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  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="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (stateless AA impl)</td></tr>
  79 <tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr>
  80 <tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
  81 <tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
  82 <tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
  83 <tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
  84 <tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
  85 <tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
  86 <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>
  87 <tr><td><a href="#dot-dom">-dot-dom</a></td><td>Print dominance tree of function to 'dot' file</td></tr>
  88 <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>
  89 <tr><td><a href="#dot-postdom">-dot-postdom</a></td><td>Print postdominance tree of function to 'dot' file</td></tr>
  90 <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>
  91 <tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr>
  92 <tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr>
  93 <tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr>
  94 <tr><td><a href="#iv-users">-iv-users</a></td><td>Induction Variable Users</td></tr>
  95 <tr><td><a href="#lazy-value-info">-lazy-value-info</a></td><td>Lazy Value Information Analysis</td></tr>
  96 <tr><td><a href="#lda">-lda</a></td><td>Loop Dependence Analysis</td></tr>
  97 <tr><td><a href="#libcall-aa">-libcall-aa</a></td><td>LibCall Alias Analysis</td></tr>
  98 <tr><td><a href="#lint">-lint</a></td><td>Statically lint-checks LLVM IR</td></tr>
  99 <tr><td><a href="#loops">-loops</a></td><td>Natural Loop Information</td></tr>
 100 <tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
 101 <tr><td><a href="#module-debuginfo">-module-debuginfo</a></td><td>Decodes module-level debug info</td></tr>
 102 <tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
 103 <tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
 104 <tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr>
 105 <tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
 106 <tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
 107 <tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr>
 108 <tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr>
 109 <tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr>
 110 <tr><td><a href="#print-dbginfo">-print-dbginfo</a></td><td>Print debug info in human readable form</td></tr>
 111 <tr><td><a href="#print-dom-info">-print-dom-info</a></td><td>Dominator Info Printer</td></tr>
 112 <tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
 113 <tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr>
 114 <tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr>
 115 <tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr>
 116 <tr><td><a href="#profile-estimator">-profile-estimator</a></td><td>Estimate profiling information</td></tr>
 117 <tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr>
 118 <tr><td><a href="#profile-verifier">-profile-verifier</a></td><td>Verify profiling information</td></tr>
 119 <tr><td><a href="#regions">-regions</a></td><td>Detect single entry single exit regions</td></tr>
 120 <tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr>
 121 <tr><td><a href="#scev-aa">-scev-aa</a></td><td>ScalarEvolution-based Alias Analysis</td></tr>
 122 <tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
 123
 124
 125 <tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr>
 126 <tr><th>Option</th><th>Name</th></tr>
 127 <tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr>
 128 <tr><td><a href="#always-inline">-always-inline</a></td><td>Inliner for always_inline functions</td></tr>
 129 <tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</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="#lowersetjmp">-lowersetjmp</a></td><td>Lower Set Jump</td></tr>
 166 <tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr>
 167 <tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr>
 168 <tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>MemCpy Optimization</td></tr>
 169 <tr><td><a href="#mergefunc">-mergefunc</a></td><td>Merge Functions</td></tr>
 170 <tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr>
 171 <tr><td><a href="#partial-inliner">-partial-inliner</a></td><td>Partial Inliner</td></tr>
 172 <tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr>
 173 <tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr>
 174 <tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr>
 175 <tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates (DT)</td></tr>
 176 <tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr>
 177 <tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr>
 178 <tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr>
 179 <tr><td><a href="#sink">-sink</a></td><td>Code sinking</td></tr>
 180 <tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments to multiple ret values</td></tr>
 181 <tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr>
 182 <tr><td><a href="#strip-dead-debug-info">-strip-dead-debug-info</a></td><td>Strip debug info for unused symbols</td></tr>
 183 <tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Strip Unused Function Prototypes</td></tr>
 184 <tr><td><a href="#strip-debug-declare">-strip-debug-declare</a></td><td>Strip all llvm.dbg.declare intrinsics</td></tr>
 185 <tr><td><a href="#strip-nondebug">-strip-nondebug</a></td><td>Strip all symbols, except dbg symbols, from a module</td></tr>
 186 <tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr>
 187 <tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr>
 188
 189
 190 <tr><th colspan="2"><b>UTILITY PASSES</b></th></tr>
 191 <tr><th>Option</th><th>Name</th></tr>
 192 <tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr>
 193 <tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr>
 194 <tr><td><a href="#instnamer">-instnamer</a></td><td>Assign names to anonymous instructions</td></tr>
 195 <tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr>
 196 <tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr>
 197 <tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr>
 198 <tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr>
 199 <tr><td><a href="#view-dom">-view-dom</a></td><td>View dominance tree of function</td></tr>
 200 <tr><td><a href="#view-dom-only">-view-dom-only</a></td><td>View dominance tree of function (with no function bodies)</td></tr>
 201 <tr><td><a href="#view-postdom">-view-postdom</a></td><td>View postdominance tree of function</td></tr>
 202 <tr><td><a href="#view-postdom-only">-view-postdom-only</a></td><td>View postdominance tree of function (with no function bodies)</td></tr>
 203 </table>
 204 </div>
 205
 206 <!-- ======================================================================= -->
 207 <div class="doc_section"> <a name="example">Analysis Passes</a></div>
 208 <div class="doc_text">
 209   <p>This section describes the LLVM Analysis Passes.</p>
 210 </div>
 211
 212 <!-------------------------------------------------------------------------- -->
 213 <div class="doc_subsection">
 214   <a name="aa-eval">-aa-eval: Exhaustive Alias Analysis Precision Evaluator</a>
 215 </div>
 216 <div class="doc_text">
 217   <p>This is a simple N^2 alias analysis accuracy evaluator.
 218   Basically, for each function in the program, it simply queries to see how the
 219   alias analysis implementation answers alias queries between each pair of
 220   pointers in the function.</p>
 221
 222   <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco
 223   Spadini, and Wojciech Stryjewski.</p>
 224 </div>
 225
 226 <!-------------------------------------------------------------------------- -->
 227 <div class="doc_subsection">
 228   <a name="basicaa">-basicaa: Basic Alias Analysis (stateless AA impl)</a>
 229 </div>
 230 <div class="doc_text">
 231   <p>
 232   This is the default implementation of the Alias Analysis interface
 233   that simply implements a few identities (two different globals cannot alias,
 234   etc), but otherwise does no analysis.
 235   </p>
 236 </div>
 237
 238 <!-------------------------------------------------------------------------- -->
 239 <div class="doc_subsection">
 240   <a name="basiccg">-basiccg: Basic CallGraph Construction</a>
 241 </div>
 242 <div class="doc_text">
 243   <p>Yet to be written.</p>
 244 </div>
 245
 246 <!-------------------------------------------------------------------------- -->
 247 <div class="doc_subsection">
 248   <a name="count-aa">-count-aa: Count Alias Analysis Query Responses</a>
 249 </div>
 250 <div class="doc_text">
 251   <p>
 252   A pass which can be used to count how many alias queries
 253   are being made and how the alias analysis implementation being used responds.
 254   </p>
 255 </div>
 256
 257 <!-------------------------------------------------------------------------- -->
 258 <div class="doc_subsection">
 259   <a name="debug-aa">-debug-aa: AA use debugger</a>
 260 </div>
 261 <div class="doc_text">
 262   <p>
 263   This simple pass checks alias analysis users to ensure that if they
 264   create a new value, they do not query AA without informing it of the value.
 265   It acts as a shim over any other AA pass you want.
 266   </p>
 267
 268   <p>
 269   Yes keeping track of every value in the program is expensive, but this is
 270   a debugging pass.
 271   </p>
 272 </div>
 273
 274 <!-------------------------------------------------------------------------- -->
 275 <div class="doc_subsection">
 276   <a name="domfrontier">-domfrontier: Dominance Frontier Construction</a>
 277 </div>
 278 <div class="doc_text">
 279   <p>
 280   This pass is a simple dominator construction algorithm for finding forward
 281   dominator frontiers.
 282   </p>
 283 </div>
 284
 285 <!-------------------------------------------------------------------------- -->
 286 <div class="doc_subsection">
 287   <a name="domtree">-domtree: Dominator Tree Construction</a>
 288 </div>
 289 <div class="doc_text">
 290   <p>
 291   This pass is a simple dominator construction algorithm for finding forward
 292   dominators.
 293   </p>
 294 </div>
 295
 296 <!-------------------------------------------------------------------------- -->
 297 <div class="doc_subsection">
 298   <a name="dot-callgraph">-dot-callgraph: Print Call Graph to 'dot' file</a>
 299 </div>
 300 <div class="doc_text">
 301   <p>
 302   This pass, only available in <code>opt</code>, prints the call graph into a
 303   <code>.dot</code> graph.  This graph can then be processed with the "dot" tool
 304   to convert it to postscript or some other suitable format.
 305   </p>
 306 </div>
 307
 308 <!-------------------------------------------------------------------------- -->
 309 <div class="doc_subsection">
 310   <a name="dot-cfg">-dot-cfg: Print CFG of function to 'dot' file</a>
 311 </div>
 312 <div class="doc_text">
 313   <p>
 314   This pass, only available in <code>opt</code>, prints the control flow graph
 315   into a <code>.dot</code> graph.  This graph can then be processed with the
 316   "dot" tool to convert it to postscript or some other suitable format.
