docs/Passes.html

   1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
   2                       "http://www.w3.org/TR/html4/strict.dtd">
   3 <html>
   4 <head>
   5   <title>LLVM's Analysis and Transform Passes</title>
   6   <link rel="stylesheet" href="llvm.css" type="text/css">
   7   <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
   8 </head>
   9 <body>
  10
  11 <!--
  12
  13 If Passes.html is up to date, the following "one-liner" should print
  14 an empty diff.
  15
  16 egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \
  17       -e '^  <a name=".*">.*</a>$' < Passes.html >html; \
  18 perl >help <<'EOT' && diff -u help html; rm -f help html
  19 open HTML, "<Passes.html" or die "open: Passes.html: $!\n";
  20 while (<HTML>) {
  21   m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next;
  22   $order{$1} = sprintf("%03d", 1 + int %order);
  23 }
  24 open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n";
  25 while (<HELP>) {
  26   m:^    -([^ ]+) +- (.*)$: or next;
  27   my $o = $order{$1};
  28   $o = "000" unless defined $o;
  29   push @x, "$o<tr><td><a href=\"#$1\">-$1</a></td><td>$2</td></tr>\n";
  30   push @y, "$o  <a name=\"$1\">$2</a>\n";
  31 }
  32 @x = map { s/^\d\d\d//; $_ } sort @x;
  33 @y = map { s/^\d\d\d//; $_ } sort @y;
  34 print @x, @y;
  35 EOT
  36
  37 This (real) one-liner can also be helpful when converting comments to HTML:
  38
  39 perl -e '$/ = undef; for (split(/\n/, <>)) { s:^ *///? ?::; print "  <p>\n" if !$on && $_ =~ /\S/; print "  </p>\n" if $on && $_ =~ /^\s*$/; print "  $_\n"; $on = ($_ =~ /\S/); } print "  </p>\n" if $on'
  40
  41   -->
  42
  43 <div class="doc_title">LLVM's Analysis and Transform Passes</div>
  44
  45 <ol>
  46   <li><a href="#intro">Introduction</a></li>
  47   <li><a href="#analyses">Analysis Passes</a>
  48   <li><a href="#transforms">Transform Passes</a></li>
  49   <li><a href="#utilities">Utility Passes</a></li>
  50 </ol>
  51
  52 <div class="doc_author">
  53   <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
  54             and Gordon Henriksen</p>
  55 </div>
  56
  57 <!-- ======================================================================= -->
  58 <div class="doc_section"> <a name="intro">Introduction</a> </div>
  59 <div class="doc_text">
  60   <p>This document serves as a high level summary of the optimization features
  61   that LLVM provides. Optimizations are implemented as Passes that traverse some
  62   portion of a program to either collect information or transform the program.
  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 </div>
  73 <div class="doc_text" >
  74 <table>
  75 <tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr>
  76 <tr><th>Option</th><th>Name</th></tr>
  77 <tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr>
  78 <tr><td><a href="#anders-aa">-anders-aa</a></td><td>Andersen's Interprocedural Alias Analysis</td></tr>
  79 <tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (default AA impl)</td></tr>
  80 <tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr>
  81 <tr><td><a href="#basicvn">-basicvn</a></td><td>Basic Value Numbering (default GVN impl)</td></tr>
  82 <tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr>
  83 <tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
  84 <tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
  85 <tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
  86 <tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
  87 <tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
  88 <tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
  89 <tr><td><a href="#dot-cfg-only">-dot-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr>
  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="#load-vn">-load-vn</a></td><td>Load Value Numbering</td></tr>
  94 <tr><td><a href="#loops">-loops</a></td><td>Natural Loop Construction</td></tr>
  95 <tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
  96 <tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
  97 <tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
  98 <tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr>
  99 <tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
 100 <tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
 101 <tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr>
 102 <tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr>
 103 <tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr>
 104 <tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
 105 <tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr>
 106 <tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr>
 107 <tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr>
 108 <tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr>
 109 <tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr>
 110 <tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
 111
 112
 113 <tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr>
 114 <tr><th>Option</th><th>Name</th></tr>
 115 <tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr>
 116 <tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr>
 117 <tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr>
 118 <tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr>
 119 <tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Prepare a function for code generation </td></tr>
 120 <tr><td><a href="#condprop">-condprop</a></td><td>Conditional Propagation</td></tr>
 121 <tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr>
 122 <tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr>
 123 <tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr>
 124 <tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr>
 125 <tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr>
 126 <tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr>
 127 <tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr>
 128 <tr><td><a href="#gcse">-gcse</a></td><td>Global Common Subexpression Elimination</td></tr>
 129 <tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr>
 130 <tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr>
 131 <tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr>
 132 <tr><td><a href="#gvnpre">-gvnpre</a></td><td>Global Value Numbering/Partial Redundancy Elimination</td></tr>
 133 <tr><td><a href="#indmemrem">-indmemrem</a></td><td>Indirect Malloc and Free Removal</td></tr>
 134 <tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr>
 135 <tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr>
 136 <tr><td><a href="#insert-block-profiling">-insert-block-profiling</a></td><td>Insert instrumentation for block profiling</td></tr>
 137 <tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr>
 138 <tr><td><a href="#insert-function-profiling">-insert-function-profiling</a></td><td>Insert instrumentation for function profiling</td></tr>
 139 <tr><td><a href="#insert-null-profiling-rs">-insert-null-profiling-rs</a></td><td>Measure profiling framework overhead</td></tr>
 140 <tr><td><a href="#insert-rs-profiling-framework">-insert-rs-profiling-framework</a></td><td>Insert random sampling instrumentation framework</td></tr>
 141 <tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr>
 142 <tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr>
 143 <tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr>
 144 <tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr>
 145 <tr><td><a href="#jump-threading">-jump-threading</a></td><td>Thread control through conditional blocks </td></tr>
 146 <tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr>
 147 <tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr>
 148 <tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Dead Loop Deletion Pass </td></tr>
 149 <tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr>
 150 <tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr>
 151 <tr><td><a href="#loop-index-split">-loop-index-split</a></td><td>Index Split Loops</td></tr>
 152 <tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr>
 153 <tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr>
 154 <tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr>
 155 <tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr>
 156 <tr><td><a href="#loopsimplify">-loopsimplify</a></td><td>Canonicalize natural loops</td></tr>
 157 <tr><td><a href="#lowerallocs">-lowerallocs</a></td><td>Lower allocations from instructions to calls</td></tr>
 158 <tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr>
 159 <tr><td><a href="#lowersetjmp">-lowersetjmp</a></td><td>Lower Set Jump</td></tr>
 160 <tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr>
 161 <tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr>
 162 <tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>Optimize use of memcpy and friends</td></tr>
 163 <tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr>
 164 <tr><td><a href="#predsimplify">-predsimplify</a></td><td>Predicate Simplifier</td></tr>
 165 <tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr>
 166 <tr><td><a href="#raiseallocs">-raiseallocs</a></td><td>Raise allocations from calls to instructions</td></tr>
 167 <tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr>
 168 <tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr>
 169 <tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates</td></tr>
 170 <tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr>
 171 <tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr>
 172 <tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr>
 173 <tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr>
 174 <tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Remove unused function declarations</td></tr>
 175 <tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments</td></tr>
 176 <tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr>
 177 <tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr>
 178
 179
 180 <tr><th colspan="2"><b>UTILITY PASSES</b></th></tr>
 181 <tr><th>Option</th><th>Name</th></tr>
 182 <tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr>
 183 <tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr>
 184 <tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr>
 185 <tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr>
 186 <tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr>
 187 <tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr>
 188 </table>
 189 </div>
 190
 191 <!-- ======================================================================= -->
 192 <div class="doc_section"> <a name="example">Analysis Passes</a></div>
 193 <div class="doc_text">
 194   <p>This section describes the LLVM Analysis Passes.</p>
 195 </div>
 196
 197 <!-------------------------------------------------------------------------- -->
 198 <div class="doc_subsection">
 199   <a name="aa-eval">Exhaustive Alias Analysis Precision Evaluator</a>
 200 </div>
 201 <div class="doc_text">
 202   <p>This is a simple N^2 alias analysis accuracy evaluator.
 203   Basically, for each function in the program, it simply queries to see how the
 204   alias analysis implementation answers alias queries between each pair of
 205   pointers in the function.</p>
 206
 207   <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco
 208   Spadini, and Wojciech Stryjewski.</p>
 209 </div>
 210
 211 <!-------------------------------------------------------------------------- -->
 212 <div class="doc_subsection">
 213   <a name="anders-aa">Andersen's Interprocedural Alias Analysis</a>
 214 </div>
 215 <div class="doc_text">
 216   <p>
 217   This is an implementation of Andersen's interprocedural alias
 218   analysis
 219   </p>
 220
 221   <p>
 222   In pointer analysis terms, this is a subset-based, flow-insensitive,
 223   field-sensitive, and context-insensitive algorithm pointer algorithm.
 224   </p>
 225
 226   <p>
 227   This algorithm is implemented as three stages:
 228   </p>
 229
 230   <ol>
 231     <li>Object identification.</li>
 232     <li>Inclusion constraint identification.</li>
 233     <li>Offline constraint graph optimization.</li>
 234     <li>Inclusion constraint solving.</li>
 235   </ol>
 236
 237   <p>
 238   The object identification stage identifies all of the memory objects in the
 239   program, which includes globals, heap allocated objects, and stack allocated
 240   objects.
