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="_static/llvm.css" type="text/css"> 7 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 8</head> 9<body> 10 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} 24open HELP, "/Release/bin/opt -help|" or die "open: opt -help: $!\n"; 25while (<HELP>) { 26 m:^ -([^ ]+) +- (.*)$: 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\">-$1: $2</a>\n"; 31} 32@x = map { s/^\d\d\d//; $_ } sort @x; 33@y = map { s/^\d\d\d//; $_ } sort @y; 34print @x, @y; 35EOT 36 37This (real) one-liner can also be helpful when converting comments to HTML: 38 39perl -e '$/ = 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 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@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="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr> 104<tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr> 105<tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr> 106<tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr> 107<tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr> 108<tr><td><a href="#print-dbginfo">-print-dbginfo</a></td><td>Print debug info in human readable form</td></tr> 109<tr><td><a href="#print-dom-info">-print-dom-info</a></td><td>Dominator Info Printer</td></tr> 110<tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr> 111<tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr> 112<tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr> 113<tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr> 114<tr><td><a href="#profile-estimator">-profile-estimator</a></td><td>Estimate profiling information</td></tr> 115<tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr> 116<tr><td><a href="#profile-verifier">-profile-verifier</a></td><td>Verify profiling information</td></tr> 117<tr><td><a href="#regions">-regions</a></td><td>Detect single entry single exit regions</td></tr> 118<tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr> 119<tr><td><a href="#scev-aa">-scev-aa</a></td><td>ScalarEvolution-based Alias Analysis</td></tr> 120<tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr> 121 122 123<tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr> 124<tr><th>Option</th><th>Name</th></tr> 125<tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr> 126<tr><td><a href="#always-inline">-always-inline</a></td><td>Inliner for always_inline functions</td></tr> 127<tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr> 128<tr><td><a href="#bb-vectorize">-bb-vectorize</a></td><td>Combine instructions to form vector instructions within basic blocks</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>A basic alias analysis pass that implements identities (two different 230 globals cannot alias, etc), but does no stateful analysis.</p> 231</div> 232 233<!-------------------------------------------------------------------------- --> 234<h3> 235 <a name="basiccg">-basiccg: Basic CallGraph Construction</a> 236</h3> 237<div> 238 <p>Yet to be written.</p> 239</div> 240 241<!-------------------------------------------------------------------------- --> 242<h3> 243 <a name="count-aa">-count-aa: Count Alias Analysis Query Responses</a> 244</h3> 245<div> 246 <p> 247 A pass which can be used to count how many alias queries 248 are being made and how the alias analysis implementation being used responds. 249 </p> 250</div> 251 252<!-------------------------------------------------------------------------- --> 253<h3> 254 <a name="debug-aa">-debug-aa: AA use debugger</a> 255</h3> 256<div> 257 <p> 258 This simple pass checks alias analysis users to ensure that if they 259 create a new value, they do not query AA without informing it of the value. 260 It acts as a shim over any other AA pass you want. 261 </p> 262 263 <p> 264 Yes keeping track of every value in the program is expensive, but this is 265 a debugging pass. 266 </p> 267</div> 268 269<!-------------------------------------------------------------------------- --> 270<h3> 271 <a name="domfrontier">-domfrontier: Dominance Frontier Construction</a> 272</h3> 273<div> 274 <p> 275 This pass is a simple dominator construction algorithm for finding forward 276 dominator frontiers. 277 </p> 278</div> 279 280<!-------------------------------------------------------------------------- --> 281<h3> 282 <a name="domtree">-domtree: Dominator Tree Construction</a> 283</h3> 284<div> 285 <p> 286 This pass is a simple dominator construction algorithm for finding forward 287 dominators. 288 </p> 289</div> 290 291<!-------------------------------------------------------------------------- --> 292<h3> 293 <a name="dot-callgraph">-dot-callgraph: Print Call Graph to 'dot' file</a> 294</h3> 295<div> 296 <p> 297 This pass, only available in <code>opt</code>, prints the call graph into a 298 <code>.dot</code> graph. This graph can then be processed with the "dot" tool 299 to convert it to postscript or some other suitable format. 300 </p> 301</div> 302 303<!-------------------------------------------------------------------------- --> 304<h3> 305 <a name="dot-cfg">-dot-cfg: Print CFG of function to 'dot' file</a> 306</h3> 307<div> 308 <p> 309 This pass, only available in <code>opt</code>, prints the control flow graph 310 into a <code>.dot</code> graph. This graph can then be processed with the 311 "dot" tool to convert it to postscript or some other suitable format. 312 </p> 313</div> 314 315<!-------------------------------------------------------------------------- --> 316<h3> 317 <a name="dot-cfg-only">-dot-cfg-only: Print CFG of function to 'dot' file (with no function bodies)</a> 318</h3> 319<div> 320 <p> 321 This pass, only available in <code>opt</code>, prints the control flow graph 322 into a <code>.dot</code> graph, omitting the function bodies. This graph can 323 then be processed with the "dot" tool to convert it to postscript or some 324 other suitable format. 325 </p> 326</div> 327 328<!-------------------------------------------------------------------------- --> 329<h3> 330 <a name="dot-dom">-dot-dom: Print dominance tree of function to 'dot' file</a> 331</h3> 332<div> 333 <p> 334 This pass, only available in <code>opt</code>, prints the dominator tree 335 into a <code>.dot</code> graph. This graph can then be processed with the 336 "dot" tool to convert it to postscript or some other suitable format. 337 </p> 338</div> 339 340<!-------------------------------------------------------------------------- --> 341<h3> 342 <a name="dot-dom-only">-dot-dom-only: Print dominance tree of function to 'dot' file (with no function bodies)</a> 343</h3> 344<div> 345 <p> 346 This pass, only available in <code>opt</code>, prints the dominator tree 347 into a <code>.dot</code> graph, omitting the function bodies. This graph can 348 then be processed with the "dot" tool to convert it to postscript or some 349 other suitable format. 350 </p> 351</div> 352 353<!-------------------------------------------------------------------------- --> 354<h3> 355 <a name="dot-postdom">-dot-postdom: Print postdominance tree of function to 'dot' file</a> 356</h3> 357<div> 358 <p> 359 This pass, only available in <code>opt</code>, prints the post dominator tree 360 into a <code>.dot</code> graph. This graph can then be processed with the 361 "dot" tool to convert it to postscript or some other suitable format. 362 </p> 363</div> 364 365<!-------------------------------------------------------------------------- --> 366<h3> 367 <a name="dot-postdom-only">-dot-postdom-only: Print postdominance tree of function to 'dot' file (with no function bodies)</a> 368</h3> 369<div> 370 <p> 371 This pass, only available in <code>opt</code>, prints the post dominator tree 372 into a <code>.dot</code> graph, omitting the function bodies. This graph can 373 then be processed with the "dot" tool to convert it to postscript or some 374 other suitable format. 375 </p> 376</div> 377 378<!-------------------------------------------------------------------------- --> 379<h3> 380 <a name="globalsmodref-aa">-globalsmodref-aa: Simple mod/ref analysis for globals</a> 381</h3> 382<div> 383 <p> 384 This simple pass provides alias and mod/ref information for global values 385 that do not have their address taken, and keeps track of whether functions 386 read or write memory (are "pure"). For this simple (but very common) case, 387 we can provide pretty accurate and useful information. 388 </p> 389</div> 390 391<!-------------------------------------------------------------------------- --> 392<h3> 393 <a name="instcount">-instcount: Counts the various types of Instructions</a> 394</h3> 395<div> 396 <p> 397 This pass collects the count of all instructions and reports them 398 </p> 399</div> 400 401<!