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  42
  43 <h1>LLVM's Analysis and Transform Passes</h1>
  44
  45 <ol>
  46   <li><a href="#intro">Introduction</a></li>
  47   <li><a href="#analyses">Analysis Passes</a>
  48   <li><a href="#transforms">Transform Passes</a></li>
  49   <li><a href="#utilities">Utility Passes</a></li>
  50 </ol>
  51
  52 <div class="doc_author">
  53   <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
  54             and Gordon Henriksen</p>
  55 </div>
  56
  57 <!-- ======================================================================= -->
  58 <h2><a name="intro">Introduction</a></h2>
  59 <div>
  60   <p>This document serves as a high level summary of the optimization features
  61   that LLVM provides. Optimizations are implemented as Passes that traverse some
  62   portion of a program to either collect information or transform the program.
  63   The table below divides the passes that LLVM provides into three categories.
  64   Analysis passes compute information that other passes can use or for debugging
  65   or program visualization purposes. Transform passes can use (or invalidate)
  66   the analysis passes. Transform passes all mutate the program in some way.
  67   Utility passes provides some utility but don't otherwise fit categorization.
  68   For example passes to extract functions to bitcode or write a module to
  69   bitcode are neither analysis nor transform passes.
  70   <p>The table below provides a quick summary of each pass and links to the more
  71   complete pass description later in the document.</p>
  72
  73 <table>
  74 <tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr>
  75 <tr><th>Option</th><th>Name</th></tr>
  76 <tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr>
  77 <tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (stateless AA impl)</td></tr>
  78 <tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr>
  79 <tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
  80 <tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
  81 <tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
  82 <tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
  83 <tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
  84 <tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
  85 <tr><td><a href="#dot-cfg-only">-dot-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr>
  86 <tr><td><a href="#dot-dom">-dot-dom</a></td><td>Print dominance tree of function to 'dot' file</td></tr>
  87 <tr><td><a href="#dot-dom-only">-dot-dom-only</a></td><td>Print dominance tree of function to 'dot' file (with no function bodies)</td></tr>
  88 <tr><td><a href="#dot-postdom">-dot-postdom</a></td><td>Print postdominance tree of function to 'dot' file</td></tr>
  89 <tr><td><a href="#dot-postdom-only">-dot-postdom-only</a></td><td>Print postdominance tree of function to 'dot' file (with no function bodies)</td></tr>
  90 <tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr>
  91 <tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr>
  92 <tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr>
  93 <tr><td><a href="#iv-users">-iv-users</a></td><td>Induction Variable Users</td></tr>
  94 <tr><td><a href="#lazy-value-info">-lazy-value-info</a></td><td>Lazy Value Information Analysis</td></tr>
  95 <tr><td><a href="#lda">-lda</a></td><td>Loop Dependence Analysis</td></tr>
  96 <tr><td><a href="#libcall-aa">-libcall-aa</a></td><td>LibCall Alias Analysis</td></tr>
  97 <tr><td><a href="#lint">-lint</a></td><td>Statically lint-checks LLVM IR</td></tr>
  98 <tr><td><a href="#loops">-loops</a></td><td>Natural Loop Information</td></tr>
  99 <tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
 100 <tr><td><a href="#module-debuginfo">-module-debuginfo</a></td><td>Decodes module-level debug info</td></tr>
 101 <tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
 102 <tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
 103 <tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr>
 104 <tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
 105 <tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
 106 <tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr>
 107 <tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr>
 108 <tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr>
 109 <tr><td><a href="#print-dbginfo">-print-dbginfo</a></td><td>Print debug info in human readable form</td></tr>
 110 <tr><td><a href="#print-dom-info">-print-dom-info</a></td><td>Dominator Info Printer</td></tr>
 111 <tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
 112 <tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr>
 113 <tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr>
 114 <tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr>
 115 <tr><td><a href="#profile-estimator">-profile-estimator</a></td><td>Estimate profiling information</td></tr>
 116 <tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr>
 117 <tr><td><a href="#profile-verifier">-profile-verifier</a></td><td>Verify profiling information</td></tr>
 118 <tr><td><a href="#regions">-regions</a></td><td>Detect single entry single exit regions</td></tr>
 119 <tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr>
 120 <tr><td><a href="#scev-aa">-scev-aa</a></td><td>ScalarEvolution-based Alias Analysis</td></tr>
 121 <tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
 122
 123
 124 <tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr>
 125 <tr><th>Option</th><th>Name</th></tr>
 126 <tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr>
 127 <tr><td><a href="#always-inline">-always-inline</a></td><td>Inliner for always_inline functions</td></tr>
 128 <tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr>
 129 <tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr>
 130 <tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr>
 131 <tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr>
 132 <tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr>
 133 <tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr>
 134 <tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr>
 135 <tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr>
 136 <tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr>
 137 <tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr>
 138 <tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr>
 139 <tr><td><a href="#functionattrs">-functionattrs</a></td><td>Deduce function attributes</td></tr>
 140 <tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr>
 141 <tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr>
 142 <tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr>
 143 <tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr>
 144 <tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr>
 145 <tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr>
 146 <tr><td><a href="#insert-optimal-edge-profiling">-insert-optimal-edge-profiling</a></td><td>Insert optimal instrumentation for edge profiling</td></tr>