 241   </p>
 242
 243   <p>
 244   The inclusion constraint identification stage finds all inclusion constraints
 245   in the program by scanning the program, looking for pointer assignments and
 246   other statements that effect the points-to graph.  For a statement like
 247   <code><var>A</var> = <var>B</var></code>, this statement is processed to
 248   indicate that <var>A</var> can point to anything that <var>B</var> can point
 249   to.  Constraints can handle copies, loads, and stores, and address taking.
 250   </p>
 251
 252   <p>
 253   The offline constraint graph optimization portion includes offline variable
 254   substitution algorithms intended to computer pointer and location
 255   equivalences.  Pointer equivalences are those pointers that will have the
 256   same points-to sets, and location equivalences are those variables that
 257   always appear together in points-to sets.
 258   </p>
 259
 260   <p>
 261   The inclusion constraint solving phase iteratively propagates the inclusion
 262   constraints until a fixed point is reached.  This is an O(<var>n</var>³)
 263   algorithm.
 264   </p>
 265
 266   <p>
 267   Function constraints are handled as if they were structs with <var>X</var>
 268   fields. Thus, an access to argument <var>X</var> of function <var>Y</var> is
 269   an access to node index <code>getNode(<var>Y</var>) + <var>X</var></code>.
 270   This representation allows handling of indirect calls without any issues.  To
 271   wit, an indirect call <code><var>Y</var>(<var>a</var>,<var>b</var>)</code> is
 272   equivalent to <code>*(<var>Y</var> + 1) = <var>a</var>, *(<var>Y</var> + 2) =
 273   <var>b</var></code>. The return node for a function <var>F</var> is always
 274   located at <code>getNode(<var>F</var>) + CallReturnPos</code>. The arguments
 275   start at <code>getNode(<var>F</var>) + CallArgPos</code>.
 276   </p>
 277 </div>
 278
 279 <!-------------------------------------------------------------------------- -->
 280 <div class="doc_subsection">
 281   <a name="basicaa">Basic Alias Analysis (default AA impl)</a>
 282 </div>
 283 <div class="doc_text">
 284   <p>
 285   This is the default implementation of the Alias Analysis interface
 286   that simply implements a few identities (two different globals cannot alias,
 287   etc), but otherwise does no analysis.
 288   </p>
 289 </div>
 290
 291 <!-------------------------------------------------------------------------- -->
 292 <div class="doc_subsection">
 293   <a name="basiccg">Basic CallGraph Construction</a>
 294 </div>
 295 <div class="doc_text">
 296   <p>Yet to be written.</p>
 297 </div>
 298
 299 <!-------------------------------------------------------------------------- -->
 300 <div class="doc_subsection">
 301   <a name="basicvn">Basic Value Numbering (default Value Numbering impl)</a>
 302 </div>
 303 <div class="doc_text">
 304   <p>
 305   This is the default implementation of the <code>ValueNumbering</code>
 306   interface.  It walks the SSA def-use chains to trivially identify
 307   lexically identical expressions.  This does not require any ahead of time
 308   analysis, so it is a very fast default implementation.
 309   </p>
 310   <p>
 311   The ValueNumbering analysis passes are mostly deprecated. They are only used
 312   by the <a href="#gcse">Global Common Subexpression Elimination pass</a>, which
 313   is deprecated by the <a href="#gvn">Global Value Numbering pass</a> (which
 314   does its value numbering on its own).
 315   </p>
 316 </div>
 317
 318 <!-------------------------------------------------------------------------- -->
 319 <div class="doc_subsection">
 320   <a name="codegenprepare">Optimize for code generation</a>
 321 </div>
 322 <div class="doc_text">
 323   <p>
 324   This pass munges the code in the input function to better prepare it for
 325   SelectionDAG-based code generation.  This works around limitations in it's
 326   basic-block-at-a-time approach.  It should eventually be removed.
 327   </p>
 328 </div>
 329
 330 <!-------------------------------------------------------------------------- -->
 331 <div class="doc_subsection">
 332   <a name="count-aa">Count Alias Analysis Query Responses</a>
 333 </div>
 334 <div class="doc_text">
 335   <p>
 336   A pass which can be used to count how many alias queries
 337   are being made and how the alias analysis implementation being used responds.
 338   </p>
 339 </div>
 340
 341 <!-------------------------------------------------------------------------- -->
 342 <div class="doc_subsection">
 343   <a name="debug-aa">AA use debugger</a>
 344 </div>
 345 <div class="doc_text">
 346   <p>
 347   This simple pass checks alias analysis users to ensure that if they
 348   create a new value, they do not query AA without informing it of the value.
 349   It acts as a shim over any other AA pass you want.
 350   </p>
 351
 352   <p>
 353   Yes keeping track of every value in the program is expensive, but this is
 354   a debugging pass.
 355   </p>
 356 </div>
 357
 358 <!-------------------------------------------------------------------------- -->
 359 <div class="doc_subsection">
 360   <a name="domfrontier">Dominance Frontier Construction</a>
 361 </div>
 362 <div class="doc_text">
 363   <p>
 364   This pass is a simple dominator construction algorithm for finding forward
 365   dominator frontiers.
 366   </p>
 367 </div>
 368
 369 <!-------------------------------------------------------------------------- -->
 370 <div class="doc_subsection">
 371   <a name="domtree">Dominator Tree Construction</a>
 372 </div>
 373 <div class="doc_text">
 374   <p>
 375   This pass is a simple dominator construction algorithm for finding forward
 376   dominators.
 377   </p>
 378 </div>
 379
 380 <!-------------------------------------------------------------------------- -->
 381 <div class="doc_subsection">
 382   <a name="dot-callgraph">Print Call Graph to 'dot' file</a>
 383 </div>
 384 <div class="doc_text">
 385   <p>
 386   This pass, only available in <code>opt</code>, prints the call graph into a
 387   <code>.dot</code> graph.  This graph can then be processed with the "dot" tool
 388   to convert it to postscript or some other suitable format.
 389   </p>
 390 </div>
 391
 392 <!-------------------------------------------------------------------------- -->
 393 <div class="doc_subsection">
 394   <a name="dot-cfg">Print CFG of function to 'dot' file</a>
 395 </div>
 396 <div class="doc_text">
 397   <p>
 398   This pass, only available in <code>opt</code>, prints the control flow graph
 399   into a <code>.dot</code> graph.  This graph can then be processed with the
 400   "dot" tool to convert it to postscript or some other suitable format.
 401   </p>
 402 </div>
 403
 404 <!-------------------------------------------------------------------------- -->
 405 <div class="doc_subsection">
 406   <a name="dot-cfg-only">Print CFG of function to 'dot' file (with no function bodies)</a>
 407 </div>
 408 <div class="doc_text">
 409   <p>
 410   This pass, only available in <code>opt</code>, prints the control flow graph
 411   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 412   then be processed with the "dot" tool to convert it to postscript or some
 413   other suitable format.
 414   </p>
 415 </div>
 416
 417 <!-------------------------------------------------------------------------- -->
 418 <div class="doc_subsection">
 419   <a name="globalsmodref-aa">Simple mod/ref analysis for globals</a>
 420 </div>
 421 <div class="doc_text">
 422   <p>
 423   This simple pass provides alias and mod/ref information for global values
 424   that do not have their address taken, and keeps track of whether functions
 425   read or write memory (are "pure").  For this simple (but very common) case,
 426   we can provide pretty accurate and useful information.
 427   </p>
 428 </div>
 429
 430 <!-------------------------------------------------------------------------- -->
 431 <div class="doc_subsection">
 432   <a name="instcount">Counts the various types of Instructions</a>
 433 </div>
 434 <div class="doc_text">
 435   <p>
 436   This pass collects the count of all instructions and reports them
 437   </p>
 438 </div>
 439
 440 <!-------------------------------------------------------------------------- -->
 441 <div class="doc_subsection">
 442   <a name="intervals">Interval Partition Construction</a>
 443 </div>
 444 <div class="doc_text">
 445   <p>
 446   This analysis calculates and represents the interval partition of a function,
 447   or a preexisting interval partition.
 448   </p>
 449
 450   <p>
 451   In this way, the interval partition may be used to reduce a flow graph down
 452   to its degenerate single node interval partition (unless it is irreducible).
 453   </p>
 454 </div>
 455
 456 <!-------------------------------------------------------------------------- -->
 457 <div class="doc_subsection">
 458   <a name="load-vn">Load Value Numbering</a>
 459 </div>
 460 <div class="doc_text">
 461   <p>
 462   This pass value numbers load and call instructions.  To do this, it finds
 463   lexically identical load instructions, and uses alias analysis to determine
 464   which loads are guaranteed to produce the same value.  To value number call
 465   instructions, it looks for calls to functions that do not write to memory
 466   which do not have intervening instructions that clobber the memory that is
 467   read from.
 468   </p>
 469
 470   <p>
 471   This pass builds off of another value numbering pass to implement value
 472   numbering for non-load and non-call instructions.  It uses Alias Analysis so
 473   that it can disambiguate the load instructions.  The more powerful these base
 474   analyses are, the more powerful the resultant value numbering will be.