-------------------------------------------------------------------------- --> 402<h3> 403 <a name="intervals">-intervals: Interval Partition Construction</a> 404</h3> 405<div> 406 <p> 407 This analysis calculates and represents the interval partition of a function, 408 or a preexisting interval partition. 409 </p> 410 411 <p> 412 In this way, the interval partition may be used to reduce a flow graph down 413 to its degenerate single node interval partition (unless it is irreducible). 414 </p> 415</div> 416 417<!-------------------------------------------------------------------------- --> 418<h3> 419 <a name="iv-users">-iv-users: Induction Variable Users</a> 420</h3> 421<div> 422 <p>Bookkeeping for "interesting" users of expressions computed from 423 induction variables.</p> 424</div> 425 426<!-------------------------------------------------------------------------- --> 427<h3> 428 <a name="lazy-value-info">-lazy-value-info: Lazy Value Information Analysis</a> 429</h3> 430<div> 431 <p>Interface for lazy computation of value constraint information.</p> 432</div> 433 434<!-------------------------------------------------------------------------- --> 435<h3> 436 <a name="lda">-lda: Loop Dependence Analysis</a> 437</h3> 438<div> 439 <p>Loop dependence analysis framework, which is used to detect dependences in 440 memory accesses in loops.</p> 441</div> 442 443<!-------------------------------------------------------------------------- --> 444<h3> 445 <a name="libcall-aa">-libcall-aa: LibCall Alias Analysis</a> 446</h3> 447<div> 448 <p>LibCall Alias Analysis.</p> 449</div> 450 451<!-------------------------------------------------------------------------- --> 452<h3> 453 <a name="lint">-lint: Statically lint-checks LLVM IR</a> 454</h3> 455<div> 456 <p>This pass statically checks for common and easily-identified constructs 457 which produce undefined or likely unintended behavior in LLVM IR.</p> 458 459 <p>It is not a guarantee of correctness, in two ways. First, it isn't 460 comprehensive. There are checks which could be done statically which are 461 not yet implemented. Some of these are indicated by TODO comments, but 462 those aren't comprehensive either. Second, many conditions cannot be 463 checked statically. This pass does no dynamic instrumentation, so it 464 can't check for all possible problems.</p> 465 466 <p>Another limitation is that it assumes all code will be executed. A store 467 through a null pointer in a basic block which is never reached is harmless, 468 but this pass will warn about it anyway.</p> 469 470 <p>Optimization passes may make conditions that this pass checks for more or 471 less obvious. If an optimization pass appears to be introducing a warning, 472 it may be that the optimization pass is merely exposing an existing 473 condition in the code.</p> 474 475 <p>This code may be run before instcombine. In many cases, instcombine checks 476 for the same kinds of things and turns instructions with undefined behavior 477 into unreachable (or equivalent). Because of this, this pass makes some 478 effort to look through bitcasts and so on. 479 </p> 480</div> 481 482<!-------------------------------------------------------------------------- --> 483<h3> 484 <a name="loops">-loops: Natural Loop Information</a> 485</h3> 486<div> 487 <p> 488 This analysis is used to identify natural loops and determine the loop depth 489 of various nodes of the CFG. Note that the loops identified may actually be 490 several natural loops that share the same header node... not just a single 491 natural loop. 492 </p> 493</div> 494 495<!-------------------------------------------------------------------------- --> 496<h3> 497 <a name="memdep">-memdep: Memory Dependence Analysis</a> 498</h3> 499<div> 500 <p> 501 An analysis that determines, for a given memory operation, what preceding 502 memory operations it depends on. It builds on alias analysis information, and 503 tries to provide a lazy, caching interface to a common kind of alias 504 information query. 505 </p> 506</div> 507 508<!-------------------------------------------------------------------------- --> 509<h3> 510 <a name="module-debuginfo">-module-debuginfo: Decodes module-level debug info</a> 511</h3> 512<div> 513 <p>This pass decodes the debug info metadata in a module and prints in a 514 (sufficiently-prepared-) human-readable form. 515 516 For example, run this pass from opt along with the -analyze option, and 517 it'll print to standard output. 518 </p> 519</div> 520 521<!-------------------------------------------------------------------------- --> 522<h3> 523 <a name="no-aa">-no-aa: No Alias Analysis (always returns 'may' alias)</a> 524</h3> 525<div> 526 <p> 527 This is the default implementation of the Alias Analysis interface. It always 528 returns "I don't know" for alias queries. NoAA is unlike other alias analysis 529 implementations, in that it does not chain to a previous analysis. As such it 530 doesn't follow many of the rules that other alias analyses must. 531 </p> 532</div> 533 534<!-------------------------------------------------------------------------- --> 535<h3> 536 <a name="no-profile">-no-profile: No Profile Information</a> 537</h3> 538<div> 539 <p> 540 The default "no profile" implementation of the abstract 541 <code>ProfileInfo</code> interface. 542 </p> 543</div> 544 545<!-------------------------------------------------------------------------- --> 546<h3> 547 <a name="postdomfrontier">-postdomfrontier: Post-Dominance Frontier Construction</a> 548</h3> 549<div> 550 <p> 551 This pass is a simple post-dominator construction algorithm for finding 552 post-dominator frontiers. 553 </p> 554</div> 555 556<!-------------------------------------------------------------------------- --> 557<h3> 558 <a name="postdomtree">-postdomtree: Post-Dominator Tree Construction</a> 559</h3> 560<div> 561 <p> 562 This pass is a simple post-dominator construction algorithm for finding 563 post-dominators. 564 </p> 565</div> 566 567<!-------------------------------------------------------------------------- --> 568<h3> 569 <a name="print-alias-sets">-print-alias-sets: Alias Set Printer</a> 570</h3> 571<div> 572 <p>Yet to be written.</p> 573</div> 574 575<!-------------------------------------------------------------------------- --> 576<h3> 577 <a name="print-callgraph">-print-callgraph: Print a call graph</a> 578</h3> 579<div> 580 <p> 581 This pass, only available in <code>opt</code>, prints the call graph to 582 standard error in a human-readable form. 583 </p> 584</div> 585 586<!-------------------------------------------------------------------------- --> 587<h3> 588 <a name="print-callgraph-sccs">-print-callgraph-sccs: Print SCCs of the Call Graph</a> 589</h3> 590<div> 591 <p> 592 This pass, only available in <code>opt</code>, prints the SCCs of the call 593 graph to standard error in a human-readable form. 594 </p> 595</div> 596 597<!-------------------------------------------------------------------------- --> 598<h3> 599 <a name="print-cfg-sccs">-print-cfg-sccs: Print SCCs of each function CFG</a> 600</h3> 601<div> 602 <p> 603 This pass, only available in <code>opt</code>, prints the SCCs of each 604 function CFG to standard error in a human-readable form. 605 </p> 606</div> 607 608<!-------------------------------------------------------------------------- --> 609<h3> 610 <a name="print-dbginfo">-print-dbginfo: Print debug info in human readable form</a> 611</h3> 612<div> 613 <p>Pass that prints instructions, and associated debug info:</p> 614 <ul> 615 616 <li>source/line/col information</li> 617 <li>original variable name</li> 618 <li>original type name</li> 619 </ul> 620</div> 621 622<!-------------------------------------------------------------------------- --> 623<h3> 624 <a name="print-dom-info">-print-dom-info: Dominator Info Printer</a> 625</h3> 626<div> 627 <p>Dominator Info Printer.</p> 628</div> 629 630<!-------------------------------------------------------------------------- --> 631<h3> 632 <a name="print-externalfnconstants">-print-externalfnconstants: Print external fn callsites passed constants</a> 633</h3> 634<div> 635 <p> 636 This pass, only available in <code>opt</code>, prints out call sites to 637 external functions that are called with constant arguments. This can be 638 useful when looking for standard library functions we should constant fold 639 or handle in alias analyses. 640 </p> 641</div> 642 643<!-------------------------------------------------------------------------- --> 644<h3> 645 <a name="print-function">-print-function: Print function to stderr</a> 646</h3> 647<div> 648 <p> 649 The <code>PrintFunctionPass</code> class is designed to be pipelined with 650 other <code>FunctionPass</code>es, and prints out the functions of the module 651 as they are processed. 