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 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="#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="#bb-vectorize">-bb-vectorize</a></td><td>Combine instructions to form vector instructions within basic blocks</td></tr> 130<tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr> 131<tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr> 132<tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr> 133<tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr> 134<tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr> 135<tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr> 136<tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr> 137<tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr> 138<tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr> 139<tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr> 140<tr><td><a href="#functionattrs">-functionattrs</a></td><td>Deduce function attributes</td></tr> 141<tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr> 142<tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr> 143<tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr> 144<tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr> 145<tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr> 146<tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr> 147<tr><td><a href="#insert-optimal-edge-profiling">-insert-optimal-edge-profiling</a></td><td>Insert optimal instrumentation for edge profiling</td></tr> 148<tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr> 149<tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr> 150<tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr> 151<tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr> 152<tr><td><a href="#jump-threading">-jump-threading</a></td><td>Jump Threading</td></tr> 153<tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr> 154<tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr> 155<tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Delete dead loops</td></tr> 156<tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr> 157<tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr> 158<tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr> 159<tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr> 160<tr><td><a href="#loop-simplify">-loop-simplify</a></td><td>Canonicalize natural loops</td></tr> 161<tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr> 162<tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr> 163<tr><td><a href="#loweratomic">-loweratomic</a></td><td>Lower atomic intrinsics to non-atomic form</td></tr> 164<tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr> 165<tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr> 166<tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr> 167<tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>MemCpy Optimization</td></tr> 168<tr><td><a href="#mergefunc">-mergefunc</a></td><td>Merge Functions</td></tr> 169<tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr> 170<tr><td><a href="#partial-inliner">-partial-inliner</a></td><td>Partial Inliner</td></tr> 171<tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr> 172<tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr> 173<tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr> 174<tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates (DT)</td></tr> 175<tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr> 176<tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr> 177<tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr> 178<tr><td><a href="#sink">-sink</a></td><td>Code sinking</td></tr> 179<tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments to multiple ret values</td></tr> 180<tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr> 181<tr><td><a href="#strip-dead-debug-info">-strip-dead-debug-info</a></td><td>Strip debug info for unused symbols</td></tr> 182<tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Strip Unused Function Prototypes</td></tr> 183<tr><td><a href="#strip-debug-declare">-strip-debug-declare</a></td><td>Strip all llvm.dbg.declare intrinsics</td></tr> 184<tr><td><a href="#strip-nondebug">-strip-nondebug</a></td><td>Strip all symbols, except dbg symbols, from a module</td></tr> 185<tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr> 186<tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr> 187 188 189<tr><th colspan="2"><b>UTILITY PASSES</b></th></tr> 190<tr><th>Option</th><th>Name</th></tr> 191<tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr> 192<tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr> 193<tr><td><a href="#instnamer">-instnamer</a></td><td>Assign names to anonymous instructions</td></tr> 194<tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr> 195<tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr> 196<tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr> 197<tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr> 198<tr><td><a href="#view-dom">-view-dom</a></td><td>View dominance tree of function</td></tr> 199<tr><td><a href="#view-dom-only">-view-dom-only</a></td><td>View dominance tree of function (with no function bodies)</td></tr> 200<tr><td><a href="#view-postdom">-view-postdom</a></td><td>View postdominance tree of function</td></tr> 201<tr><td><a href="#view-postdom-only">-view-postdom-only</a></td><td>View postdominance tree of function (with no function bodies)</td></tr> 202</table> 203 204</div> 205 206<!-- ======================================================================= --> 207<h2><a name="analyses">Analysis Passes</a></h2> 208<div> 209 <p>This section describes the LLVM Analysis Passes.</p> 210 211<!-------------------------------------------------------------------------- --> 212<h3> 213 <a name="aa-eval">-aa-eval: Exhaustive Alias Analysis Precision Evaluator</a> 214</h3> 215<div> 216 <p>This is a simple N^2 alias analysis accuracy evaluator. 217 Basically, for each function in the program, it simply queries to see how the 218 alias analysis implementation answers alias queries between each pair of 219 pointers in the function.</p> 220 221 <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco 222 Spadini, and Wojciech Stryjewski.</p> 223</div> 224 225<!-------------------------------------------------------------------------- --> 226<h3> 227 <a name="basicaa">-basicaa: Basic Alias Analysis (stateless AA impl)</a> 228</h3> 229<div> 230 <p>A basic alias analysis pass that implements identities (two different 231 globals cannot alias, etc), but does no stateful analysis.</p> 232</div> 233 234<!-------------------------------------------------------------------------- --> 235<h3> 236 <a name="basiccg">-basiccg: Basic CallGraph Construction</a> 237</h3> 238<div> 239 <p>Yet to be written.</p> 240</div> 241 242<!-------------------------------------------------------------------------- --> 243<h3> 244 <a name="count-aa">-count-aa: Count Alias Analysis Query Responses</a> 245</h3> 246<div> 247 <p> 248 A pass which can be used to count how many alias queries 249 are being made and how the alias analysis implementation being used responds. 250 </p> 251</div> 252 253<!-------------------------------------------------------------------------- --> 254<h3> 255 <a name="debug-aa">-debug-aa: AA use debugger</a> 256</h3> 257<div> 258 <p> 259 This simple pass checks alias analysis users to ensure that if they 260 create a new value, they do not query AA without informing it of the value. 261 It acts as a shim over any other AA pass you want. 262 </p> 263 264 <p> 265 Yes keeping track of every value in the program is expensive, but this is 266 a debugging pass. 267 </p> 268</div> 269 270<!-------------------------------------------------------------------------- --> 271<h3> 272 <a name="domfrontier">-domfrontier: Dominance Frontier Construction</a> 273</h3> 274<div> 275 <p> 276 This pass is a simple dominator construction algorithm for finding forward 277 dominator frontiers. 278 </p> 279</div> 280 281<!-------------------------------------------------------------------------- --> 282<h3> 283 <a name="domtree">-domtree: Dominator Tree Construction</a> 284</h3> 285<div> 286 <p> 287 This pass is a simple dominator construction algorithm for finding forward 288 dominators. 289 </p> 290</div> 291 292<!-------------------------------------------------------------------------- --> 293<h3> 294 <a name="dot-callgraph">-dot-callgraph: Print Call Graph to 'dot' file</a> 295</h3> 296<div> 297 <p> 298 This pass, only available in <code>opt</code>, prints the call graph into a 299 <code>.dot</code> graph. This graph can then be processed with the "dot" tool 300 to convert it to postscript or some other suitable format. 301 </p> 302</div> 303 304<!-------------------------------------------------------------------------- --> 305<h3> 306 <a name="dot-cfg">-dot-cfg: Print CFG of function to 'dot' file</a> 307</h3> 308<div> 309 <p> 310 This pass, only available in <code>opt</code>, prints the control flow graph 311 into a <code>.dot</code> graph. This graph can then be processed with the 312 "dot" tool to convert it to postscript or some other suitable format. 313 </p> 314</div> 315 316<!