 475   </p>
 476 </div>
 477
 478 <!-------------------------------------------------------------------------- -->
 479 <div class="doc_subsection">
 480   <a name="loops">Natural Loop Construction</a>
 481 </div>
 482 <div class="doc_text">
 483   <p>
 484   This analysis is used to identify natural loops and determine the loop depth
 485   of various nodes of the CFG.  Note that the loops identified may actually be
 486   several natural loops that share the same header node... not just a single
 487   natural loop.
 488   </p>
 489 </div>
 490
 491 <!-------------------------------------------------------------------------- -->
 492 <div class="doc_subsection">
 493   <a name="memdep">Memory Dependence Analysis</a>
 494 </div>
 495 <div class="doc_text">
 496   <p>
 497   An analysis that determines, for a given memory operation, what preceding
 498   memory operations it depends on.  It builds on alias analysis information, and
 499   tries to provide a lazy, caching interface to a common kind of alias
 500   information query.
 501   </p>
 502 </div>
 503
 504 <!-------------------------------------------------------------------------- -->
 505 <div class="doc_subsection">
 506   <a name="no-aa">No Alias Analysis (always returns 'may' alias)</a>
 507 </div>
 508 <div class="doc_text">
 509   <p>
 510   Always returns "I don't know" for alias queries.  NoAA is unlike other alias
 511   analysis implementations, in that it does not chain to a previous analysis. As
 512   such it doesn't follow many of the rules that other alias analyses must.
 513   </p>
 514 </div>
 515
 516 <!-------------------------------------------------------------------------- -->
 517 <div class="doc_subsection">
 518   <a name="no-profile">No Profile Information</a>
 519 </div>
 520 <div class="doc_text">
 521   <p>
 522   The default "no profile" implementation of the abstract
 523   <code>ProfileInfo</code> interface.
 524   </p>
 525 </div>
 526
 527 <!-------------------------------------------------------------------------- -->
 528 <div class="doc_subsection">
 529   <a name="postdomfrontier">Post-Dominance Frontier Construction</a>
 530 </div>
 531 <div class="doc_text">
 532   <p>
 533   This pass is a simple post-dominator construction algorithm for finding
 534   post-dominator frontiers.
 535   </p>
 536 </div>
 537
 538 <!-------------------------------------------------------------------------- -->
 539 <div class="doc_subsection">
 540   <a name="postdomtree">Post-Dominator Tree Construction</a>
 541 </div>
 542 <div class="doc_text">
 543   <p>
 544   This pass is a simple post-dominator construction algorithm for finding
 545   post-dominators.
 546   </p>
 547 </div>
 548
 549 <!-------------------------------------------------------------------------- -->
 550 <div class="doc_subsection">
 551   <a name="print-alias-sets">Alias Set Printer</a>
 552 </div>
 553 <div class="doc_text">
 554   <p>Yet to be written.</p>
 555 </div>
 556
 557 <!-------------------------------------------------------------------------- -->
 558 <div class="doc_subsection">
 559   <a name="print-callgraph">Print a call graph</a>
 560 </div>
 561 <div class="doc_text">
 562   <p>
 563   This pass, only available in <code>opt</code>, prints the call graph to
 564   standard output in a human-readable form.
 565   </p>
 566 </div>
 567
 568 <!-------------------------------------------------------------------------- -->
 569 <div class="doc_subsection">
 570   <a name="print-callgraph-sccs">Print SCCs of the Call Graph</a>
 571 </div>
 572 <div class="doc_text">
 573   <p>
 574   This pass, only available in <code>opt</code>, prints the SCCs of the call
 575   graph to standard output in a human-readable form.
 576   </p>
 577 </div>
 578
 579 <!-------------------------------------------------------------------------- -->
 580 <div class="doc_subsection">
 581   <a name="print-cfg-sccs">Print SCCs of each function CFG</a>
 582 </div>
 583 <div class="doc_text">
 584   <p>
 585   This pass, only available in <code>opt</code>, prints the SCCs of each
 586   function CFG to standard output in a human-readable form.
 587   </p>
 588 </div>
 589
 590 <!-------------------------------------------------------------------------- -->
 591 <div class="doc_subsection">
 592   <a name="print-externalfnconstants">Print external fn callsites passed constants</a>
 593 </div>
 594 <div class="doc_text">
 595   <p>
 596   This pass, only available in <code>opt</code>, prints out call sites to
 597   external functions that are called with constant arguments.  This can be
 598   useful when looking for standard library functions we should constant fold
 599   or handle in alias analyses.
 600   </p>
 601 </div>
 602
 603 <!-------------------------------------------------------------------------- -->
 604 <div class="doc_subsection">
 605   <a name="print-function">Print function to stderr</a>
 606 </div>
 607 <div class="doc_text">
 608   <p>
 609   The <code>PrintFunctionPass</code> class is designed to be pipelined with
 610   other <code>FunctionPass</code>es, and prints out the functions of the module
 611   as they are processed.
 612   </p>
 613 </div>
 614
 615 <!-------------------------------------------------------------------------- -->
 616 <div class="doc_subsection">
 617   <a name="print-module">Print module to stderr</a>
 618 </div>
 619 <div class="doc_text">
 620   <p>
 621   This pass simply prints out the entire module when it is executed.
 622   </p>
 623 </div>
 624
 625 <!-------------------------------------------------------------------------- -->
 626 <div class="doc_subsection">
 627   <a name="print-used-types">Find Used Types</a>
 628 </div>
 629 <div class="doc_text">
 630   <p>
 631   This pass is used to seek out all of the types in use by the program.  Note
 632   that this analysis explicitly does not include types only used by the symbol
 633   table.
 634 </div>
 635
 636 <!-------------------------------------------------------------------------- -->
 637 <div class="doc_subsection">
 638   <a name="profile-loader">Load profile information from llvmprof.out</a>
 639 </div>
 640 <div class="doc_text">
 641   <p>
 642   A concrete implementation of profiling information that loads the information
 643   from a profile dump file.
 644   </p>
 645 </div>
 646
 647 <!-------------------------------------------------------------------------- -->
 648 <div class="doc_subsection">
 649   <a name="scalar-evolution">Scalar Evolution Analysis</a>
 650 </div>
 651 <div class="doc_text">
 652   <p>
 653   The <code>ScalarEvolution</code> analysis can be used to analyze and
 654   catagorize scalar expressions in loops.  It specializes in recognizing general
 655   induction variables, representing them with the abstract and opaque
 656   <code>SCEV</code> class.  Given this analysis, trip counts of loops and other
 657   important properties can be obtained.
 658   </p>
 659
 660   <p>
 661   This analysis is primarily useful for induction variable substitution and
 662   strength reduction.
 663   </p>
 664 </div>
 665
 666 <!-------------------------------------------------------------------------- -->
 667 <div class="doc_subsection">
 668   <a name="targetdata">Target Data Layout</a>
 669 </div>
 670 <div class="doc_text">
 671   <p>Provides other passes access to information on how the size and alignment
 672   required by the the target ABI for various data types.</p>
 673 </div>
 674
 675 <!-- ======================================================================= -->
 676 <div class="doc_section"> <a name="transform">Transform Passes</a></div>
 677 <div class="doc_text">
 678   <p>This section describes the LLVM Transform Passes.</p>
 679 </div>
 680
 681 <!-------------------------------------------------------------------------- -->
 682 <div class="doc_subsection">
 683   <a name="adce">Aggressive Dead Code Elimination</a>
 684 </div>
 685 <div class="doc_text">
 686   <p>ADCE aggressively tries to eliminate code. This pass is similar to
 687   <a href="#dce">DCE</a> but it assumes that values are dead until proven
 688   otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to
 689   the liveness of values.</p>
 690 </div>
 691
 692 <!-------------------------------------------------------------------------- -->
 693 <div class="doc_subsection">
 694   <a name="argpromotion">Promote 'by reference' arguments to scalars</a>
 695 </div>
 696 <div class="doc_text">
 697   <p>
 698   This pass promotes "by reference" arguments to be "by value" arguments.  In
 699   practice, this means looking for internal functions that have pointer
 700   arguments.  If it can prove, through the use of alias analysis, that an
 701   argument is *only* loaded, then it can pass the value into the function
 702   instead of the address of the value.  This can cause recursive simplification
 703   of code and lead to the elimination of allocas (especially in C++ template
 704   code like the STL).
 705   </p>
 706
 707   <p>
 708   This pass also handles aggregate arguments that are passed into a function,
 709   scalarizing them if the elements of the aggregate are only loaded.  Note that
 710   it refuses to scalarize aggregates which would require passing in more than
 711   three operands to the function, because passing thousands of operands for a
 712   large array or structure is unprofitable!
 713   </p>
 714
 715   <p>
 716   Note that this transformation could also be done for arguments that are only
 717   stored to (returning the value instead), but does not currently.  This case
 718   would be best handled when and if LLVM starts supporting multiple return
 719   values from functions.
 720   </p>
 721 </div>
 722
 723 <!-------------------------------------------------------------------------- -->
 724 <div class="doc_subsection">
 725   <a name="block-placement">Profile Guided Basic Block Placement</a>
 726 </div>
 727 <div class="doc_text">
 728   <p>This pass is a very simple profile guided basic block placement algorithm.