 317   </p>
 318 </div>
 319
 320 <!-------------------------------------------------------------------------- -->
 321 <div class="doc_subsection">
 322   <a name="dot-cfg-only">-dot-cfg-only: Print CFG of function to 'dot' file (with no function bodies)</a>
 323 </div>
 324 <div class="doc_text">
 325   <p>
 326   This pass, only available in <code>opt</code>, prints the control flow graph
 327   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 328   then be processed with the "dot" tool to convert it to postscript or some
 329   other suitable format.
 330   </p>
 331 </div>
 332
 333 <!-------------------------------------------------------------------------- -->
 334 <div class="doc_subsection">
 335   <a name="dot-dom">-dot-dom: Print dominance tree of function to 'dot' file</a>
 336 </div>
 337 <div class="doc_text">
 338   <p>
 339   This pass, only available in <code>opt</code>, prints the dominator tree
 340   into a <code>.dot</code> graph.  This graph can then be processed with the
 341   "dot" tool to convert it to postscript or some other suitable format.
 342   </p>
 343 </div>
 344
 345 <!-------------------------------------------------------------------------- -->
 346 <div class="doc_subsection">
 347   <a name="dot-dom-only">-dot-dom-only: Print dominance tree of function to 'dot' file (with no function bodies)</a>
 348 </div>
 349 <div class="doc_text">
 350   <p>
 351   This pass, only available in <code>opt</code>, prints the dominator tree
 352   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 353   then be processed with the "dot" tool to convert it to postscript or some
 354   other suitable format.
 355   </p>
 356 </div>
 357
 358 <!-------------------------------------------------------------------------- -->
 359 <div class="doc_subsection">
 360   <a name="dot-postdom">-dot-postdom: Print postdominance tree of function to 'dot' file</a>
 361 </div>
 362 <div class="doc_text">
 363   <p>
 364   This pass, only available in <code>opt</code>, prints the post dominator tree
 365   into a <code>.dot</code> graph.  This graph can then be processed with the
 366   "dot" tool to convert it to postscript or some other suitable format.
 367   </p>
 368 </div>
 369
 370 <!-------------------------------------------------------------------------- -->
 371 <div class="doc_subsection">
 372   <a name="dot-postdom-only">-dot-postdom-only: Print postdominance tree of function to 'dot' file (with no function bodies)</a>
 373 </div>
 374 <div class="doc_text">
 375   <p>
 376   This pass, only available in <code>opt</code>, prints the post dominator tree
 377   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 378   then be processed with the "dot" tool to convert it to postscript or some
 379   other suitable format.
 380   </p>
 381 </div>
 382
 383 <!-------------------------------------------------------------------------- -->
 384 <div class="doc_subsection">
 385   <a name="globalsmodref-aa">-globalsmodref-aa: Simple mod/ref analysis for globals</a>
 386 </div>
 387 <div class="doc_text">
 388   <p>
 389   This simple pass provides alias and mod/ref information for global values
 390   that do not have their address taken, and keeps track of whether functions
 391   read or write memory (are "pure").  For this simple (but very common) case,
 392   we can provide pretty accurate and useful information.
 393   </p>
 394 </div>
 395
 396 <!-------------------------------------------------------------------------- -->
 397 <div class="doc_subsection">
 398   <a name="instcount">-instcount: Counts the various types of Instructions</a>
 399 </div>
 400 <div class="doc_text">
 401   <p>
 402   This pass collects the count of all instructions and reports them
 403   </p>
 404 </div>
 405
 406 <!-------------------------------------------------------------------------- -->
 407 <div class="doc_subsection">
 408   <a name="intervals">-intervals: Interval Partition Construction</a>
 409 </div>
 410 <div class="doc_text">
 411   <p>
 412   This analysis calculates and represents the interval partition of a function,
 413   or a preexisting interval partition.
 414   </p>
 415
 416   <p>
 417   In this way, the interval partition may be used to reduce a flow graph down
 418   to its degenerate single node interval partition (unless it is irreducible).
 419   </p>
 420 </div>
 421
 422 <!-------------------------------------------------------------------------- -->
 423 <div class="doc_subsection">
 424   <a name="iv-users">-iv-users: Induction Variable Users</a>
 425 </div>
 426 <div class="doc_text">
 427   <p>Bookkeeping for "interesting" users of expressions computed from
 428   induction variables.</p>
 429 </div>
 430
 431 <!-------------------------------------------------------------------------- -->
 432 <div class="doc_subsection">
 433   <a name="lazy-value-info">-lazy-value-info: Lazy Value Information Analysis</a>
 434 </div>
 435 <div class="doc_text">
 436   <p>Interface for lazy computation of value constraint information.</p>
 437 </div>
 438
 439 <!-------------------------------------------------------------------------- -->
 440 <div class="doc_subsection">
 441   <a name="lda">-lda: Loop Dependence Analysis</a>
 442 </div>
 443 <div class="doc_text">
 444   <p>Loop dependence analysis framework, which is used to detect dependences in
 445   memory accesses in loops.</p>
 446 </div>
 447
 448 <!-------------------------------------------------------------------------- -->
 449 <div class="doc_subsection">
 450   <a name="libcall-aa">-libcall-aa: LibCall Alias Analysis</a>
 451 </div>
 452 <div class="doc_text">
 453   <p>LibCall Alias Analysis.</p>
 454 </div>
 455
 456 <!-------------------------------------------------------------------------- -->
 457 <div class="doc_subsection">
 458   <a name="lint">-lint: Statically lint-checks LLVM IR</a>
 459 </div>
 460 <div class="doc_text">
 461   <p>This pass statically checks for common and easily-identified constructs
 462   which produce undefined or likely unintended behavior in LLVM IR.</p>
 463
 464   <p>It is not a guarantee of correctness, in two ways. First, it isn't
 465   comprehensive. There are checks which could be done statically which are
 466   not yet implemented. Some of these are indicated by TODO comments, but
 467   those aren't comprehensive either. Second, many conditions cannot be
 468   checked statically. This pass does no dynamic instrumentation, so it
 469   can't check for all possible problems.</p>
 470
 471   <p>Another limitation is that it assumes all code will be executed. A store
 472   through a null pointer in a basic block which is never reached is harmless,
 473   but this pass will warn about it anyway.</p>
 474
 475   <p>Optimization passes may make conditions that this pass checks for more or
 476   less obvious. If an optimization pass appears to be introducing a warning,
 477   it may be that the optimization pass is merely exposing an existing
 478   condition in the code.</p>
 479
 480   <p>This code may be run before instcombine. In many cases, instcombine checks
 481   for the same kinds of things and turns instructions with undefined behavior
 482   into unreachable (or equivalent). Because of this, this pass makes some
 483   effort to look through bitcasts and so on.
 484   </p>
 485 </div>
 486
 487 <!-------------------------------------------------------------------------- -->
 488 <div class="doc_subsection">
 489   <a name="loops">-loops: Natural Loop Information</a>
 490 </div>
 491 <div class="doc_text">
 492   <p>
 493   This analysis is used to identify natural loops and determine the loop depth
 494   of various nodes of the CFG.  Note that the loops identified may actually be
 495   several natural loops that share the same header node... not just a single
 496   natural loop.
 497   </p>
 498 </div>
 499
 500 <!-------------------------------------------------------------------------- -->
 501 <div class="doc_subsection">
 502   <a name="memdep">-memdep: Memory Dependence Analysis</a>
 503 </div>
 504 <div class="doc_text">
 505   <p>
 506   An analysis that determines, for a given memory operation, what preceding
 507   memory operations it depends on.  It builds on alias analysis information, and
 508   tries to provide a lazy, caching interface to a common kind of alias
 509   information query.
 510   </p>
 511 </div>
 512
 513 <!-------------------------------------------------------------------------- -->
 514 <div class="doc_subsection">
 515   <a name="module-debuginfo">-module-debuginfo: Decodes module-level debug info</a>
 516 </div>
 517 <div class="doc_text">
 518   <p>This pass decodes the debug info metadata in a module and prints in a
 519  (sufficiently-prepared-) human-readable form.
 520
 521  For example, run this pass from opt along with the -analyze option, and
 522  it'll print to standard output.
 523   </p>
 524 </div>
 525
 526 <!-------------------------------------------------------------------------- -->
 527 <div class="doc_subsection">
 528   <a name="no-aa">-no-aa: No Alias Analysis (always returns 'may' alias)</a>
 529 </div>
 530 <div class="doc_text">
 531   <p>
 532   Always returns "I don't know" for alias queries.  NoAA is unlike other alias
 533   analysis implementations, in that it does not chain to a previous analysis. As
 534   such it doesn't follow many of the rules that other alias analyses must.
 535   </p>
 536 </div>
 537
 538 <!-------------------------------------------------------------------------- -->
 539 <div class="doc_subsection">
 540   <a name="no-profile">-no-profile: No Profile Information</a>
 541 </div>
 542 <div class="doc_text">
 543   <p>
 544   The default "no profile" implementation of the abstract
 545   <code>ProfileInfo</code> interface.
 546   </p>
 547 </div>
 548
 549 <!-------------------------------------------------------------------------- -->
 550 <div class="doc_subsection">
 551   <a name="postdomfrontier">-postdomfrontier: Post-Dominance Frontier Construction</a>
 552 </div>
 553 <div class="doc_text">
 554   <p>
 555   This pass is a simple post-dominator construction algorithm for finding
 556   post-dominator frontiers.
 557   </p>
 558 </div>
 559
 560 <!-------------------------------------------------------------------------- -->
 561 <div class="doc_subsection">
 562   <a name="postdomtree">-postdomtree: Post-Dominator Tree Construction</a>
 563 </div>
 564 <div class="doc_text">
 565   <p>
 566   This pass is a simple post-dominator construction algorithm for finding
 567   post-dominators.