-------------------------------------------------------------------------- --> 317<h3> 318 <a name="dot-cfg-only">-dot-cfg-only: Print CFG of function to 'dot' file (with no function bodies)</a> 319</h3> 320<div> 321 <p> 322 This pass, only available in <code>opt</code>, prints the control flow graph 323 into a <code>.dot</code> graph, omitting the function bodies. This graph can 324 then be processed with the "dot" tool to convert it to postscript or some 325 other suitable format. 326 </p> 327</div> 328 329<!-------------------------------------------------------------------------- --> 330<h3> 331 <a name="dot-dom">-dot-dom: Print dominance tree of function to 'dot' file</a> 332</h3> 333<div> 334 <p> 335 This pass, only available in <code>opt</code>, prints the dominator tree 336 into a <code>.dot</code> graph. This graph can then be processed with the 337 "dot" tool to convert it to postscript or some other suitable format. 338 </p> 339</div> 340 341<!-------------------------------------------------------------------------- --> 342<h3> 343 <a name="dot-dom-only">-dot-dom-only: Print dominance tree of function to 'dot' file (with no function bodies)</a> 344</h3> 345<div> 346 <p> 347 This pass, only available in <code>opt</code>, prints the dominator tree 348 into a <code>.dot</code> graph, omitting the function bodies. This graph can 349 then be processed with the "dot" tool to convert it to postscript or some 350 other suitable format. 351 </p> 352</div> 353 354<!-------------------------------------------------------------------------- --> 355<h3> 356 <a name="dot-postdom">-dot-postdom: Print postdominance tree of function to 'dot' file</a> 357</h3> 358<div> 359 <p> 360 This pass, only available in <code>opt</code>, prints the post dominator tree 361 into a <code>.dot</code> graph. This graph can then be processed with the 362 "dot" tool to convert it to postscript or some other suitable format. 363 </p> 364</div> 365 366<!-------------------------------------------------------------------------- --> 367<h3> 368 <a name="dot-postdom-only">-dot-postdom-only: Print postdominance tree of function to 'dot' file (with no function bodies)</a> 369</h3> 370<div> 371 <p> 372 This pass, only available in <code>opt</code>, prints the post dominator tree 373 into a <code>.dot</code> graph, omitting the function bodies. This graph can 374 then be processed with the "dot" tool to convert it to postscript or some 375 other suitable format. 376 </p> 377</div> 378 379<!-------------------------------------------------------------------------- --> 380<h3> 381 <a name="globalsmodref-aa">-globalsmodref-aa: Simple mod/ref analysis for globals</a> 382</h3> 383<div> 384 <p> 385 This simple pass provides alias and mod/ref information for global values 386 that do not have their address taken, and keeps track of whether functions 387 read or write memory (are "pure"). For this simple (but very common) case, 388 we can provide pretty accurate and useful information. 389 </p> 390</div> 391 392<!-------------------------------------------------------------------------- --> 393<h3> 394 <a name="instcount">-instcount: Counts the various types of Instructions</a> 395</h3> 396<div> 397 <p> 398 This pass collects the count of all instructions and reports them 399 </p> 400</div> 401 402<!-------------------------------------------------------------------------- --> 403<h3> 404 <a name="intervals">-intervals: Interval Partition Construction</a> 405</h3> 406<div> 407 <p> 408 This analysis calculates and represents the interval partition of a function, 409 or a preexisting interval partition. 410 </p> 411 412 <p> 413 In this way, the interval partition may be used to reduce a flow graph down 414 to its degenerate single node interval partition (unless it is irreducible). 415 </p> 416</div> 417 418<!-------------------------------------------------------------------------- --> 419<h3> 420 <a name="iv-users">-iv-users: Induction Variable Users</a> 421</h3> 422<div> 423 <p>Bookkeeping for "interesting" users of expressions computed from 424 induction variables.</p> 425</div> 426 427<!-------------------------------------------------------------------------- --> 428<h3> 429 <a name="lazy-value-info">-lazy-value-info: Lazy Value Information Analysis</a> 430</h3> 431<div> 432 <p>Interface for lazy computation of value constraint information.</p> 433</div> 434 435<!-------------------------------------------------------------------------- --> 436<h3> 437 <a name="lda">-lda: Loop Dependence Analysis</a> 438</h3> 439<div> 440 <p>Loop dependence analysis framework, which is used to detect dependences in 441 memory accesses in loops.</p> 442</div> 443 444<!-------------------------------------------------------------------------- --> 445<h3> 446 <a name="libcall-aa">-libcall-aa: LibCall Alias Analysis</a> 447</h3> 448<div> 449 <p>LibCall Alias Analysis.</p> 450</div> 451 452<!-------------------------------------------------------------------------- --> 453<h3> 454 <a name="lint">-lint: Statically lint-checks LLVM IR</a> 455</h3> 456<div> 457 <p>This pass statically checks for common and easily-identified constructs 458 which produce undefined or likely unintended behavior in LLVM IR.</p> 459 460 <p>It is not a guarantee of correctness, in two ways. First, it isn't 461 comprehensive. There are checks which could be done statically which are 462 not yet implemented. Some of these are indicated by TODO comments, but 463 those aren't comprehensive either. Second, many conditions cannot be 464 checked statically. This pass does no dynamic instrumentation, so it 465 can't check for all possible problems.</p> 466 467 <p>Another limitation is that it assumes all code will be executed. A store 468 through a null pointer in a basic block which is never reached is harmless, 469 but this pass will warn about it anyway.</p> 470 471 <p>Optimization passes may make conditions that this pass checks for more or 472 less obvious. If an optimization pass appears to be introducing a warning, 473 it may be that the optimization pass is merely exposing an existing 474 condition in the code.</p> 475 476 <p>This code may be run before instcombine. In many cases, instcombine checks 477 for the same kinds of things and turns instructions with undefined behavior 478 into unreachable (or equivalent). Because of this, this pass makes some 479 effort to look through bitcasts and so on. 480 </p> 481</div> 482 483<!-------------------------------------------------------------------------- --> 484<h3> 485 <a name="loops">-loops: Natural Loop Information</a> 486</h3> 487<div> 488 <p> 489 This analysis is used to identify natural loops and determine the loop depth 490 of various nodes of the CFG. Note that the loops identified may actually be 491 several natural loops that share the same header node... not just a single 492 natural loop. 493 </p> 494</div> 495 496<!-------------------------------------------------------------------------- --> 497<h3> 498 <a name="memdep">-memdep: Memory Dependence Analysis</a> 499</h3> 500<div> 501 <p> 502 An analysis that determines, for a given memory operation, what preceding 503 memory operations it depends on. It builds on alias analysis information, and 504 tries to provide a lazy, caching interface to a common kind of alias 505 information query. 506 </p> 507</div> 508 509<!-------------------------------------------------------------------------- --> 510<h3> 511 <a name="module-debuginfo">-module-debuginfo: Decodes module-level debug info</a> 512</h3> 513<div> 514 <p>This pass decodes the debug info metadata in a module and prints in a 515 (sufficiently-prepared-) human-readable form. 516 517 For example, run this pass from opt along with the -analyze option, and 518 it'll print to standard output. 519 </p> 520</div> 521 522<!-------------------------------------------------------------------------- --> 523<h3> 524 <a name="no-aa">-no-aa: No Alias Analysis (always returns 'may' alias)</a> 525</h3> 526<div> 527 <p> 528 This is the default implementation of the Alias Analysis interface. It always 529 returns "I don't know" for alias queries. NoAA is unlike other alias analysis 530 implementations, in that it does not chain to a previous analysis. As such it 531 doesn't follow many of the rules that other alias analyses must. 532 </p> 533</div> 534 535<!-------------------------------------------------------------------------- --> 536<h3> 537 <a name="no-profile">-no-profile: No Profile Information</a> 538</h3> 539<div> 540 <p> 541 The default "no profile" implementation of the abstract 542 <code>ProfileInfo</code> interface. 543 </p> 544</div> 545 546<!-------------------------------------------------------------------------- --> 547<h3> 548 <a name="postdomfrontier">-postdomfrontier: Post-Dominance Frontier Construction</a> 549</h3> 550<div> 551 <p> 552 This pass is a simple post-dominator construction algorithm for finding 553 post-dominator frontiers. 554 </p> 555</div> 556 557<!-------------------------------------------------------------------------- --> 558<h3> 559 <a name="postdomtree">-postdomtree: Post-Dominator Tree Construction</a> 560</h3> 561<div> 562 <p> 563 This pass is a simple post-dominator construction algorithm for finding 564 post-dominators. 565 </p> 566</div> 567 568<!