 729   The idea is to put frequently executed blocks together at the start of the
 730   function and hopefully increase the number of fall-through conditional
 731   branches.  If there is no profile information for a particular function, this
 732   pass basically orders blocks in depth-first order.</p>
 733 </div>
 734
 735 <!-------------------------------------------------------------------------- -->
 736 <div class="doc_subsection">
 737   <a name="break-crit-edges">Break critical edges in CFG</a>
 738 </div>
 739 <div class="doc_text">
 740   <p>
 741   Break all of the critical edges in the CFG by inserting a dummy basic block.
 742   It may be "required" by passes that cannot deal with critical edges. This
 743   transformation obviously invalidates the CFG, but can update forward dominator
 744   (set, immediate dominators, tree, and frontier) information.
 745   </p>
 746 </div>
 747
 748 <!-------------------------------------------------------------------------- -->
 749 <div class="doc_subsection">
 750   <a name="codegenprepare">Prepare a function for code generation</a>
 751 </div>
 752 <div class="doc_text">
 753   This pass munges the code in the input function to better prepare it for
 754   SelectionDAG-based code generation. This works around limitations in it's
 755   basic-block-at-a-time approach. It should eventually be removed.
 756 </div>
 757
 758 <!-------------------------------------------------------------------------- -->
 759 <div class="doc_subsection">
 760   <a name="condprop">Conditional Propagation</a>
 761 </div>
 762 <div class="doc_text">
 763   <p>This pass propagates information about conditional expressions through the
 764   program, allowing it to eliminate conditional branches in some cases.</p>
 765 </div>
 766
 767 <!-------------------------------------------------------------------------- -->
 768 <div class="doc_subsection">
 769   <a name="constmerge">Merge Duplicate Global Constants</a>
 770 </div>
 771 <div class="doc_text">
 772   <p>
 773   Merges duplicate global constants together into a single constant that is
 774   shared.  This is useful because some passes (ie TraceValues) insert a lot of
 775   string constants into the program, regardless of whether or not an existing
 776   string is available.
 777   </p>
 778 </div>
 779
 780 <!-------------------------------------------------------------------------- -->
 781 <div class="doc_subsection">
 782   <a name="constprop">Simple constant propagation</a>
 783 </div>
 784 <div class="doc_text">
 785   <p>This file implements constant propagation and merging. It looks for
 786   instructions involving only constant operands and replaces them with a
 787   constant value instead of an instruction. For example:</p>
 788   <blockquote><pre>add i32 1, 2</pre></blockquote>
 789   <p>becomes</p>
 790   <blockquote><pre>i32 3</pre></blockquote>
 791   <p>NOTE: this pass has a habit of making definitions be dead.  It is a good
 792   idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass
 793   sometime after running this pass.</p>
 794 </div>
 795
 796 <!-------------------------------------------------------------------------- -->
 797 <div class="doc_subsection">
 798   <a name="dce">Dead Code Elimination</a>
 799 </div>
 800 <div class="doc_text">
 801   <p>
 802   Dead code elimination is similar to <a href="#die">dead instruction
 803   elimination</a>, but it rechecks instructions that were used by removed
 804   instructions to see if they are newly dead.
 805   </p>
 806 </div>
 807
 808 <!-------------------------------------------------------------------------- -->
 809 <div class="doc_subsection">
 810   <a name="deadargelim">Dead Argument Elimination</a>
 811 </div>
 812 <div class="doc_text">
 813   <p>
 814   This pass deletes dead arguments from internal functions.  Dead argument
 815   elimination removes arguments which are directly dead, as well as arguments
 816   only passed into function calls as dead arguments of other functions.  This
 817   pass also deletes dead arguments in a similar way.
 818   </p>
 819
 820   <p>
 821   This pass is often useful as a cleanup pass to run after aggressive
 822   interprocedural passes, which add possibly-dead arguments.
 823   </p>
 824 </div>
 825
 826 <!-------------------------------------------------------------------------- -->
 827 <div class="doc_subsection">
 828   <a name="deadtypeelim">Dead Type Elimination</a>
 829 </div>
 830 <div class="doc_text">
 831   <p>
 832   This pass is used to cleanup the output of GCC.  It eliminate names for types
 833   that are unused in the entire translation unit, using the <a
 834   href="#findusedtypes">find used types</a> pass.
 835   </p>
 836 </div>
 837
 838 <!-------------------------------------------------------------------------- -->
 839 <div class="doc_subsection">
 840   <a name="die">Dead Instruction Elimination</a>
 841 </div>
 842 <div class="doc_text">
 843   <p>
 844   Dead instruction elimination performs a single pass over the function,
 845   removing instructions that are obviously dead.
 846   </p>
 847 </div>
 848
 849 <!-------------------------------------------------------------------------- -->
 850 <div class="doc_subsection">
 851   <a name="dse">Dead Store Elimination</a>
 852 </div>
 853 <div class="doc_text">
 854   <p>
 855   A trivial dead store elimination that only considers basic-block local
 856   redundant stores.
 857   </p>
 858 </div>
 859
 860 <!-------------------------------------------------------------------------- -->
 861 <div class="doc_subsection">
 862   <a name="gcse">Global Common Subexpression Elimination</a>
 863 </div>
 864 <div class="doc_text">
 865   <p>
 866   This pass is designed to be a very quick global transformation that
 867   eliminates global common subexpressions from a function.  It does this by
 868   using an existing value numbering analysis pass to identify the common
 869   subexpressions, eliminating them when possible.
 870   </p>
 871   <p>
 872   This pass is deprecated by the <a href="#gvn">Global Value Numbering pass</a>
 873   (which does a better job with its own value numbering).
 874   </p>
 875 </div>
 876
 877 <!-------------------------------------------------------------------------- -->
 878 <div class="doc_subsection">
 879   <a name="globaldce">Dead Global Elimination</a>
 880 </div>
 881 <div class="doc_text">
 882   <p>
 883   This transform is designed to eliminate unreachable internal globals from the
 884   program.  It uses an aggressive algorithm, searching out globals that are
 885   known to be alive.  After it finds all of the globals which are needed, it
 886   deletes whatever is left over.  This allows it to delete recursive chunks of
 887   the program which are unreachable.
 888   </p>
 889 </div>
 890
 891 <!-------------------------------------------------------------------------- -->
 892 <div class="doc_subsection">
 893   <a name="globalopt">Global Variable Optimizer</a>
 894 </div>
 895 <div class="doc_text">
 896   <p>
 897   This pass transforms simple global variables that never have their address
 898   taken.  If obviously true, it marks read/write globals as constant, deletes
 899   variables only stored to, etc.
 900   </p>
 901 </div>
 902
 903 <!-------------------------------------------------------------------------- -->
 904 <div class="doc_subsection">
 905   <a name="gvn">Global Value Numbering</a>
 906 </div>
 907 <div class="doc_text">
 908   <p>
 909   This pass performs global value numbering to eliminate fully redundant
 910   instructions.  It also performs simple dead load elimination.
 911   </p>
 912   <p>
 913   Note that this pass does the value numbering itself, it does not use the
 914   ValueNumbering analysis passes.
 915   </p>
 916 </div>
 917
 918 <!-------------------------------------------------------------------------- -->
 919 <div class="doc_subsection">
 920   <a name="gvnpre">Global Value Numbering/Partial Redundancy Elimination</a>
 921 </div>
 922 <div class="doc_text">
 923   <p>
 924   This pass performs a hybrid of global value numbering and partial redundancy
 925   elimination, known as GVN-PRE.  It performs partial redundancy elimination on
 926   values, rather than lexical expressions, allowing a more comprehensive view
 927   the optimization.  It replaces redundant values with uses of earlier
 928   occurences of the same value.  While this is beneficial in that it eliminates
 929   unneeded computation, it also increases register pressure by creating large
 930   live ranges, and should be used with caution on platforms that are very
 931   sensitive to register pressure.
 932   </p>
 933   <p>
 934   Note that this pass does the value numbering itself, it does not use the
 935   ValueNumbering analysis passes.
 936   </p>
 937 </div>
 938
 939 <!-------------------------------------------------------------------------- -->
 940 <div class="doc_subsection">
 941   <a name="indmemrem">Indirect Malloc and Free Removal</a>
 942 </div>
 943 <div class="doc_text">
 944   <p>
 945   This pass finds places where memory allocation functions may escape into
 946   indirect land.  Some transforms are much easier (aka possible) only if free
 947   or malloc are not called indirectly.
 948   </p>
 949
 950   <p>
 951   Thus find places where the address of memory functions are taken and construct
 952   bounce functions with direct calls of those functions.
 953   </p>
 954 </div>
 955
 956 <!-------------------------------------------------------------------------- -->
 957 <div class="doc_subsection">
 958   <a name="indvars">Canonicalize Induction Variables</a>
 959 </div>
 960 <div class="doc_text">
 961   <p>
 962   This transformation analyzes and transforms the induction variables (and
 963   computations derived from them) into simpler forms suitable for subsequent
 964   analysis and transformation.