652 </p> 653</div> 654 655<!-------------------------------------------------------------------------- --> 656<h3> 657 <a name="print-module">-print-module: Print module to stderr</a> 658</h3> 659<div> 660 <p> 661 This pass simply prints out the entire module when it is executed. 662 </p> 663</div> 664 665<!-------------------------------------------------------------------------- --> 666<h3> 667 <a name="print-used-types">-print-used-types: Find Used Types</a> 668</h3> 669<div> 670 <p> 671 This pass is used to seek out all of the types in use by the program. Note 672 that this analysis explicitly does not include types only used by the symbol 673 table. 674</div> 675 676<!-------------------------------------------------------------------------- --> 677<h3> 678 <a name="profile-estimator">-profile-estimator: Estimate profiling information</a> 679</h3> 680<div> 681 <p>Profiling information that estimates the profiling information 682 in a very crude and unimaginative way. 683 </p> 684</div> 685 686<!-------------------------------------------------------------------------- --> 687<h3> 688 <a name="profile-loader">-profile-loader: Load profile information from llvmprof.out</a> 689</h3> 690<div> 691 <p> 692 A concrete implementation of profiling information that loads the information 693 from a profile dump file. 694 </p> 695</div> 696 697<!-------------------------------------------------------------------------- --> 698<h3> 699 <a name="profile-verifier">-profile-verifier: Verify profiling information</a> 700</h3> 701<div> 702 <p>Pass that checks profiling information for plausibility.</p> 703</div> 704<h3> 705 <a name="regions">-regions: Detect single entry single exit regions</a> 706</h3> 707<div> 708 <p> 709 The <code>RegionInfo</code> pass detects single entry single exit regions in a 710 function, where a region is defined as any subgraph that is connected to the 711 remaining graph at only two spots. Furthermore, an hierarchical region tree is 712 built. 713 </p> 714</div> 715 716<!-------------------------------------------------------------------------- --> 717<h3> 718 <a name="scalar-evolution">-scalar-evolution: Scalar Evolution Analysis</a> 719</h3> 720<div> 721 <p> 722 The <code>ScalarEvolution</code> analysis can be used to analyze and 723 catagorize scalar expressions in loops. It specializes in recognizing general 724 induction variables, representing them with the abstract and opaque 725 <code>SCEV</code> class. Given this analysis, trip counts of loops and other 726 important properties can be obtained. 727 </p> 728 729 <p> 730 This analysis is primarily useful for induction variable substitution and 731 strength reduction. 732 </p> 733</div> 734 735<!-------------------------------------------------------------------------- --> 736<h3> 737 <a name="scev-aa">-scev-aa: ScalarEvolution-based Alias Analysis</a> 738</h3> 739<div> 740 <p>Simple alias analysis implemented in terms of ScalarEvolution queries. 741 742 This differs from traditional loop dependence analysis in that it tests 743 for dependencies within a single iteration of a loop, rather than 744 dependencies between different iterations. 745 746 ScalarEvolution has a more complete understanding of pointer arithmetic 747 than BasicAliasAnalysis' collection of ad-hoc analyses. 748 </p> 749</div> 750 751<!-------------------------------------------------------------------------- --> 752<h3> 753 <a name="targetdata">-targetdata: Target Data Layout</a> 754</h3> 755<div> 756 <p>Provides other passes access to information on how the size and alignment 757 required by the target ABI for various data types.</p> 758</div> 759 760</div> 761 762<!-- ======================================================================= --> 763<h2><a name="transforms">Transform Passes</a></h2> 764<div> 765 <p>This section describes the LLVM Transform Passes.</p> 766 767<!-------------------------------------------------------------------------- --> 768<h3> 769 <a name="adce">-adce: Aggressive Dead Code Elimination</a> 770</h3> 771<div> 772 <p>ADCE aggressively tries to eliminate code. This pass is similar to 773 <a href="#dce">DCE</a> but it assumes that values are dead until proven 774 otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to 775 the liveness of values.</p> 776</div> 777 778<!-------------------------------------------------------------------------- --> 779<h3> 780 <a name="always-inline">-always-inline: Inliner for always_inline functions</a> 781</h3> 782<div> 783 <p>A custom inliner that handles only functions that are marked as 784 "always inline".</p> 785</div> 786 787<!-------------------------------------------------------------------------- --> 788<h3> 789 <a name="argpromotion">-argpromotion: Promote 'by reference' arguments to scalars</a> 790</h3> 791<div> 792 <p> 793 This pass promotes "by reference" arguments to be "by value" arguments. In 794 practice, this means looking for internal functions that have pointer 795 arguments. If it can prove, through the use of alias analysis, that an 796 argument is *only* loaded, then it can pass the value into the function 797 instead of the address of the value. This can cause recursive simplification 798 of code and lead to the elimination of allocas (especially in C++ template 799 code like the STL). 800 </p> 801 802 <p> 803 This pass also handles aggregate arguments that are passed into a function, 804 scalarizing them if the elements of the aggregate are only loaded. Note that 805 it refuses to scalarize aggregates which would require passing in more than 806 three operands to the function, because passing thousands of operands for a 807 large array or structure is unprofitable! 808 </p> 809 810 <p> 811 Note that this transformation could also be done for arguments that are only 812 stored to (returning the value instead), but does not currently. This case 813 would be best handled when and if LLVM starts supporting multiple return 814 values from functions. 815 </p> 816</div> 817 818<!-------------------------------------------------------------------------- --> 819<h3> 820 <a name="bb-vectorize">-bb-vectorize: Basic-Block Vectorization</a> 821</h3> 822<div> 823 <p>This pass combines instructions inside basic blocks to form vector 824 instructions. It iterates over each basic block, attempting to pair 825 compatible instructions, repeating this process until no additional 826 pairs are selected for vectorization. When the outputs of some pair 827 of compatible instructions are used as inputs by some other pair of 828 compatible instructions, those pairs are part of a potential 829 vectorization chain. Instruction pairs are only fused into vector 830 instructions when they are part of a chain longer than some 831 threshold length. Moreover, the pass attempts to find the best 832 possible chain for each pair of compatible instructions. These 833 heuristics are intended to prevent vectorization in cases where 834 it would not yield a performance increase of the resulting code. 835 </p> 836</div> 837 838<!-------------------------------------------------------------------------- --> 839<h3> 840 <a name="block-placement">-block-placement: Profile Guided Basic Block Placement</a> 841</h3> 842<div> 843 <p>This pass is a very simple profile guided basic block placement algorithm. 844 The idea is to put frequently executed blocks together at the start of the 845 function and hopefully increase the number of fall-through conditional 846 branches. If there is no profile information for a particular function, this 847 pass basically orders blocks in depth-first order.</p> 848</div> 849 850<!-------------------------------------------------------------------------- --> 851<h3> 852 <a name="break-crit-edges">-break-crit-edges: Break critical edges in CFG</a> 853</h3> 854<div> 855 <p> 856 Break all of the critical edges in the CFG by inserting a dummy basic block. 857 It may be "required" by passes that cannot deal with critical edges. This 858 transformation obviously invalidates the CFG, but can update forward dominator 859 (set, immediate dominators, tree, and frontier) information. 860 </p> 861</div> 862 863<!-------------------------------------------------------------------------- --> 864<h3> 865 <a name="codegenprepare">-codegenprepare: Optimize for code generation</a> 866</h3> 867<div> 868 This pass munges the code in the input function to better prepare it for 869 SelectionDAG-based code generation. This works around limitations in it's 870 basic-block-at-a-time approach. It should eventually be removed. 871</div> 872 873<!