-------------------------------------------------------------------------- --> 569<h3> 570 <a name="print-alias-sets">-print-alias-sets: Alias Set Printer</a> 571</h3> 572<div> 573 <p>Yet to be written.</p> 574</div> 575 576<!-------------------------------------------------------------------------- --> 577<h3> 578 <a name="print-callgraph">-print-callgraph: Print a call graph</a> 579</h3> 580<div> 581 <p> 582 This pass, only available in <code>opt</code>, prints the call graph to 583 standard error in a human-readable form. 584 </p> 585</div> 586 587<!-------------------------------------------------------------------------- --> 588<h3> 589 <a name="print-callgraph-sccs">-print-callgraph-sccs: Print SCCs of the Call Graph</a> 590</h3> 591<div> 592 <p> 593 This pass, only available in <code>opt</code>, prints the SCCs of the call 594 graph to standard error in a human-readable form. 595 </p> 596</div> 597 598<!-------------------------------------------------------------------------- --> 599<h3> 600 <a name="print-cfg-sccs">-print-cfg-sccs: Print SCCs of each function CFG</a> 601</h3> 602<div> 603 <p> 604 This pass, only available in <code>opt</code>, prints the SCCs of each 605 function CFG to standard error in a human-readable form. 606 </p> 607</div> 608 609<!-------------------------------------------------------------------------- --> 610<h3> 611 <a name="print-dbginfo">-print-dbginfo: Print debug info in human readable form</a> 612</h3> 613<div> 614 <p>Pass that prints instructions, and associated debug info:</p> 615 <ul> 616 617 <li>source/line/col information</li> 618 <li>original variable name</li> 619 <li>original type name</li> 620 </ul> 621</div> 622 623<!-------------------------------------------------------------------------- --> 624<h3> 625 <a name="print-dom-info">-print-dom-info: Dominator Info Printer</a> 626</h3> 627<div> 628 <p>Dominator Info Printer.</p> 629</div> 630 631<!-------------------------------------------------------------------------- --> 632<h3> 633 <a name="print-externalfnconstants">-print-externalfnconstants: Print external fn callsites passed constants</a> 634</h3> 635<div> 636 <p> 637 This pass, only available in <code>opt</code>, prints out call sites to 638 external functions that are called with constant arguments. This can be 639 useful when looking for standard library functions we should constant fold 640 or handle in alias analyses. 641 </p> 642</div> 643 644<!-------------------------------------------------------------------------- --> 645<h3> 646 <a name="print-function">-print-function: Print function to stderr</a> 647</h3> 648<div> 649 <p> 650 The <code>PrintFunctionPass</code> class is designed to be pipelined with 651 other <code>FunctionPass</code>es, and prints out the functions of the module 652 as they are processed. 653 </p> 654</div> 655 656<!-------------------------------------------------------------------------- --> 657<h3> 658 <a name="print-module">-print-module: Print module to stderr</a> 659</h3> 660<div> 661 <p> 662 This pass simply prints out the entire module when it is executed. 663 </p> 664</div> 665 666<!-------------------------------------------------------------------------- --> 667<h3> 668 <a name="print-used-types">-print-used-types: Find Used Types</a> 669</h3> 670<div> 671 <p> 672 This pass is used to seek out all of the types in use by the program. Note 673 that this analysis explicitly does not include types only used by the symbol 674 table. 675</div> 676 677<!-------------------------------------------------------------------------- --> 678<h3> 679 <a name="profile-estimator">-profile-estimator: Estimate profiling information</a> 680</h3> 681<div> 682 <p>Profiling information that estimates the profiling information 683 in a very crude and unimaginative way. 684 </p> 685</div> 686 687<!-------------------------------------------------------------------------- --> 688<h3> 689 <a name="profile-loader">-profile-loader: Load profile information from llvmprof.out</a> 690</h3> 691<div> 692 <p> 693 A concrete implementation of profiling information that loads the information 694 from a profile dump file. 695 </p> 696</div> 697 698<!-------------------------------------------------------------------------- --> 699<h3> 700 <a name="profile-verifier">-profile-verifier: Verify profiling information</a> 701</h3> 702<div> 703 <p>Pass that checks profiling information for plausibility.</p> 704</div> 705<h3> 706 <a name="regions">-regions: Detect single entry single exit regions</a> 707</h3> 708<div> 709 <p> 710 The <code>RegionInfo</code> pass detects single entry single exit regions in a 711 function, where a region is defined as any subgraph that is connected to the 712 remaining graph at only two spots. Furthermore, an hierarchical region tree is 713 built. 714 </p> 715</div> 716 717<!-------------------------------------------------------------------------- --> 718<h3> 719 <a name="scalar-evolution">-scalar-evolution: Scalar Evolution Analysis</a> 720</h3> 721<div> 722 <p> 723 The <code>ScalarEvolution</code> analysis can be used to analyze and 724 catagorize scalar expressions in loops. It specializes in recognizing general 725 induction variables, representing them with the abstract and opaque 726 <code>SCEV</code> class. Given this analysis, trip counts of loops and other 727 important properties can be obtained. 728 </p> 729 730 <p> 731 This analysis is primarily useful for induction variable substitution and 732 strength reduction. 733 </p> 734</div> 735 736<!-------------------------------------------------------------------------- --> 737<h3> 738 <a name="scev-aa">-scev-aa: ScalarEvolution-based Alias Analysis</a> 739</h3> 740<div> 741 <p>Simple alias analysis implemented in terms of ScalarEvolution queries. 742 743 This differs from traditional loop dependence analysis in that it tests 744 for dependencies within a single iteration of a loop, rather than 745 dependencies between different iterations. 746 747 ScalarEvolution has a more complete understanding of pointer arithmetic 748 than BasicAliasAnalysis' collection of ad-hoc analyses. 749 </p> 750</div> 751 752<!-------------------------------------------------------------------------- --> 753<h3> 754 <a name="targetdata">-targetdata: Target Data Layout</a> 755</h3> 756<div> 757 <p>Provides other passes access to information on how the size and alignment 758 required by the the target ABI for various data types.</p> 759</div> 760 761</div> 762 763<!-- ======================================================================= --> 764<h2><a name="transforms">Transform Passes</a></h2> 765<div> 766 <p>This section describes the LLVM Transform Passes.</p> 767 768<!-------------------------------------------------------------------------- --> 769<h3> 770 <a name="adce">-adce: Aggressive Dead Code Elimination</a> 771</h3> 772<div> 773 <p>ADCE aggressively tries to eliminate code. This pass is similar to 774 <a href="#dce">DCE</a> but it assumes that values are dead until proven 775 otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to 776 the liveness of values.</p> 777</div> 778 779<!-------------------------------------------------------------------------- --> 780<h3> 781 <a name="always-inline">-always-inline: Inliner for always_inline functions</a> 782</h3> 783<div> 784 <p>A custom inliner that handles only functions that are marked as 785 "always inline".</p> 786</div> 787 788<!-------------------------------------------------------------------------- --> 789<h3> 790 <a name="argpromotion">-argpromotion: Promote 'by reference' arguments to scalars</a> 791</h3> 792<div> 793 <p> 794 This pass promotes "by reference" arguments to be "by value" arguments. In 795 practice, this means looking for internal functions that have pointer 796 arguments. If it can prove, through the use of alias analysis, that an 797 argument is *only* loaded, then it can pass the value into the function 798 instead of the address of the value. This can cause recursive simplification 799 of code and lead to the elimination of allocas (especially in C++ template 800 code like the STL). 801 </p> 802 803 <p> 804 This pass also handles aggregate arguments that are passed into a function, 805 scalarizing them if the elements of the aggregate are only loaded. Note that 806 it refuses to scalarize aggregates which would require passing in more than 807 three operands to the function, because passing thousands of operands for a 808 large array or structure is unprofitable! 809 </p> 810 811 <p> 812 Note that this transformation could also be done for arguments that are only 813 stored to (returning the value instead), but does not currently. This case 814 would be best handled when and if LLVM starts supporting multiple return 815 values from functions. 816 </p> 817</div> 818 819<!