 965   </p>
 966
 967   <p>
 968   This transformation makes the following changes to each loop with an
 969   identifiable induction variable:
 970   </p>
 971
 972   <ol>
 973     <li>All loops are transformed to have a <em>single</em> canonical
 974         induction variable which starts at zero and steps by one.</li>
 975     <li>The canonical induction variable is guaranteed to be the first PHI node
 976         in the loop header block.</li>
 977     <li>Any pointer arithmetic recurrences are raised to use array
 978         subscripts.</li>
 979   </ol>
 980
 981   <p>
 982   If the trip count of a loop is computable, this pass also makes the following
 983   changes:
 984   </p>
 985
 986   <ol>
 987     <li>The exit condition for the loop is canonicalized to compare the
 988         induction value against the exit value.  This turns loops like:
 989         <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote>
 990         into
 991         <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li>
 992     <li>Any use outside of the loop of an expression derived from the indvar
 993         is changed to compute the derived value outside of the loop, eliminating
 994         the dependence on the exit value of the induction variable.  If the only
 995         purpose of the loop is to compute the exit value of some derived
 996         expression, this transformation will make the loop dead.</li>
 997   </ol>
 998
 999   <p>
1000   This transformation should be followed by strength reduction after all of the
1001   desired loop transformations have been performed.  Additionally, on targets
1002   where it is profitable, the loop could be transformed to count down to zero
1003   (the "do loop" optimization).
1004   </p>
1005 </div>
1006
1007 <!-------------------------------------------------------------------------- -->
1008 <div class="doc_subsection">
1009   <a name="inline">Function Integration/Inlining</a>
1010 </div>
1011 <div class="doc_text">
1012   <p>
1013   Bottom-up inlining of functions into callees.
1014   </p>
1015 </div>
1016
1017 <!-------------------------------------------------------------------------- -->
1018 <div class="doc_subsection">
1019   <a name="insert-block-profiling">Insert instrumentation for block profiling</a>
1020 </div>
1021 <div class="doc_text">
1022   <p>
1023   This pass instruments the specified program with counters for basic block
1024   profiling, which counts the number of times each basic block executes.  This
1025   is the most basic form of profiling, which can tell which blocks are hot, but
1026   cannot reliably detect hot paths through the CFG.
1027   </p>
1028
1029   <p>
1030   Note that this implementation is very naïve.  Control equivalent regions of
1031   the CFG should not require duplicate counters, but it does put duplicate
1032   counters in.
1033   </p>
1034 </div>
1035
1036 <!-------------------------------------------------------------------------- -->
1037 <div class="doc_subsection">
1038   <a name="insert-edge-profiling">Insert instrumentation for edge profiling</a>
1039 </div>
1040 <div class="doc_text">
1041   <p>
1042   This pass instruments the specified program with counters for edge profiling.
1043   Edge profiling can give a reasonable approximation of the hot paths through a
1044   program, and is used for a wide variety of program transformations.
1045   </p>
1046
1047   <p>
1048   Note that this implementation is very naïve.  It inserts a counter for
1049   <em>every</em> edge in the program, instead of using control flow information
1050   to prune the number of counters inserted.
1051   </p>
1052 </div>
1053
1054 <!-------------------------------------------------------------------------- -->
1055 <div class="doc_subsection">
1056   <a name="insert-function-profiling">Insert instrumentation for function profiling</a>
1057 </div>
1058 <div class="doc_text">
1059   <p>
1060   This pass instruments the specified program with counters for function
1061   profiling, which counts the number of times each function is called.
1062   </p>
1063 </div>
1064
1065 <!-------------------------------------------------------------------------- -->
1066 <div class="doc_subsection">
1067   <a name="insert-null-profiling-rs">Measure profiling framework overhead</a>
1068 </div>
1069 <div class="doc_text">
1070   <p>
1071   The basic profiler that does nothing.  It is the default profiler and thus
1072   terminates <code>RSProfiler</code> chains.  It is useful for  measuring
1073   framework overhead.
1074   </p>
1075 </div>
1076
1077 <!-------------------------------------------------------------------------- -->
1078 <div class="doc_subsection">
1079   <a name="insert-rs-profiling-framework">Insert random sampling instrumentation framework</a>
1080 </div>
1081 <div class="doc_text">
1082   <p>
1083   The second stage of the random-sampling instrumentation framework, duplicates
1084   all instructions in a function, ignoring the profiling code, then connects the
1085   two versions together at the entry and at backedges.  At each connection point
1086   a choice is made as to whether to jump to the profiled code (take a sample) or
1087   execute the unprofiled code.
1088   </p>
1089
1090   <p>
1091   After this pass, it is highly recommended to run<a href="#mem2reg">mem2reg</a>
1092   and <a href="#adce">adce</a>. <a href="#instcombine">instcombine</a>,
1093   <a href="#load-vn">load-vn</a>, <a href="#gdce">gdce</a>, and
1094   <a href="#dse">dse</a> also are good to run afterwards.
1095   </p>
1096 </div>
1097
1098 <!-------------------------------------------------------------------------- -->
1099 <div class="doc_subsection">
1100   <a name="instcombine">Combine redundant instructions</a>
1101 </div>
1102 <div class="doc_text">
1103   <p>
1104   Combine instructions to form fewer, simple
1105   instructions.  This pass does not modify the CFG This pass is where algebraic
1106   simplification happens.
1107   </p>
1108
1109   <p>
1110   This pass combines things like:
1111   </p>
1112
1113 <blockquote><pre
1114 >%Y = add i32 %X, 1
1115 %Z = add i32 %Y, 1</pre></blockquote>
1116
1117   <p>
1118   into:
1119   </p>
1120
1121 <blockquote><pre
1122 >%Z = add i32 %X, 2</pre></blockquote>
1123
1124   <p>
1125   This is a simple worklist driven algorithm.
1126   </p>
1127
1128   <p>
1129   This pass guarantees that the following canonicalizations are performed on
1130   the program:
1131   </p>
1132
1133   <ul>
1134     <li>If a binary operator has a constant operand, it is moved to the right-
1135         hand side.</li>
1136     <li>Bitwise operators with constant operands are always grouped so that
1137         shifts are performed first, then <code>or</code>s, then
1138         <code>and</code>s, then <code>xor</code>s.</li>
1139     <li>Compare instructions are converted from <code>&lt;</code>,
1140         <code>&gt;</code>, <code>≤</code>, or <code>≥</code> to
1141         <code>=</code> or <code>≠</code> if possible.</li>
1142     <li>All <code>cmp</code> instructions on boolean values are replaced with
1143         logical operations.</li>
1144     <li><code>add <var>X</var>, <var>X</var></code> is represented as
1145         <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li>
1146     <li>Multiplies with a constant power-of-two argument are transformed into
1147         shifts.</li>
1148     <li>… etc.</li>
1149   </ul>
1150 </div>
1151
1152 <!-------------------------------------------------------------------------- -->
1153 <div class="doc_subsection">
1154   <a name="internalize">Internalize Global Symbols</a>
1155 </div>
1156 <div class="doc_text">
1157   <p>
1158   This pass loops over all of the functions in the input module, looking for a
1159   main function.  If a main function is found, all other functions and all
1160   global variables with initializers are marked as internal.
1161   </p>
1162 </div>
1163
1164 <!-------------------------------------------------------------------------- -->
1165 <div class="doc_subsection">
1166   <a name="ipconstprop">Interprocedural constant propagation</a>
1167 </div>
1168 <div class="doc_text">
1169   <p>
1170   This pass implements an <em>extremely</em> simple interprocedural constant
1171   propagation pass.  It could certainly be improved in many different ways,
1172   like using a worklist.  This pass makes arguments dead, but does not remove
1173   them.  The existing dead argument elimination pass should be run after this
1174   to clean up the mess.
1175   </p>
1176 </div>
1177
1178 <!-------------------------------------------------------------------------- -->
1179 <div class="doc_subsection">
1180   <a name="ipsccp">Interprocedural Sparse Conditional Constant Propagation</a>
1181 </div>
1182 <div class="doc_text">
1183   <p>
1184   An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
1185   Propagation</a>.
1186   </p>
1187 </div>
1188
1189 <!-------------------------------------------------------------------------- -->
1190 <div class="doc_subsection">
1191   <a name="jump-threading">Thread control through conditional blocks</a>
1192 </div>
1193 <div class="doc_text">
1194   <p>
1195   Jump threading tries to find distinct threads of control flow running through
1196   a basic block. This pass looks at blocks that have multiple predecessors and
1197   multiple successors.  If one or more of the predecessors of the block can be
1198   proven to always cause a jump to one of the successors, we forward the edge
1199   from the predecessor to the successor by duplicating the contents of this
1200   block.
1201   </p>
1202   <p>
1203   An example of when this can occur is code like this:
1204   </p>
1205
1206   <pre
1207 >if () { ...
1208   X = 4;
1209 }
1210 if (X &lt; 3) {</pre>
1211
1212   <p>
1213   In this case, the unconditional branch at the end of the first if can be
1214   revectored to the false side of the second if.
1215   </p>
1216 </div>
1217
1218 <!-------------------------------------------------------------------------- -->
1219 <div class="doc_subsection">
1220   <a name="lcssa">Loop-Closed SSA Form Pass</a>
1221 </div>
1222 <div class="doc_text">
1223   <p>
1224   This pass transforms loops by placing phi nodes at the end of the loops for
1225   all values that are live across the loop boundary.  For example, it turns
1226   the left into the right code:
1227   </p>
1228
1229   <pre
1230 >for (...)                for (...)