-------------------------------------------------------------------------- --> 874<h3> 875 <a name="constmerge">-constmerge: Merge Duplicate Global Constants</a> 876</h3> 877<div> 878 <p> 879 Merges duplicate global constants together into a single constant that is 880 shared. This is useful because some passes (ie TraceValues) insert a lot of 881 string constants into the program, regardless of whether or not an existing 882 string is available. 883 </p> 884</div> 885 886<!-------------------------------------------------------------------------- --> 887<h3> 888 <a name="constprop">-constprop: Simple constant propagation</a> 889</h3> 890<div> 891 <p>This file implements constant propagation and merging. It looks for 892 instructions involving only constant operands and replaces them with a 893 constant value instead of an instruction. For example:</p> 894 <blockquote><pre>add i32 1, 2</pre></blockquote> 895 <p>becomes</p> 896 <blockquote><pre>i32 3</pre></blockquote> 897 <p>NOTE: this pass has a habit of making definitions be dead. It is a good 898 idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass 899 sometime after running this pass.</p> 900</div> 901 902<!-------------------------------------------------------------------------- --> 903<h3> 904 <a name="dce">-dce: Dead Code Elimination</a> 905</h3> 906<div> 907 <p> 908 Dead code elimination is similar to <a href="#die">dead instruction 909 elimination</a>, but it rechecks instructions that were used by removed 910 instructions to see if they are newly dead. 911 </p> 912</div> 913 914<!-------------------------------------------------------------------------- --> 915<h3> 916 <a name="deadargelim">-deadargelim: Dead Argument Elimination</a> 917</h3> 918<div> 919 <p> 920 This pass deletes dead arguments from internal functions. Dead argument 921 elimination removes arguments which are directly dead, as well as arguments 922 only passed into function calls as dead arguments of other functions. This 923 pass also deletes dead arguments in a similar way. 924 </p> 925 926 <p> 927 This pass is often useful as a cleanup pass to run after aggressive 928 interprocedural passes, which add possibly-dead arguments. 929 </p> 930</div> 931 932<!-------------------------------------------------------------------------- --> 933<h3> 934 <a name="deadtypeelim">-deadtypeelim: Dead Type Elimination</a> 935</h3> 936<div> 937 <p> 938 This pass is used to cleanup the output of GCC. It eliminate names for types 939 that are unused in the entire translation unit, using the <a 940 href="#findusedtypes">find used types</a> pass. 941 </p> 942</div> 943 944<!-------------------------------------------------------------------------- --> 945<h3> 946 <a name="die">-die: Dead Instruction Elimination</a> 947</h3> 948<div> 949 <p> 950 Dead instruction elimination performs a single pass over the function, 951 removing instructions that are obviously dead. 952 </p> 953</div> 954 955<!-------------------------------------------------------------------------- --> 956<h3> 957 <a name="dse">-dse: Dead Store Elimination</a> 958</h3> 959<div> 960 <p> 961 A trivial dead store elimination that only considers basic-block local 962 redundant stores. 963 </p> 964</div> 965 966<!-------------------------------------------------------------------------- --> 967<h3> 968 <a name="functionattrs">-functionattrs: Deduce function attributes</a> 969</h3> 970<div> 971 <p>A simple interprocedural pass which walks the call-graph, looking for 972 functions which do not access or only read non-local memory, and marking them 973 readnone/readonly. In addition, it marks function arguments (of pointer type) 974 'nocapture' if a call to the function does not create any copies of the pointer 975 value that outlive the call. This more or less means that the pointer is only 976 dereferenced, and not returned from the function or stored in a global. 977 This pass is implemented as a bottom-up traversal of the call-graph. 978 </p> 979</div> 980 981<!-------------------------------------------------------------------------- --> 982<h3> 983 <a name="globaldce">-globaldce: Dead Global Elimination</a> 984</h3> 985<div> 986 <p> 987 This transform is designed to eliminate unreachable internal globals from the 988 program. It uses an aggressive algorithm, searching out globals that are 989 known to be alive. After it finds all of the globals which are needed, it 990 deletes whatever is left over. This allows it to delete recursive chunks of 991 the program which are unreachable. 992 </p> 993</div> 994 995<!-------------------------------------------------------------------------- --> 996<h3> 997 <a name="globalopt">-globalopt: Global Variable Optimizer</a> 998</h3> 999<div> 1000 <p> 1001 This pass transforms simple global variables that never have their address 1002 taken. If obviously true, it marks read/write globals as constant, deletes 1003 variables only stored to, etc. 1004 </p> 1005</div> 1006 1007<!-------------------------------------------------------------------------- --> 1008<h3> 1009 <a name="gvn">-gvn: Global Value Numbering</a> 1010</h3> 1011<div> 1012 <p> 1013 This pass performs global value numbering to eliminate fully and partially 1014 redundant instructions. It also performs redundant load elimination. 1015 </p> 1016</div> 1017 1018<!-------------------------------------------------------------------------- --> 1019<h3> 1020 <a name="indvars">-indvars: Canonicalize Induction Variables</a> 1021</h3> 1022<div> 1023 <p> 1024 This transformation analyzes and transforms the induction variables (and 1025 computations derived from them) into simpler forms suitable for subsequent 1026 analysis and transformation. 1027 </p> 1028 1029 <p> 1030 This transformation makes the following changes to each loop with an 1031 identifiable induction variable: 1032 </p> 1033 1034 <ol> 1035 <li>All loops are transformed to have a <em>single</em> canonical 1036 induction variable which starts at zero and steps by one.</li> 1037 <li>The canonical induction variable is guaranteed to be the first PHI node 1038 in the loop header block.</li> 1039 <li>Any pointer arithmetic recurrences are raised to use array 1040 subscripts.</li> 1041 </ol> 1042 1043 <p> 1044 If the trip count of a loop is computable, this pass also makes the following 1045 changes: 1046 </p> 1047 1048 <ol> 1049 <li>The exit condition for the loop is canonicalized to compare the 1050 induction value against the exit value. This turns loops like: 1051 <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote> 1052 into 1053 <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li> 1054 <li>Any use outside of the loop of an expression derived from the indvar 1055 is changed to compute the derived value outside of the loop, eliminating 1056 the dependence on the exit value of the induction variable. If the only 1057 purpose of the loop is to compute the exit value of some derived 1058 expression, this transformation will make the loop dead.</li> 1059 </ol> 1060 1061 <p> 1062 This transformation should be followed by strength reduction after all of the 1063 desired loop transformations have been performed. Additionally, on targets 1064 where it is profitable, the loop could be transformed to count down to zero 1065 (the "do loop" optimization). 1066 </p> 1067</div> 1068 1069<!-------------------------------------------------------------------------- --> 1070<h3> 1071 <a name="inline">-inline: Function Integration/Inlining</a> 1072</h3> 1073<div> 1074 <p> 1075 Bottom-up inlining of functions into callees. 1076 </p> 1077</div> 1078 1079<!-------------------------------------------------------------------------- --> 1080<h3> 1081 <a name="insert-edge-profiling">-insert-edge-profiling: Insert instrumentation for edge profiling</a> 1082</h3> 1083<div> 1084 <p> 1085 This pass instruments the specified program with counters for edge profiling. 1086 Edge profiling can give a reasonable approximation of the hot paths through a 1087 program, and is used for a wide variety of program transformations. 1088 </p> 1089 1090 <p> 1091 Note that this implementation is very naïve. It inserts a counter for 1092 <em>every</em> edge in the program, instead of using control flow information 1093 to prune the number of counters inserted. 1094 </p> 1095</div> 1096 1097<!-------------------------------------------------------------------------- --> 1098<h3> 1099 <a name="insert-optimal-edge-profiling">-insert-optimal-edge-profiling: Insert optimal instrumentation for edge profiling</a> 1100</h3> 1101<div> 1102 <p>This pass instruments the specified program with counters for edge profiling. 1103 Edge profiling can give a reasonable approximation of the hot paths through a 1104 program, and is used for a wide variety of program transformations. 