 147 <tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr>
 148 <tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr>
 149 <tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr>
 150 <tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr>
 151 <tr><td><a href="#jump-threading">-jump-threading</a></td><td>Jump Threading</td></tr>
 152 <tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr>
 153 <tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr>
 154 <tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Delete dead loops</td></tr>
 155 <tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr>
 156 <tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr>
 157 <tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr>
 158 <tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr>
 159 <tr><td><a href="#loop-simplify">-loop-simplify</a></td><td>Canonicalize natural loops</td></tr>
 160 <tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr>
 161 <tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr>
 162 <tr><td><a href="#loweratomic">-loweratomic</a></td><td>Lower atomic intrinsics to non-atomic form</td></tr>
 163 <tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr>
 164 <tr><td><a href="#lowersetjmp">-lowersetjmp</a></td><td>Lower Set Jump</td></tr>
 165 <tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr>
 166 <tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr>
 167 <tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>MemCpy Optimization</td></tr>
 168 <tr><td><a href="#mergefunc">-mergefunc</a></td><td>Merge Functions</td></tr>
 169 <tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr>
 170 <tr><td><a href="#partial-inliner">-partial-inliner</a></td><td>Partial Inliner</td></tr>
 171 <tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr>
 172 <tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr>
 173 <tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr>
 174 <tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates (DT)</td></tr>
 175 <tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr>
 176 <tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr>
 177 <tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr>
 178 <tr><td><a href="#sink">-sink</a></td><td>Code sinking</td></tr>
 179 <tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments to multiple ret values</td></tr>
 180 <tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr>
 181 <tr><td><a href="#strip-dead-debug-info">-strip-dead-debug-info</a></td><td>Strip debug info for unused symbols</td></tr>
 182 <tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Strip Unused Function Prototypes</td></tr>
 183 <tr><td><a href="#strip-debug-declare">-strip-debug-declare</a></td><td>Strip all llvm.dbg.declare intrinsics</td></tr>
 184 <tr><td><a href="#strip-nondebug">-strip-nondebug</a></td><td>Strip all symbols, except dbg symbols, from a module</td></tr>
 185 <tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr>
 186 <tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr>
 187
 188
 189 <tr><th colspan="2"><b>UTILITY PASSES</b></th></tr>
 190 <tr><th>Option</th><th>Name</th></tr>
 191 <tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr>
 192 <tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr>
 193 <tr><td><a href="#instnamer">-instnamer</a></td><td>Assign names to anonymous instructions</td></tr>
 194 <tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr>
 195 <tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr>
 196 <tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr>
 197 <tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr>
 198 <tr><td><a href="#view-dom">-view-dom</a></td><td>View dominance tree of function</td></tr>
 199 <tr><td><a href="#view-dom-only">-view-dom-only</a></td><td>View dominance tree of function (with no function bodies)</td></tr>
 200 <tr><td><a href="#view-postdom">-view-postdom</a></td><td>View postdominance tree of function</td></tr>
 201 <tr><td><a href="#view-postdom-only">-view-postdom-only</a></td><td>View postdominance tree of function (with no function bodies)</td></tr>
 202 </table>
 203
 204 </div>
 205
 206 <!-- ======================================================================= -->
 207 <h2><a name="analyses">Analysis Passes</a></h2>
 208 <div>
 209   <p>This section describes the LLVM Analysis Passes.</p>
 210
 211 <!-------------------------------------------------------------------------- -->
 212 <h3>
 213   <a name="aa-eval">-aa-eval: Exhaustive Alias Analysis Precision Evaluator</a>
 214 </h3>
 215 <div>
 216   <p>This is a simple N^2 alias analysis accuracy evaluator.
 217   Basically, for each function in the program, it simply queries to see how the
 218   alias analysis implementation answers alias queries between each pair of
 219   pointers in the function.</p>
 220
 221   <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco
 222   Spadini, and Wojciech Stryjewski.</p>
 223 </div>
 224
 225 <!-------------------------------------------------------------------------- -->
 226 <h3>
 227   <a name="basicaa">-basicaa: Basic Alias Analysis (stateless AA impl)</a>
 228 </h3>
 229 <div>
 230   <p>
 231   This is the default implementation of the Alias Analysis interface
 232   that simply implements a few identities (two different globals cannot alias,
 233   etc), but otherwise does no analysis.
 234   </p>
 235 </div>
 236
 237 <!-------------------------------------------------------------------------- -->
 238 <h3>
 239   <a name="basiccg">-basiccg: Basic CallGraph Construction</a>
 240 </h3>
 241 <div>
 242   <p>Yet to be written.</p>
 243 </div>
 244
 245 <!-------------------------------------------------------------------------- -->
 246 <h3>
 247   <a name="count-aa">-count-aa: Count Alias Analysis Query Responses</a>
 248 </h3>
 249 <div>
 250   <p>
 251   A pass which can be used to count how many alias queries
 252   are being made and how the alias analysis implementation being used responds.
 253   </p>
 254 </div>
 255
 256 <!-------------------------------------------------------------------------- -->
 257 <h3>
 258   <a name="debug-aa">-debug-aa: AA use debugger</a>
 259 </h3>
 260 <div>
 261   <p>
 262   This simple pass checks alias analysis users to ensure that if they
 263   create a new value, they do not query AA without informing it of the value.
 264   It acts as a shim over any other AA pass you want.
 265   </p>
 266
 267   <p>
 268   Yes keeping track of every value in the program is expensive, but this is
 269   a debugging pass.
 270   </p>
 271 </div>
 272
 273 <!-------------------------------------------------------------------------- -->
 274 <h3>
 275   <a name="domfrontier">-domfrontier: Dominance Frontier Construction</a>
 276 </h3>
 277 <div>
 278   <p>
 279   This pass is a simple dominator construction algorithm for finding forward
 280   dominator frontiers.