1231   if (c)                   if (c)
1232     X1 = ...                 X1 = ...
1233   else                     else
1234     X2 = ...                 X2 = ...
1235   X3 = phi(X1, X2)         X3 = phi(X1, X2)
1236 ... = X3 + 4              X4 = phi(X3)
1237                           ... = X4 + 4</pre>
1238
1239   <p>
1240   This is still valid LLVM; the extra phi nodes are purely redundant, and will
1241   be trivially eliminated by <code>InstCombine</code>.  The major benefit of
1242   this transformation is that it makes many other loop optimizations, such as
1243   LoopUnswitching, simpler.
1244   </p>
1245 </div>
1246
1247 <!-------------------------------------------------------------------------- -->
1248 <div class="doc_subsection">
1249   <a name="licm">Loop Invariant Code Motion</a>
1250 </div>
1251 <div class="doc_text">
1252   <p>
1253   This pass performs loop invariant code motion, attempting to remove as much
1254   code from the body of a loop as possible.  It does this by either hoisting
1255   code into the preheader block, or by sinking code to the exit blocks if it is
1256   safe.  This pass also promotes must-aliased memory locations in the loop to
1257   live in registers, thus hoisting and sinking "invariant" loads and stores.
1258   </p>
1259
1260   <p>
1261   This pass uses alias analysis for two purposes:
1262   </p>
1263
1264   <ul>
1265     <li>Moving loop invariant loads and calls out of loops.  If we can determine
1266         that a load or call inside of a loop never aliases anything stored to,
1267         we can hoist it or sink it like any other instruction.</li>
1268     <li>Scalar Promotion of Memory - If there is a store instruction inside of
1269         the loop, we try to move the store to happen AFTER the loop instead of
1270         inside of the loop.  This can only happen if a few conditions are true:
1271         <ul>
1272           <li>The pointer stored through is loop invariant.</li>
1273           <li>There are no stores or loads in the loop which <em>may</em> alias
1274               the pointer.  There are no calls in the loop which mod/ref the
1275               pointer.</li>
1276         </ul>
1277         If these conditions are true, we can promote the loads and stores in the
1278         loop of the pointer to use a temporary alloca'd variable.  We then use
1279         the mem2reg functionality to construct the appropriate SSA form for the
1280         variable.</li>
1281   </ul>
1282 </div>
1283 <!-------------------------------------------------------------------------- -->
1284 <div class="doc_subsection">
1285   <a name="loop-deletion">Dead Loop Deletion Pass</a>
1286 </div>
1287 <div class="doc_text">
1288   <p>
1289   This file implements the Dead Loop Deletion Pass.  This pass is responsible
1290   for eliminating loops with non-infinite computable trip counts that have no
1291   side effects or volatile instructions, and do not contribute to the
1292   computation of the function's return value.
1293   </p>
1294 </div>
1295
1296 <!-------------------------------------------------------------------------- -->
1297 <div class="doc_subsection">
1298   <a name="loop-extract">Extract loops into new functions</a>
1299 </div>
1300 <div class="doc_text">
1301   <p>
1302   A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to
1303   extract each top-level loop into its own new function. If the loop is the
1304   <em>only</em> loop in a given function, it is not touched. This is a pass most
1305   useful for debugging via bugpoint.
1306   </p>
1307 </div>
1308
1309 <!-------------------------------------------------------------------------- -->
1310 <div class="doc_subsection">
1311   <a name="loop-extract-single">Extract at most one loop into a new function</a>
1312 </div>
1313 <div class="doc_text">
1314   <p>
1315   Similar to <a href="#loop-extract">Extract loops into new functions</a>,
1316   this pass extracts one natural loop from the program into a function if it
1317   can. This is used by bugpoint.
1318   </p>
1319 </div>
1320
1321 <!-------------------------------------------------------------------------- -->
1322 <div class="doc_subsection">
1323   <a name="loop-index-split">Index Split Loops</a>
1324 </div>
1325 <div class="doc_text">
1326   <p>
1327   This pass divides loop's iteration range by spliting loop such that each
1328   individual loop is executed efficiently.
1329   </p>
1330 </div>
1331
1332 <!-------------------------------------------------------------------------- -->
1333 <div class="doc_subsection">
1334   <a name="loop-reduce">Loop Strength Reduction</a>
1335 </div>
1336 <div class="doc_text">
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 <div class="doc_subsection">
1348   <a name="loop-rotate">Rotate Loops</a>
1349 </div>
1350 <div class="doc_text">
1351   <p>A simple loop rotation transformation.</p>
1352 </div>
1353
1354 <!-------------------------------------------------------------------------- -->
1355 <div class="doc_subsection">
1356   <a name="loop-unroll">Unroll loops</a>
1357 </div>
1358 <div class="doc_text">
1359   <p>
1360   This pass implements a simple loop unroller.  It works best when loops have
1361   been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass,
1362   allowing it to determine the trip counts of loops easily.
1363   </p>
1364 </div>
1365
1366 <!-------------------------------------------------------------------------- -->
1367 <div class="doc_subsection">
1368   <a name="loop-unswitch">Unswitch loops</a>
1369 </div>
1370 <div class="doc_text">
1371   <p>
1372   This pass transforms loops that contain branches on loop-invariant conditions
1373   to have multiple loops.  For example, it turns the left into the right code:
1374   </p>
1375
1376   <pre
1377 >for (...)                  if (lic)
1378   A                          for (...)
1379   if (lic)                     A; B; C
1380     B                      else
1381   C                          for (...)
1382                                A; C</pre>
1383
1384   <p>
1385   This can increase the size of the code exponentially (doubling it every time
1386   a loop is unswitched) so we only unswitch if the resultant code will be
1387   smaller than a threshold.
1388   </p>
1389
1390   <p>
1391   This pass expects LICM to be run before it to hoist invariant conditions out
1392   of the loop, to make the unswitching opportunity obvious.
1393   </p>
1394 </div>
1395
1396 <!-------------------------------------------------------------------------- -->
1397 <div class="doc_subsection">
1398   <a name="loopsimplify">Canonicalize natural loops</a>
1399 </div>
1400 <div class="doc_text">
1401   <p>
1402   This pass performs several transformations to transform natural loops into a
1403   simpler form, which makes subsequent analyses and transformations simpler and
1404   more effective.
1405   </p>
1406
1407   <p>
1408   Loop pre-header insertion guarantees that there is a single, non-critical
1409   entry edge from outside of the loop to the loop header.  This simplifies a
1410   number of analyses and transformations, such as LICM.
1411   </p>
1412
1413   <p>
1414   Loop exit-block insertion guarantees that all exit blocks from the loop
1415   (blocks which are outside of the loop that have predecessors inside of the
1416   loop) only have predecessors from inside of the loop (and are thus dominated
1417   by the loop header).  This simplifies transformations such as store-sinking
1418   that are built into LICM.
1419   </p>
1420
1421   <p>
1422   This pass also guarantees that loops will have exactly one backedge.
1423   </p>
1424
1425   <p>
1426   Note that the simplifycfg pass will clean up blocks which are split out but
1427   end up being unnecessary, so usage of this pass should not pessimize
1428   generated code.
1429   </p>
1430
1431   <p>
1432   This pass obviously modifies the CFG, but updates loop information and
1433   dominator information.
1434   </p>
1435 </div>
1436
1437 <!-------------------------------------------------------------------------- -->
1438 <div class="doc_subsection">
1439   <a name="lowerallocs">Lower allocations from instructions to calls</a>
1440 </div>
1441 <div class="doc_text">
1442   <p>
1443   Turn <tt>malloc</tt> and <tt>free</tt> instructions into <tt>@malloc</tt> and
1444   <tt>@free</tt> calls.
1445   </p>
1446
1447   <p>
1448   This is a target-dependent tranformation because it depends on the size of
1449   data types and alignment constraints.
1450   </p>
1451 </div>
1452
1453 <!-------------------------------------------------------------------------- -->
1454 <div class="doc_subsection">
1455   <a name="lowerinvoke">Lower invoke and unwind, for unwindless code generators</a>
1456 </div>
1457 <div class="doc_text">
1458   <p>
1459   This transformation is designed for use by code generators which do not yet
1460   support stack unwinding.  This pass supports two models of exception handling
1461   lowering, the 'cheap' support and the 'expensive' support.
1462   </p>
1463
1464   <p>
1465   'Cheap' exception handling support gives the program the ability to execute
1466   any program which does not "throw an exception", by turning 'invoke'
1467   instructions into calls and by turning 'unwind' instructions into calls to
1468   abort().  If the program does dynamically use the unwind instruction, the
1469   program will print a message then abort.
1470   </p>
1471
1472   <p>
1473   'Expensive' exception handling support gives the full exception handling
1474   support to the program at the cost of making the 'invoke' instruction
1475   really expensive.  It basically inserts setjmp/longjmp calls to emulate the
1476   exception handling as necessary.