1105 </p> 1106</div> 1107 1108<!-------------------------------------------------------------------------- --> 1109<h3> 1110 <a name="instcombine">-instcombine: Combine redundant instructions</a> 1111</h3> 1112<div> 1113 <p> 1114 Combine instructions to form fewer, simple 1115 instructions. This pass does not modify the CFG This pass is where algebraic 1116 simplification happens. 1117 </p> 1118 1119 <p> 1120 This pass combines things like: 1121 </p> 1122 1123<blockquote><pre 1124>%Y = add i32 %X, 1 1125%Z = add i32 %Y, 1</pre></blockquote> 1126 1127 <p> 1128 into: 1129 </p> 1130 1131<blockquote><pre 1132>%Z = add i32 %X, 2</pre></blockquote> 1133 1134 <p> 1135 This is a simple worklist driven algorithm. 1136 </p> 1137 1138 <p> 1139 This pass guarantees that the following canonicalizations are performed on 1140 the program: 1141 </p> 1142 1143 <ul> 1144 <li>If a binary operator has a constant operand, it is moved to the right- 1145 hand side.</li> 1146 <li>Bitwise operators with constant operands are always grouped so that 1147 shifts are performed first, then <code>or</code>s, then 1148 <code>and</code>s, then <code>xor</code>s.</li> 1149 <li>Compare instructions are converted from <code><</code>, 1150 <code>></code>, <code>≤</code>, or <code>≥</code> to 1151 <code>=</code> or <code>≠</code> if possible.</li> 1152 <li>All <code>cmp</code> instructions on boolean values are replaced with 1153 logical operations.</li> 1154 <li><code>add <var>X</var>, <var>X</var></code> is represented as 1155 <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li> 1156 <li>Multiplies with a constant power-of-two argument are transformed into 1157 shifts.</li> 1158 <li>… etc.</li> 1159 </ul> 1160</div> 1161 1162<!-------------------------------------------------------------------------- --> 1163<h3> 1164 <a name="internalize">-internalize: Internalize Global Symbols</a> 1165</h3> 1166<div> 1167 <p> 1168 This pass loops over all of the functions in the input module, looking for a 1169 main function. If a main function is found, all other functions and all 1170 global variables with initializers are marked as internal. 1171 </p> 1172</div> 1173 1174<!-------------------------------------------------------------------------- --> 1175<h3> 1176 <a name="ipconstprop">-ipconstprop: Interprocedural constant propagation</a> 1177</h3> 1178<div> 1179 <p> 1180 This pass implements an <em>extremely</em> simple interprocedural constant 1181 propagation pass. It could certainly be improved in many different ways, 1182 like using a worklist. This pass makes arguments dead, but does not remove 1183 them. The existing dead argument elimination pass should be run after this 1184 to clean up the mess. 1185 </p> 1186</div> 1187 1188<!-------------------------------------------------------------------------- --> 1189<h3> 1190 <a name="ipsccp">-ipsccp: Interprocedural Sparse Conditional Constant Propagation</a> 1191</h3> 1192<div> 1193 <p> 1194 An interprocedural variant of <a href="#sccp">Sparse Conditional Constant 1195 Propagation</a>. 1196 </p> 1197</div> 1198 1199<!-------------------------------------------------------------------------- --> 1200<h3> 1201 <a name="jump-threading">-jump-threading: Jump Threading</a> 1202</h3> 1203<div> 1204 <p> 1205 Jump threading tries to find distinct threads of control flow running through 1206 a basic block. This pass looks at blocks that have multiple predecessors and 1207 multiple successors. If one or more of the predecessors of the block can be 1208 proven to always cause a jump to one of the successors, we forward the edge 1209 from the predecessor to the successor by duplicating the contents of this 1210 block. 1211 </p> 1212 <p> 1213 An example of when this can occur is code like this: 1214 </p> 1215 1216 <pre 1217>if () { ... 1218 X = 4; 1219} 1220if (X < 3) {</pre> 1221 1222 <p> 1223 In this case, the unconditional branch at the end of the first if can be 1224 revectored to the false side of the second if. 1225 </p> 1226</div> 1227 1228<!-------------------------------------------------------------------------- --> 1229<h3> 1230 <a name="lcssa">-lcssa: Loop-Closed SSA Form Pass</a> 1231</h3> 1232<div> 1233 <p> 1234 This pass transforms loops by placing phi nodes at the end of the loops for 1235 all values that are live across the loop boundary. For example, it turns 1236 the left into the right code: 1237 </p> 1238 1239 <pre 1240>for (...) for (...) 1241 if (c) if (c) 1242 X1 = ... X1 = ... 1243 else else 1244 X2 = ... X2 = ... 1245 X3 = phi(X1, X2) X3 = phi(X1, X2) 1246... = X3 + 4 X4 = phi(X3) 1247 ... = X4 + 4</pre> 1248 1249 <p> 1250 This is still valid LLVM; the extra phi nodes are purely redundant, and will 1251 be trivially eliminated by <code>InstCombine</code>. The major benefit of 1252 this transformation is that it makes many other loop optimizations, such as 1253 LoopUnswitching, simpler. 1254 </p> 1255</div> 1256 1257<!-------------------------------------------------------------------------- --> 1258<h3> 1259 <a name="licm">-licm: Loop Invariant Code Motion</a> 1260</h3> 1261<div> 1262 <p> 1263 This pass performs loop invariant code motion, attempting to remove as much 1264 code from the body of a loop as possible. It does this by either hoisting 1265 code into the preheader block, or by sinking code to the exit blocks if it is 1266 safe. This pass also promotes must-aliased memory locations in the loop to 1267 live in registers, thus hoisting and sinking "invariant" loads and stores. 1268 </p> 1269 1270 <p> 1271 This pass uses alias analysis for two purposes: 1272 </p> 1273 1274 <ul> 1275 <li>Moving loop invariant loads and calls out of loops. If we can determine 1276 that a load or call inside of a loop never aliases anything stored to, 1277 we can hoist it or sink it like any other instruction.</li> 1278 <li>Scalar Promotion of Memory - If there is a store instruction inside of 1279 the loop, we try to move the store to happen AFTER the loop instead of 1280 inside of the loop. This can only happen if a few conditions are true: 1281 <ul> 1282 <li>The pointer stored through is loop invariant.</li> 1283 <li>There are no stores or loads in the loop which <em>may</em> alias 1284 the pointer. There are no calls in the loop which mod/ref the 1285 pointer.</li> 1286 </ul> 1287 If these conditions are true, we can promote the loads and stores in the 1288 loop of the pointer to use a temporary alloca'd variable. We then use 1289 the mem2reg functionality to construct the appropriate SSA form for the 1290 variable.</li> 1291 </ul> 1292</div> 1293 1294<!-------------------------------------------------------------------------- --> 1295<h3> 1296 <a name="loop-deletion">-loop-deletion: Delete dead loops</a> 1297</h3> 1298<div> 1299 <p> 1300 This file implements the Dead Loop Deletion Pass. This pass is responsible 1301 for eliminating loops with non-infinite computable trip counts that have no 1302 side effects or volatile instructions, and do not contribute to the 1303 computation of the function's return value. 1304 </p> 1305</div> 1306 1307<!-------------------------------------------------------------------------- --> 1308<h3> 1309 <a name="loop-extract">-loop-extract: Extract loops into new functions</a> 1310</h3> 1311<div> 1312 <p> 1313 A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to 1314 extract each top-level loop into its own new function. If the loop is the 1315 <em>only</em> loop in a given function, it is not touched. This is a pass most 1316 useful for debugging via bugpoint. 1317 </p> 1318</div> 1319 1320<!-------------------------------------------------------------------------- --> 1321<h3> 1322 <a name="loop-extract-single">-loop-extract-single: Extract at most one loop into a new function</a> 1323</h3> 1324<div> 1325 <p> 1326 Similar to <a href="#loop-extract">Extract loops into new functions</a>, 1327 this pass extracts one natural loop from the program into a function if it 1328 can. This is used by bugpoint. 1329 </p> 1330</div> 1331 1332<!-------------------------------------------------------------------------- --> 1333<h3> 1334 <a name="loop-reduce">-loop-reduce: Loop Strength Reduction</a> 1335</h3> 1336<div> 1337 <p> 1338 This pass performs a strength reduction on array references inside loops that 1339 have as one or more of their components the loop induction variable. This is 1340 accomplished by creating a new value to hold the initial value of the array 1341 access for the first iteration, and then creating a new GEP instruction in 1342 the loop to increment the value by the appropriate amount. 1343 </p> 1344</div> 1345 1346<!