 568   </p>
 569 </div>
 570
 571 <!-------------------------------------------------------------------------- -->
 572 <div class="doc_subsection">
 573   <a name="print-alias-sets">-print-alias-sets: Alias Set Printer</a>
 574 </div>
 575 <div class="doc_text">
 576   <p>Yet to be written.</p>
 577 </div>
 578
 579 <!-------------------------------------------------------------------------- -->
 580 <div class="doc_subsection">
 581   <a name="print-callgraph">-print-callgraph: Print a call graph</a>
 582 </div>
 583 <div class="doc_text">
 584   <p>
 585   This pass, only available in <code>opt</code>, prints the call graph to
 586   standard error in a human-readable form.
 587   </p>
 588 </div>
 589
 590 <!-------------------------------------------------------------------------- -->
 591 <div class="doc_subsection">
 592   <a name="print-callgraph-sccs">-print-callgraph-sccs: Print SCCs of the Call Graph</a>
 593 </div>
 594 <div class="doc_text">
 595   <p>
 596   This pass, only available in <code>opt</code>, prints the SCCs of the call
 597   graph to standard error in a human-readable form.
 598   </p>
 599 </div>
 600
 601 <!-------------------------------------------------------------------------- -->
 602 <div class="doc_subsection">
 603   <a name="print-cfg-sccs">-print-cfg-sccs: Print SCCs of each function CFG</a>
 604 </div>
 605 <div class="doc_text">
 606   <p>
 607   This pass, only available in <code>opt</code>, prints the SCCs of each
 608   function CFG to standard error in a human-readable form.
 609   </p>
 610 </div>
 611
 612 <!-------------------------------------------------------------------------- -->
 613 <div class="doc_subsection">
 614   <a name="print-dbginfo">-print-dbginfo: Print debug info in human readable form</a>
 615 </div>
 616 <div class="doc_text">
 617   <p>Pass that prints instructions, and associated debug info:</p>
 618   <ul>
 619
 620   <li>source/line/col information</li>
 621   <li>original variable name</li>
 622   <li>original type name</li>
 623   </ul>
 624 </div>
 625
 626 <!-------------------------------------------------------------------------- -->
 627 <div class="doc_subsection">
 628   <a name="print-dom-info">-print-dom-info: Dominator Info Printer</a>
 629 </div>
 630 <div class="doc_text">
 631   <p>Dominator Info Printer.</p>
 632 </div>
 633
 634 <!-------------------------------------------------------------------------- -->
 635 <div class="doc_subsection">
 636   <a name="print-externalfnconstants">-print-externalfnconstants: Print external fn callsites passed constants</a>
 637 </div>
 638 <div class="doc_text">
 639   <p>
 640   This pass, only available in <code>opt</code>, prints out call sites to
 641   external functions that are called with constant arguments.  This can be
 642   useful when looking for standard library functions we should constant fold
 643   or handle in alias analyses.
 644   </p>
 645 </div>
 646
 647 <!-------------------------------------------------------------------------- -->
 648 <div class="doc_subsection">
 649   <a name="print-function">-print-function: Print function to stderr</a>
 650 </div>
 651 <div class="doc_text">
 652   <p>
 653   The <code>PrintFunctionPass</code> class is designed to be pipelined with
 654   other <code>FunctionPass</code>es, and prints out the functions of the module
 655   as they are processed.
 656   </p>
 657 </div>
 658
 659 <!-------------------------------------------------------------------------- -->
 660 <div class="doc_subsection">
 661   <a name="print-module">-print-module: Print module to stderr</a>
 662 </div>
 663 <div class="doc_text">
 664   <p>
 665   This pass simply prints out the entire module when it is executed.
 666   </p>
 667 </div>
 668
 669 <!-------------------------------------------------------------------------- -->
 670 <div class="doc_subsection">
 671   <a name="print-used-types">-print-used-types: Find Used Types</a>
 672 </div>
 673 <div class="doc_text">
 674   <p>
 675   This pass is used to seek out all of the types in use by the program.  Note
 676   that this analysis explicitly does not include types only used by the symbol
 677   table.
 678 </div>
 679
 680 <!-------------------------------------------------------------------------- -->
 681 <div class="doc_subsection">
 682   <a name="profile-estimator">-profile-estimator: Estimate profiling information</a>
 683 </div>
 684 <div class="doc_text">
 685   <p>Profiling information that estimates the profiling information
 686   in a very crude and unimaginative way.
 687   </p>
 688 </div>
 689
 690 <!-------------------------------------------------------------------------- -->
 691 <div class="doc_subsection">
 692   <a name="profile-loader">-profile-loader: Load profile information from llvmprof.out</a>
 693 </div>
 694 <div class="doc_text">
 695   <p>
 696   A concrete implementation of profiling information that loads the information
 697   from a profile dump file.
 698   </p>
 699 </div>
 700
 701 <!-------------------------------------------------------------------------- -->
 702 <div class="doc_subsection">
 703   <a name="profile-verifier">-profile-verifier: Verify profiling information</a>
 704 </div>
 705 <div class="doc_text">
 706   <p>Pass that checks profiling information for plausibility.</p>
 707 </div>
 708 <div class="doc_subsection">
 709   <a name="regions">-regions: Detect single entry single exit regions</a>
 710 </div>
 711 <div class="doc_text">
 712   <p>
 713   The <code>RegionInfo</code> pass detects single entry single exit regions in a
 714   function, where a region is defined as any subgraph that is connected to the
 715   remaining graph at only two spots. Furthermore, an hierarchical region tree is
 716   built.
 717   </p>
 718 </div>
 719
 720 <!-------------------------------------------------------------------------- -->
 721 <div class="doc_subsection">
 722   <a name="scalar-evolution">-scalar-evolution: Scalar Evolution Analysis</a>
 723 </div>
 724 <div class="doc_text">
 725   <p>
 726   The <code>ScalarEvolution</code> analysis can be used to analyze and
 727   catagorize scalar expressions in loops.  It specializes in recognizing general
 728   induction variables, representing them with the abstract and opaque
 729   <code>SCEV</code> class.  Given this analysis, trip counts of loops and other
 730   important properties can be obtained.
 731   </p>
 732
 733   <p>
 734   This analysis is primarily useful for induction variable substitution and
 735   strength reduction.
 736   </p>
 737 </div>
 738
 739 <!-------------------------------------------------------------------------- -->
 740 <div class="doc_subsection">
 741   <a name="scev-aa">-scev-aa: ScalarEvolution-based Alias Analysis</a>
 742 </div>
 743 <div class="doc_text">
 744   <p>Simple alias analysis implemented in terms of ScalarEvolution queries.
 745
 746   This differs from traditional loop dependence analysis in that it tests
 747   for dependencies within a single iteration of a loop, rather than
 748   dependencies between different iterations.
 749
 750   ScalarEvolution has a more complete understanding of pointer arithmetic
 751   than BasicAliasAnalysis' collection of ad-hoc analyses.
 752   </p>
 753 </div>
 754
 755 <!-------------------------------------------------------------------------- -->
 756 <div class="doc_subsection">
 757   <a name="targetdata">-targetdata: Target Data Layout</a>
 758 </div>
 759 <div class="doc_text">
 760   <p>Provides other passes access to information on how the size and alignment
 761   required by the the target ABI for various data types.</p>
 762 </div>
 763
 764 <!-- ======================================================================= -->
 765 <div class="doc_section"> <a name="transform">Transform Passes</a></div>
 766 <div class="doc_text">
 767   <p>This section describes the LLVM Transform Passes.</p>
 768 </div>
 769
 770 <!-------------------------------------------------------------------------- -->
 771 <div class="doc_subsection">
 772   <a name="adce">-adce: Aggressive Dead Code Elimination</a>
 773 </div>
 774 <div class="doc_text">
 775   <p>ADCE aggressively tries to eliminate code. This pass is similar to
 776   <a href="#dce">DCE</a> but it assumes that values are dead until proven
 777   otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to
 778   the liveness of values.</p>
 779 </div>
 780
 781 <!-------------------------------------------------------------------------- -->
 782 <div class="doc_subsection">
 783   <a name="always-inline">-always-inline: Inliner for always_inline functions</a>
 784 </div>
 785 <div class="doc_text">
 786   <p>A custom inliner that handles only functions that are marked as
 787   "always inline".</p>
 788 </div>
 789
 790 <!-------------------------------------------------------------------------- -->
 791 <div class="doc_subsection">
 792   <a name="argpromotion">-argpromotion: Promote 'by reference' arguments to scalars</a>
 793 </div>
 794 <div class="doc_text">
 795   <p>
 796   This pass promotes "by reference" arguments to be "by value" arguments.  In
 797   practice, this means looking for internal functions that have pointer
 798   arguments.  If it can prove, through the use of alias analysis, that an
 799   argument is *only* loaded, then it can pass the value into the function
 800   instead of the address of the value.  This can cause recursive simplification
 801   of code and lead to the elimination of allocas (especially in C++ template
 802   code like the STL).
 803   </p>
 804
 805   <p>
 806   This pass also handles aggregate arguments that are passed into a function,
 807   scalarizing them if the elements of the aggregate are only loaded.  Note that
 808   it refuses to scalarize aggregates which would require passing in more than
 809   three operands to the function, because passing thousands of operands for a
 810   large array or structure is unprofitable!
 811   </p>
 812
 813   <p>
 814   Note that this transformation could also be done for arguments that are only
 815   stored to (returning the value instead), but does not currently.  This case
 816   would be best handled when and if LLVM starts supporting multiple return
 817   values from functions.
