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| author | Dmitri Gribenko <gribozavr@gmail.com> | 2012-12-11 15:29:37 +0000 |
|---|---|---|
| committer | Dmitri Gribenko <gribozavr@gmail.com> | 2012-12-11 15:29:37 +0000 |
| commit | e4b3e9445f0bbd4ecf2c5da6add13ec2423fe2ef (patch) | |
| tree | 2f77ba1478660c5412bf2c3bb4d9700b90e8e344 /docs/Passes.html | |
| parent | f38833c7adbdc58b8862835b22b0859c00e6796c (diff) | |
| download | external_llvm-e4b3e9445f0bbd4ecf2c5da6add13ec2423fe2ef.zip external_llvm-e4b3e9445f0bbd4ecf2c5da6add13ec2423fe2ef.tar.gz external_llvm-e4b3e9445f0bbd4ecf2c5da6add13ec2423fe2ef.tar.bz2 | |
Documentation: convert Passes.html to reST.
Since now we have an autogenerated TOC, a manually written table of all passes
was removed.
Patch by Anthony Mykhailenko with small fixes by me.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169867 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs/Passes.html')
| -rw-r--r-- | docs/Passes.html | 2025 |
1 files changed, 0 insertions, 2025 deletions
diff --git a/docs/Passes.html b/docs/Passes.html deleted file mode 100644 index 7bffc54..0000000 --- a/docs/Passes.html +++ /dev/null @@ -1,2025 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> -<head> - <title>LLVM's Analysis and Transform Passes</title> - <link rel="stylesheet" href="_static/llvm.css" type="text/css"> - <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> -</head> -<body> - -<!-- - -If Passes.html is up to date, the following "one-liner" should print -an empty diff. - -egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \ - -e '^ <a name=".*">.*</a>$' < Passes.html >html; \ -perl >help <<'EOT' && diff -u help html; rm -f help html -open HTML, "<Passes.html" or die "open: Passes.html: $!\n"; -while (<HTML>) { - m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next; - $order{$1} = sprintf("%03d", 1 + int %order); -} -open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n"; -while (<HELP>) { - m:^ -([^ ]+) +- (.*)$: or next; - my $o = $order{$1}; - $o = "000" unless defined $o; - push @x, "$o<tr><td><a href=\"#$1\">-$1</a></td><td>$2</td></tr>\n"; - push @y, "$o <a name=\"$1\">-$1: $2</a>\n"; -} -@x = map { s/^\d\d\d//; $_ } sort @x; -@y = map { s/^\d\d\d//; $_ } sort @y; -print @x, @y; -EOT - -This (real) one-liner can also be helpful when converting comments to HTML: - -perl -e '$/ = undef; for (split(/\n/, <>)) { s:^ *///? ?::; print " <p>\n" if !$on && $_ =~ /\S/; print " </p>\n" if $on && $_ =~ /^\s*$/; print " $_\n"; $on = ($_ =~ /\S/); } print " </p>\n" if $on' - - --> - -<h1>LLVM's Analysis and Transform Passes</h1> - -<ol> - <li><a href="#intro">Introduction</a></li> - <li><a href="#analyses">Analysis Passes</a> - <li><a href="#transforms">Transform Passes</a></li> - <li><a href="#utilities">Utility Passes</a></li> -</ol> - -<div class="doc_author"> - <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a> - and Gordon Henriksen</p> -</div> - -<!-- ======================================================================= --> -<h2><a name="intro">Introduction</a></h2> -<div> - <p>This document serves as a high level summary of the optimization features - that LLVM provides. Optimizations are implemented as Passes that traverse some - portion of a program to either collect information or transform the program. - The table below divides the passes that LLVM provides into three categories. - Analysis passes compute information that other passes can use or for debugging - or program visualization purposes. Transform passes can use (or invalidate) - the analysis passes. Transform passes all mutate the program in some way. - Utility passes provides some utility but don't otherwise fit categorization. - For example passes to extract functions to bitcode or write a module to - bitcode are neither analysis nor transform passes. - <p>The table below provides a quick summary of each pass and links to the more - complete pass description later in the document.</p> - -<table> -<tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr> -<tr><th>Option</th><th>Name</th></tr> -<tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr> -<tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (stateless AA impl)</td></tr> -<tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr> -<tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr> -<tr><td><a href="#da">-da</a></td><td>Dependence Analysis</td></tr> -<tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr> -<tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr> -<tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr> -<tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr> -<tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr> -<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> -<tr><td><a href="#dot-dom">-dot-dom</a></td><td>Print dominance tree of function to 'dot' file</td></tr> -<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> -<tr><td><a href="#dot-postdom">-dot-postdom</a></td><td>Print postdominance tree of function to 'dot' file</td></tr> -<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> -<tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr> -<tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr> -<tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr> -<tr><td><a href="#iv-users">-iv-users</a></td><td>Induction Variable Users</td></tr> -<tr><td><a href="#lazy-value-info">-lazy-value-info</a></td><td>Lazy Value Information Analysis</td></tr> -<tr><td><a href="#libcall-aa">-libcall-aa</a></td><td>LibCall Alias Analysis</td></tr> -<tr><td><a href="#lint">-lint</a></td><td>Statically lint-checks