-------------------------------------------------------------------------- --> 1347<h3> 1348 <a name="loop-rotate">-loop-rotate: Rotate Loops</a> 1349</h3> 1350<div> 1351 <p>A simple loop rotation transformation.</p> 1352</div> 1353 1354<!-------------------------------------------------------------------------- --> 1355<h3> 1356 <a name="loop-simplify">-loop-simplify: Canonicalize natural loops</a> 1357</h3> 1358<div> 1359 <p> 1360 This pass performs several transformations to transform natural loops into a 1361 simpler form, which makes subsequent analyses and transformations simpler and 1362 more effective. 1363 </p> 1364 1365 <p> 1366 Loop pre-header insertion guarantees that there is a single, non-critical 1367 entry edge from outside of the loop to the loop header. This simplifies a 1368 number of analyses and transformations, such as LICM. 1369 </p> 1370 1371 <p> 1372 Loop exit-block insertion guarantees that all exit blocks from the loop 1373 (blocks which are outside of the loop that have predecessors inside of the 1374 loop) only have predecessors from inside of the loop (and are thus dominated 1375 by the loop header). This simplifies transformations such as store-sinking 1376 that are built into LICM. 1377 </p> 1378 1379 <p> 1380 This pass also guarantees that loops will have exactly one backedge. 1381 </p> 1382 1383 <p> 1384 Note that the simplifycfg pass will clean up blocks which are split out but 1385 end up being unnecessary, so usage of this pass should not pessimize 1386 generated code. 1387 </p> 1388 1389 <p> 1390 This pass obviously modifies the CFG, but updates loop information and 1391 dominator information. 1392 </p> 1393</div> 1394 1395<!-------------------------------------------------------------------------- --> 1396<h3> 1397 <a name="loop-unroll">-loop-unroll: Unroll loops</a> 1398</h3> 1399<div> 1400 <p> 1401 This pass implements a simple loop unroller. It works best when loops have 1402 been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass, 1403 allowing it to determine the trip counts of loops easily. 1404 </p> 1405</div> 1406 1407<!-------------------------------------------------------------------------- --> 1408<h3> 1409 <a name="loop-unswitch">-loop-unswitch: Unswitch loops</a> 1410</h3> 1411<div> 1412 <p> 1413 This pass transforms loops that contain branches on loop-invariant conditions 1414 to have multiple loops. For example, it turns the left into the right code: 1415 </p> 1416 1417 <pre 1418>for (...) if (lic) 1419 A for (...) 1420 if (lic) A; B; C 1421 B else 1422 C for (...) 1423 A; C</pre> 1424 1425 <p> 1426 This can increase the size of the code exponentially (doubling it every time 1427 a loop is unswitched) so we only unswitch if the resultant code will be 1428 smaller than a threshold. 1429 </p> 1430 1431 <p> 1432 This pass expects LICM to be run before it to hoist invariant conditions out 1433 of the loop, to make the unswitching opportunity obvious. 1434 </p> 1435</div> 1436 1437<!-------------------------------------------------------------------------- --> 1438<h3> 1439 <a name="loweratomic">-loweratomic: Lower atomic intrinsics to non-atomic form</a> 1440</h3> 1441<div> 1442 <p> 1443 This pass lowers atomic intrinsics to non-atomic form for use in a known 1444 non-preemptible environment. 1445 </p> 1446 1447 <p> 1448 The pass does not verify that the environment is non-preemptible (in 1449 general this would require knowledge of the entire call graph of the 1450 program including any libraries which may not be available in bitcode form); 1451 it simply lowers every atomic intrinsic. 1452 </p> 1453</div> 1454 1455<!-------------------------------------------------------------------------- --> 1456<h3> 1457 <a name="lowerinvoke">-lowerinvoke: Lower invoke and unwind, for unwindless code generators</a> 1458</h3> 1459<div> 1460 <p> 1461 This transformation is designed for use by code generators which do not yet 1462 support stack unwinding. This pass supports two models of exception handling 1463 lowering, the 'cheap' support and the 'expensive' support. 1464 </p> 1465 1466 <p> 1467 'Cheap' exception handling support gives the program the ability to execute 1468 any program which does not "throw an exception", by turning 'invoke' 1469 instructions into calls and by turning 'unwind' instructions into calls to 1470 abort(). If the program does dynamically use the unwind instruction, the 1471 program will print a message then abort. 1472 </p> 1473 1474 <p> 1475 'Expensive' exception handling support gives the full exception handling 1476 support to the program at the cost of making the 'invoke' instruction 1477 really expensive. It basically inserts setjmp/longjmp calls to emulate the 1478 exception handling as necessary. 1479 </p> 1480 1481 <p> 1482 Because the 'expensive' support slows down programs a lot, and EH is only 1483 used for a subset of the programs, it must be specifically enabled by the 1484 <tt>-enable-correct-eh-support</tt> option. 1485 </p> 1486 1487 <p> 1488 Note that after this pass runs the CFG is not entirely accurate (exceptional 1489 control flow edges are not correct anymore) so only very simple things should 1490 be done after the lowerinvoke pass has run (like generation of native code). 1491 This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't 1492 support the invoke instruction yet" lowering pass. 1493 </p> 1494</div> 1495 1496<!-------------------------------------------------------------------------- --> 1497<h3> 1498 <a name="lowerswitch">-lowerswitch: Lower SwitchInst's to branches</a> 1499</h3> 1500<div> 1501 <p> 1502 Rewrites <tt>switch</tt> instructions with a sequence of branches, which 1503 allows targets to get away with not implementing the switch instruction until 1504 it is convenient. 1505 </p> 1506</div> 1507 1508<!-------------------------------------------------------------------------- --> 1509<h3> 1510 <a name="mem2reg">-mem2reg: Promote Memory to Register</a> 1511</h3> 1512<div> 1513 <p> 1514 This file promotes memory references to be register references. It promotes 1515 <tt>alloca</tt> instructions which only have <tt>load</tt>s and 1516 <tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator 1517 frontiers to place <tt>phi</tt> nodes, then traversing the function in 1518 depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as 1519 appropriate. This is just the standard SSA construction algorithm to construct 1520 "pruned" SSA form. 1521 </p> 1522</div> 1523 1524<!-------------------------------------------------------------------------- --> 1525<h3> 1526 <a name="memcpyopt">-memcpyopt: MemCpy Optimization</a> 1527</h3> 1528<div> 1529 <p> 1530 This pass performs various transformations related to eliminating memcpy 1531 calls, or transforming sets of stores into memset's. 1532 </p> 1533</div> 1534 1535<!-------------------------------------------------------------------------- --> 1536<h3> 1537 <a name="mergefunc">-mergefunc: Merge Functions</a> 1538</h3> 1539<div> 1540 <p>This pass looks for equivalent functions that are mergable and folds them. 1541 1542 A hash is computed from the function, based on its type and number of 1543 basic blocks. 1544 1545 Once all hashes are computed, we perform an expensive equality comparison 1546 on each function pair. This takes n^2/2 comparisons per bucket, so it's 1547 important that the hash function be high quality. The equality comparison 1548 iterates through each instruction in each basic block. 1549 1550 When a match is found the functions are folded. If both functions are 1551 overridable, we move the functionality into a new internal function and 1552 leave two overridable thunks to it. 1553 </p> 1554</div> 1555 1556<!-------------------------------------------------------------------------- --> 1557<h3> 1558 <a name="mergereturn">-mergereturn: Unify function exit nodes</a> 1559</h3> 1560<div> 1561 <p> 1562 Ensure that functions have at most one <tt>ret</tt> instruction in them. 1563 Additionally, it keeps track of which node is the new exit node of the CFG. 1564 </p> 1565</div> 1566 1567<!-------------------------------------------------------------------------- --> 1568<h3> 1569 <a name="partial-inliner">-partial-inliner: Partial Inliner</a> 1570</h3> 1571<div> 1572 <p>This pass performs partial inlining, typically by inlining an if 1573 statement that surrounds the body of the function. 1574 </p> 1575</div> 1576 1577<!-------------------------------------------------------------------------- --> 1578<h3> 1579 <a name="prune-eh">-prune-eh: Remove unused exception handling info</a> 1580</h3> 1581<div> 1582 <p> 1583 This file implements a simple interprocedural pass which walks the call-graph, 1584 turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and 1585 only if the callee cannot throw an exception. It implements this as a 1586 bottom-up traversal of the call-graph. 