1477   </p>
1478
1479   <p>
1480   Because the 'expensive' support slows down programs a lot, and EH is only
1481   used for a subset of the programs, it must be specifically enabled by the
1482   <tt>-enable-correct-eh-support</tt> option.
1483   </p>
1484
1485   <p>
1486   Note that after this pass runs the CFG is not entirely accurate (exceptional
1487   control flow edges are not correct anymore) so only very simple things should
1488   be done after the lowerinvoke pass has run (like generation of native code).
1489   This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't
1490   support the invoke instruction yet" lowering pass.
1491   </p>
1492 </div>
1493
1494 <!-------------------------------------------------------------------------- -->
1495 <div class="doc_subsection">
1496   <a name="lowersetjmp">Lower Set Jump</a>
1497 </div>
1498 <div class="doc_text">
1499   <p>
1500    Lowers <tt>setjmp</tt> and <tt>longjmp</tt> to use the LLVM invoke and unwind
1501    instructions as necessary.
1502   </p>
1503
1504   <p>
1505    Lowering of <tt>longjmp</tt> is fairly trivial. We replace the call with a
1506    call to the LLVM library function <tt>__llvm_sjljeh_throw_longjmp()</tt>.
1507    This unwinds the stack for us calling all of the destructors for
1508    objects allocated on the stack.
1509   </p>
1510
1511   <p>
1512    At a <tt>setjmp</tt> call, the basic block is split and the <tt>setjmp</tt>
1513    removed. The calls in a function that have a <tt>setjmp</tt> are converted to
1514    invoke where the except part checks to see if it's a <tt>longjmp</tt>
1515    exception and, if so, if it's handled in the function. If it is, then it gets
1516    the value returned by the <tt>longjmp</tt> and goes to where the basic block
1517    was split. <tt>invoke</tt> instructions are handled in a similar fashion with
1518    the original except block being executed if it isn't a <tt>longjmp</tt>
1519    except that is handled by that function.
1520   </p>
1521 </div>
1522
1523 <!-------------------------------------------------------------------------- -->
1524 <div class="doc_subsection">
1525   <a name="lowerswitch">Lower SwitchInst's to branches</a>
1526 </div>
1527 <div class="doc_text">
1528   <p>
1529   Rewrites <tt>switch</tt> instructions with a sequence of branches, which
1530   allows targets to get away with not implementing the switch instruction until
1531   it is convenient.
1532   </p>
1533 </div>
1534
1535 <!-------------------------------------------------------------------------- -->
1536 <div class="doc_subsection">
1537   <a name="mem2reg">Promote Memory to Register</a>
1538 </div>
1539 <div class="doc_text">
1540   <p>
1541   This file promotes memory references to be register references.  It promotes
1542   <tt>alloca</tt> instructions which only have <tt>load</tt>s and
1543   <tt>store</tt>s as uses.  An <tt>alloca</tt> is transformed by using dominator
1544   frontiers to place <tt>phi</tt> nodes, then traversing the function in
1545   depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as
1546   appropriate. This is just the standard SSA construction algorithm to construct
1547   "pruned" SSA form.
1548   </p>
1549 </div>
1550
1551 <!-------------------------------------------------------------------------- -->
1552 <div class="doc_subsection">
1553   <a name="memcpyopt">Optimize use of memcpy and friend</a>
1554 </div>
1555 <div class="doc_text">
1556   <p>
1557   This pass performs various transformations related to eliminating memcpy
1558   calls, or transforming sets of stores into memset's.
1559   </p>
1560 </div>
1561
1562 <!-------------------------------------------------------------------------- -->
1563 <div class="doc_subsection">
1564   <a name="mergereturn">Unify function exit nodes</a>
1565 </div>
1566 <div class="doc_text">
1567   <p>
1568   Ensure that functions have at most one <tt>ret</tt> instruction in them.
1569   Additionally, it keeps track of which node is the new exit node of the CFG.
1570   </p>
1571 </div>
1572
1573 <!-------------------------------------------------------------------------- -->
1574 <div class="doc_subsection">
1575   <a name="predsimplify">Predicate Simplifier</a>
1576 </div>
1577 <div class="doc_text">
1578   <p>
1579   Path-sensitive optimizer. In a branch where <tt>x == y</tt>, replace uses of
1580   <tt>x</tt> with <tt>y</tt>. Permits further optimization, such as the
1581   elimination of the unreachable call:
1582   </p>
1583
1584 <blockquote><pre
1585 >void test(int *p, int *q)
1586 {
1587   if (p != q)
1588     return;
1589
1590   if (*p != *q)
1591     foo(); // unreachable
1592 }</pre></blockquote>
1593 </div>
1594
1595 <!-------------------------------------------------------------------------- -->
1596 <div class="doc_subsection">
1597   <a name="prune-eh">Remove unused exception handling info</a>
1598 </div>
1599 <div class="doc_text">
1600   <p>
1601   This file implements a simple interprocedural pass which walks the call-graph,
1602   turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and
1603   only if the callee cannot throw an exception. It implements this as a
1604   bottom-up traversal of the call-graph.
1605   </p>
1606 </div>
1607
1608 <!-------------------------------------------------------------------------- -->
1609 <div class="doc_subsection">
1610   <a name="raiseallocs">Raise allocations from calls to instructions</a>
1611 </div>
1612 <div class="doc_text">
1613   <p>
1614   Converts <tt>@malloc</tt> and <tt>@free</tt> calls to <tt>malloc</tt> and
1615   <tt>free</tt> instructions.
1616   </p>
1617 </div>
1618
1619 <!-------------------------------------------------------------------------- -->
1620 <div class="doc_subsection">
1621   <a name="reassociate">Reassociate expressions</a>
1622 </div>
1623 <div class="doc_text">
1624   <p>
1625   This pass reassociates commutative expressions in an order that is designed
1626   to promote better constant propagation, GCSE, LICM, PRE, etc.
1627   </p>
1628
1629   <p>
1630   For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5)
1631   </p>
1632
1633   <p>
1634   In the implementation of this algorithm, constants are assigned rank = 0,
1635   function arguments are rank = 1, and other values are assigned ranks
1636   corresponding to the reverse post order traversal of current function
1637   (starting at 2), which effectively gives values in deep loops higher rank
1638   than values not in loops.
1639   </p>
1640 </div>
1641
1642 <!-------------------------------------------------------------------------- -->
1643 <div class="doc_subsection">
1644   <a name="reg2mem">Demote all values to stack slots</a>
1645 </div>
1646 <div class="doc_text">
1647   <p>
1648   This file demotes all registers to memory references.  It is intented to be
1649   the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>.  By converting to
1650   <tt>load</tt> instructions, the only values live accross basic blocks are
1651   <tt>alloca</tt> instructions and <tt>load</tt> instructions before
1652   <tt>phi</tt> nodes. It is intended that this should make CFG hacking much
1653   easier. To make later hacking easier, the entry block is split into two, such
1654   that all introduced <tt>alloca</tt> instructions (and nothing else) are in the
1655   entry block.
1656   </p>
1657 </div>
1658
1659 <!-------------------------------------------------------------------------- -->
1660 <div class="doc_subsection">
1661   <a name="scalarrepl">Scalar Replacement of Aggregates</a>
1662 </div>
1663 <div class="doc_text">
1664   <p>
1665   The well-known scalar replacement of aggregates transformation.  This
1666   transform breaks up <tt>alloca</tt> instructions of aggregate type (structure
1667   or array) into individual <tt>alloca</tt> instructions for each member if
1668   possible.  Then, if possible, it transforms the individual <tt>alloca</tt>
1669   instructions into nice clean scalar SSA form.
1670   </p>
1671
1672   <p>
1673   This combines a simple scalar replacement of aggregates algorithm with the <a
1674   href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact,
1675   especially for C++ programs.  As such, iterating between <tt>scalarrepl</tt>,
1676   then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to
1677   promote works well.
1678   </p>
1679 </div>
1680
1681 <!-------------------------------------------------------------------------- -->
1682 <div class="doc_subsection">
1683   <a name="sccp">Sparse Conditional Constant Propagation</a>
1684 </div>
1685 <div class="doc_text">
1686   <p>
1687   Sparse conditional constant propagation and merging, which can be summarized
1688   as:
1689   </p>
1690
1691   <ol>
1692     <li>Assumes values are constant unless proven otherwise</li>
1693     <li>Assumes BasicBlocks are dead unless proven otherwise</li>
1694     <li>Proves values to be constant, and replaces them with constants</li>
1695     <li>Proves conditional branches to be unconditional</li>
1696   </ol>
1697
1698   <p>
1699   Note that this pass has a habit of making definitions be dead.  It is a good
1700   idea to to run a DCE pass sometime after running this pass.
1701   </p>
1702 </div>
1703
1704 <!-------------------------------------------------------------------------- -->
1705 <div class="doc_subsection">
1706   <a name="simplify-libcalls">Simplify well-known library calls</a>
1707 </div>
1708 <div class="doc_text">
1709   <p>
1710   Applies a variety of small optimizations for calls to specific well-known
1711   function calls (e.g. runtime library functions). For example, a call
1712    <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be
1713    transformed into simply <tt>return 3</tt>.