-------------------------------------------------------------------------- --> 820<h3> 821 <a name="bb-vectorize">-bb-vectorize: Basic-Block Vectorization</a> 822</h3> 823<div> 824 <p>This pass combines instructions inside basic blocks to form vector 825 instructions. It iterates over each basic block, attempting to pair 826 compatible instructions, repeating this process until no additional 827 pairs are selected for vectorization. When the outputs of some pair 828 of compatible instructions are used as inputs by some other pair of 829 compatible instructions, those pairs are part of a potential 830 vectorization chain. Instruction pairs are only fused into vector 831 instructions when they are part of a chain longer than some 832 threshold length. Moreover, the pass attempts to find the best 833 possible chain for each pair of compatible instructions. These 834 heuristics are intended to prevent vectorization in cases where 835 it would not yield a performance increase of the resulting code. 836 </p> 837</div> 838 839<!-------------------------------------------------------------------------- --> 840<h3> 841 <a name="block-placement">-block-placement: Profile Guided Basic Block Placement</a> 842</h3> 843<div> 844 <p>This pass is a very simple profile guided basic block placement algorithm. 845 The idea is to put frequently executed blocks together at the start of the 846 function and hopefully increase the number of fall-through conditional 847 branches. If there is no profile information for a particular function, this 848 pass basically orders blocks in depth-first order.</p> 849</div> 850 851<!-------------------------------------------------------------------------- --> 852<h3> 853 <a name="break-crit-edges">-break-crit-edges: Break critical edges in CFG</a> 854</h3> 855<div> 856 <p> 857 Break all of the critical edges in the CFG by inserting a dummy basic block. 858 It may be "required" by passes that cannot deal with critical edges. This 859 transformation obviously invalidates the CFG, but can update forward dominator 860 (set, immediate dominators, tree, and frontier) information. 861 </p> 862</div> 863 864<!-------------------------------------------------------------------------- --> 865<h3> 866 <a name="codegenprepare">-codegenprepare: Optimize for code generation</a> 867</h3> 868<div> 869 This pass munges the code in the input function to better prepare it for 870 SelectionDAG-based code generation. This works around limitations in it's 871 basic-block-at-a-time approach. It should eventually be removed. 872</div> 873 874<!-------------------------------------------------------------------------- --> 875<h3> 876 <a name="constmerge">-constmerge: Merge Duplicate Global Constants</a> 877</h3> 878<div> 879 <p> 880 Merges duplicate global constants together into a single constant that is 881 shared. This is useful because some passes (ie TraceValues) insert a lot of 882 string constants into the program, regardless of whether or not an existing 883 string is available. 884 </p> 885</div> 886 887<!-------------------------------------------------------------------------- --> 888<h3> 889 <a name="constprop">-constprop: Simple constant propagation</a> 890</h3> 891<div> 892 <p>This file implements constant propagation and merging. It looks for 893 instructions involving only constant operands and replaces them with a 894 constant value instead of an instruction. For example:</p> 895 <blockquote><pre>add i32 1, 2</pre></blockquote> 896 <p>becomes</p> 897 <blockquote><pre>i32 3</pre></blockquote> 898 <p>NOTE: this pass has a habit of making definitions be dead. It is a good 899 idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass 900 sometime after running this pass.</p> 901</div> 902 903<!-------------------------------------------------------------------------- --> 904<h3> 905 <a name="dce">-dce: Dead Code Elimination</a> 906</h3> 907<div> 908 <p> 909 Dead code elimination is similar to <a href="#die">dead instruction 910 elimination</a>, but it rechecks instructions that were used by removed 911 instructions to see if they are newly dead. 912 </p> 913</div> 914 915<!-------------------------------------------------------------------------- --> 916<h3> 917 <a name="deadargelim">-deadargelim: Dead Argument Elimination</a> 918</h3> 919<div> 920 <p> 921 This pass deletes dead arguments from internal functions. Dead argument 922 elimination removes arguments which are directly dead, as well as arguments 923 only passed into function calls as dead arguments of other functions. This 924 pass also deletes dead arguments in a similar way. 925 </p> 926 927 <p> 928 This pass is often useful as a cleanup pass to run after aggressive 929 interprocedural passes, which add possibly-dead arguments. 930 </p> 931</div> 932 933<!-------------------------------------------------------------------------- --> 934<h3> 935 <a name="deadtypeelim">-deadtypeelim: Dead Type Elimination</a> 936</h3> 937<div> 938 <p> 939 This pass is used to cleanup the output of GCC. It eliminate names for types 940 that are unused in the entire translation unit, using the <a 941 href="#findusedtypes">find used types</a> pass. 942 </p> 943</div> 944 945<!-------------------------------------------------------------------------- --> 946<h3> 947 <a name="die">-die: Dead Instruction Elimination</a> 948</h3> 949<div> 950 <p> 951 Dead instruction elimination performs a single pass over the function, 952 removing instructions that are obviously dead. 953 </p> 954</div> 955 956<!-------------------------------------------------------------------------- --> 957<h3> 958 <a name="dse">-dse: Dead Store Elimination</a> 959</h3> 960<div> 961 <p> 962 A trivial dead store elimination that only considers basic-block local 963 redundant stores. 964 </p> 965</div> 966 967<!-------------------------------------------------------------------------- --> 968<h3> 969 <a name="functionattrs">-functionattrs: Deduce function attributes</a> 970</h3> 971<div> 972 <p>A simple interprocedural pass which walks the call-graph, looking for 973 functions which do not access or only read non-local memory, and marking them 974 readnone/readonly. In addition, it marks function arguments (of pointer type) 975 'nocapture' if a call to the function does not create any copies of the pointer 976 value that outlive the call. This more or less means that the pointer is only 977 dereferenced, and not returned from the function or stored in a global. 978 This pass is implemented as a bottom-up traversal of the call-graph. 979 </p> 980</div> 981 982<!-------------------------------------------------------------------------- --> 983<h3> 984 <a name="globaldce">-globaldce: Dead Global Elimination</a> 985</h3> 986<div> 987 <p> 988 This transform is designed to eliminate unreachable internal globals from the 989 program. It uses an aggressive algorithm, searching out globals that are 990 known to be alive. After it finds all of the globals which are needed, it 991 deletes whatever is left over. This allows it to delete recursive chunks of 992 the program which are unreachable. 993 </p> 994</div> 995 996<!-------------------------------------------------------------------------- --> 997<h3> 998 <a name="globalopt">-globalopt: Global Variable Optimizer</a> 999</h3> 1000<div> 1001 <p> 1002 This pass transforms simple global variables that never have their address 1003 taken. If obviously true, it marks read/write globals as constant, deletes 1004 variables only stored to, etc. 1005 </p> 1006</div> 1007 1008<!-------------------------------------------------------------------------- --> 1009<h3> 1010 <a name="gvn">-gvn: Global Value Numbering</a> 1011</h3> 1012<div> 1013 <p> 1014 This pass performs global value numbering to eliminate fully and partially 1015 redundant instructions. It also performs redundant load elimination. 1016 </p> 1017</div> 1018 1019<!-------------------------------------------------------------------------- --> 1020<h3> 1021 <a name="indvars">-indvars: Canonicalize Induction Variables</a> 1022</h3> 1023<div> 1024 <p> 1025 This transformation analyzes and transforms the induction variables (and 1026 computations derived from them) into simpler forms suitable for subsequent 1027 analysis and transformation. 1028 </p> 1029 1030 <p> 1031 This transformation makes the following changes to each loop with an 1032 identifiable induction variable: 1033 </p> 1034 1035 <ol> 1036 <li>All loops are transformed to have a <em>single</em> canonical 1037 induction variable which starts at zero and steps by one.</li> 1038 <li>The canonical induction variable is guaranteed to be the first PHI node 1039 in the loop header block.</li> 1040 <li>Any pointer arithmetic recurrences are raised to use array 1041 subscripts.</li> 1042 </ol> 1043 1044 <p> 1045 If the trip count of a loop is computable, this pass also makes the following 1046 changes: 1047 </p> 1048 1049 <ol> 1050 <li>The exit condition for the loop is canonicalized to compare the 1051 induction value against the exit value. This turns loops like: 1052 <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote> 1053 into 1054 <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li> 1055 <li>Any use outside of the loop of an expression derived from the indvar 1056 is changed to compute the derived value outside of the loop, eliminating 1057 the dependence on the exit value of the induction variable. If the only 1058 purpose of the loop is to compute the exit value of some derived 1059 expression, this transformation will make the loop dead.</li> 1060 </ol> 1061 1062 <p> 1063 This transformation should be followed by strength reduction after all of the 1064 desired loop transformations have been performed. Additionally, on targets 1065 where it is profitable, the loop could be transformed to count down to zero 1066 (the "do loop" optimization). 