1587 </p> 1588</div> 1589 1590<!-------------------------------------------------------------------------- --> 1591<h3> 1592 <a name="reassociate">-reassociate: Reassociate expressions</a> 1593</h3> 1594<div> 1595 <p> 1596 This pass reassociates commutative expressions in an order that is designed 1597 to promote better constant propagation, GCSE, LICM, PRE, etc. 1598 </p> 1599 1600 <p> 1601 For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5) 1602 </p> 1603 1604 <p> 1605 In the implementation of this algorithm, constants are assigned rank = 0, 1606 function arguments are rank = 1, and other values are assigned ranks 1607 corresponding to the reverse post order traversal of current function 1608 (starting at 2), which effectively gives values in deep loops higher rank 1609 than values not in loops. 1610 </p> 1611</div> 1612 1613<!-------------------------------------------------------------------------- --> 1614<h3> 1615 <a name="reg2mem">-reg2mem: Demote all values to stack slots</a> 1616</h3> 1617<div> 1618 <p> 1619 This file demotes all registers to memory references. It is intended to be 1620 the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to 1621 <tt>load</tt> instructions, the only values live across basic blocks are 1622 <tt>alloca</tt> instructions and <tt>load</tt> instructions before 1623 <tt>phi</tt> nodes. It is intended that this should make CFG hacking much 1624 easier. To make later hacking easier, the entry block is split into two, such 1625 that all introduced <tt>alloca</tt> instructions (and nothing else) are in the 1626 entry block. 1627 </p> 1628</div> 1629 1630<!-------------------------------------------------------------------------- --> 1631<h3> 1632 <a name="scalarrepl">-scalarrepl: Scalar Replacement of Aggregates (DT)</a> 1633</h3> 1634<div> 1635 <p> 1636 The well-known scalar replacement of aggregates transformation. This 1637 transform breaks up <tt>alloca</tt> instructions of aggregate type (structure 1638 or array) into individual <tt>alloca</tt> instructions for each member if 1639 possible. Then, if possible, it transforms the individual <tt>alloca</tt> 1640 instructions into nice clean scalar SSA form. 1641 </p> 1642 1643 <p> 1644 This combines a simple scalar replacement of aggregates algorithm with the <a 1645 href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact, 1646 especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>, 1647 then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to 1648 promote works well. 1649 </p> 1650</div> 1651 1652<!-------------------------------------------------------------------------- --> 1653<h3> 1654 <a name="sccp">-sccp: Sparse Conditional Constant Propagation</a> 1655</h3> 1656<div> 1657 <p> 1658 Sparse conditional constant propagation and merging, which can be summarized 1659 as: 1660 </p> 1661 1662 <ol> 1663 <li>Assumes values are constant unless proven otherwise</li> 1664 <li>Assumes BasicBlocks are dead unless proven otherwise</li> 1665 <li>Proves values to be constant, and replaces them with constants</li> 1666 <li>Proves conditional branches to be unconditional</li> 1667 </ol> 1668 1669 <p> 1670 Note that this pass has a habit of making definitions be dead. It is a good 1671 idea to to run a DCE pass sometime after running this pass. 1672 </p> 1673</div> 1674 1675<!-------------------------------------------------------------------------- --> 1676<h3> 1677 <a name="simplify-libcalls">-simplify-libcalls: Simplify well-known library calls</a> 1678</h3> 1679<div> 1680 <p> 1681 Applies a variety of small optimizations for calls to specific well-known 1682 function calls (e.g. runtime library functions). For example, a call 1683 <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be 1684 transformed into simply <tt>return 3</tt>. 1685 </p> 1686</div> 1687 1688<!-------------------------------------------------------------------------- --> 1689<h3> 1690 <a name="simplifycfg">-simplifycfg: Simplify the CFG</a> 1691</h3> 1692<div> 1693 <p> 1694 Performs dead code elimination and basic block merging. Specifically: 1695 </p> 1696 1697 <ol> 1698 <li>Removes basic blocks with no predecessors.</li> 1699 <li>Merges a basic block into its predecessor if there is only one and the 1700 predecessor only has one successor.</li> 1701 <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li> 1702 <li>Eliminates a basic block that only contains an unconditional 1703 branch.</li> 1704 </ol> 1705</div> 1706 1707<!-------------------------------------------------------------------------- --> 1708<h3> 1709 <a name="sink">-sink: Code sinking</a> 1710</h3> 1711<div> 1712 <p>This pass moves instructions into successor blocks, when possible, so that 1713 they aren't executed on paths where their results aren't needed. 1714 </p> 1715</div> 1716 1717<!-------------------------------------------------------------------------- --> 1718<h3> 1719 <a name="sretpromotion">-sretpromotion: Promote sret arguments to multiple ret values</a> 1720</h3> 1721<div> 1722 <p> 1723 This pass finds functions that return a struct (using a pointer to the struct 1724 as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and 1725 replaces them with a new function that simply returns each of the elements of 1726 that struct (using multiple return values). 1727 </p> 1728 1729 <p> 1730 This pass works under a number of conditions: 1731 </p> 1732 1733 <ul> 1734 <li>The returned struct must not contain other structs</li> 1735 <li>The returned struct must only be used to load values from</li> 1736 <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li> 1737 </ul> 1738</div> 1739 1740<!-------------------------------------------------------------------------- --> 1741<h3> 1742 <a name="strip">-strip: Strip all symbols from a module</a> 1743</h3> 1744<div> 1745 <p> 1746 performs code stripping. this transformation can delete: 1747 </p> 1748 1749 <ol> 1750 <li>names for virtual registers</li> 1751 <li>symbols for internal globals and functions</li> 1752 <li>debug information</li> 1753 </ol> 1754 1755 <p> 1756 note that this transformation makes code much less readable, so it should 1757 only be used in situations where the <tt>strip</tt> utility would be used, 1758 such as reducing code size or making it harder to reverse engineer code. 1759 </p> 1760</div> 1761 1762<!-------------------------------------------------------------------------- --> 1763<h3> 1764 <a name="strip-dead-debug-info">-strip-dead-debug-info: Strip debug info for unused symbols</a> 1765</h3> 1766<div> 1767 <p> 1768 performs code stripping. this transformation can delete: 1769 </p> 1770 1771 <ol> 1772 <li>names for virtual registers</li> 1773 <li>symbols for internal globals and functions</li> 1774 <li>debug information</li> 1775 </ol> 1776 1777 <p> 1778 note that this transformation makes code much less readable, so it should 1779 only be used in situations where the <tt>strip</tt> utility would be used, 1780 such as reducing code size or making it harder to reverse engineer code. 1781 </p> 1782</div> 1783 1784<!-------------------------------------------------------------------------- --> 1785<h3> 1786 <a name="strip-dead-prototypes">-strip-dead-prototypes: Strip Unused Function Prototypes</a> 1787</h3> 1788<div> 1789 <p> 1790 This pass loops over all of the functions in the input module, looking for 1791 dead declarations and removes them. Dead declarations are declarations of 1792 functions for which no implementation is available (i.e., declarations for 1793 unused library functions). 1794 </p> 1795</div> 1796 1797<!-------------------------------------------------------------------------- --> 1798<h3> 1799 <a name="strip-debug-declare">-strip-debug-declare: Strip all llvm.dbg.declare intrinsics</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="strip-nondebug">-strip-nondebug: Strip all symbols, except dbg symbols, from a module</a> 1818</h3> 1819<div> 1820 <p>This pass implements code stripping. Specifically, it can delete:</p> 1821 <ul> 1822 <li>names for virtual registers</li> 1823 <li>symbols for internal globals and functions</li> 1824 <li>debug information</li> 1825 </ul> 1826 <p> 1827 Note that this transformation makes code much less readable, so it should 1828 only be used in situations where the 'strip' utility would be used, such as 1829 reducing code size or making it harder to reverse engineer code. 1830 </p> 1831</div> 1832 1833<!