 281   </p>
 282 </div>
 283
 284 <!-------------------------------------------------------------------------- -->
 285 <h3>
 286   <a name="domtree">-domtree: Dominator Tree Construction</a>
 287 </h3>
 288 <div>
 289   <p>
 290   This pass is a simple dominator construction algorithm for finding forward
 291   dominators.
 292   </p>
 293 </div>
 294
 295 <!-------------------------------------------------------------------------- -->
 296 <h3>
 297   <a name="dot-callgraph">-dot-callgraph: Print Call Graph to 'dot' file</a>
 298 </h3>
 299 <div>
 300   <p>
 301   This pass, only available in <code>opt</code>, prints the call graph into a
 302   <code>.dot</code> graph.  This graph can then be processed with the "dot" tool
 303   to convert it to postscript or some other suitable format.
 304   </p>
 305 </div>
 306
 307 <!-------------------------------------------------------------------------- -->
 308 <h3>
 309   <a name="dot-cfg">-dot-cfg: Print CFG of function to 'dot' file</a>
 310 </h3>
 311 <div>
 312   <p>
 313   This pass, only available in <code>opt</code>, prints the control flow graph
 314   into a <code>.dot</code> graph.  This graph can then be processed with the
 315   "dot" tool to convert it to postscript or some other suitable format.
 316   </p>
 317 </div>
 318
 319 <!-------------------------------------------------------------------------- -->
 320 <h3>
 321   <a name="dot-cfg-only">-dot-cfg-only: Print CFG of function to 'dot' file (with no function bodies)</a>
 322 </h3>
 323 <div>
 324   <p>
 325   This pass, only available in <code>opt</code>, prints the control flow graph
 326   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 327   then be processed with the "dot" tool to convert it to postscript or some
 328   other suitable format.
 329   </p>
 330 </div>
 331
 332 <!-------------------------------------------------------------------------- -->
 333 <h3>
 334   <a name="dot-dom">-dot-dom: Print dominance tree of function to 'dot' file</a>
 335 </h3>
 336 <div>
 337   <p>
 338   This pass, only available in <code>opt</code>, prints the dominator tree
 339   into a <code>.dot</code> graph.  This graph can then be processed with the
 340   "dot" tool to convert it to postscript or some other suitable format.
 341   </p>
 342 </div>
 343
 344 <!-------------------------------------------------------------------------- -->
 345 <h3>
 346   <a name="dot-dom-only">-dot-dom-only: Print dominance tree of function to 'dot' file (with no function bodies)</a>
 347 </h3>
 348 <div>
 349   <p>
 350   This pass, only available in <code>opt</code>, prints the dominator tree
 351   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 352   then be processed with the "dot" tool to convert it to postscript or some
 353   other suitable format.
 354   </p>
 355 </div>
 356
 357 <!-------------------------------------------------------------------------- -->
 358 <h3>
 359   <a name="dot-postdom">-dot-postdom: Print postdominance tree of function to 'dot' file</a>
 360 </h3>
 361 <div>
 362   <p>
 363   This pass, only available in <code>opt</code>, prints the post dominator tree
 364   into a <code>.dot</code> graph.  This graph can then be processed with the
 365   "dot" tool to convert it to postscript or some other suitable format.
 366   </p>
 367 </div>
 368
 369 <!-------------------------------------------------------------------------- -->
 370 <h3>
 371   <a name="dot-postdom-only">-dot-postdom-only: Print postdominance tree of function to 'dot' file (with no function bodies)</a>
 372 </h3>
 373 <div>
 374   <p>
 375   This pass, only available in <code>opt</code>, prints the post dominator tree
 376   into a <code>.dot</code> graph, omitting the function bodies.  This graph can
 377   then be processed with the "dot" tool to convert it to postscript or some
 378   other suitable format.
 379   </p>
 380 </div>
 381
 382 <!-------------------------------------------------------------------------- -->
 383 <h3>
 384   <a name="globalsmodref-aa">-globalsmodref-aa: Simple mod/ref analysis for globals</a>
 385 </h3>
 386 <div>
 387   <p>
 388   This simple pass provides alias and mod/ref information for global values
 389   that do not have their address taken, and keeps track of whether functions
 390   read or write memory (are "pure").  For this simple (but very common) case,
 391   we can provide pretty accurate and useful information.
 392   </p>
 393 </div>
 394
 395 <!-------------------------------------------------------------------------- -->
 396 <h3>
 397   <a name="instcount">-instcount: Counts the various types of Instructions</a>
 398 </h3>
 399 <div>
 400   <p>
 401   This pass collects the count of all instructions and reports them
 402   </p>
 403 </div>
 404
 405 <!-------------------------------------------------------------------------- -->
 406 <h3>
 407   <a name="intervals">-intervals: Interval Partition Construction</a>
 408 </h3>
 409 <div>
 410   <p>
 411   This analysis calculates and represents the interval partition of a function,
 412   or a preexisting interval partition.
 413   </p>
 414
 415   <p>
 416   In this way, the interval partition may be used to reduce a flow graph down
 417   to its degenerate single node interval partition (unless it is irreducible).