1067 </p> 1068</div> 1069 1070<!-------------------------------------------------------------------------- --> 1071<h3> 1072 <a name="inline">-inline: Function Integration/Inlining</a> 1073</h3> 1074<div> 1075 <p> 1076 Bottom-up inlining of functions into callees. 1077 </p> 1078</div> 1079 1080<!-------------------------------------------------------------------------- --> 1081<h3> 1082 <a name="insert-edge-profiling">-insert-edge-profiling: Insert instrumentation for edge profiling</a> 1083</h3> 1084<div> 1085 <p> 1086 This pass instruments the specified program with counters for edge profiling. 1087 Edge profiling can give a reasonable approximation of the hot paths through a 1088 program, and is used for a wide variety of program transformations. 1089 </p> 1090 1091 <p> 1092 Note that this implementation is very naïve. It inserts a counter for 1093 <em>every</em> edge in the program, instead of using control flow information 1094 to prune the number of counters inserted. 1095 </p> 1096</div> 1097 1098<!-------------------------------------------------------------------------- --> 1099<h3> 1100 <a name="insert-optimal-edge-profiling">-insert-optimal-edge-profiling: Insert optimal instrumentation for edge profiling</a> 1101</h3> 1102<div> 1103 <p>This pass instruments the specified program with counters for edge profiling. 1104 Edge profiling can give a reasonable approximation of the hot paths through a 1105 program, and is used for a wide variety of program transformations. 1106 </p> 1107</div> 1108 1109<!-------------------------------------------------------------------------- --> 1110<h3> 1111 <a name="instcombine">-instcombine: Combine redundant instructions</a> 1112</h3> 1113<div> 1114 <p> 1115 Combine instructions to form fewer, simple 1116 instructions. This pass does not modify the CFG This pass is where algebraic 1117 simplification happens. 1118 </p> 1119 1120 <p> 1121 This pass combines things like: 1122 </p> 1123 1124<blockquote><pre 1125>%Y = add i32 %X, 1 1126%Z = add i32 %Y, 1</pre></blockquote> 1127 1128 <p> 1129 into: 1130 </p> 1131 1132<blockquote><pre 1133>%Z = add i32 %X, 2</pre></blockquote> 1134 1135 <p> 1136 This is a simple worklist driven algorithm. 1137 </p> 1138 1139 <p> 1140 This pass guarantees that the following canonicalizations are performed on 1141 the program: 1142 </p> 1143 1144 <ul> 1145 <li>If a binary operator has a constant operand, it is moved to the right- 1146 hand side.</li> 1147 <li>Bitwise operators with constant operands are always grouped so that 1148 shifts are performed first, then <code>or</code>s, then 1149 <code>and</code>s, then <code>xor</code>s.</li> 1150 <li>Compare instructions are converted from <code><</code>, 1151 <code>></code>, <code>≤</code>, or <code>≥</code> to 1152 <code>=</code> or <code>≠</code> if possible.</li> 1153 <li>All <code>cmp</code> instructions on boolean values are replaced with 1154 logical operations.</li> 1155 <li><code>add <var>X</var>, <var>X</var></code> is represented as 1156 <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li> 1157 <li>Multiplies with a constant power-of-two argument are transformed into 1158 shifts.</li> 1159 <li>… etc.</li> 1160 </ul> 1161</div> 1162 1163<!-------------------------------------------------------------------------- --> 1164<h3> 1165 <a name="internalize">-internalize: Internalize Global Symbols</a> 1166</h3> 1167<div> 1168 <p> 1169 This pass loops over all of the functions in the input module, looking for a 1170 main function. If a main function is found, all other functions and all 1171 global variables with initializers are marked as internal. 1172 </p> 1173</div> 1174 1175<!-------------------------------------------------------------------------- --> 1176<h3> 1177 <a name="ipconstprop">-ipconstprop: Interprocedural constant propagation</a> 1178</h3> 1179<div> 1180 <p> 1181 This pass implements an <em>extremely</em> simple interprocedural constant 1182 propagation pass. It could certainly be improved in many different ways, 1183 like using a worklist. This pass makes arguments dead, but does not remove 1184 them. The existing dead argument elimination pass should be run after this 1185 to clean up the mess. 1186 </p> 1187</div> 1188 1189<!-------------------------------------------------------------------------- --> 1190<h3> 1191 <a name="ipsccp">-ipsccp: Interprocedural Sparse Conditional Constant Propagation</a> 1192</h3> 1193<div> 1194 <p> 1195 An interprocedural variant of <a href="#sccp">Sparse Conditional Constant 1196 Propagation</a>. 1197 </p> 1198</div> 1199 1200<!-------------------------------------------------------------------------- --> 1201<h3> 1202 <a name="jump-threading">-jump-threading: Jump Threading</a> 1203</h3> 1204<div> 1205 <p> 1206 Jump threading tries to find distinct threads of control flow running through 1207 a basic block. This pass looks at blocks that have multiple predecessors and 1208 multiple successors. If one or more of the predecessors of the block can be 1209 proven to always cause a jump to one of the successors, we forward the edge 1210 from the predecessor to the successor by duplicating the contents of this 1211 block. 1212 </p> 1213 <p> 1214 An example of when this can occur is code like this: 1215 </p> 1216 1217 <pre 1218>if () { ... 1219 X = 4; 1220} 1221if (X < 3) {</pre> 1222 1223 <p> 1224 In this case, the unconditional branch at the end of the first if can be 1225 revectored to the false side of the second if. 1226 </p> 1227</div> 1228 1229<!-------------------------------------------------------------------------- --> 1230<h3> 1231 <a name="lcssa">-lcssa: Loop-Closed SSA Form Pass</a> 1232</h3> 1233<div> 1234 <p> 1235 This pass transforms loops by placing phi nodes at the end of the loops for 1236 all values that are live across the loop boundary. For example, it turns 1237 the left into the right code: 1238 </p> 1239 1240 <pre 1241>for (...) for (...) 1242 if (c) if (c) 1243 X1 = ... X1 = ... 1244 else else 1245 X2 = ... X2 = ... 1246 X3 = phi(X1, X2) X3 = phi(X1, X2) 1247... = X3 + 4 X4 = phi(X3) 1248 ... = X4 + 4</pre> 1249 1250 <p> 1251 This is still valid LLVM; the extra phi nodes are purely redundant, and will 1252 be trivially eliminated by <code>InstCombine</code>. The major benefit of 1253 this transformation is that it makes many other loop optimizations, such as 1254 LoopUnswitching, simpler. 1255 </p> 1256</div> 1257 1258<!-------------------------------------------------------------------------- --> 1259<h3> 1260 <a name="licm">-licm: Loop Invariant Code Motion</a> 1261</h3> 1262<div> 1263 <p> 1264 This pass performs loop invariant code motion, attempting to remove as much 1265 code from the body of a loop as possible. It does this by either hoisting 1266 code into the preheader block, or by sinking code to the exit blocks if it is 1267 safe. This pass also promotes must-aliased memory locations in the loop to 1268 live in registers, thus hoisting and sinking "invariant" loads and stores. 1269 </p> 1270 1271 <p> 1272 This pass uses alias analysis for two purposes: 1273 </p> 1274 1275 <ul> 1276 <li>Moving loop invariant loads and calls out of loops. If we can determine 1277 that a load or call inside of a loop never aliases anything stored to, 1278 we can hoist it or sink it like any other instruction.</li> 1279 <li>Scalar Promotion of Memory - If there is a store instruction inside of 1280 the loop, we try to move the store to happen AFTER the loop instead of 1281 inside of the loop. This can only happen if a few conditions are true: 1282 <ul> 1283 <li>The pointer stored through is loop invariant.</li> 1284 <li>There are no stores or loads in the loop which <em>may</em> alias 1285 the pointer. There are no calls in the loop which mod/ref the 1286 pointer.</li> 1287 </ul> 1288 If these conditions are true, we can promote the loads and stores in the 1289 loop of the pointer to use a temporary alloca'd variable. We then use 1290 the mem2reg functionality to construct the appropriate SSA form for the 1291 variable.</li> 1292 </ul> 1293</div> 1294 1295<!-------------------------------------------------------------------------- --> 1296<h3> 1297 <a name="loop-deletion">-loop-deletion: Delete dead loops</a> 1298</h3> 1299<div> 1300 <p> 1301 This file implements the Dead Loop Deletion Pass. This pass is responsible 1302 for eliminating loops with non-infinite computable trip counts that have no 1303 side effects or volatile instructions, and do not contribute to the 1304 computation of the function's return value. 1305 </p> 1306</div> 1307 1308<!-------------------------------------------------------------------------- --> 1309<h3> 1310 <a name="loop-extract">-loop-extract: Extract loops into new functions</a> 1311</h3> 1312<div> 1313 <p> 1314 A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to 1315 extract each top-level loop into its own new function. If the loop is the 1316 <em>only</em> loop in a given function, it is not touched. This is a pass most 1317 useful for debugging via bugpoint. 1318 </p> 1319</div> 1320 1321<!