 818   </p>
 819 </div>
 820
 821 <!-------------------------------------------------------------------------- -->
 822 <div class="doc_subsection">
 823   <a name="block-placement">-block-placement: Profile Guided Basic Block Placement</a>
 824 </div>
 825 <div class="doc_text">
 826   <p>This pass is a very simple profile guided basic block placement algorithm.
 827   The idea is to put frequently executed blocks together at the start of the
 828   function and hopefully increase the number of fall-through conditional
 829   branches.  If there is no profile information for a particular function, this
 830   pass basically orders blocks in depth-first order.</p>
 831 </div>
 832
 833 <!-------------------------------------------------------------------------- -->
 834 <div class="doc_subsection">
 835   <a name="break-crit-edges">-break-crit-edges: Break critical edges in CFG</a>
 836 </div>
 837 <div class="doc_text">
 838   <p>
 839   Break all of the critical edges in the CFG by inserting a dummy basic block.
 840   It may be "required" by passes that cannot deal with critical edges. This
 841   transformation obviously invalidates the CFG, but can update forward dominator
 842   (set, immediate dominators, tree, and frontier) information.
 843   </p>
 844 </div>
 845
 846 <!-------------------------------------------------------------------------- -->
 847 <div class="doc_subsection">
 848   <a name="codegenprepare">-codegenprepare: Optimize for code generation</a>
 849 </div>
 850 <div class="doc_text">
 851   This pass munges the code in the input function to better prepare it for
 852   SelectionDAG-based code generation. This works around limitations in it's
 853   basic-block-at-a-time approach. It should eventually be removed.
 854 </div>
 855
 856 <!-------------------------------------------------------------------------- -->
 857 <div class="doc_subsection">
 858   <a name="constmerge">-constmerge: Merge Duplicate Global Constants</a>
 859 </div>
 860 <div class="doc_text">
 861   <p>
 862   Merges duplicate global constants together into a single constant that is
 863   shared.  This is useful because some passes (ie TraceValues) insert a lot of
 864   string constants into the program, regardless of whether or not an existing
 865   string is available.
 866   </p>
 867 </div>
 868
 869 <!-------------------------------------------------------------------------- -->
 870 <div class="doc_subsection">
 871   <a name="constprop">-constprop: Simple constant propagation</a>
 872 </div>
 873 <div class="doc_text">
 874   <p>This file implements constant propagation and merging. It looks for
 875   instructions involving only constant operands and replaces them with a
 876   constant value instead of an instruction. For example:</p>
 877   <blockquote><pre>add i32 1, 2</pre></blockquote>
 878   <p>becomes</p>
 879   <blockquote><pre>i32 3</pre></blockquote>
 880   <p>NOTE: this pass has a habit of making definitions be dead.  It is a good
 881   idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass
 882   sometime after running this pass.</p>
 883 </div>
 884
 885 <!-------------------------------------------------------------------------- -->
 886 <div class="doc_subsection">
 887   <a name="dce">-dce: Dead Code Elimination</a>
 888 </div>
 889 <div class="doc_text">
 890   <p>
 891   Dead code elimination is similar to <a href="#die">dead instruction
 892   elimination</a>, but it rechecks instructions that were used by removed
 893   instructions to see if they are newly dead.
 894   </p>
 895 </div>
 896
 897 <!-------------------------------------------------------------------------- -->
 898 <div class="doc_subsection">
 899   <a name="deadargelim">-deadargelim: Dead Argument Elimination</a>
 900 </div>
 901 <div class="doc_text">
 902   <p>
 903   This pass deletes dead arguments from internal functions.  Dead argument
 904   elimination removes arguments which are directly dead, as well as arguments
 905   only passed into function calls as dead arguments of other functions.  This
 906   pass also deletes dead arguments in a similar way.
 907   </p>
 908
 909   <p>
 910   This pass is often useful as a cleanup pass to run after aggressive
 911   interprocedural passes, which add possibly-dead arguments.
 912   </p>
 913 </div>
 914
 915 <!-------------------------------------------------------------------------- -->
 916 <div class="doc_subsection">
 917   <a name="deadtypeelim">-deadtypeelim: Dead Type Elimination</a>
 918 </div>
 919 <div class="doc_text">
 920   <p>
 921   This pass is used to cleanup the output of GCC.  It eliminate names for types
 922   that are unused in the entire translation unit, using the <a
 923   href="#findusedtypes">find used types</a> pass.
 924   </p>
 925 </div>
 926
 927 <!-------------------------------------------------------------------------- -->
 928 <div class="doc_subsection">
 929   <a name="die">-die: Dead Instruction Elimination</a>
 930 </div>
 931 <div class="doc_text">
 932   <p>
 933   Dead instruction elimination performs a single pass over the function,
 934   removing instructions that are obviously dead.
 935   </p>
 936 </div>
 937
 938 <!-------------------------------------------------------------------------- -->
 939 <div class="doc_subsection">
 940   <a name="dse">-dse: Dead Store Elimination</a>
 941 </div>
 942 <div class="doc_text">
 943   <p>
 944   A trivial dead store elimination that only considers basic-block local
 945   redundant stores.
 946   </p>
 947 </div>
 948
 949 <!-------------------------------------------------------------------------- -->
 950 <div class="doc_subsection">
 951   <a name="functionattrs">-functionattrs: Deduce function attributes</a>
 952 </div>
 953 <div class="doc_text">
 954   <p>A simple interprocedural pass which walks the call-graph, looking for
 955   functions which do not access or only read non-local memory, and marking them
 956   readnone/readonly.  In addition, it marks function arguments (of pointer type)
 957   'nocapture' if a call to the function does not create any copies of the pointer
 958   value that outlive the call. This more or less means that the pointer is only
 959   dereferenced, and not returned from the function or stored in a global.
 960   This pass is implemented as a bottom-up traversal of the call-graph.
 961   </p>
 962 </div>
 963
 964 <!-------------------------------------------------------------------------- -->
 965 <div class="doc_subsection">
 966   <a name="globaldce">-globaldce: Dead Global Elimination</a>
 967 </div>
 968 <div class="doc_text">
 969   <p>
 970   This transform is designed to eliminate unreachable internal globals from the
 971   program.  It uses an aggressive algorithm, searching out globals that are
 972   known to be alive.  After it finds all of the globals which are needed, it
 973   deletes whatever is left over.  This allows it to delete recursive chunks of
 974   the program which are unreachable.
 975   </p>
 976 </div>
 977
 978 <!-------------------------------------------------------------------------- -->
 979 <div class="doc_subsection">
 980   <a name="globalopt">-globalopt: Global Variable Optimizer</a>
 981 </div>
 982 <div class="doc_text">
 983   <p>
 984   This pass transforms simple global variables that never have their address
 985   taken.  If obviously true, it marks read/write globals as constant, deletes
 986   variables only stored to, etc.
 987   </p>
 988 </div>
 989
 990 <!-------------------------------------------------------------------------- -->
 991 <div class="doc_subsection">
 992   <a name="gvn">-gvn: Global Value Numbering</a>
 993 </div>
 994 <div class="doc_text">
 995   <p>
 996   This pass performs global value numbering to eliminate fully and partially
 997   redundant instructions.  It also performs redundant load elimination.
 998   </p>
 999 </div>
1000
1001 <!-------------------------------------------------------------------------- -->
1002 <div class="doc_subsection">
1003   <a name="indvars">-indvars: Canonicalize Induction Variables</a>
1004 </div>
1005 <div class="doc_text">
1006   <p>
1007   This transformation analyzes and transforms the induction variables (and
1008   computations derived from them) into simpler forms suitable for subsequent
1009   analysis and transformation.
1010   </p>
1011
1012   <p>
1013   This transformation makes the following changes to each loop with an
1014   identifiable induction variable:
1015   </p>
1016
1017   <ol>
1018     <li>All loops are transformed to have a <em>single</em> canonical
1019         induction variable which starts at zero and steps by one.</li>
1020     <li>The canonical induction variable is guaranteed to be the first PHI node
1021         in the loop header block.</li>
1022     <li>Any pointer arithmetic recurrences are raised to use array
1023         subscripts.</li>
1024   </ol>
1025
1026   <p>
1027   If the trip count of a loop is computable, this pass also makes the following
1028   changes:
1029   </p>
1030
1031   <ol>
1032     <li>The exit condition for the loop is canonicalized to compare the
1033         induction value against the exit value.  This turns loops like:
1034         <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote>
1035         into
1036         <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li>
1037     <li>Any use outside of the loop of an expression derived from the indvar
1038         is changed to compute the derived value outside of the loop, eliminating
1039         the dependence on the exit value of the induction variable.  If the only
1040         purpose of the loop is to compute the exit value of some derived
1041         expression, this transformation will make the loop dead.</li>
1042   </ol>
1043
1044   <p>
1045   This transformation should be followed by strength reduction after all of the
1046   desired loop transformations have been performed.  Additionally, on targets
1047   where it is profitable, the loop could be transformed to count down to zero
1048   (the "do loop" optimization).
1049   </p>
1050 </div>
1051
1052 <!-------------------------------------------------------------------------- -->
1053 <div class="doc_subsection">
1054   <a name="inline">-inline: Function Integration/Inlining</a>
1055 </div>
1056 <div class="doc_text">
1057   <p>
1058   Bottom-up inlining of functions into callees.
1059   </p>
1060 </div>
1061
1062 <!-------------------------------------------------------------------------- -->
1063 <div class="doc_subsection">
1064   <a name="insert-edge-profiling">-insert-edge-profiling: Insert instrumentation for edge profiling</a>
1065 </div>
1066 <div class="doc_text">
1067   <p>
1068   This pass instruments the specified program with counters for edge profiling.
1069   Edge profiling can give a reasonable approximation of the hot paths through a
1070   program, and is used for a wide variety of program transformations.