 418   </p>
 419 </div>
 420
 421 <!-------------------------------------------------------------------------- -->
 422 <h3>
 423   <a name="iv-users">-iv-users: Induction Variable Users</a>
 424 </h3>
 425 <div>
 426   <p>Bookkeeping for "interesting" users of expressions computed from
 427   induction variables.</p>
 428 </div>
 429
 430 <!-------------------------------------------------------------------------- -->
 431 <h3>
 432   <a name="lazy-value-info">-lazy-value-info: Lazy Value Information Analysis</a>
 433 </h3>
 434 <div>
 435   <p>Interface for lazy computation of value constraint information.</p>
 436 </div>
 437
 438 <!-------------------------------------------------------------------------- -->
 439 <h3>
 440   <a name="lda">-lda: Loop Dependence Analysis</a>
 441 </h3>
 442 <div>
 443   <p>Loop dependence analysis framework, which is used to detect dependences in
 444   memory accesses in loops.</p>
 445 </div>
 446
 447 <!-------------------------------------------------------------------------- -->
 448 <h3>
 449   <a name="libcall-aa">-libcall-aa: LibCall Alias Analysis</a>
 450 </h3>
 451 <div>
 452   <p>LibCall Alias Analysis.</p>
 453 </div>
 454
 455 <!-------------------------------------------------------------------------- -->
 456 <h3>
 457   <a name="lint">-lint: Statically lint-checks LLVM IR</a>
 458 </h3>
 459 <div>
 460   <p>This pass statically checks for common and easily-identified constructs
 461   which produce undefined or likely unintended behavior in LLVM IR.</p>
 462
 463   <p>It is not a guarantee of correctness, in two ways. First, it isn't
 464   comprehensive. There are checks which could be done statically which are
 465   not yet implemented. Some of these are indicated by TODO comments, but
 466   those aren't comprehensive either. Second, many conditions cannot be
 467   checked statically. This pass does no dynamic instrumentation, so it
 468   can't check for all possible problems.</p>
 469
 470   <p>Another limitation is that it assumes all code will be executed. A store
 471   through a null pointer in a basic block which is never reached is harmless,
 472   but this pass will warn about it anyway.</p>
 473
 474   <p>Optimization passes may make conditions that this pass checks for more or
 475   less obvious. If an optimization pass appears to be introducing a warning,
 476   it may be that the optimization pass is merely exposing an existing
 477   condition in the code.</p>
 478
 479   <p>This code may be run before instcombine. In many cases, instcombine checks
 480   for the same kinds of things and turns instructions with undefined behavior
 481   into unreachable (or equivalent). Because of this, this pass makes some
 482   effort to look through bitcasts and so on.
 483   </p>
 484 </div>
 485
 486 <!-------------------------------------------------------------------------- -->
 487 <h3>
 488   <a name="loops">-loops: Natural Loop Information</a>
 489 </h3>
 490 <div>
 491   <p>
 492   This analysis is used to identify natural loops and determine the loop depth
 493   of various nodes of the CFG.  Note that the loops identified may actually be
 494   several natural loops that share the same header node... not just a single
 495   natural loop.
 496   </p>
 497 </div>
 498
 499 <!-------------------------------------------------------------------------- -->
 500 <h3>
 501   <a name="memdep">-memdep: Memory Dependence Analysis</a>
 502 </h3>
 503 <div>
 504   <p>
 505   An analysis that determines, for a given memory operation, what preceding
 506   memory operations it depends on.  It builds on alias analysis information, and
 507   tries to provide a lazy, caching interface to a common kind of alias
 508   information query.
 509   </p>
 510 </div>
 511
 512 <!-------------------------------------------------------------------------- -->
 513 <h3>
 514   <a name="module-debuginfo">-module-debuginfo: Decodes module-level debug info</a>
 515 </h3>
 516 <div>
 517   <p>This pass decodes the debug info metadata in a module and prints in a
 518  (sufficiently-prepared-) human-readable form.
 519
 520  For example, run this pass from opt along with the -analyze option, and
 521  it'll print to standard output.
 522   </p>
 523 </div>
 524
 525 <!-------------------------------------------------------------------------- -->
 526 <h3>
 527   <a name="no-aa">-no-aa: No Alias Analysis (always returns 'may' alias)</a>
 528 </h3>
 529 <div>
 530   <p>
 531   Always returns "I don't know" for alias queries.  NoAA is unlike other alias
 532   analysis implementations, in that it does not chain to a previous analysis. As
 533   such it doesn't follow many of the rules that other alias analyses must.
 534   </p>
 535 </div>
 536
 537 <!-------------------------------------------------------------------------- -->
 538 <h3>
 539   <a name="no-profile">-no-profile: No Profile Information</a>
 540 </h3>
 541 <div>
 542   <p>
 543   The default "no profile" implementation of the abstract
 544   <code>ProfileInfo</code> interface.
 545   </p>
 546 </div>
 547
 548 <!-------------------------------------------------------------------------- -->
 549 <h3>
 550   <a name="postdomfrontier">-postdomfrontier: Post-Dominance Frontier Construction</a>
 551 </h3>
 552 <div>
 553   <p>
 554   This pass is a simple post-dominator construction algorithm for finding
 555   post-dominator frontiers.
 556   </p>
 557 </div>
 558
 559 <!-------------------------------------------------------------------------- -->
 560 <h3>
 561   <a name="postdomtree">-postdomtree: Post-Dominator Tree Construction</a>
 562 </h3>
 563 <div>
 564   <p>
 565   This pass is a simple post-dominator construction algorithm for finding
 566   post-dominators.
 567   </p>
 568 </div>
 569
 570 <!-------------------------------------------------------------------------- -->
 571 <h3>
 572   <a name="print-alias-sets">-print-alias-sets: Alias Set Printer</a>
 573 </h3>
 574 <div>
 575   <p>Yet to be written.</p>
 576 </div>
 577
 578 <!-------------------------------------------------------------------------- -->
 579 <h3>
 580   <a name="print-callgraph">-print-callgraph: Print a call graph</a>
 581 </h3>
 582 <div>
 583   <p>
 584   This pass, only available in <code>opt</code>, prints the call graph to
 585   standard error in a human-readable form.