-------------------------------------------------------------------------- --> 1322<h3> 1323 <a name="loop-extract-single">-loop-extract-single: Extract at most one loop into a new function</a> 1324</h3> 1325<div> 1326 <p> 1327 Similar to <a href="#loop-extract">Extract loops into new functions</a>, 1328 this pass extracts one natural loop from the program into a function if it 1329 can. This is used by bugpoint. 1330 </p> 1331</div> 1332 1333<!-------------------------------------------------------------------------- --> 1334<h3> 1335 <a name="loop-reduce">-loop-reduce: Loop Strength Reduction</a> 1336</h3> 1337<div> 1338 <p> 1339 This pass performs a strength reduction on array references inside loops that 1340 have as one or more of their components the loop induction variable. This is 1341 accomplished by creating a new value to hold the initial value of the array 1342 access for the first iteration, and then creating a new GEP instruction in 1343 the loop to increment the value by the appropriate amount. 1344 </p> 1345</div> 1346 1347<!-------------------------------------------------------------------------- --> 1348<h3> 1349 <a name="loop-rotate">-loop-rotate: Rotate Loops</a> 1350</h3> 1351<div> 1352 <p>A simple loop rotation transformation.</p> 1353</div> 1354 1355<!-------------------------------------------------------------------------- --> 1356<h3> 1357 <a name="loop-simplify">-loop-simplify: Canonicalize natural loops</a> 1358</h3> 1359<div> 1360 <p> 1361 This pass performs several transformations to transform natural loops into a 1362 simpler form, which makes subsequent analyses and transformations simpler and 1363 more effective. 1364 </p> 1365 1366 <p> 1367 Loop pre-header insertion guarantees that there is a single, non-critical 1368 entry edge from outside of the loop to the loop header. This simplifies a 1369 number of analyses and transformations, such as LICM. 1370 </p> 1371 1372 <p> 1373 Loop exit-block insertion guarantees that all exit blocks from the loop 1374 (blocks which are outside of the loop that have predecessors inside of the 1375 loop) only have predecessors from inside of the loop (and are thus dominated 1376 by the loop header). This simplifies transformations such as store-sinking 1377 that are built into LICM. 1378 </p> 1379 1380 <p> 1381 This pass also guarantees that loops will have exactly one backedge. 1382 </p> 1383 1384 <p> 1385 Note that the simplifycfg pass will clean up blocks which are split out but 1386 end up being unnecessary, so usage of this pass should not pessimize 1387 generated code. 1388 </p> 1389 1390 <p> 1391 This pass obviously modifies the CFG, but updates loop information and 1392 dominator information. 1393 </p> 1394</div> 1395 1396<!-------------------------------------------------------------------------- --> 1397<h3> 1398 <a name="loop-unroll">-loop-unroll: Unroll loops</a> 1399</h3> 1400<div> 1401 <p> 1402 This pass implements a simple loop unroller. It works best when loops have 1403 been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass, 1404 allowing it to determine the trip counts of loops easily. 1405 </p> 1406</div> 1407 1408<!-------------------------------------------------------------------------- --> 1409<h3> 1410 <a name="loop-unswitch">-loop-unswitch: Unswitch loops</a> 1411</h3> 1412<div> 1413 <p> 1414 This pass transforms loops that contain branches on loop-invariant conditions 1415 to have multiple loops. For example, it turns the left into the right code: 1416 </p> 1417 1418 <pre 1419>for (...) if (lic) 1420 A for (...) 1421 if (lic) A; B; C 1422 B else 1423 C for (...) 1424 A; C</pre> 1425 1426 <p> 1427 This can increase the size of the code exponentially (doubling it every time 1428 a loop is unswitched) so we only unswitch if the resultant code will be 1429 smaller than a threshold. 1430 </p> 1431 1432 <p> 1433 This pass expects LICM to be run before it to hoist invariant conditions out 1434 of the loop, to make the unswitching opportunity obvious. 1435 </p> 1436</div> 1437 1438<!-------------------------------------------------------------------------- --> 1439<h3> 1440 <a name="loweratomic">-loweratomic: Lower atomic intrinsics to non-atomic form</a> 1441</h3> 1442<div> 1443 <p> 1444 This pass lowers atomic intrinsics to non-atomic form for use in a known 1445 non-preemptible environment. 1446 </p> 1447 1448 <p> 1449 The pass does not verify that the environment is non-preemptible (in 1450 general this would require knowledge of the entire call graph of the 1451 program including any libraries which may not be available in bitcode form); 1452 it simply lowers every atomic intrinsic. 1453 </p> 1454</div> 1455 1456<!-------------------------------------------------------------------------- --> 1457<h3> 1458 <a name="lowerinvoke">-lowerinvoke: Lower invoke and unwind, for unwindless code generators</a> 1459</h3> 1460<div> 1461 <p> 1462 This transformation is designed for use by code generators which do not yet 1463 support stack unwinding. This pass supports two models of exception handling 1464 lowering, the 'cheap' support and the 'expensive' support. 1465 </p> 1466 1467 <p> 1468 'Cheap' exception handling support gives the program the ability to execute 1469 any program which does not "throw an exception", by turning 'invoke' 1470 instructions into calls and by turning 'unwind' instructions into calls to 1471 abort(). If the program does dynamically use the unwind instruction, the 1472 program will print a message then abort. 1473 </p> 1474 1475 <p> 1476 'Expensive' exception handling support gives the full exception handling 1477 support to the program at the cost of making the 'invoke' instruction 1478 really expensive. It basically inserts setjmp/longjmp calls to emulate the 1479 exception handling as necessary. 1480 </p> 1481 1482 <p> 1483 Because the 'expensive' support slows down programs a lot, and EH is only 1484 used for a subset of the programs, it must be specifically enabled by the 1485 <tt>-enable-correct-eh-support</tt> option. 1486 </p> 1487 1488 <p> 1489 Note that after this pass runs the CFG is not entirely accurate (exceptional 1490 control flow edges are not correct anymore) so only very simple things should 1491 be done after the lowerinvoke pass has run (like generation of native code). 1492 This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't 1493 support the invoke instruction yet" lowering pass. 1494 </p> 1495</div> 1496 1497<!-------------------------------------------------------------------------- --> 1498<h3> 1499 <a name="lowerswitch">-lowerswitch: Lower SwitchInst's to branches</a> 1500</h3> 1501<div> 1502 <p> 1503 Rewrites <tt>switch</tt> instructions with a sequence of branches, which 1504 allows targets to get away with not implementing the switch instruction until 1505 it is convenient. 1506 </p> 1507</div> 1508 1509<!-------------------------------------------------------------------------- --> 1510<h3> 1511 <a name="mem2reg">-mem2reg: Promote Memory to Register</a> 1512</h3> 1513<div> 1514 <p> 1515 This file promotes memory references to be register references. It promotes 1516 <tt>alloca</tt> instructions which only have <tt>load</tt>s and 1517 <tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator 1518 frontiers to place <tt>phi</tt> nodes, then traversing the function in 1519 depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as 1520 appropriate. This is just the standard SSA construction algorithm to construct 1521 "pruned" SSA form. 1522 </p> 1523</div> 1524 1525<!-------------------------------------------------------------------------- --> 1526<h3> 1527 <a name="memcpyopt">-memcpyopt: MemCpy Optimization</a> 1528</h3> 1529<div> 1530 <p> 1531 This pass performs various transformations related to eliminating memcpy 1532 calls, or transforming sets of stores into memset's. 1533 </p> 1534</div> 1535 1536<!-------------------------------------------------------------------------- --> 1537<h3> 1538 <a name="mergefunc">-mergefunc: Merge Functions</a> 1539</h3> 1540<div> 1541 <p>This pass looks for equivalent functions that are mergable and folds them. 1542 1543 A hash is computed from the function, based on its type and number of 1544 basic blocks. 1545 1546 Once all hashes are computed, we perform an expensive equality comparison 1547 on each function pair. This takes n^2/2 comparisons per bucket, so it's 1548 important that the hash function be high quality. The equality comparison 1549 iterates through each instruction in each basic block. 1550 1551 When a match is found the functions are folded. If both functions are 1552 overridable, we move the functionality into a new internal function and 1553 leave two overridable thunks to it. 1554 </p> 1555</div> 1556 1557<!-------------------------------------------------------------------------- --> 1558<h3> 1559 <a name="mergereturn">-mergereturn: Unify function exit nodes</a> 1560</h3> 1561<div> 1562 <p> 1563 Ensure that functions have at most one <tt>ret</tt> instruction in them. 1564 Additionally, it keeps track of which node is the new exit node of the CFG. 1565 </p> 1566</div> 1567 1568<!-------------------------------------------------------------------------- --> 1569<h3> 1570 <a name="partial-inliner">-partial-inliner: Partial Inliner</a> 1571</h3> 1572<div> 1573 <p>This pass performs partial inlining, typically by inlining an if 1574 statement that surrounds the body of the function. 