1071   </p>
1072
1073   <p>
1074   Note that this implementation is very naïve.  It inserts a counter for
1075   <em>every</em> edge in the program, instead of using control flow information
1076   to prune the number of counters inserted.
1077   </p>
1078 </div>
1079
1080 <!-------------------------------------------------------------------------- -->
1081 <div class="doc_subsection">
1082   <a name="insert-optimal-edge-profiling">-insert-optimal-edge-profiling: Insert optimal instrumentation for edge profiling</a>
1083 </div>
1084 <div class="doc_text">
1085   <p>This pass instruments the specified program with counters for edge profiling.
1086   Edge profiling can give a reasonable approximation of the hot paths through a
1087   program, and is used for a wide variety of program transformations.
1088   </p>
1089 </div>
1090
1091 <!-------------------------------------------------------------------------- -->
1092 <div class="doc_subsection">
1093   <a name="instcombine">-instcombine: Combine redundant instructions</a>
1094 </div>
1095 <div class="doc_text">
1096   <p>
1097   Combine instructions to form fewer, simple
1098   instructions.  This pass does not modify the CFG This pass is where algebraic
1099   simplification happens.
1100   </p>
1101
1102   <p>
1103   This pass combines things like:
1104   </p>
1105
1106 <blockquote><pre
1107 >%Y = add i32 %X, 1
1108 %Z = add i32 %Y, 1</pre></blockquote>
1109
1110   <p>
1111   into:
1112   </p>
1113
1114 <blockquote><pre
1115 >%Z = add i32 %X, 2</pre></blockquote>
1116
1117   <p>
1118   This is a simple worklist driven algorithm.
1119   </p>
1120
1121   <p>
1122   This pass guarantees that the following canonicalizations are performed on
1123   the program:
1124   </p>
1125
1126   <ul>
1127     <li>If a binary operator has a constant operand, it is moved to the right-
1128         hand side.</li>
1129     <li>Bitwise operators with constant operands are always grouped so that
1130         shifts are performed first, then <code>or</code>s, then
1131         <code>and</code>s, then <code>xor</code>s.</li>
1132     <li>Compare instructions are converted from <code>&lt;</code>,
1133         <code>&gt;</code>, <code>≤</code>, or <code>≥</code> to
1134         <code>=</code> or <code>≠</code> if possible.</li>
1135     <li>All <code>cmp</code> instructions on boolean values are replaced with
1136         logical operations.</li>
1137     <li><code>add <var>X</var>, <var>X</var></code> is represented as
1138         <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li>
1139     <li>Multiplies with a constant power-of-two argument are transformed into
1140         shifts.</li>
1141     <li>… etc.</li>
1142   </ul>
1143 </div>
1144
1145 <!-------------------------------------------------------------------------- -->
1146 <div class="doc_subsection">
1147   <a name="internalize">-internalize: Internalize Global Symbols</a>
1148 </div>
1149 <div class="doc_text">
1150   <p>
1151   This pass loops over all of the functions in the input module, looking for a
1152   main function.  If a main function is found, all other functions and all
1153   global variables with initializers are marked as internal.
1154   </p>
1155 </div>
1156
1157 <!-------------------------------------------------------------------------- -->
1158 <div class="doc_subsection">
1159   <a name="ipconstprop">-ipconstprop: Interprocedural constant propagation</a>
1160 </div>
1161 <div class="doc_text">
1162   <p>
1163   This pass implements an <em>extremely</em> simple interprocedural constant
1164   propagation pass.  It could certainly be improved in many different ways,
1165   like using a worklist.  This pass makes arguments dead, but does not remove
1166   them.  The existing dead argument elimination pass should be run after this
1167   to clean up the mess.
1168   </p>
1169 </div>
1170
1171 <!-------------------------------------------------------------------------- -->
1172 <div class="doc_subsection">
1173   <a name="ipsccp">-ipsccp: Interprocedural Sparse Conditional Constant Propagation</a>
1174 </div>
1175 <div class="doc_text">
1176   <p>
1177   An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
1178   Propagation</a>.
1179   </p>
1180 </div>
1181
1182 <!-------------------------------------------------------------------------- -->
1183 <div class="doc_subsection">
1184   <a name="jump-threading">-jump-threading: Jump Threading</a>
1185 </div>
1186 <div class="doc_text">
1187   <p>
1188   Jump threading tries to find distinct threads of control flow running through
1189   a basic block. This pass looks at blocks that have multiple predecessors and
1190   multiple successors.  If one or more of the predecessors of the block can be
1191   proven to always cause a jump to one of the successors, we forward the edge
1192   from the predecessor to the successor by duplicating the contents of this
1193   block.
1194   </p>
1195   <p>
1196   An example of when this can occur is code like this:
1197   </p>
1198
1199   <pre
1200 >if () { ...
1201   X = 4;
1202 }
1203 if (X &lt; 3) {</pre>
1204
1205   <p>
1206   In this case, the unconditional branch at the end of the first if can be
1207   revectored to the false side of the second if.
1208   </p>
1209 </div>
1210
1211 <!-------------------------------------------------------------------------- -->
1212 <div class="doc_subsection">
1213   <a name="lcssa">-lcssa: Loop-Closed SSA Form Pass</a>
1214 </div>
1215 <div class="doc_text">
1216   <p>
1217   This pass transforms loops by placing phi nodes at the end of the loops for
1218   all values that are live across the loop boundary.  For example, it turns
1219   the left into the right code:
1220   </p>
1221
1222   <pre
1223 >for (...)                for (...)
1224   if (c)                   if (c)
1225     X1 = ...                 X1 = ...
1226   else                     else
1227     X2 = ...                 X2 = ...
1228   X3 = phi(X1, X2)         X3 = phi(X1, X2)
1229 ... = X3 + 4              X4 = phi(X3)
1230                           ... = X4 + 4</pre>
1231
1232   <p>
1233   This is still valid LLVM; the extra phi nodes are purely redundant, and will
1234   be trivially eliminated by <code>InstCombine</code>.  The major benefit of
1235   this transformation is that it makes many other loop optimizations, such as
1236   LoopUnswitching, simpler.
1237   </p>
1238 </div>
1239
1240 <!-------------------------------------------------------------------------- -->
1241 <div class="doc_subsection">
1242   <a name="licm">-licm: Loop Invariant Code Motion</a>
1243 </div>
1244 <div class="doc_text">
1245   <p>
1246   This pass performs loop invariant code motion, attempting to remove as much
1247   code from the body of a loop as possible.  It does this by either hoisting
1248   code into the preheader block, or by sinking code to the exit blocks if it is
1249   safe.  This pass also promotes must-aliased memory locations in the loop to
1250   live in registers, thus hoisting and sinking "invariant" loads and stores.
1251   </p>
1252
1253   <p>
1254   This pass uses alias analysis for two purposes:
1255   </p>
1256
1257   <ul>
1258     <li>Moving loop invariant loads and calls out of loops.  If we can determine
1259         that a load or call inside of a loop never aliases anything stored to,
1260         we can hoist it or sink it like any other instruction.</li>
1261     <li>Scalar Promotion of Memory - If there is a store instruction inside of
1262         the loop, we try to move the store to happen AFTER the loop instead of
1263         inside of the loop.  This can only happen if a few conditions are true:
1264         <ul>
1265           <li>The pointer stored through is loop invariant.</li>
1266           <li>There are no stores or loads in the loop which <em>may</em> alias
1267               the pointer.  There are no calls in the loop which mod/ref the
1268               pointer.</li>
1269         </ul>
1270         If these conditions are true, we can promote the loads and stores in the
1271         loop of the pointer to use a temporary alloca'd variable.  We then use
1272         the mem2reg functionality to construct the appropriate SSA form for the
1273         variable.</li>
1274   </ul>
1275 </div>
1276
1277 <!-------------------------------------------------------------------------- -->
1278 <div class="doc_subsection">
1279   <a name="loop-deletion">-loop-deletion: Delete dead loops</a>
1280 </div>
1281 <div class="doc_text">
1282   <p>
1283   This file implements the Dead Loop Deletion Pass.  This pass is responsible
1284   for eliminating loops with non-infinite computable trip counts that have no
1285   side effects or volatile instructions, and do not contribute to the
1286   computation of the function's return value.
1287   </p>
1288 </div>
1289
1290 <!-------------------------------------------------------------------------- -->
1291 <div class="doc_subsection">
1292   <a name="loop-extract">-loop-extract: Extract loops into new functions</a>
1293 </div>
1294 <div class="doc_text">
1295   <p>
1296   A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to
1297   extract each top-level loop into its own new function. If the loop is the
1298   <em>only</em> loop in a given function, it is not touched. This is a pass most
1299   useful for debugging via bugpoint.
1300   </p>
1301 </div>
1302
1303 <!-------------------------------------------------------------------------- -->
1304 <div class="doc_subsection">
1305   <a name="loop-extract-single">-loop-extract-single: Extract at most one loop into a new function</a>
1306 </div>
1307 <div class="doc_text">
1308   <p>
1309   Similar to <a href="#loop-extract">Extract loops into new functions</a>,
1310   this pass extracts one natural loop from the program into a function if it
1311   can. This is used by bugpoint.