 586   </p>
 587 </div>
 588
 589 <!-------------------------------------------------------------------------- -->
 590 <h3>
 591   <a name="print-callgraph-sccs">-print-callgraph-sccs: Print SCCs of the Call Graph</a>
 592 </h3>
 593 <div>
 594   <p>
 595   This pass, only available in <code>opt</code>, prints the SCCs of the call
 596   graph to standard error in a human-readable form.
 597   </p>
 598 </div>
 599
 600 <!-------------------------------------------------------------------------- -->
 601 <h3>
 602   <a name="print-cfg-sccs">-print-cfg-sccs: Print SCCs of each function CFG</a>
 603 </h3>
 604 <div>
 605   <p>
 606   This pass, only available in <code>opt</code>, prints the SCCs of each
 607   function CFG to standard error in a human-readable form.
 608   </p>
 609 </div>
 610
 611 <!-------------------------------------------------------------------------- -->
 612 <h3>
 613   <a name="print-dbginfo">-print-dbginfo: Print debug info in human readable form</a>
 614 </h3>
 615 <div>
 616   <p>Pass that prints instructions, and associated debug info:</p>
 617   <ul>
 618
 619   <li>source/line/col information</li>
 620   <li>original variable name</li>
 621   <li>original type name</li>
 622   </ul>
 623 </div>
 624
 625 <!-------------------------------------------------------------------------- -->
 626 <h3>
 627   <a name="print-dom-info">-print-dom-info: Dominator Info Printer</a>
 628 </h3>
 629 <div>
 630   <p>Dominator Info Printer.</p>
 631 </div>
 632
 633 <!-------------------------------------------------------------------------- -->
 634 <h3>
 635   <a name="print-externalfnconstants">-print-externalfnconstants: Print external fn callsites passed constants</a>
 636 </h3>
 637 <div>
 638   <p>
 639   This pass, only available in <code>opt</code>, prints out call sites to
 640   external functions that are called with constant arguments.  This can be
 641   useful when looking for standard library functions we should constant fold
 642   or handle in alias analyses.
 643   </p>
 644 </div>
 645
 646 <!-------------------------------------------------------------------------- -->
 647 <h3>
 648   <a name="print-function">-print-function: Print function to stderr</a>
 649 </h3>
 650 <div>
 651   <p>
 652   The <code>PrintFunctionPass</code> class is designed to be pipelined with
 653   other <code>FunctionPass</code>es, and prints out the functions of the module
 654   as they are processed.
 655   </p>
 656 </div>
 657
 658 <!-------------------------------------------------------------------------- -->
 659 <h3>
 660   <a name="print-module">-print-module: Print module to stderr</a>
 661 </h3>
 662 <div>
 663   <p>
 664   This pass simply prints out the entire module when it is executed.
 665   </p>
 666 </div>
 667
 668 <!-------------------------------------------------------------------------- -->
 669 <h3>
 670   <a name="print-used-types">-print-used-types: Find Used Types</a>
 671 </h3>
 672 <div>
 673   <p>
 674   This pass is used to seek out all of the types in use by the program.  Note
 675   that this analysis explicitly does not include types only used by the symbol
 676   table.
 677 </div>
 678
 679 <!-------------------------------------------------------------------------- -->
 680 <h3>
 681   <a name="profile-estimator">-profile-estimator: Estimate profiling information</a>
 682 </h3>
 683 <div>
 684   <p>Profiling information that estimates the profiling information
 685   in a very crude and unimaginative way.
 686   </p>
 687 </div>
 688
 689 <!-------------------------------------------------------------------------- -->
 690 <h3>
 691   <a name="profile-loader">-profile-loader: Load profile information from llvmprof.out</a>
 692 </h3>
 693 <div>
 694   <p>
 695   A concrete implementation of profiling information that loads the information
 696   from a profile dump file.
 697   </p>
 698 </div>
 699
 700 <!-------------------------------------------------------------------------- -->
 701 <h3>
 702   <a name="profile-verifier">-profile-verifier: Verify profiling information</a>
 703 </h3>
 704 <div>
 705   <p>Pass that checks profiling information for plausibility.</p>
 706 </div>
 707 <h3>
 708   <a name="regions">-regions: Detect single entry single exit regions</a>
 709 </h3>
 710 <div>
 711   <p>
 712   The <code>RegionInfo</code> pass detects single entry single exit regions in a
 713   function, where a region is defined as any subgraph that is connected to the
 714   remaining graph at only two spots. Furthermore, an hierarchical region tree is
 715   built.
 716   </p>
 717 </div>
 718
 719 <!-------------------------------------------------------------------------- -->
 720 <h3>
 721   <a name="scalar-evolution">-scalar-evolution: Scalar Evolution Analysis</a>
 722 </h3>
 723 <div>
 724   <p>
 725   The <code>ScalarEvolution</code> analysis can be used to analyze and
 726   catagorize scalar expressions in loops.  It specializes in recognizing general
 727   induction variables, representing them with the abstract and opaque
 728   <code>SCEV</code> class.  Given this analysis, trip counts of loops and other
 729   important properties can be obtained.
 730   </p>
 731
 732   <p>
 733   This analysis is primarily useful for induction variable substitution and
 734   strength reduction.
 735   </p>
 736 </div>
 737
 738 <!-------------------------------------------------------------------------- -->
 739 <h3>
 740   <a name="scev-aa">-scev-aa: ScalarEvolution-based Alias Analysis</a>
 741 </h3>
 742 <div>
 743   <p>Simple alias analysis implemented in terms of ScalarEvolution queries.
 744
 745   This differs from traditional loop dependence analysis in that it tests
 746   for dependencies within a single iteration of a loop, rather than
 747   dependencies between different iterations.