1714   </p>
1715 </div>
1716
1717 <!-------------------------------------------------------------------------- -->
1718 <div class="doc_subsection">
1719   <a name="simplifycfg">Simplify the CFG</a>
1720 </div>
1721 <div class="doc_text">
1722   <p>
1723   Performs dead code elimination and basic block merging. Specifically:
1724   </p>
1725
1726   <ol>
1727     <li>Removes basic blocks with no predecessors.</li>
1728     <li>Merges a basic block into its predecessor if there is only one and the
1729         predecessor only has one successor.</li>
1730     <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li>
1731     <li>Eliminates a basic block that only contains an unconditional
1732         branch.</li>
1733   </ol>
1734 </div>
1735
1736 <!-------------------------------------------------------------------------- -->
1737 <div class="doc_subsection">
1738   <a name="strip">Strip all symbols from a module</a>
1739 </div>
1740 <div class="doc_text">
1741   <p>
1742   Performs code stripping. This transformation can delete:
1743   </p>
1744
1745   <ol>
1746     <li>names for virtual registers</li>
1747     <li>symbols for internal globals and functions</li>
1748     <li>debug information</li>
1749   </ol>
1750
1751   <p>
1752   Note that this transformation makes code much less readable, so it should
1753   only be used in situations where the <tt>strip</tt> utility would be used,
1754   such as reducing code size or making it harder to reverse engineer code.
1755   </p>
1756 </div>
1757
1758 <!-------------------------------------------------------------------------- -->
1759 <div class="doc_subsection">
1760   <a name="strip-dead-prototypes">Remove unused function declarations</a>
1761 </div>
1762 <div class="doc_text">
1763   <p>
1764   This pass loops over all of the functions in the input module, looking for
1765   dead declarations and removes them. Dead declarations are declarations of
1766   functions for which no implementation is available (i.e., declarations for
1767   unused library functions).
1768   </p>
1769 </div>
1770
1771 <!-------------------------------------------------------------------------- -->
1772 <div class="doc_subsection">
1773   <a name="sretpromotion">Promote sret arguments</a>
1774 </div>
1775 <div class="doc_text">
1776   <p>
1777   This pass finds functions that return a struct (using a pointer to the struct
1778   as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and
1779   replaces them with a new function that simply returns each of the elements of
1780   that struct (using multiple return values).
1781   </p>
1782
1783   <p>
1784   This pass works under a number of conditions:
1785   </p>
1786
1787   <ul>
1788   <li>The returned struct must not contain other structs</li>
1789   <li>The returned struct must only be used to load values from</li>
1790   <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li>
1791   </ul>
1792 </div>
1793
1794 <!-------------------------------------------------------------------------- -->
1795 <div class="doc_subsection">
1796   <a name="tailcallelim">Tail Call Elimination</a>
1797 </div>
1798 <div class="doc_text">
1799   <p>
1800   This file transforms calls of the current function (self recursion) followed
1801   by a return instruction with a branch to the entry of the function, creating
1802   a loop.  This pass also implements the following extensions to the basic
1803   algorithm:
1804   </p>
1805
1806   <ul>
1807   <li>Trivial instructions between the call and return do not prevent the
1808       transformation from taking place, though currently the analysis cannot
1809       support moving any really useful instructions (only dead ones).
1810   <li>This pass transforms functions that are prevented from being tail
1811       recursive by an associative expression to use an accumulator variable,
1812       thus compiling the typical naive factorial or <tt>fib</tt> implementation
1813       into efficient code.
1814   <li>TRE is performed if the function returns void, if the return
1815       returns the result returned by the call, or if the function returns a
1816       run-time constant on all exits from the function.  It is possible, though
1817       unlikely, that the return returns something else (like constant 0), and
1818       can still be TRE'd.  It can be TRE'd if <em>all other</em> return
1819       instructions in the function return the exact same value.
1820   <li>If it can prove that callees do not access theier caller stack frame,
1821       they are marked as eligible for tail call elimination (by the code
1822       generator).
1823   </ul>
1824 </div>
1825
1826 <!-------------------------------------------------------------------------- -->
1827 <div class="doc_subsection">
1828   <a name="tailduplicate">Tail Duplication</a>
1829 </div>
1830 <div class="doc_text">
1831   <p>
1832   This pass performs a limited form of tail duplication, intended to simplify
1833   CFGs by removing some unconditional branches.  This pass is necessary to
1834   straighten out loops created by the C front-end, but also is capable of
1835   making other code nicer.  After this pass is run, the CFG simplify pass
1836   should be run to clean up the mess.
1837   </p>
1838 </div>
1839
1840 <!-- ======================================================================= -->
1841 <div class="doc_section"> <a name="transform">Utility Passes</a></div>
1842 <div class="doc_text">
1843   <p>This section describes the LLVM Utility Passes.</p>
1844 </div>
1845
1846 <!-------------------------------------------------------------------------- -->
1847 <div class="doc_subsection">
1848   <a name="deadarghaX0r">Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
1849 </div>
1850 <div class="doc_text">
1851   <p>
1852   Same as dead argument elimination, but deletes arguments to functions which
1853   are external.  This is only for use by <a
1854   href="Bugpoint.html">bugpoint</a>.</p>
1855 </div>
1856
1857 <!-------------------------------------------------------------------------- -->
1858 <div class="doc_subsection">
1859   <a name="extract-blocks">Extract Basic Blocks From Module (for bugpoint use)</a>
1860 </div>
1861 <div class="doc_text">
1862   <p>
1863   This pass is used by bugpoint to extract all blocks from the module into their
1864   own functions.</p>
1865 </div>
1866
1867 <!-------------------------------------------------------------------------- -->
1868 <div class="doc_subsection">
1869   <a name="preverify">Preliminary module verification</a>
1870 </div>
1871 <div class="doc_text">
1872   <p>
1873   Ensures that the module is in the form required by the <a
1874   href="#verifier">Module Verifier</a> pass.
1875   </p>
1876
1877   <p>
1878   Running the verifier runs this pass automatically, so there should be no need
1879   to use it directly.
1880   </p>
1881 </div>
1882
1883 <!-------------------------------------------------------------------------- -->
1884 <div class="doc_subsection">
1885   <a name="verify">Module Verifier</a>
1886 </div>
1887 <div class="doc_text">
1888   <p>
1889   Verifies an LLVM IR code. This is useful to run after an optimization which is
1890   undergoing testing. Note that <tt>llvm-as</tt> verifies its input before
1891   emitting bitcode, and also that malformed bitcode is likely to make LLVM
1892   crash. All language front-ends are therefore encouraged to verify their output
1893   before performing optimizing transformations.
1894   </p>
1895
1896   <ul>
1897     <li>Both of a binary operator's parameters are of the same type.</li>
1898     <li>Verify that the indices of mem access instructions match other
1899         operands.</li>
1900     <li>Verify that arithmetic and other things are only performed on
1901         first-class types.  Verify that shifts and logicals only happen on
1902         integrals f.e.</li>
1903     <li>All of the constants in a switch statement are of the correct type.</li>
1904     <li>The code is in valid SSA form.</li>
1905     <li>It should be illegal to put a label into any other type (like a
1906         structure) or to return one. [except constant arrays!]</li>
1907     <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is
1908         invalid.</li>
1909     <li>PHI nodes must have an entry for each predecessor, with no extras.</li>
1910     <li>PHI nodes must be the first thing in a basic block, all grouped
1911         together.</li>
1912     <li>PHI nodes must have at least one entry.</li>
1913     <li>All basic blocks should only end with terminator insts, not contain
1914         them.</li>
1915     <li>The entry node to a function must not have predecessors.</li>
1916     <li>All Instructions must be embedded into a basic block.</li>
1917     <li>Functions cannot take a void-typed parameter.</li>
1918     <li>Verify that a function's argument list agrees with its declared
1919         type.</li>
1920     <li>It is illegal to specify a name for a void value.</li>
1921     <li>It is illegal to have a internal global value with no initializer.</li>
1922     <li>It is illegal to have a ret instruction that returns a value that does
1923         not agree with the function return value type.</li>
1924     <li>Function call argument types match the function prototype.</li>
1925     <li>All other things that are tested by asserts spread about the code.</li>
1926   </ul>
1927
1928   <p>
1929   Note that this does not provide full security verification (like Java), but
1930   instead just tries to ensure that code is well-formed.
1931   </p>
1932 </div>
1933
1934 <!-------------------------------------------------------------------------- -->
1935 <div class="doc_subsection">
1936   <a name="view-cfg">View CFG of function</a>
1937 </div>
1938 <div class="doc_text">
1939   <p>
1940   Displays the control flow graph using the GraphViz tool.
1941   </p>
1942 </div>
1943
1944 <!-------------------------------------------------------------------------- -->
1945 <div class="doc_subsection">
1946   <a name="view-cfg-only">View CFG of function (with no function bodies)</a>
1947 </div>
1948 <div class="doc_text">
1949   <p>
1950   Displays the control flow graph using the GraphViz tool, but omitting function
1951   bodies.
1952   </p>
1953 </div>
1954
1955 <!-- *********************************************************************** -->
1956
1957 <hr>
1958 <address>
1959   <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
1960   src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
1961   <a href="http://validator.w3.org/check/referer"><img
1962   src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
1963
1964   <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
1965   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
1966   Last modified: $Date$
1967 </address>
1968
1969 </body>
1970 </html>