1312   </p>
1313 </div>
1314
1315 <!-------------------------------------------------------------------------- -->
1316 <div class="doc_subsection">
1317   <a name="loop-reduce">-loop-reduce: Loop Strength Reduction</a>
1318 </div>
1319 <div class="doc_text">
1320   <p>
1321   This pass performs a strength reduction on array references inside loops that
1322   have as one or more of their components the loop induction variable.  This is
1323   accomplished by creating a new value to hold the initial value of the array
1324   access for the first iteration, and then creating a new GEP instruction in
1325   the loop to increment the value by the appropriate amount.
1326   </p>
1327 </div>
1328
1329 <!-------------------------------------------------------------------------- -->
1330 <div class="doc_subsection">
1331   <a name="loop-rotate">-loop-rotate: Rotate Loops</a>
1332 </div>
1333 <div class="doc_text">
1334   <p>A simple loop rotation transformation.</p>
1335 </div>
1336
1337 <!-------------------------------------------------------------------------- -->
1338 <div class="doc_subsection">
1339   <a name="loop-simplify">-loop-simplify: Canonicalize natural loops</a>
1340 </div>
1341 <div class="doc_text">
1342   <p>
1343   This pass performs several transformations to transform natural loops into a
1344   simpler form, which makes subsequent analyses and transformations simpler and
1345   more effective.
1346   </p>
1347
1348   <p>
1349   Loop pre-header insertion guarantees that there is a single, non-critical
1350   entry edge from outside of the loop to the loop header.  This simplifies a
1351   number of analyses and transformations, such as LICM.
1352   </p>
1353
1354   <p>
1355   Loop exit-block insertion guarantees that all exit blocks from the loop
1356   (blocks which are outside of the loop that have predecessors inside of the
1357   loop) only have predecessors from inside of the loop (and are thus dominated
1358   by the loop header).  This simplifies transformations such as store-sinking
1359   that are built into LICM.
1360   </p>
1361
1362   <p>
1363   This pass also guarantees that loops will have exactly one backedge.
1364   </p>
1365
1366   <p>
1367   Note that the simplifycfg pass will clean up blocks which are split out but
1368   end up being unnecessary, so usage of this pass should not pessimize
1369   generated code.
1370   </p>
1371
1372   <p>
1373   This pass obviously modifies the CFG, but updates loop information and
1374   dominator information.
1375   </p>
1376 </div>
1377
1378 <!-------------------------------------------------------------------------- -->
1379 <div class="doc_subsection">
1380   <a name="loop-unroll">-loop-unroll: Unroll loops</a>
1381 </div>
1382 <div class="doc_text">
1383   <p>
1384   This pass implements a simple loop unroller.  It works best when loops have
1385   been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass,
1386   allowing it to determine the trip counts of loops easily.
1387   </p>
1388 </div>
1389
1390 <!-------------------------------------------------------------------------- -->
1391 <div class="doc_subsection">
1392   <a name="loop-unswitch">-loop-unswitch: Unswitch loops</a>
1393 </div>
1394 <div class="doc_text">
1395   <p>
1396   This pass transforms loops that contain branches on loop-invariant conditions
1397   to have multiple loops.  For example, it turns the left into the right code:
1398   </p>
1399
1400   <pre
1401 >for (...)                  if (lic)
1402   A                          for (...)
1403   if (lic)                     A; B; C
1404     B                      else
1405   C                          for (...)
1406                                A; C</pre>
1407
1408   <p>
1409   This can increase the size of the code exponentially (doubling it every time
1410   a loop is unswitched) so we only unswitch if the resultant code will be
1411   smaller than a threshold.
1412   </p>
1413
1414   <p>
1415   This pass expects LICM to be run before it to hoist invariant conditions out
1416   of the loop, to make the unswitching opportunity obvious.
1417   </p>
1418 </div>
1419
1420 <!-------------------------------------------------------------------------- -->
1421 <div class="doc_subsection">
1422   <a name="loweratomic">-loweratomic: Lower atomic intrinsics to non-atomic form</a>
1423 </div>
1424 <div class="doc_text">
1425   <p>
1426   This pass lowers atomic intrinsics to non-atomic form for use in a known
1427   non-preemptible environment.
1428   </p>
1429
1430   <p>
1431   The pass does not verify that the environment is non-preemptible (in
1432   general this would require knowledge of the entire call graph of the
1433   program including any libraries which may not be available in bitcode form);
1434   it simply lowers every atomic intrinsic.
1435   </p>
1436 </div>
1437
1438 <!-------------------------------------------------------------------------- -->
1439 <div class="doc_subsection">
1440   <a name="lowerinvoke">-lowerinvoke: Lower invoke and unwind, for unwindless code generators</a>
1441 </div>
1442 <div class="doc_text">
1443   <p>
1444   This transformation is designed for use by code generators which do not yet
1445   support stack unwinding.  This pass supports two models of exception handling
1446   lowering, the 'cheap' support and the 'expensive' support.
1447   </p>
1448
1449   <p>
1450   'Cheap' exception handling support gives the program the ability to execute
1451   any program which does not "throw an exception", by turning 'invoke'
1452   instructions into calls and by turning 'unwind' instructions into calls to
1453   abort().  If the program does dynamically use the unwind instruction, the
1454   program will print a message then abort.
1455   </p>
1456
1457   <p>
1458   'Expensive' exception handling support gives the full exception handling
1459   support to the program at the cost of making the 'invoke' instruction
1460   really expensive.  It basically inserts setjmp/longjmp calls to emulate the
1461   exception handling as necessary.
1462   </p>
1463
1464   <p>
1465   Because the 'expensive' support slows down programs a lot, and EH is only
1466   used for a subset of the programs, it must be specifically enabled by the
1467   <tt>-enable-correct-eh-support</tt> option.
1468   </p>
1469
1470   <p>
1471   Note that after this pass runs the CFG is not entirely accurate (exceptional
1472   control flow edges are not correct anymore) so only very simple things should
1473   be done after the lowerinvoke pass has run (like generation of native code).
1474   This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't
1475   support the invoke instruction yet" lowering pass.
1476   </p>
1477 </div>
1478
1479 <!-------------------------------------------------------------------------- -->
1480 <div class="doc_subsection">
1481   <a name="lowersetjmp">-lowersetjmp: Lower Set Jump</a>
1482 </div>
1483 <div class="doc_text">
1484   <p>
1485    Lowers <tt>setjmp</tt> and <tt>longjmp</tt> to use the LLVM invoke and unwind
1486    instructions as necessary.
1487   </p>
1488
1489   <p>
1490    Lowering of <tt>longjmp</tt> is fairly trivial. We replace the call with a
1491    call to the LLVM library function <tt>__llvm_sjljeh_throw_longjmp()</tt>.
1492    This unwinds the stack for us calling all of the destructors for
1493    objects allocated on the stack.
1494   </p>
1495
1496   <p>
1497    At a <tt>setjmp</tt> call, the basic block is split and the <tt>setjmp</tt>
1498    removed. The calls in a function that have a <tt>setjmp</tt> are converted to
1499    invoke where the except part checks to see if it's a <tt>longjmp</tt>
1500    exception and, if so, if it's handled in the function. If it is, then it gets
1501    the value returned by the <tt>longjmp</tt> and goes to where the basic block
1502    was split. <tt>invoke</tt> instructions are handled in a similar fashion with
1503    the original except block being executed if it isn't a <tt>longjmp</tt>
1504    except that is handled by that function.
1505   </p>
1506 </div>
1507
1508 <!-------------------------------------------------------------------------- -->
1509 <div class="doc_subsection">
1510   <a name="lowerswitch">-lowerswitch: Lower SwitchInst's to branches</a>
1511 </div>
1512 <div class="doc_text">
1513   <p>
1514   Rewrites <tt>switch</tt> instructions with a sequence of branches, which
1515   allows targets to get away with not implementing the switch instruction until
1516   it is convenient.
1517   </p>
1518 </div>
1519
1520 <!-------------------------------------------------------------------------- -->
1521 <div class="doc_subsection">
1522   <a name="mem2reg">-mem2reg: Promote Memory to Register</a>
1523 </div>
1524 <div class="doc_text">
1525   <p>
1526   This file promotes memory references to be register references.  It promotes
1527   <tt>alloca</tt> instructions which only have <tt>load</tt>s and
1528   <tt>store</tt>s as uses.  An <tt>alloca</tt> is transformed by using dominator
1529   frontiers to place <tt>phi</tt> nodes, then traversing the function in
1530   depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as
1531   appropriate. This is just the standard SSA construction algorithm to construct
1532   "pruned" SSA form.
1533   </p>
1534 </div>
1535
1536 <!-------------------------------------------------------------------------- -->
1537 <div class="doc_subsection">
1538   <a name="memcpyopt">-memcpyopt: MemCpy Optimization</a>
1539 </div>
1540 <div class="doc_text">
1541   <p>
1542   This pass performs various transformations related to eliminating memcpy
1543   calls, or transforming sets of stores into memset's.
1544   </p>
1545 </div>
1546
1547 <!-------------------------------------------------------------------------- -->
1548 <div class="doc_subsection">
1549   <a name="mergefunc">-mergefunc: Merge Functions</a>
1550 </div>
1551 <div class="doc_text">
1552   <p>This pass looks for equivalent functions that are mergable and folds them.
1553
1554   A hash is computed from the function, based on its type and number of
1555   basic blocks.
1556
1557   Once all hashes are computed, we perform an expensive equality comparison
1558   on each function pair. This takes n^2/2 comparisons per bucket, so it's
1559   important that the hash function be high quality. The equality comparison
1560   iterates through each instruction in each basic block.
1561
1562   When a match is found the functions are folded. If both functions are
1563   overridable, we move the functionality into a new internal function and
1564   leave two overridable thunks to it.
