970 <a name="view-cfg">-view-cfg: View CFG of function</a> 1971 </h3> 1972 <div> 1973 <p> 1974 Displays the control flow graph using the GraphViz tool. 1975 </p> 1976 </div> 1977 1978 <!-------------------------------------------------------------------------- --> 1979 <h3> 1980 <a name="view-cfg-only">-view-cfg-only: View CFG of function (with no function bodies)</a> 1981 </h3> 1982 <div> 1983 <p> 1984 Displays the control flow graph using the GraphViz tool, but omitting function 1985 bodies. 1986 </p> 1987 </div> 1988 1989 <!-------------------------------------------------------------------------- --> 1990 <h3> 1991 <a name="view-dom">-view-dom: View dominance tree of function</a> 1992 </h3> 1993 <div> 1994 <p> 1995 Displays the dominator tree using the GraphViz tool. 1996 </p> 1997 </div> 1998 1999 <!-------------------------------------------------------------------------- --> 2000 <h3> 2001 <a name="view-dom-only">-view-dom-only: View dominance tree of function (with no function bodies)</a> 2002 </h3> 2003 <div> 2004 <p> 2005 Displays the dominator tree using the GraphViz tool, but omitting function 2006 bodies. 2007 </p> 2008 </div> 2009 2010 <!-------------------------------------------------------------------------- --> 2011 <h3> 2012 <a name="view-postdom">-view-postdom: View postdominance tree of function</a> 2013 </h3> 2014 <div> 2015 <p> 2016 Displays the post dominator tree using the GraphViz tool. 2017 </p> 2018 </div> 2019 2020 <!-------------------------------------------------------------------------- --> 2021 <h3> 2022 <a name="view-postdom-only">-view-postdom-only: View postdominance tree of function (with no function bodies)</a> 2023 </h3> 2024 <div> 2025 <p> 2026 Displays the post dominator tree using the GraphViz tool, but omitting 2027 function bodies. 2028 </p> 2029 </div> 2030 2031 </div> 2032 2033 <!-- *********************************************************************** --> 2034 2035 <hr> 2036 <address> 2037 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 2038 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 2039 <a href="http://validator.w3.org/check/referer"><img 2040 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 2041 2042 <a href="mailto:rspencer (a] x10sys.com">Reid Spencer</a><br> 2043 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 2044 Last modified: $Date: 2011-08-03 18:18:20 -0400 (Wed, 03 Aug 2011) $ 2045 </address> 2046 2047 </body> 2048 </html> 2049