LLVM IR</td></tr> -<tr><td><a href="#loops">-loops</a></td><td>Natural Loop Information</td></tr> -<tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr> -<tr><td><a href="#module-debuginfo">-module-debuginfo</a></td><td>Decodes module-level debug info</td></tr> -<tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr> -<tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr> -<tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr> -<tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr> -<tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr> -<tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr> -<tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr> -<tr><td><a href="#print-dbginfo">-print-dbginfo</a></td><td>Print debug info in human readable form</td></tr> -<tr><td><a href="#print-dom-info">-print-dom-info</a></td><td>Dominator Info Printer</td></tr> -<tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr> -<tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr> -<tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr> -<tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr> -<tr><td><a href="#profile-estimator">-profile-estimator</a></td><td>Estimate profiling information</td></tr> -<tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr> -<tr><td><a href="#profile-verifier">-profile-verifier</a></td><td>Verify profiling information</td></tr> -<tr><td><a href="#regions">-regions</a></td><td>Detect single entry single exit regions</td></tr> -<tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr> -<tr><td><a href="#scev-aa">-scev-aa</a></td><td>ScalarEvolution-based Alias Analysis</td></tr> -<tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr> - - -<tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr> -<tr><th>Option</th><th>Name</th></tr> -<tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr> -<tr><td><a href="#always-inline">-always-inline</a></td><td>Inliner for always_inline functions</td></tr> -<tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr> -<tr><td><a href="#bb-vectorize">-bb-vectorize</a></td><td>Combine instructions to form vector instructions within basic blocks</td></tr> -<tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr> -<tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr> -<tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr> -<tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr> -<tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr> -<tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr> -<tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr> -<tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr> -<tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr> -<tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr> -<tr><td><a href="#functionattrs">-functionattrs</a></td><td>Deduce function attributes</td></tr> -<tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr> -<tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr> -<tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr> -<tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr> -<tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr> -<tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr> -<tr><td><a href="#insert-optimal-edge-profiling">-insert-optimal-edge-profiling</a></td><td>Insert optimal instrumentation for edge profiling</td></tr> -<tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr> -<tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr> -<tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr> -<tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr> -<tr><td><a href="#jump-threading">-jump-threading</a></td><td>Jump Threading</td></tr> -<tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr> -<tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr> -<tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Delete dead loops</td></tr> -<tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr> -<tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr> -<tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr> -<tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr> -<tr><td><a href="#loop-simplify">-loop-simplify</a></td><td>Canonicalize natural loops</td></tr> -<tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr> -<tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr> -<tr><td><a href="#loweratomic">-loweratomic</a></td><td>Lower atomic intrinsics to non-atomic form</td></tr> -<tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr> -<tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr> -<tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr> -<tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>MemCpy Optimization</td></tr> -<tr><td><a href="#mergefunc">-mergefunc</a></td><td>Merge Functions</td></tr> -<tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr> -<tr><td><a href="#partial-inliner">-partial-inliner</a></td><td>Partial Inliner</td></tr> -<tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr> -<tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr> -<tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr> -<tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates (DT)</td></tr> -<tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr> -<tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr> -<tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr> -<tr><td><a href="#sink">-sink</a></td><td>Code sinking</td></tr> -<tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr> -<tr><td><a href="#strip-dead-debug-info">-strip-dead-debug-info</a></td><td>Strip debug info for unused