1565   </p>
1566 </div>
1567
1568 <!-------------------------------------------------------------------------- -->
1569 <div class="doc_subsection">
1570   <a name="mergereturn">-mergereturn: Unify function exit nodes</a>
1571 </div>
1572 <div class="doc_text">
1573   <p>
1574   Ensure that functions have at most one <tt>ret</tt> instruction in them.
1575   Additionally, it keeps track of which node is the new exit node of the CFG.
1576   </p>
1577 </div>
1578
1579 <!-------------------------------------------------------------------------- -->
1580 <div class="doc_subsection">
1581   <a name="partial-inliner">-partial-inliner: Partial Inliner</a>
1582 </div>
1583 <div class="doc_text">
1584   <p>This pass performs partial inlining, typically by inlining an if
1585   statement that surrounds the body of the function.
1586   </p>
1587 </div>
1588
1589 <!-------------------------------------------------------------------------- -->
1590 <div class="doc_subsection">
1591   <a name="prune-eh">-prune-eh: Remove unused exception handling info</a>
1592 </div>
1593 <div class="doc_text">
1594   <p>
1595   This file implements a simple interprocedural pass which walks the call-graph,
1596   turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and
1597   only if the callee cannot throw an exception. It implements this as a
1598   bottom-up traversal of the call-graph.
1599   </p>
1600 </div>
1601
1602 <!-------------------------------------------------------------------------- -->
1603 <div class="doc_subsection">
1604   <a name="reassociate">-reassociate: Reassociate expressions</a>
1605 </div>
1606 <div class="doc_text">
1607   <p>
1608   This pass reassociates commutative expressions in an order that is designed
1609   to promote better constant propagation, GCSE, LICM, PRE, etc.
1610   </p>
1611
1612   <p>
1613   For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5)
1614   </p>
1615
1616   <p>
1617   In the implementation of this algorithm, constants are assigned rank = 0,
1618   function arguments are rank = 1, and other values are assigned ranks
1619   corresponding to the reverse post order traversal of current function
1620   (starting at 2), which effectively gives values in deep loops higher rank
1621   than values not in loops.
1622   </p>
1623 </div>
1624
1625 <!-------------------------------------------------------------------------- -->
1626 <div class="doc_subsection">
1627   <a name="reg2mem">-reg2mem: Demote all values to stack slots</a>
1628 </div>
1629 <div class="doc_text">
1630   <p>
1631   This file demotes all registers to memory references.  It is intented to be
1632   the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>.  By converting to
1633   <tt>load</tt> instructions, the only values live across basic blocks are
1634   <tt>alloca</tt> instructions and <tt>load</tt> instructions before
1635   <tt>phi</tt> nodes. It is intended that this should make CFG hacking much
1636   easier. To make later hacking easier, the entry block is split into two, such
1637   that all introduced <tt>alloca</tt> instructions (and nothing else) are in the
1638   entry block.
1639   </p>
1640 </div>
1641
1642 <!-------------------------------------------------------------------------- -->
1643 <div class="doc_subsection">
1644   <a name="scalarrepl">-scalarrepl: Scalar Replacement of Aggregates (DT)</a>
1645 </div>
1646 <div class="doc_text">
1647   <p>
1648   The well-known scalar replacement of aggregates transformation.  This
1649   transform breaks up <tt>alloca</tt> instructions of aggregate type (structure
1650   or array) into individual <tt>alloca</tt> instructions for each member if
1651   possible.  Then, if possible, it transforms the individual <tt>alloca</tt>
1652   instructions into nice clean scalar SSA form.
1653   </p>
1654
1655   <p>
1656   This combines a simple scalar replacement of aggregates algorithm with the <a
1657   href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact,
1658   especially for C++ programs.  As such, iterating between <tt>scalarrepl</tt>,
1659   then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to
1660   promote works well.
1661   </p>
1662 </div>
1663
1664 <!-------------------------------------------------------------------------- -->
1665 <div class="doc_subsection">
1666   <a name="sccp">-sccp: Sparse Conditional Constant Propagation</a>
1667 </div>
1668 <div class="doc_text">
1669   <p>
1670   Sparse conditional constant propagation and merging, which can be summarized
1671   as:
1672   </p>
1673
1674   <ol>
1675     <li>Assumes values are constant unless proven otherwise</li>
1676     <li>Assumes BasicBlocks are dead unless proven otherwise</li>
1677     <li>Proves values to be constant, and replaces them with constants</li>
1678     <li>Proves conditional branches to be unconditional</li>
1679   </ol>
1680
1681   <p>
1682   Note that this pass has a habit of making definitions be dead.  It is a good
1683   idea to to run a DCE pass sometime after running this pass.
1684   </p>
1685 </div>
1686
1687 <!-------------------------------------------------------------------------- -->
1688 <div class="doc_subsection">
1689   <a name="simplify-libcalls">-simplify-libcalls: Simplify well-known library calls</a>
1690 </div>
1691 <div class="doc_text">
1692   <p>
1693   Applies a variety of small optimizations for calls to specific well-known
1694   function calls (e.g. runtime library functions). For example, a call
1695    <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be
1696    transformed into simply <tt>return 3</tt>.
1697   </p>
1698 </div>
1699
1700 <!-------------------------------------------------------------------------- -->
1701 <div class="doc_subsection">
1702   <a name="simplifycfg">-simplifycfg: Simplify the CFG</a>
1703 </div>
1704 <div class="doc_text">
1705   <p>
1706   Performs dead code elimination and basic block merging. Specifically:
1707   </p>
1708
1709   <ol>
1710     <li>Removes basic blocks with no predecessors.</li>
1711     <li>Merges a basic block into its predecessor if there is only one and the
1712         predecessor only has one successor.</li>
1713     <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li>
1714     <li>Eliminates a basic block that only contains an unconditional
1715         branch.</li>
1716   </ol>
1717 </div>
1718
1719 <!-------------------------------------------------------------------------- -->
1720 <div class="doc_subsection">
1721   <a name="sink">-sink: Code sinking</a>
1722 </div>
1723 <div class="doc_text">
1724   <p>This pass moves instructions into successor blocks, when possible, so that
1725  they aren't executed on paths where their results aren't needed.
1726   </p>
1727 </div>
1728
1729 <!-------------------------------------------------------------------------- -->
1730 <div class="doc_subsection">
1731   <a name="sretpromotion">-sretpromotion: Promote sret arguments to multiple ret values</a>
1732 </div>
1733 <div class="doc_text">
1734   <p>
1735   This pass finds functions that return a struct (using a pointer to the struct
1736   as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and
1737   replaces them with a new function that simply returns each of the elements of
1738   that struct (using multiple return values).
1739   </p>
1740
1741   <p>
1742   This pass works under a number of conditions:
1743   </p>
1744
1745   <ul>
1746   <li>The returned struct must not contain other structs</li>
1747   <li>The returned struct must only be used to load values from</li>
1748   <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li>
1749   </ul>
1750 </div>
1751
1752 <!-------------------------------------------------------------------------- -->
1753 <div class="doc_subsection">
1754   <a name="strip">-strip: Strip all symbols from a module</a>
1755 </div>
1756 <div class="doc_text">
1757   <p>
1758   performs code stripping. this transformation can delete:
1759   </p>
1760
1761   <ol>
1762     <li>names for virtual registers</li>
1763     <li>symbols for internal globals and functions</li>
1764     <li>debug information</li>
1765   </ol>
1766
1767   <p>
1768   note that this transformation makes code much less readable, so it should
1769   only be used in situations where the <tt>strip</tt> utility would be used,
1770   such as reducing code size or making it harder to reverse engineer code.
1771   </p>
1772 </div>
1773
1774 <!-------------------------------------------------------------------------- -->
1775 <div class="doc_subsection">
1776   <a name="strip-dead-debug-info">-strip-dead-debug-info: Strip debug info for unused symbols</a>
1777 </div>
1778 <div class="doc_text">
1779   <p>
1780   performs code stripping. this transformation can delete:
1781   </p>
1782
1783   <ol>
1784     <li>names for virtual registers</li>
1785     <li>symbols for internal globals and functions</li>
1786     <li>debug information</li>
1787   </ol>
1788
1789   <p>
1790   note that this transformation makes code much less readable, so it should
1791   only be used in situations where the <tt>strip</tt> utility would be used,
1792   such as reducing code size or making it harder to reverse engineer code.
1793   </p>
1794 </div>
1795
1796 <!-------------------------------------------------------------------------- -->
1797 <div class="doc_subsection">
1798   <a name="strip-dead-prototypes">-strip-dead-prototypes: Strip Unused Function Prototypes</a>
1799 </div>
1800 <div class="doc_text">
1801   <p>
1802   This pass loops over all of the functions in the input module, looking for
1803   dead declarations and removes them. Dead declarations are declarations of
1804   functions for which no implementation is available (i.e., declarations for
1805   unused library functions).
1806   </p>
1807 </div>
1808
1809 <!-------------------------------------------------------------------------- -->
1810 <div class="doc_subsection">
1811   <a name="strip-debug-declare">-strip-debug-declare: Strip all llvm.dbg.declare intrinsics</a>
1812 </div>
1813 <div class="doc_text">
1814   <p>This pass implements code stripping. Specifically, it can delete:</p>
1815   <ul>
1816   <li>names for virtual registers</li>
1817   <li>symbols for internal globals and functions</li>
1818   <li>debug information</li>
1819   </ul>
1820   <p>
1821   Note that this transformation makes code much less readable, so it should
1822   only be used in situations where the 'strip' utility would be used, such as
1823   reducing code size or making it harder to reverse engineer code.