 748
 749   ScalarEvolution has a more complete understanding of pointer arithmetic
 750   than BasicAliasAnalysis' collection of ad-hoc analyses.
 751   </p>
 752 </div>
 753
 754 <!-------------------------------------------------------------------------- -->
 755 <h3>
 756   <a name="targetdata">-targetdata: Target Data Layout</a>
 757 </h3>
 758 <div>
 759   <p>Provides other passes access to information on how the size and alignment
 760   required by the the target ABI for various data types.</p>
 761 </div>
 762
 763 </div>
 764
 765 <!-- ======================================================================= -->
 766 <h2><a name="transforms">Transform Passes</a></h2>
 767 <div>
 768   <p>This section describes the LLVM Transform Passes.</p>
 769
 770 <!-------------------------------------------------------------------------- -->
 771 <h3>
 772   <a name="adce">-adce: Aggressive Dead Code Elimination</a>
 773 </h3>
 774 <div>
 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 <h3>
 783   <a name="always-inline">-always-inline: Inliner for always_inline functions</a>
 784 </h3>
 785 <div>
 786   <p>A custom inliner that handles only functions that are marked as
 787   "always inline".</p>
 788 </div>
 789
 790 <!-------------------------------------------------------------------------- -->
 791 <h3>
 792   <a name="argpromotion">-argpromotion: Promote 'by reference' arguments to scalars</a>
 793 </h3>
 794 <div>
 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 <h3>
 823   <a name="block-placement">-block-placement: Profile Guided Basic Block Placement</a>
 824 </h3>
 825 <div>
 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 <h3>
 835   <a name="break-crit-edges">-break-crit-edges: Break critical edges in CFG</a>
 836 </h3>
 837 <div>
 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 <h3>
 848   <a name="codegenprepare">-codegenprepare: Optimize for code generation</a>
 849 </h3>
 850 <div>
 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 <h3>
 858   <a name="constmerge">-constmerge: Merge Duplicate Global Constants</a>
 859 </h3>
 860 <div>
 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 <h3>
 871   <a name="constprop">-constprop: Simple constant propagation</a>
 872 </h3>
 873 <div>
 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 <h3>
 887   <a name="dce">-dce: Dead Code Elimination</a>
 888 </h3>
 889 <div>
 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 <h3>
 899   <a name="deadargelim">-deadargelim: Dead Argument Elimination</a>
 900 </h3>
 901 <div>
 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 <h3>
 917   <a name="deadtypeelim">-deadtypeelim: Dead Type Elimination</a>
 918 </h3>
 919 <div>
 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 <h3>
 929   <a name="die">-die: Dead Instruction Elimination</a>
 930 </h3>
 931 <div>
 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 <h3>
 940   <a name="dse">-dse: Dead Store Elimination</a>
 941 </h3>
 942 <div>
 943   <p>
 944   A trivial dead store elimination that only considers basic-block local
 945   redundant stores.
 946   </p>
 947 </div>
 948
 949 <!-------------------------------------------------------------------------- -->
 950 <h3>
 951   <a name="functionattrs">-functionattrs: Deduce function attributes</a>
 952 </h3>
 953 <div>
 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 <h3>
 966   <a name="globaldce">-globaldce: Dead Global Elimination</a>
 967 </h3>
 968 <div>
 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 <h3>
 980   <a name="globalopt">-globalopt: Global Variable Optimizer</a>
 981 </h3>
 982 <div>
 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 <h3>
 992   <a name="gvn">-gvn: Global Value Numbering</a>
 993 </h3>
 994 <div>
 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 <h3>
1003   <a name="indvars">-indvars: Canonicalize Induction Variables</a>
1004 </h3>
1005 <div>
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 <h3>
1054   <a name="inline">-inline: Function Integration/Inlining</a>
1055 </h3>
1056 <div>
1057   <p>
1058   Bottom-up inlining of functions into callees.
1059   </p>
1060 </div>
1061
1062 <!-------------------------------------------------------------------------- -->
1063 <h3>
1064   <a name="insert-edge-profiling">-insert-edge-profiling: Insert instrumentation for edge profiling</a>
1065 </h3>
1066 <div>
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 <h3>
1082   <a name="insert-optimal-edge-profiling">-insert-optimal-edge-profiling: Insert optimal instrumentation for edge profiling</a>
1083 </h3>
1084 <div>
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 <h3>
1093   <a name="instcombine">-instcombine: Combine redundant instructions</a>
1094 </h3>
1095 <div>
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 <h3>
1147   <a name="internalize">-internalize: Internalize Global Symbols</a>
1148 </h3>
1149 <div>
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 <h3>
1159   <a name="ipconstprop">-ipconstprop: Interprocedural constant propagation</a>
1160 </h3>
1161 <div>
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 <h3>
1173   <a name="ipsccp">-ipsccp: Interprocedural Sparse Conditional Constant Propagation</a>