symbols</td></tr> -<tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Strip Unused Function Prototypes</td></tr> -<tr><td><a href="#strip-debug-declare">-strip-debug-declare</a></td><td>Strip all llvm.dbg.declare intrinsics</td></tr> -<tr><td><a href="#strip-nondebug">-strip-nondebug</a></td><td>Strip all symbols, except dbg symbols, from a module</td></tr> -<tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr> - - -<tr><th colspan="2"><b>UTILITY PASSES</b></th></tr> -<tr><th>Option</th><th>Name</th></tr> -<tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr> -<tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr> -<tr><td><a href="#instnamer">-instnamer</a></td><td>Assign names to anonymous instructions</td></tr> -<tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr> -<tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr> -<tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr> -<tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr> -<tr><td><a href="#view-dom">-view-dom</a></td><td>View dominance tree of function</td></tr> -<tr><td><a href="#view-dom-only">-view-dom-only</a></td><td>View dominance tree of function (with no function bodies)</td></tr> -<tr><td><a href="#view-postdom">-view-postdom</a></td><td>View postdominance tree of function</td></tr> -<tr><td><a href="#view-postdom-only">-view-postdom-only</a></td><td>View postdominance tree of function (with no function bodies)</td></tr> -</table> - -</div> - -<!-- ======================================================================= --> -<h2><a name="analyses">Analysis Passes</a></h2> -<div> - <p>This section describes the LLVM Analysis Passes.</p> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="aa-eval">-aa-eval: Exhaustive Alias Analysis Precision Evaluator</a> -</h3> -<div> - <p>This is a simple N^2 alias analysis accuracy evaluator. - Basically, for each function in the program, it simply queries to see how the - alias analysis implementation answers alias queries between each pair of - pointers in the function.</p> - - <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco - Spadini, and Wojciech Stryjewski.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="basicaa">-basicaa: Basic Alias Analysis (stateless AA impl)</a> -</h3> -<div> - <p>A basic alias analysis pass that implements identities (two different - globals cannot alias, etc), but does no stateful analysis.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="basiccg">-basiccg: Basic CallGraph Construction</a> -</h3> -<div> - <p>Yet to be written.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="count-aa">-count-aa: Count Alias Analysis Query Responses</a> -</h3> -<div> - <p> - A pass which can be used to count how many alias queries - are being made and how the alias analysis implementation being used responds. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="da">-da: Dependence Analysis</a> -</h3> -<div> - <p>Dependence analysis framework, which is used to detect dependences in - memory accesses.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="debug-aa">-debug-aa: AA use debugger</a> -</h3> -<div> - <p> - This simple pass checks alias analysis users to ensure that if they - create a new value, they do not query AA without informing it of the value. - It acts as a shim over any other AA pass you want. - </p> - - <p> - Yes keeping track of every value in the program is expensive, but this is - a debugging pass. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="domfrontier">-domfrontier: Dominance Frontier Construction</a> -</h3> -<div> - <p> - This pass is a simple dominator construction algorithm for finding forward - dominator frontiers. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="domtree">-domtree: Dominator Tree Construction</a> -</h3> -<div> - <p> - This pass is a simple dominator construction algorithm for finding forward - dominators. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dot-callgraph">-dot-callgraph: Print Call Graph to 'dot' file</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the call graph into a - <code>.dot</code> graph. This graph can then be processed with the "dot" tool - to convert it to postscript or some other suitable format. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dot-cfg">-dot-cfg: Print CFG of function to 'dot' file</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the control flow graph - into a <code>.dot</code> graph. This graph can then be processed with the - "dot" tool to convert it to postscript or some other suitable format. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dot-cfg-only">-dot-cfg-only: Print CFG of function to 'dot' file (with no function bodies)</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the control flow graph - into a <code>.dot</code> graph, omitting the function bodies. This graph can - then be processed with the "dot" tool to convert it to postscript or some - other suitable format. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dot-dom">-dot-dom: Print dominance tree of function to 'dot' file</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the dominator tree - into a <code>.dot</code> graph. This graph can then be processed with the - "dot" tool to convert it to postscript or some other suitable format. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dot-dom-only">-dot-dom-only: Print dominance tree of function to 'dot' file (with no function bodies)</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the dominator tree - into a <code>.dot</code> graph, omitting the function bodies. This graph can - then be processed with the "dot" tool to convert it to postscript or some - other suitable format. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dot-postdom">-dot-postdom: Print postdominance tree of function to 'dot' file</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the post dominator tree - into a <code>.dot</code> graph. This graph can then be processed with the - "dot" tool to convert it to postscript or some other suitable format. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dot-postdom-only">-dot-postdom-only: Print postdominance tree of function to 'dot' file (with no function bodies)</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the post dominator tree - into a <code>.dot</code> graph, omitting the function bodies. This graph can - then be processed with the "dot" tool to convert it to postscript or some - other suitable format. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="globalsmodref-aa">-globalsmodref-aa: Simple mod/ref analysis for globals</a> -</h3> -<div> - <p> - This simple pass provides alias and mod/ref information for global values - that do not have their address taken, and keeps track of whether functions - read or write memory (are "pure"). For this simple (but very common) case, - we can provide pretty accurate and useful information. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="instcount">-instcount: Counts the various types of Instructions</a> -</h3> -<div> - <p> - This pass collects the count of all instructions and reports them - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="intervals">-intervals: Interval Partition Construction</a> -</h3> -<div> - <p> - This analysis calculates and represents the interval partition of a function, - or a preexisting interval partition. - </p> - - <p> - In this way, the interval partition may be used to reduce a flow graph down - to its degenerate single node interval partition (unless it is irreducible). - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="iv-users">-iv-users: Induction Variable Users</a> -</h3> -<div> - <p>Bookkeeping for "interesting" users of expressions computed from - induction variables.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="lazy-value-info">-lazy-value-info: Lazy Value Information Analysis</a> -</h3> -<div> - <p>Interface for lazy computation of value constraint information.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="libcall-aa">-libcall-aa: LibCall Alias Analysis</a> -</h3> -<div> - <p>LibCall Alias Analysis.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="lint">-lint: Statically lint-checks LLVM IR</a> -</h3> -<div> - <p>This pass statically checks for common and easily-identified constructs - which produce undefined or likely unintended behavior in LLVM IR.</p> - - <p>It is not a guarantee of correctness, in two ways. First, it isn't - comprehensive. There are checks which could be done statically which are - not yet implemented. Some of these are indicated by TODO comments, but - those aren't comprehensive either. Second, many conditions cannot be - checked statically. This pass does no dynamic instrumentation, so it - can't check for all possible problems.</p> - - <p>Another limitation is that it assumes all code will be executed. A store - through a null pointer in a basic block which is never reached is harmless, - but this pass will warn about it anyway.</p> - - <p>Optimization passes may make conditions that this pass checks for more or - less obvious. If an optimization pass appears to be introducing a warning, - it may be that the optimization pass is merely exposing an existing - condition in the code.</p> - - <p>This code may be run before instcombine. In many cases, instcombine checks - for the same kinds of things and turns instructions with undefined behavior - into unreachable (or equivalent). Because of this, this pass makes some - effort to look through bitcasts and so on. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loops">-loops: Natural Loop Information</a> -</h3> -<div> - <p> - This analysis is used to identify natural loops and determine the loop depth - of various nodes of the CFG. Note that the loops identified may actually be - several natural loops that share the same header node... not just a single - natural loop. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="memdep">-memdep: Memory Dependence Analysis</a> -</h3> -<div> - <p> - An analysis that determines, for a given memory operation, what preceding - memory operations it depends on. It builds on alias analysis information, and - tries to provide a lazy, caching interface to a common kind of alias - information query. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="module-debuginfo">-module-debuginfo: Decodes module-level debug info</a> -</h3> -<div> - <p>This pass decodes the debug info metadata in a module and prints in a - (sufficiently-prepared-) human-readable form. - - For example, run this pass from opt along with the -analyze option, and - it'll print to standard output. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="no-aa">-no-aa: No Alias Analysis (always returns 'may' alias)</a> -</h3> -<div> - <p> - This is the default implementation of the Alias Analysis interface. It always - returns "I don't know" for alias queries. NoAA is unlike other alias analysis - implementations, in that it does not chain to a previous analysis. As such it - doesn't follow many of the rules that other alias analyses must. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="no-profile">-no-profile: No Profile Information</a> -</h3> -<div> - <p> - The default "no profile" implementation of the abstract - <code>ProfileInfo</code> interface. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="postdomfrontier">-postdomfrontier: Post-Dominance Frontier Construction</a> -</h3> -<div> - <p> - This pass is a simple post-dominator construction algorithm for finding - post-dominator frontiers. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="postdomtree">-postdomtree: Post-Dominator Tree Construction</a> -</h3> -<div> - <p> - This pass is a simple post-dominator construction algorithm for finding - post-dominators. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-alias-sets">-print-alias-sets: Alias Set Printer</a> -</h3> -<div> - <p>Yet to be written.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-callgraph">-print-callgraph: Print a call graph</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the call graph to - standard error in a human-readable form. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-callgraph-sccs">-print-callgraph-sccs: Print SCCs of the Call Graph</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the SCCs of the call - graph to standard error in a human-readable form. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-cfg-sccs">-print-cfg-sccs: Print SCCs of each function CFG</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints the SCCs of each - function CFG to standard error in a human-readable form. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-dbginfo">-print-dbginfo: Print debug info in human readable form</a> -</h3> -<div> - <p>Pass that prints instructions, and associated debug info:</p> - <ul> - - <li>source/line/col information</li> - <li>original variable name</li> - <li>original type name</li> - </ul> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-dom-info">-print-dom-info: Dominator Info Printer</a> -</h3> -<div> - <p>Dominator Info Printer.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-externalfnconstants">-print-externalfnconstants: Print external fn callsites passed constants</a> -</h3> -<div> - <p> - This pass, only available in <code>opt</code>, prints out call sites to - external functions that are called with constant arguments. This can be - useful when looking for standard library functions we should constant fold - or handle in alias analyses. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-function">-print-function: Print function to stderr</a> -</h3> -<div> - <p> - The <code>PrintFunctionPass</code> class is designed to be pipelined with - other <code>FunctionPass</code>es, and prints out the functions of the module - as they are processed. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-module">-print-module: Print module to stderr</a> -</h3> -<div> - <p> - This pass simply prints out the entire module when it is executed. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="print-used-types">-print-used-types: Find Used Types</a> -</h3> -<div> - <p> - This pass is used to seek out all of the types in use by the program. Note - that this analysis explicitly does not include types only used by the symbol - table. -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="profile-estimator">-profile-estimator: Estimate profiling information</a> -</h3> -<div> - <p>Profiling information that estimates the profiling information - in a very crude and unimaginative way. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="profile-loader">-profile-loader: Load profile information from llvmprof.out</a> -</h3> -<div> - <p> - A concrete implementation of profiling information that loads the information - from a profile dump file. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="profile-verifier">-profile-verifier: Verify profiling information</a> -</h3> -<div> - <p>Pass that checks profiling information for plausibility.</p> -</div> -<h3> - <a name="regions">-regions: Detect single entry single exit regions</a> -</h3> -<div> - <p> - The <code>RegionInfo</code> pass detects single entry single exit regions in a - function, where a region is defined as any subgraph that is connected to the - remaining graph at only two spots. Furthermore, an hierarchical region tree is - built. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="scalar-evolution">-scalar-evolution: Scalar Evolution Analysis</a> -</h3> -<div> - <p> - The <code>ScalarEvolution</code> analysis can be used to analyze and - catagorize scalar expressions in loops. It specializes in recognizing general - induction variables, representing them with the abstract and opaque - <code>SCEV</code> class. Given this analysis, trip counts of loops and other - important properties can be obtained. - </p> - - <p> - This analysis is primarily useful for induction variable substitution and - strength reduction. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="scev-aa">-scev-aa: ScalarEvolution-based Alias Analysis</a> -</h3> -<div> - <p>Simple alias analysis implemented in terms of ScalarEvolution queries. - - This differs from traditional loop dependence analysis in that it tests - for dependencies within a single iteration of a loop, rather than - dependencies between different iterations. - - ScalarEvolution has a more complete understanding of pointer arithmetic - than BasicAliasAnalysis' collection of ad-hoc analyses. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="targetdata">-targetdata: Target Data Layout</a> -</h3> -<div> - <p>Provides other passes access to information on how the size and alignment - required by the target ABI for various data types.</p> -</div> - -</div> - -<!-- ======================================================================= --> -<h2><a name="transforms">Transform Passes</a></h2> -<div> - <p>This section describes the LLVM Transform Passes.