-------------------------------------------------------------------------- --> 1834<h3> 1835 <a name="tailcallelim">-tailcallelim: Tail Call Elimination</a> 1836</h3> 1837<div> 1838 <p> 1839 This file transforms calls of the current function (self recursion) followed 1840 by a return instruction with a branch to the entry of the function, creating 1841 a loop. This pass also implements the following extensions to the basic 1842 algorithm: 1843 </p> 1844 1845 <ul> 1846 <li>Trivial instructions between the call and return do not prevent the 1847 transformation from taking place, though currently the analysis cannot 1848 support moving any really useful instructions (only dead ones). 1849 <li>This pass transforms functions that are prevented from being tail 1850 recursive by an associative expression to use an accumulator variable, 1851 thus compiling the typical naive factorial or <tt>fib</tt> implementation 1852 into efficient code. 1853 <li>TRE is performed if the function returns void, if the return 1854 returns the result returned by the call, or if the function returns a 1855 run-time constant on all exits from the function. It is possible, though 1856 unlikely, that the return returns something else (like constant 0), and 1857 can still be TRE'd. It can be TRE'd if <em>all other</em> return 1858 instructions in the function return the exact same value. 1859 <li>If it can prove that callees do not access theier caller stack frame, 1860 they are marked as eligible for tail call elimination (by the code 1861 generator). 1862 </ul> 1863</div> 1864 1865<!-------------------------------------------------------------------------- --> 1866<h3> 1867 <a name="tailduplicate">-tailduplicate: Tail Duplication</a> 1868</h3> 1869<div> 1870 <p> 1871 This pass performs a limited form of tail duplication, intended to simplify 1872 CFGs by removing some unconditional branches. This pass is necessary to 1873 straighten out loops created by the C front-end, but also is capable of 1874 making other code nicer. After this pass is run, the CFG simplify pass 1875 should be run to clean up the mess. 1876 </p> 1877</div> 1878 1879</div> 1880 1881<!-- ======================================================================= --> 1882<h2><a name="utilities">Utility Passes</a></h2> 1883<div> 1884 <p>This section describes the LLVM Utility Passes.</p> 1885 1886<!-------------------------------------------------------------------------- --> 1887<h3> 1888 <a name="deadarghaX0r">-deadarghaX0r: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a> 1889</h3> 1890<div> 1891 <p> 1892 Same as dead argument elimination, but deletes arguments to functions which 1893 are external. This is only for use by <a 1894 href="Bugpoint.html">bugpoint</a>.</p> 1895</div> 1896 1897<!-------------------------------------------------------------------------- --> 1898<h3> 1899 <a name="extract-blocks">-extract-blocks: Extract Basic Blocks From Module (for bugpoint use)</a> 1900</h3> 1901<div> 1902 <p> 1903 This pass is used by bugpoint to extract all blocks from the module into their 1904 own functions.</p> 1905</div> 1906 1907<!-------------------------------------------------------------------------- --> 1908<h3> 1909 <a name="instnamer">-instnamer: Assign names to anonymous instructions</a> 1910</h3> 1911<div> 1912 <p>This is a little utility pass that gives instructions names, this is mostly 1913 useful when diffing the effect of an optimization because deleting an 1914 unnamed instruction can change all other instruction numbering, making the 1915 diff very noisy. 1916 </p> 1917</div> 1918 1919<!-------------------------------------------------------------------------- --> 1920<h3> 1921 <a name="preverify">-preverify: Preliminary module verification</a> 1922</h3> 1923<div> 1924 <p> 1925 Ensures that the module is in the form required by the <a 1926 href="#verifier">Module Verifier</a> pass. 1927 </p> 1928 1929 <p> 1930 Running the verifier runs this pass automatically, so there should be no need 1931 to use it directly. 1932 </p> 1933</div> 1934 1935<!-------------------------------------------------------------------------- --> 1936<h3> 1937 <a name="verify">-verify: Module Verifier</a> 1938</h3> 1939<div> 1940 <p> 1941 Verifies an LLVM IR code. This is useful to run after an optimization which is 1942 undergoing testing. Note that <tt>llvm-as</tt> verifies its input before 1943 emitting bitcode, and also that malformed bitcode is likely to make LLVM 1944 crash. All language front-ends are therefore encouraged to verify their output 1945 before performing optimizing transformations. 1946 </p> 1947 1948 <ul> 1949 <li>Both of a binary operator's parameters are of the same type.</li> 1950 <li>Verify that the indices of mem access instructions match other 1951 operands.</li> 1952 <li>Verify that arithmetic and other things are only performed on 1953 first-class types. Verify that shifts and logicals only happen on 1954 integrals f.e.</li> 1955 <li>All of the constants in a switch statement are of the correct type.</li> 1956 <li>The code is in valid SSA form.</li> 1957 <li>It is illegal to put a label into any other type (like a structure) or 1958 to return one.</li> 1959 <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is 1960 invalid.</li> 1961 <li>PHI nodes must have an entry for each predecessor, with no extras.</li> 1962 <li>PHI nodes must be the first thing in a basic block, all grouped 1963 together.</li> 1964 <li>PHI nodes must have at least one entry.</li> 1965 <li>All basic blocks should only end with terminator insts, not contain 1966 them.</li> 1967 <li>The entry node to a function must not have predecessors.</li> 1968 <li>All Instructions must be embedded into a basic block.</li> 1969 <li>Functions cannot take a void-typed parameter.</li> 1970 <li>Verify that a function's argument list agrees with its declared 1971 type.</li> 1972 <li>It is illegal to specify a name for a void value.</li> 1973 <li>It is illegal to have an internal global value with no initializer.</li> 1974 <li>It is illegal to have a ret instruction that returns a value that does 1975 not agree with the function return value type.</li> 1976 <li>Function call argument types match the function prototype.</li> 1977 <li>All other things that are tested by asserts spread about the code.</li> 1978 </ul> 1979 1980 <p> 1981 Note that this does not provide full security verification (like Java), but 1982 instead just tries to ensure that code is well-formed. 1983 </p> 1984</div> 1985 1986<!-------------------------------------------------------------------------- --> 1987<h3> 1988 <a name="view-cfg">-view-cfg: View CFG of function</a> 1989</h3> 1990<div> 1991 <p> 1992 Displays the control flow graph using the GraphViz tool. 1993 </p> 1994</div> 1995 1996<!-------------------------------------------------------------------------- --> 1997<h3> 1998 <a name="view-cfg-only">-view-cfg-only: View CFG of function (with no function bodies)</a> 1999</h3> 2000<div> 2001 <p> 2002 Displays the control flow graph using the GraphViz tool, but omitting function 2003 bodies. 2004 </p> 2005</div> 2006 2007<!-------------------------------------------------------------------------- --> 2008<h3> 2009 <a name="view-dom">-view-dom: View dominance tree of function</a> 2010</h3> 2011<div> 2012 <p> 2013 Displays the dominator tree using the GraphViz tool. 2014 </p> 2015</div> 2016 2017<!-------------------------------------------------------------------------- --> 2018<h3> 2019 <a name="view-dom-only">-view-dom-only: View dominance tree of function (with no function bodies)</a> 2020</h3> 2021<div> 2022 <p> 2023 Displays the dominator tree using the GraphViz tool, but omitting function 2024 bodies. 2025 </p> 2026</div> 2027 2028<!-------------------------------------------------------------------------- --> 2029<h3> 2030 <a name="view-postdom">-view-postdom: View postdominance tree of function</a> 2031</h3> 2032<div> 2033 <p> 2034 Displays the post dominator tree using the GraphViz tool. 2035 </p> 2036</div> 2037 2038<!-------------------------------------------------------------------------- --> 2039<h3> 2040 <a name="view-postdom-only">-view-postdom-only: View postdominance tree of function (with no function bodies)</a> 2041</h3> 2042<div> 2043 <p> 2044 Displays the post dominator tree using the GraphViz tool, but omitting 2045 function bodies. 2046 </p> 2047</div> 2048 2049</div> 2050 2051<!-- *********************************************************************** --> 2052 2053<hr> 2054<address> 2055 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 2056 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 2057 <a href="http://validator.w3.org/check/referer"><img 2058 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 2059 2060 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br> 2061 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 2062 Last modified: $Date$ 2063</address> 2064 2065</body> 2066</html> 2067