1174 </h3>
1175 <div>
1176   <p>
1177   An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
1178   Propagation</a>.
1179   </p>
1180 </div>
1181
1182 <!-------------------------------------------------------------------------- -->
1183 <h3>
1184   <a name="jump-threading">-jump-threading: Jump Threading</a>
1185 </h3>
1186 <div>
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 <h3>
1213   <a name="lcssa">-lcssa: Loop-Closed SSA Form Pass</a>
1214 </h3>
1215 <div>
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 <h3>
1242   <a name="licm">-licm: Loop Invariant Code Motion</a>
1243 </h3>
1244 <div>
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 <h3>
1279   <a name="loop-deletion">-loop-deletion: Delete dead loops</a>
1280 </h3>
1281 <div>
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 <h3>
1292   <a name="loop-extract">-loop-extract: Extract loops into new functions</a>
1293 </h3>
1294 <div>
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 <h3>
1305   <a name="loop-extract-single">-loop-extract-single: Extract at most one loop into a new function</a>
1306 </h3>
1307 <div>
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 <h3>
1317   <a name="loop-reduce">-loop-reduce: Loop Strength Reduction</a>
1318 </h3>
1319 <div>
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 <h3>
1331   <a name="loop-rotate">-loop-rotate: Rotate Loops</a>
1332 </h3>
1333 <div>
1334   <p>A simple loop rotation transformation.</p>
1335 </div>
1336
1337 <!-------------------------------------------------------------------------- -->
1338 <h3>
1339   <a name="loop-simplify">-loop-simplify: Canonicalize natural loops</a>
1340 </h3>
1341 <div>
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 <h3>
1380   <a name="loop-unroll">-loop-unroll: Unroll loops</a>
1381 </h3>
1382 <div>
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 <h3>
1392   <a name="loop-unswitch">-loop-unswitch: Unswitch loops</a>
1393 </h3>
1394 <div>
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 <h3>
1422   <a name="loweratomic">-loweratomic: Lower atomic intrinsics to non-atomic form</a>
1423 </h3>
1424 <div>
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 <h3>
1440   <a name="lowerinvoke">-lowerinvoke: Lower invoke and unwind, for unwindless code generators</a>
1441 </h3>
1442 <div>
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 <h3>
1481   <a name="lowersetjmp">-lowersetjmp: Lower Set Jump</a>
1482 </h3>
1483 <div>
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 <h3>
1510   <a name="lowerswitch">-lowerswitch: Lower SwitchInst's to branches</a>
1511 </h3>
1512 <div>
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 <h3>
1522   <a name="mem2reg">-mem2reg: Promote Memory to Register</a>
1523 </h3>
1524 <div>
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 <h3>
1538   <a name="memcpyopt">-memcpyopt: MemCpy Optimization</a>
1539 </h3>
1540 <div>
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 <h3>
1549   <a name="mergefunc">-mergefunc: Merge Functions</a>
1550 </h3>
1551 <div>
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 <h3>
1570   <a name="mergereturn">-mergereturn: Unify function exit nodes</a>
1571 </h3>
1572 <div>
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 <h3>
1581   <a name="partial-inliner">-partial-inliner: Partial Inliner</a>
1582 </h3>
1583 <div>
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 <h3>
1591   <a name="prune-eh">-prune-eh: Remove unused exception handling info</a>
1592 </h3>
1593 <div>
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 <h3>
1604   <a name="reassociate">-reassociate: Reassociate expressions</a>
1605 </h3>
1606 <div>
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 <h3>
1627   <a name="reg2mem">-reg2mem: Demote all values to stack slots</a>
1628 </h3>
1629 <div>
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 <h3>
1644   <a name="scalarrepl">-scalarrepl: Scalar Replacement of Aggregates (DT)</a>
1645 </h3>
1646 <div>
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 <h3>
1666   <a name="sccp">-sccp: Sparse Conditional Constant Propagation</a>
1667 </h3>
1668 <div>
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 <h3>
1689   <a name="simplify-libcalls">-simplify-libcalls: Simplify well-known library calls</a>
1690 </h3>
1691 <div>
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 <h3>
1702   <a name="simplifycfg">-simplifycfg: Simplify the CFG</a>
1703 </h3>
1704 <div>
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 <h3>
1721   <a name="sink">-sink: Code sinking</a>
1722 </h3>
1723 <div>
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 <h3>
1731   <a name="sretpromotion">-sretpromotion: Promote sret arguments to multiple ret values</a>
1732 </h3>
1733 <div>
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 <h3>
1754   <a name="strip">-strip: Strip all symbols from a module</a>
1755 </h3>
1756 <div>
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 <h3>
1776   <a name="strip-dead-debug-info">-strip-dead-debug-info: Strip debug info for unused symbols</a>
1777 </h3>
1778 <div>
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 <h3>
1798   <a name="strip-dead-prototypes">-strip-dead-prototypes: Strip Unused Function Prototypes</a>
1799 </h3>
1800 <div>
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 <h3>
1811   <a name="strip-debug-declare">-strip-debug-declare: Strip all llvm.dbg.declare intrinsics</a>
1812 </h3>
1813 <div>
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 <h3>
1829   <a name="strip-nondebug">-strip-nondebug: Strip all symbols, except dbg symbols, from a module</a>
1830 </h3>
1831 <div>
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 <h3>
1847   <a name="tailcallelim">-tailcallelim: Tail Call Elimination</a>
1848 </h3>
1849 <div>
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 <h3>
1879   <a name="tailduplicate">-tailduplicate: Tail Duplication</a>
1880 </h3>
1881 <div>
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 </div>
1892
1893 <!-- ======================================================================= -->
1894 <h2><a name="utilities">Utility Passes</a></h2>
1895 <div>
1896   <p>This section describes the LLVM Utility Passes.</p>
1897
1898 <!-------------------------------------------------------------------------- -->
1899 <h3>
1900   <a name="deadarghaX0r">-deadarghaX0r: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
1901 </h3>
1902 <div>
1903   <p>
1904   Same as dead argument elimination, but deletes arguments to functions which
1905   are external.  This is only for use by <a
1906   href="Bugpoint.html">bugpoint</a>.</p>
1907 </div>
1908
1909 <!-------------------------------------------------------------------------- -->
1910 <h3>
1911   <a name="extract-blocks">-extract-blocks: Extract Basic Blocks From Module (for bugpoint use)</a>
1912 </h3>
1913 <div>
1914   <p>
1915   This pass is used by bugpoint to extract all blocks from the module into their
1916   own functions.</p>
1917 </div>
1918
1919 <!-------------------------------------------------------------------------- -->
1920 <h3>
1921   <a name="instnamer">-instnamer: Assign names to anonymous instructions</a>
1922 </h3>
1923 <div>
1924   <p>This is a little utility pass that gives instructions names, this is mostly
1925  useful when diffing the effect of an optimization because deleting an
1926  unnamed instruction can change all other instruction numbering, making the
1927  diff very noisy.