1575 </p> 1576</div> 1577 1578<!-------------------------------------------------------------------------- --> 1579<h3> 1580 <a name="prune-eh">-prune-eh: Remove unused exception handling info</a> 1581</h3> 1582<div> 1583 <p> 1584 This file implements a simple interprocedural pass which walks the call-graph, 1585 turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and 1586 only if the callee cannot throw an exception. It implements this as a 1587 bottom-up traversal of the call-graph. 1588 </p> 1589</div> 1590 1591<!-------------------------------------------------------------------------- --> 1592<h3> 1593 <a name="reassociate">-reassociate: Reassociate expressions</a> 1594</h3> 1595<div> 1596 <p> 1597 This pass reassociates commutative expressions in an order that is designed 1598 to promote better constant propagation, GCSE, LICM, PRE, etc. 1599 </p> 1600 1601 <p> 1602 For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5) 1603 </p> 1604 1605 <p> 1606 In the implementation of this algorithm, constants are assigned rank = 0, 1607 function arguments are rank = 1, and other values are assigned ranks 1608 corresponding to the reverse post order traversal of current function 1609 (starting at 2), which effectively gives values in deep loops higher rank 1610 than values not in loops. 1611 </p> 1612</div> 1613 1614<!-------------------------------------------------------------------------- --> 1615<h3> 1616 <a name="reg2mem">-reg2mem: Demote all values to stack slots</a> 1617</h3> 1618<div> 1619 <p> 1620 This file demotes all registers to memory references. It is intented to be 1621 the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to 1622 <tt>load</tt> instructions, the only values live across basic blocks are 1623 <tt>alloca</tt> instructions and <tt>load</tt> instructions before 1624 <tt>phi</tt> nodes. It is intended that this should make CFG hacking much 1625 easier. To make later hacking easier, the entry block is split into two, such 1626 that all introduced <tt>alloca</tt> instructions (and nothing else) are in the 1627 entry block. 1628 </p> 1629</div> 1630 1631<!-------------------------------------------------------------------------- --> 1632<h3> 1633 <a name="scalarrepl">-scalarrepl: Scalar Replacement of Aggregates (DT)</a> 1634</h3> 1635<div> 1636 <p> 1637 The well-known scalar replacement of aggregates transformation. This 1638 transform breaks up <tt>alloca</tt> instructions of aggregate type (structure 1639 or array) into individual <tt>alloca</tt> instructions for each member if 1640 possible. Then, if possible, it transforms the individual <tt>alloca</tt> 1641 instructions into nice clean scalar SSA form. 1642 </p> 1643 1644 <p> 1645 This combines a simple scalar replacement of aggregates algorithm with the <a 1646 href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact, 1647 especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>, 1648 then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to 1649 promote works well. 1650 </p> 1651</div> 1652 1653<!-------------------------------------------------------------------------- --> 1654<h3> 1655 <a name="sccp">-sccp: Sparse Conditional Constant Propagation</a> 1656</h3> 1657<div> 1658 <p> 1659 Sparse conditional constant propagation and merging, which can be summarized 1660 as: 1661 </p> 1662 1663 <ol> 1664 <li>Assumes values are constant unless proven otherwise</li> 1665 <li>Assumes BasicBlocks are dead unless proven otherwise</li> 1666 <li>Proves values to be constant, and replaces them with constants</li> 1667 <li>Proves conditional branches to be unconditional</li> 1668 </ol> 1669 1670 <p> 1671 Note that this pass has a habit of making definitions be dead. It is a good 1672 idea to to run a DCE pass sometime after running this pass. 1673 </p> 1674</div> 1675 1676<!-------------------------------------------------------------------------- --> 1677<h3> 1678 <a name="simplify-libcalls">-simplify-libcalls: Simplify well-known library calls</a> 1679</h3> 1680<div> 1681 <p> 1682 Applies a variety of small optimizations for calls to specific well-known 1683 function calls (e.g. runtime library functions). For example, a call 1684 <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be 1685 transformed into simply <tt>return 3</tt>. 1686 </p> 1687</div> 1688 1689<!-------------------------------------------------------------------------- --> 1690<h3> 1691 <a name="simplifycfg">-simplifycfg: Simplify the CFG</a> 1692</h3> 1693<div> 1694 <p> 1695 Performs dead code elimination and basic block merging. Specifically: 1696 </p> 1697 1698 <ol> 1699 <li>Removes basic blocks with no predecessors.</li> 1700 <li>Merges a basic block into its predecessor if there is only one and the 1701 predecessor only has one successor.</li> 1702 <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li> 1703 <li>Eliminates a basic block that only contains an unconditional 1704 branch.</li> 1705 </ol> 1706</div> 1707 1708<!-------------------------------------------------------------------------- --> 1709<h3> 1710 <a name="sink">-sink: Code sinking</a> 1711</h3> 1712<div> 1713 <p>This pass moves instructions into successor blocks, when possible, so that 1714 they aren't executed on paths where their results aren't needed. 1715 </p> 1716</div> 1717 1718<!-------------------------------------------------------------------------- --> 1719<h3> 1720 <a name="sretpromotion">-sretpromotion: Promote sret arguments to multiple ret values</a> 1721</h3> 1722<div> 1723 <p> 1724 This pass finds functions that return a struct (using a pointer to the struct 1725 as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and 1726 replaces them with a new function that simply returns each of the elements of 1727 that struct (using multiple return values). 1728 </p> 1729 1730 <p> 1731 This pass works under a number of conditions: 1732 </p> 1733 1734 <ul> 1735 <li>The returned struct must not contain other structs</li> 1736 <li>The returned struct must only be used to load values from</li> 1737 <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li> 1738 </ul> 1739</div> 1740 1741<!-------------------------------------------------------------------------- --> 1742<h3> 1743 <a name="strip">-strip: Strip all symbols from a module</a> 1744</h3> 1745<div> 1746 <p> 1747 performs code stripping. this transformation can delete: 1748 </p> 1749 1750 <ol> 1751 <li>names for virtual registers</li> 1752 <li>symbols for internal globals and functions</li> 1753 <li>debug information</li> 1754 </ol> 1755 1756 <p> 1757 note that this transformation makes code much less readable, so it should 1758 only be used in situations where the <tt>strip</tt> utility would be used, 1759 such as reducing code size or making it harder to reverse engineer code. 1760 </p> 1761</div> 1762 1763<!-------------------------------------------------------------------------- --> 1764<h3> 1765 <a name="strip-dead-debug-info">-strip-dead-debug-info: Strip debug info for unused symbols</a> 1766</h3> 1767<div> 1768 <p> 1769 performs code stripping. this transformation can delete: 1770 </p> 1771 1772 <ol> 1773 <li>names for virtual registers</li> 1774 <li>symbols for internal globals and functions</li> 1775 <li>debug information</li> 1776 </ol> 1777 1778 <p> 1779 note that this transformation makes code much less readable, so it should 1780 only be used in situations where the <tt>strip</tt> utility would be used, 1781 such as reducing code size or making it harder to reverse engineer code. 1782 </p> 1783</div> 1784 1785<!-------------------------------------------------------------------------- --> 1786<h3> 1787 <a name="strip-dead-prototypes">-strip-dead-prototypes: Strip Unused Function Prototypes</a> 1788</h3> 1789<div> 1790 <p> 1791 This pass loops over all of the functions in the input module, looking for 1792 dead declarations and removes them. Dead declarations are declarations of 1793 functions for which no implementation is available (i.e., declarations for 1794 unused library functions). 1795 </p> 1796</div> 1797 1798<!-------------------------------------------------------------------------- --> 1799<h3> 1800 <a name="strip-debug-declare">-strip-debug-declare: Strip all llvm.dbg.declare intrinsics</a> 1801</h3> 1802<div> 1803 <p>This pass implements code stripping. Specifically, it can delete:</p> 1804 <ul> 1805 <li>names for virtual registers</li> 1806 <li>symbols for internal globals and functions</li> 1807 <li>debug information</li> 1808 </ul> 1809 <p> 1810 Note that this transformation makes code much less readable, so it should 1811 only be used in situations where the 'strip' utility would be used, such as 1812 reducing code size or making it harder to reverse engineer code. 1813 </p> 1814</div> 1815 1816<!-------------------------------------------------------------------------- --> 1817<h3> 1818 <a name="strip-nondebug">-strip-nondebug: Strip all symbols, except dbg symbols, from a module</a> 1819</h3> 1820<div> 1821 <p>This pass implements code stripping. Specifically, it can delete:</p> 1822 <ul> 1823 <li>names for virtual registers</li> 1824 <li>symbols for internal globals and functions</li> 1825 <li>debug information</li> 1826 </ul> 1827 <p> 1828 Note that this transformation makes code much less readable, so it should 1829 only be used in situations where the 'strip' utility would be used, such as 1830 reducing code size or making it harder to reverse engineer code. 1831 </p> 1832</div> 1833 1834<!