1824   </p>
1825 </div>
1826
1827 <!-------------------------------------------------------------------------- -->
1828 <div class="doc_subsection">
1829   <a name="strip-nondebug">-strip-nondebug: Strip all symbols, except dbg symbols, from a module</a>
1830 </div>
1831 <div class="doc_text">
1832   <p>This pass implements code stripping. Specifically, it can delete:</p>
1833   <ul>
1834   <li>names for virtual registers</li>
1835   <li>symbols for internal globals and functions</li>
1836   <li>debug information</li>
1837   </ul>
1838   <p>
1839   Note that this transformation makes code much less readable, so it should
1840   only be used in situations where the 'strip' utility would be used, such as
1841   reducing code size or making it harder to reverse engineer code.
1842   </p>
1843 </div>
1844
1845 <!-------------------------------------------------------------------------- -->
1846 <div class="doc_subsection">
1847   <a name="tailcallelim">-tailcallelim: Tail Call Elimination</a>
1848 </div>
1849 <div class="doc_text">
1850   <p>
1851   This file transforms calls of the current function (self recursion) followed
1852   by a return instruction with a branch to the entry of the function, creating
1853   a loop.  This pass also implements the following extensions to the basic
1854   algorithm:
1855   </p>
1856
1857   <ul>
1858   <li>Trivial instructions between the call and return do not prevent the
1859       transformation from taking place, though currently the analysis cannot
1860       support moving any really useful instructions (only dead ones).
1861   <li>This pass transforms functions that are prevented from being tail
1862       recursive by an associative expression to use an accumulator variable,
1863       thus compiling the typical naive factorial or <tt>fib</tt> implementation
1864       into efficient code.
1865   <li>TRE is performed if the function returns void, if the return
1866       returns the result returned by the call, or if the function returns a
1867       run-time constant on all exits from the function.  It is possible, though
1868       unlikely, that the return returns something else (like constant 0), and
1869       can still be TRE'd.  It can be TRE'd if <em>all other</em> return
1870       instructions in the function return the exact same value.
1871   <li>If it can prove that callees do not access theier caller stack frame,
1872       they are marked as eligible for tail call elimination (by the code
1873       generator).
1874   </ul>
1875 </div>
1876
1877 <!-------------------------------------------------------------------------- -->
1878 <div class="doc_subsection">
1879   <a name="tailduplicate">-tailduplicate: Tail Duplication</a>
1880 </div>
1881 <div class="doc_text">
1882   <p>
1883   This pass performs a limited form of tail duplication, intended to simplify
1884   CFGs by removing some unconditional branches.  This pass is necessary to
1885   straighten out loops created by the C front-end, but also is capable of
1886   making other code nicer.  After this pass is run, the CFG simplify pass
1887   should be run to clean up the mess.
1888   </p>
1889 </div>
1890
1891 <!-- ======================================================================= -->
1892 <div class="doc_section"> <a name="transform">Utility Passes</a></div>
1893 <div class="doc_text">
1894   <p>This section describes the LLVM Utility Passes.</p>
1895 </div>
1896
1897 <!-------------------------------------------------------------------------- -->
1898 <div class="doc_subsection">
1899   <a name="deadarghaX0r">-deadarghaX0r: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
1900 </div>
1901 <div class="doc_text">
1902   <p>
1903   Same as dead argument elimination, but deletes arguments to functions which
1904   are external.  This is only for use by <a
1905   href="Bugpoint.html">bugpoint</a>.</p>
1906 </div>
1907
1908 <!-------------------------------------------------------------------------- -->
1909 <div class="doc_subsection">
1910   <a name="extract-blocks">-extract-blocks: Extract Basic Blocks From Module (for bugpoint use)</a>
1911 </div>
1912 <div class="doc_text">
1913   <p>
1914   This pass is used by bugpoint to extract all blocks from the module into their
1915   own functions.</p>
1916 </div>
1917
1918 <!-------------------------------------------------------------------------- -->
1919 <div class="doc_subsection">
1920   <a name="instnamer">-instnamer: Assign names to anonymous instructions</a>
1921 </div>
1922 <div class="doc_text">
1923   <p>This is a little utility pass that gives instructions names, this is mostly
1924  useful when diffing the effect of an optimization because deleting an
1925  unnamed instruction can change all other instruction numbering, making the
1926  diff very noisy.
1927   </p>
1928 </div>
1929
1930 <!-------------------------------------------------------------------------- -->
1931 <div class="doc_subsection">
1932   <a name="preverify">-preverify: Preliminary module verification</a>
1933 </div>
1934 <div class="doc_text">
1935   <p>
1936   Ensures that the module is in the form required by the <a
1937   href="#verifier">Module Verifier</a> pass.
1938   </p>
1939
1940   <p>
1941   Running the verifier runs this pass automatically, so there should be no need
1942   to use it directly.
1943   </p>
1944 </div>
1945
1946 <!-------------------------------------------------------------------------- -->
1947 <div class="doc_subsection">
1948   <a name="verify">-verify: Module Verifier</a>
1949 </div>
1950 <div class="doc_text">
1951   <p>
1952   Verifies an LLVM IR code. This is useful to run after an optimization which is
1953   undergoing testing. Note that <tt>llvm-as</tt> verifies its input before
1954   emitting bitcode, and also that malformed bitcode is likely to make LLVM
1955   crash. All language front-ends are therefore encouraged to verify their output
1956   before performing optimizing transformations.
1957   </p>
1958
1959   <ul>
1960     <li>Both of a binary operator's parameters are of the same type.</li>
1961     <li>Verify that the indices of mem access instructions match other
1962         operands.</li>
1963     <li>Verify that arithmetic and other things are only performed on
1964         first-class types.  Verify that shifts and logicals only happen on
1965         integrals f.e.</li>
1966     <li>All of the constants in a switch statement are of the correct type.</li>
1967     <li>The code is in valid SSA form.</li>
1968     <li>It is illegal to put a label into any other type (like a structure) or
1969         to return one.</li>
1970     <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is
1971         invalid.</li>
1972     <li>PHI nodes must have an entry for each predecessor, with no extras.</li>
1973     <li>PHI nodes must be the first thing in a basic block, all grouped
1974         together.</li>
1975     <li>PHI nodes must have at least one entry.</li>
1976     <li>All basic blocks should only end with terminator insts, not contain
1977         them.</li>
1978     <li>The entry node to a function must not have predecessors.</li>
1979     <li>All Instructions must be embedded into a basic block.</li>
1980     <li>Functions cannot take a void-typed parameter.</li>
1981     <li>Verify that a function's argument list agrees with its declared
1982         type.</li>
1983     <li>It is illegal to specify a name for a void value.</li>
1984     <li>It is illegal to have a internal global value with no initializer.</li>
1985     <li>It is illegal to have a ret instruction that returns a value that does
1986         not agree with the function return value type.</li>
1987     <li>Function call argument types match the function prototype.</li>
1988     <li>All other things that are tested by asserts spread about the code.</li>
1989   </ul>
1990
1991   <p>
1992   Note that this does not provide full security verification (like Java), but
1993   instead just tries to ensure that code is well-formed.
1994   </p>
1995 </div>
1996
1997 <!-------------------------------------------------------------------------- -->
1998 <div class="doc_subsection">
1999   <a name="view-cfg">-view-cfg: View CFG of function</a>
2000 </div>
2001 <div class="doc_text">
2002   <p>
2003   Displays the control flow graph using the GraphViz tool.
2004   </p>
2005 </div>
2006
2007 <!-------------------------------------------------------------------------- -->
2008 <div class="doc_subsection">
2009   <a name="view-cfg-only">-view-cfg-only: View CFG of function (with no function bodies)</a>
2010 </div>
2011 <div class="doc_text">
2012   <p>
2013   Displays the control flow graph using the GraphViz tool, but omitting function
2014   bodies.
2015   </p>
2016 </div>
2017
2018 <!-------------------------------------------------------------------------- -->
2019 <div class="doc_subsection">
2020   <a name="view-dom">-view-dom: View dominance tree of function</a>
2021 </div>
2022 <div class="doc_text">
2023   <p>
2024   Displays the dominator tree using the GraphViz tool.
2025   </p>
2026 </div>
2027
2028 <!-------------------------------------------------------------------------- -->
2029 <div class="doc_subsection">
2030   <a name="view-dom-only">-view-dom-only: View dominance tree of function (with no function bodies)</a>
2031 </div>
2032 <div class="doc_text">
2033   <p>
2034   Displays the dominator tree using the GraphViz tool, but omitting function
2035   bodies.
2036   </p>
2037 </div>
2038
2039 <!-------------------------------------------------------------------------- -->
2040 <div class="doc_subsection">
2041   <a name="view-postdom">-view-postdom: View postdominance tree of function</a>
2042 </div>
2043 <div class="doc_text">
2044   <p>
2045   Displays the post dominator tree using the GraphViz tool.
2046   </p>
2047 </div>
2048
2049 <!-------------------------------------------------------------------------- -->
2050 <div class="doc_subsection">
2051   <a name="view-postdom-only">-view-postdom-only: View postdominance tree of function (with no function bodies)</a>
2052 </div>
2053 <div class="doc_text">
2054   <p>
2055   Displays the post dominator tree using the GraphViz tool, but omitting
2056   function bodies.
2057   </p>
2058 </div>
2059
2060 <!-- *********************************************************************** -->
2061
2062 <hr>
2063 <address>
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2068
2069   <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
2070   <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
2071   Last modified: $Date$
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