1928   </p>
1929 </div>
1930
1931 <!-------------------------------------------------------------------------- -->
1932 <h3>
1933   <a name="preverify">-preverify: Preliminary module verification</a>
1934 </h3>
1935 <div>
1936   <p>
1937   Ensures that the module is in the form required by the <a
1938   href="#verifier">Module Verifier</a> pass.
1939   </p>
1940
1941   <p>
1942   Running the verifier runs this pass automatically, so there should be no need
1943   to use it directly.
1944   </p>
1945 </div>
1946
1947 <!-------------------------------------------------------------------------- -->
1948 <h3>
1949   <a name="verify">-verify: Module Verifier</a>
1950 </h3>
1951 <div>
1952   <p>
1953   Verifies an LLVM IR code. This is useful to run after an optimization which is
1954   undergoing testing. Note that <tt>llvm-as</tt> verifies its input before
1955   emitting bitcode, and also that malformed bitcode is likely to make LLVM
1956   crash. All language front-ends are therefore encouraged to verify their output
1957   before performing optimizing transformations.
1958   </p>
1959
1960   <ul>
1961     <li>Both of a binary operator's parameters are of the same type.</li>
1962     <li>Verify that the indices of mem access instructions match other
1963         operands.</li>
1964     <li>Verify that arithmetic and other things are only performed on
1965         first-class types.  Verify that shifts and logicals only happen on
1966         integrals f.e.</li>
1967     <li>All of the constants in a switch statement are of the correct type.</li>
1968     <li>The code is in valid SSA form.</li>
1969     <li>It is illegal to put a label into any other type (like a structure) or
1970         to return one.</li>
1971     <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is
1972         invalid.</li>
1973     <li>PHI nodes must have an entry for each predecessor, with no extras.</li>
1974     <li>PHI nodes must be the first thing in a basic block, all grouped
1975         together.</li>
1976     <li>PHI nodes must have at least one entry.</li>
1977     <li>All basic blocks should only end with terminator insts, not contain
1978         them.</li>
1979     <li>The entry node to a function must not have predecessors.</li>
1980     <li>All Instructions must be embedded into a basic block.</li>
1981     <li>Functions cannot take a void-typed parameter.</li>
1982     <li>Verify that a function's argument list agrees with its declared
1983         type.</li>
1984     <li>It is illegal to specify a name for a void value.</li>
1985     <li>It is illegal to have a internal global value with no initializer.</li>
1986     <li>It is illegal to have a ret instruction that returns a value that does
1987         not agree with the function return value type.</li>
1988     <li>Function call argument types match the function prototype.</li>
1989     <li>All other things that are tested by asserts spread about the code.</li>
1990   </ul>
1991
1992   <p>
1993   Note that this does not provide full security verification (like Java), but
1994   instead just tries to ensure that code is well-formed.
1995   </p>
1996 </div>
1997
1998 <!-------------------------------------------------------------------------- -->
1999 <h3>
2000   <a name="view-cfg">-view-cfg: View CFG of function</a>
2001 </h3>
2002 <div>
2003   <p>
2004   Displays the control flow graph using the GraphViz tool.
2005   </p>
2006 </div>
2007
2008 <!-------------------------------------------------------------------------- -->
2009 <h3>
2010   <a name="view-cfg-only">-view-cfg-only: View CFG of function (with no function bodies)</a>
2011 </h3>
2012 <div>
2013   <p>
2014   Displays the control flow graph using the GraphViz tool, but omitting function
2015   bodies.
2016   </p>
2017 </div>
2018
2019 <!-------------------------------------------------------------------------- -->
2020 <h3>
2021   <a name="view-dom">-view-dom: View dominance tree of function</a>
2022 </h3>
2023 <div>
2024   <p>
2025   Displays the dominator tree using the GraphViz tool.
2026   </p>
2027 </div>
2028
2029 <!-------------------------------------------------------------------------- -->
2030 <h3>
2031   <a name="view-dom-only">-view-dom-only: View dominance tree of function (with no function bodies)</a>
2032 </h3>
2033 <div>
2034   <p>
2035   Displays the dominator tree using the GraphViz tool, but omitting function
2036   bodies.
2037   </p>
2038 </div>
2039
2040 <!-------------------------------------------------------------------------- -->
2041 <h3>
2042   <a name="view-postdom">-view-postdom: View postdominance tree of function</a>
2043 </h3>
2044 <div>
2045   <p>
2046   Displays the post dominator tree using the GraphViz tool.
2047   </p>
2048 </div>
2049
2050 <!-------------------------------------------------------------------------- -->
2051 <h3>
2052   <a name="view-postdom-only">-view-postdom-only: View postdominance tree of function (with no function bodies)</a>
2053 </h3>
2054 <div>
2055   <p>
2056   Displays the post dominator tree using the GraphViz tool, but omitting
2057   function bodies.
2058   </p>
2059 </div>
2060
2061 </div>
2062
2063 <!-- *********************************************************************** -->
2064
2065 <hr>
2066 <address>
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2071
2072   <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
2073   <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
2074   Last modified: $Date$
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