-------------------------------------------------------------------------- --> 1835<h3> 1836 <a name="tailcallelim">-tailcallelim: Tail Call Elimination</a> 1837</h3> 1838<div> 1839 <p> 1840 This file transforms calls of the current function (self recursion) followed 1841 by a return instruction with a branch to the entry of the function, creating 1842 a loop. This pass also implements the following extensions to the basic 1843 algorithm: 1844 </p> 1845 1846 <ul> 1847 <li>Trivial instructions between the call and return do not prevent the 1848 transformation from taking place, though currently the analysis cannot 1849 support moving any really useful instructions (only dead ones). 1850 <li>This pass transforms functions that are prevented from being tail 1851 recursive by an associative expression to use an accumulator variable, 1852 thus compiling the typical naive factorial or <tt>fib</tt> implementation 1853 into efficient code. 1854 <li>TRE is performed if the function returns void, if the return 1855 returns the result returned by the call, or if the function returns a 1856 run-time constant on all exits from the function. It is possible, though 1857 unlikely, that the return returns something else (like constant 0), and 1858 can still be TRE'd. It can be TRE'd if <em>all other</em> return 1859 instructions in the function return the exact same value. 1860 <li>If it can prove that callees do not access theier caller stack frame, 1861 they are marked as eligible for tail call elimination (by the code 1862 generator). 1863 </ul> 1864</div> 1865 1866<!-------------------------------------------------------------------------- --> 1867<h3> 1868 <a name="tailduplicate">-tailduplicate: Tail Duplication</a> 1869</h3> 1870<div> 1871 <p> 1872 This pass performs a limited form of tail duplication, intended to simplify 1873 CFGs by removing some unconditional branches. This pass is necessary to 1874 straighten out loops created by the C front-end, but also is capable of 1875 making other code nicer. After this pass is run, the CFG simplify pass 1876 should be run to clean up the mess. 1877 </p> 1878</div> 1879 1880</div> 1881 1882<!-- ======================================================================= --> 1883<h2><a name="utilities">Utility Passes</a></h2> 1884<div> 1885 <p>This section describes the LLVM Utility Passes.</p> 1886 1887<!-------------------------------------------------------------------------- --> 1888<h3> 1889 <a name="deadarghaX0r">-deadarghaX0r: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a> 1890</h3> 1891<div> 1892 <p> 1893 Same as dead argument elimination, but deletes arguments to functions which 1894 are external. This is only for use by <a 1895 href="Bugpoint.html">bugpoint</a>.</p> 1896</div> 1897 1898<!-------------------------------------------------------------------------- --> 1899<h3> 1900 <a name="extract-blocks">-extract-blocks: Extract Basic Blocks From Module (for bugpoint use)</a> 1901</h3> 1902<div> 1903 <p> 1904 This pass is used by bugpoint to extract all blocks from the module into their 1905 own functions.</p> 1906</div> 1907 1908<!-------------------------------------------------------------------------- --> 1909<h3> 1910 <a name="instnamer">-instnamer: Assign names to anonymous instructions</a> 1911</h3> 1912<div> 1913 <p>This is a little utility pass that gives instructions names, this is mostly 1914 useful when diffing the effect of an optimization because deleting an 1915 unnamed instruction can change all other instruction numbering, making the 1916 diff very noisy. 1917 </p> 1918</div> 1919 1920<!-------------------------------------------------------------------------- --> 1921<h3> 1922 <a name="preverify">-preverify: Preliminary module verification</a> 1923</h3> 1924<div> 1925 <p> 1926 Ensures that the module is in the form required by the <a 1927 href="#verifier">Module Verifier</a> pass. 1928 </p> 1929 1930 <p> 1931 Running the verifier runs this pass automatically, so there should be no need 1932 to use it directly. 1933 </p> 1934</div> 1935 1936<!-------------------------------------------------------------------------- --> 1937<h3> 1938 <a name="verify">-verify: Module Verifier</a> 1939</h3> 1940<div> 1941 <p> 1942 Verifies an LLVM IR code. This is useful to run after an optimization which is 1943 undergoing testing. Note that <tt>llvm-as</tt> verifies its input before 1944 emitting bitcode, and also that malformed bitcode is likely to make LLVM 1945 crash. All language front-ends are therefore encouraged to verify their output 1946 before performing optimizing transformations. 1947 </p> 1948 1949 <ul> 1950 <li>Both of a binary operator's parameters are of the same type.</li> 1951 <li>Verify that the indices of mem access instructions match other 1952 operands.</li> 1953 <li>Verify that arithmetic and other things are only performed on 1954 first-class types. Verify that shifts and logicals only happen on 1955 integrals f.e.</li> 1956 <li>All of the constants in a switch statement are of the correct type.</li> 1957 <li>The code is in valid SSA form.</li> 1958 <li>It is illegal to put a label into any other type (like a structure) or 1959 to return one.</li> 1960 <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is 1961 invalid.</li> 1962 <li>PHI nodes must have an entry for each predecessor, with no extras.</li> 1963 <li>PHI nodes must be the first thing in a basic block, all grouped 1964 together.</li> 1965 <li>PHI nodes must have at least one entry.</li> 1966 <li>All basic blocks should only end with terminator insts, not contain 1967 them.</li> 1968 <li>The entry node to a function must not have predecessors.</li> 1969 <li>All Instructions must be embedded into a basic block.</li> 1970 <li>Functions cannot take a void-typed parameter.</li> 1971 <li>Verify that a function's argument list agrees with its declared 1972 type.</li> 1973 <li>It is illegal to specify a name for a void value.</li> 1974 <li>It is illegal to have a internal global value with no initializer.</li> 1975 <li>It is illegal to have a ret instruction that returns a value that does 1976 not agree with the function return value type.</li> 1977 <li>Function call argument types match the function prototype.</li> 1978 <li>All other things that are tested by asserts spread about the code.</li> 1979 </ul> 1980 1981 <p> 1982 Note that this does not provide full security verification (like Java), but 1983 instead just tries to ensure that code is well-formed. 1984 </p> 1985</div> 1986 1987<!-------------------------------------------------------------------------- --> 1988<h3> 1989 <a name="view-cfg">-view-cfg: View CFG of function</a> 1990</h3> 1991<div> 1992 <p> 1993 Displays the control flow graph using the GraphViz tool. 1994 </p> 1995</div> 1996 1997<!-------------------------------------------------------------------------- --> 1998<h3> 1999 <a name="view-cfg-only">-view-cfg-only: View CFG of function (with no function bodies)</a> 2000</h3> 2001<div> 2002 <p> 2003 Displays the control flow graph using the GraphViz tool, but omitting function 2004 bodies. 2005 </p> 2006</div> 2007 2008<!-------------------------------------------------------------------------- --> 2009<h3> 2010 <a name="view-dom">-view-dom: View dominance tree of function</a> 2011</h3> 2012<div> 2013 <p> 2014 Displays the dominator tree using the GraphViz tool. 2015 </p> 2016</div> 2017 2018<!-------------------------------------------------------------------------- --> 2019<h3> 2020 <a name="view-dom-only">-view-dom-only: View dominance tree of function (with no function bodies)</a> 2021</h3> 2022<div> 2023 <p> 2024 Displays the dominator tree using the GraphViz tool, but omitting function 2025 bodies. 2026 </p> 2027</div> 2028 2029<!-------------------------------------------------------------------------- --> 2030<h3> 2031 <a name="view-postdom">-view-postdom: View postdominance tree of function</a> 2032</h3> 2033<div> 2034 <p> 2035 Displays the post dominator tree using the GraphViz tool. 2036 </p> 2037</div> 2038 2039<!-------------------------------------------------------------------------- --> 2040<h3> 2041 <a name="view-postdom-only">-view-postdom-only: View postdominance tree of function (with no function bodies)</a> 2042</h3> 2043<div> 2044 <p> 2045 Displays the post dominator tree using the GraphViz tool, but omitting 2046 function bodies. 2047 </p> 2048</div> 2049 2050</div> 2051 2052<!-- *********************************************************************** --> 2053 2054<hr> 2055<address> 2056 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 2057 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 2058 <a href="http://validator.w3.org/check/referer"><img 2059 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 2060 2061 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br> 2062 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 2063 Last modified: $Date$ 2064</address> 2065 2066</body> 2067</html> 2068