</p> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="adce">-adce: Aggressive Dead Code Elimination</a> -</h3> -<div> - <p>ADCE aggressively tries to eliminate code. This pass is similar to - <a href="#dce">DCE</a> but it assumes that values are dead until proven - otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to - the liveness of values.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="always-inline">-always-inline: Inliner for always_inline functions</a> -</h3> -<div> - <p>A custom inliner that handles only functions that are marked as - "always inline".</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="argpromotion">-argpromotion: Promote 'by reference' arguments to scalars</a> -</h3> -<div> - <p> - This pass promotes "by reference" arguments to be "by value" arguments. In - practice, this means looking for internal functions that have pointer - arguments. If it can prove, through the use of alias analysis, that an - argument is *only* loaded, then it can pass the value into the function - instead of the address of the value. This can cause recursive simplification - of code and lead to the elimination of allocas (especially in C++ template - code like the STL). - </p> - - <p> - This pass also handles aggregate arguments that are passed into a function, - scalarizing them if the elements of the aggregate are only loaded. Note that - it refuses to scalarize aggregates which would require passing in more than - three operands to the function, because passing thousands of operands for a - large array or structure is unprofitable! - </p> - - <p> - Note that this transformation could also be done for arguments that are only - stored to (returning the value instead), but does not currently. This case - would be best handled when and if LLVM starts supporting multiple return - values from functions. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="bb-vectorize">-bb-vectorize: Basic-Block Vectorization</a> -</h3> -<div> - <p>This pass combines instructions inside basic blocks to form vector - instructions. It iterates over each basic block, attempting to pair - compatible instructions, repeating this process until no additional - pairs are selected for vectorization. When the outputs of some pair - of compatible instructions are used as inputs by some other pair of - compatible instructions, those pairs are part of a potential - vectorization chain. Instruction pairs are only fused into vector - instructions when they are part of a chain longer than some - threshold length. Moreover, the pass attempts to find the best - possible chain for each pair of compatible instructions. These - heuristics are intended to prevent vectorization in cases where - it would not yield a performance increase of the resulting code. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="block-placement">-block-placement: Profile Guided Basic Block Placement</a> -</h3> -<div> - <p>This pass is a very simple profile guided basic block placement algorithm. - The idea is to put frequently executed blocks together at the start of the - function and hopefully increase the number of fall-through conditional - branches. If there is no profile information for a particular function, this - pass basically orders blocks in depth-first order.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="break-crit-edges">-break-crit-edges: Break critical edges in CFG</a> -</h3> -<div> - <p> - Break all of the critical edges in the CFG by inserting a dummy basic block. - It may be "required" by passes that cannot deal with critical edges. This - transformation obviously invalidates the CFG, but can update forward dominator - (set, immediate dominators, tree, and frontier) information. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="codegenprepare">-codegenprepare: Optimize for code generation</a> -</h3> -<div> - This pass munges the code in the input function to better prepare it for - SelectionDAG-based code generation. This works around limitations in it's - basic-block-at-a-time approach. It should eventually be removed. -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="constmerge">-constmerge: Merge Duplicate Global Constants</a> -</h3> -<div> - <p> - Merges duplicate global constants together into a single constant that is - shared. This is useful because some passes (ie TraceValues) insert a lot of - string constants into the program, regardless of whether or not an existing - string is available. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="constprop">-constprop: Simple constant propagation</a> -</h3> -<div> - <p>This file implements constant propagation and merging. It looks for - instructions involving only constant operands and replaces them with a - constant value instead of an instruction. For example:</p> - <blockquote><pre>add i32 1, 2</pre></blockquote> - <p>becomes</p> - <blockquote><pre>i32 3</pre></blockquote> - <p>NOTE: this pass has a habit of making definitions be dead. It is a good - idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass - sometime after running this pass.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dce">-dce: Dead Code Elimination</a> -</h3> -<div> - <p> - Dead code elimination is similar to <a href="#die">dead instruction - elimination</a>, but it rechecks instructions that were used by removed - instructions to see if they are newly dead. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="deadargelim">-deadargelim: Dead Argument Elimination</a> -</h3> -<div> - <p> - This pass deletes dead arguments from internal functions. Dead argument - elimination removes arguments which are directly dead, as well as arguments - only passed into function calls as dead arguments of other functions. This - pass also deletes dead arguments in a similar way. - </p> - - <p> - This pass is often useful as a cleanup pass to run after aggressive - interprocedural passes, which add possibly-dead arguments. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="deadtypeelim">-deadtypeelim: Dead Type Elimination</a> -</h3> -<div> - <p> - This pass is used to cleanup the output of GCC. It eliminate names for types - that are unused in the entire translation unit, using the <a - href="#findusedtypes">find used types</a> pass. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="die">-die: Dead Instruction Elimination</a> -</h3> -<div> - <p> - Dead instruction elimination performs a single pass over the function, - removing instructions that are obviously dead. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="dse">-dse: Dead Store Elimination</a> -</h3> -<div> - <p> - A trivial dead store elimination that only considers basic-block local - redundant stores. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="functionattrs">-functionattrs: Deduce function attributes</a> -</h3> -<div> - <p>A simple interprocedural pass which walks the call-graph, looking for - functions which do not access or only read non-local memory, and marking them - readnone/readonly. In addition, it marks function arguments (of pointer type) - 'nocapture' if a call to the function does not create any copies of the pointer - value that outlive the call. This more or less means that the pointer is only - dereferenced, and not returned from the function or stored in a global. - This pass is implemented as a bottom-up traversal of the call-graph. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="globaldce">-globaldce: Dead Global Elimination</a> -</h3> -<div> - <p> - This transform is designed to eliminate unreachable internal globals from the - program. It uses an aggressive algorithm, searching out globals that are - known to be alive. After it finds all of the globals which are needed, it - deletes whatever is left over. This allows it to delete recursive chunks of - the program which are unreachable. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="globalopt">-globalopt: Global Variable Optimizer</a> -</h3> -<div> - <p> - This pass transforms simple global variables that never have their address - taken. If obviously true, it marks read/write globals as constant, deletes - variables only stored to, etc. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="gvn">-gvn: Global Value Numbering</a> -</h3> -<div> - <p> - This pass performs global value numbering to eliminate fully and partially - redundant instructions. It also performs redundant load elimination. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="indvars">-indvars: Canonicalize Induction Variables</a> -</h3> -<div> - <p> - This transformation analyzes and transforms the induction variables (and - computations derived from them) into simpler forms suitable for subsequent - analysis and transformation. - </p> - - <p> - This transformation makes the following changes to each loop with an - identifiable induction variable: - </p> - - <ol> - <li>All loops are transformed to have a <em>single</em> canonical - induction variable which starts at zero and steps by one.</li> - <li>The canonical induction variable is guaranteed to be the first PHI node - in the loop header block.</li> - <li>Any pointer arithmetic recurrences are raised to use array - subscripts.</li> - </ol> - - <p> - If the trip count of a loop is computable, this pass also makes the following - changes: - </p> - - <ol> - <li>The exit condition for the loop is canonicalized to compare the - induction value against the exit value. This turns loops like: - <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote> - into - <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li> - <li>Any use outside of the loop of an expression derived from the indvar - is changed to compute the derived value outside of the loop, eliminating - the dependence on the exit value of the induction variable. If the only - purpose of the loop is to compute the exit value of some derived - expression, this transformation will make the loop dead.</li> - </ol> - - <p> - This transformation should be followed by strength reduction after all of the - desired loop transformations have been performed. Additionally, on targets - where it is profitable, the loop could be transformed to count down to zero - (the "do loop" optimization). - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="inline">-inline: Function Integration/Inlining</a> -</h3> -<div> - <p> - Bottom-up inlining of functions into callees. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="insert-edge-profiling">-insert-edge-profiling: Insert instrumentation for edge profiling</a> -</h3> -<div> - <p> - This pass instruments the specified program with counters for edge profiling. - Edge profiling can give a reasonable approximation of the hot paths through a - program, and is used for a wide variety of program transformations. - </p> - - <p> - Note that this implementation is very naïve. It inserts a counter for - <em>every</em> edge in the program, instead of using control flow information - to prune the number of counters inserted. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="insert-optimal-edge-profiling">-insert-optimal-edge-profiling: Insert optimal instrumentation for edge profiling</a> -</h3> -<div> - <p>This pass instruments the specified program with counters for edge profiling. - Edge profiling can give a reasonable approximation of the hot paths through a - program, and is used for a wide variety of program transformations. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="instcombine">-instcombine: Combine redundant instructions</a> -</h3> -<div> - <p> - Combine instructions to form fewer, simple - instructions. This pass does not modify the CFG This pass is where algebraic - simplification happens. - </p> - - <p> - This pass combines things like: - </p> - -<blockquote><pre ->%Y = add i32 %X, 1 -%Z = add i32 %Y, 1</pre></blockquote> - - <p> - into: - </p> - -<blockquote><pre ->%Z = add i32 %X, 2</pre></blockquote> - - <p> - This is a simple worklist driven algorithm. - </p> - - <p> - This pass guarantees that the following canonicalizations are performed on - the program: - </p> - - <ul> - <li>If a binary operator has a constant operand, it is moved to the right- - hand side.</li> - <li>Bitwise operators with constant operands are always grouped so that - shifts are performed first, then <code>or</code>s, then - <code>and</code>s, then <code>xor</code>s.</li> - <li>Compare instructions are converted from <code><</code>, - <code>></code>, <code>≤</code>, or <code>≥</code> to - <code>=</code> or <code>≠</code> if possible.</li> - <li>All <code>cmp</code> instructions on boolean values are replaced with - logical operations.</li> - <li><code>add <var>X</var>, <var>X</var></code> is represented as - <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li> - <li>Multiplies with a constant power-of-two argument are transformed into - shifts.</li> - <li>… etc.</li> - </ul> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="internalize">-internalize: Internalize Global Symbols</a> -</h3> -<div> - <p> - This pass loops over all of the functions in the input module, looking for a - main function. If a main function is found, all other functions and all - global variables with initializers are marked as internal. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="ipconstprop">-ipconstprop: Interprocedural constant propagation</a> -</h3> -<div> - <p> - This pass implements an <em>extremely</em> simple interprocedural constant - propagation pass. It could certainly be improved in many different ways, - like using a worklist. This pass makes arguments dead, but does not remove - them. The existing dead argument elimination pass should be run after this - to clean up the mess. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="ipsccp">-ipsccp: Interprocedural Sparse Conditional Constant Propagation</a> -</h3> -<div> - <p> - An interprocedural variant of <a href="#sccp">Sparse Conditional Constant - Propagation</a>. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="jump-threading">-jump-threading: Jump Threading</a> -</h3> -<div> - <p> - Jump threading tries to find distinct threads of control flow running through - a basic block. This pass looks at blocks that have multiple predecessors and - multiple successors. If one or more of the predecessors of the block can be - proven to always cause a jump to one of the successors, we forward the edge - from the predecessor to the successor by duplicating the contents of this - block. - </p> - <p> - An example of when this can occur is code like this: - </p> - - <pre ->if () { ... - X = 4; -} -if (X < 3) {</pre> - - <p> - In this case, the unconditional branch at the end of the first if can be - revectored to the false side of the second if. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="lcssa">-lcssa: Loop-Closed SSA Form Pass</a> -</h3> -<div> - <p> - This pass transforms loops by placing phi nodes at the end of the loops for - all values that are live across the loop boundary. For example, it turns - the left into the right code: - </p> - - <pre ->for (...) for (...) - if (c) if (c) - X1 = ... X1 = ... - else else - X2 = ... X2 = ... - X3 = phi(X1, X2) X3 = phi(X1, X2) -... = X3 + 4 X4 = phi(X3) - ... = X4 + 4</pre> - - <p> - This is still valid LLVM; the extra phi nodes are purely redundant, and will - be trivially eliminated by <code>InstCombine</code>. The major benefit of - this transformation is that it makes many other loop optimizations, such as - LoopUnswitching, simpler. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="licm">-licm: Loop Invariant Code Motion</a> -</h3> -<div> - <p> - This pass performs loop invariant code motion, attempting to remove as much - code from the body of a loop as possible. It does this by either hoisting - code into the preheader block, or by sinking code to the exit blocks if it is - safe. This pass also promotes must-aliased memory locations in the loop to - live in registers, thus hoisting and sinking "invariant" loads and stores. - </p> - - <p> - This pass uses alias analysis for two purposes: - </p> - - <ul> - <li>Moving loop invariant loads and calls out of loops. If we can determine - that a load or call inside of a loop never aliases anything stored to, - we can hoist it or sink it like any other instruction.</li> - <li>Scalar Promotion of Memory - If there is a store instruction inside of - the loop, we try to move the store to happen AFTER the loop instead of - inside of the loop. This can only happen if a few conditions are true: - <ul> - <li>The pointer stored through is loop invariant.</li> - <li>There are no stores or loads in the loop which <em>may</em> alias - the pointer. There are no calls in the loop which mod/ref the - pointer.</li> - </ul> - If these conditions are true, we can promote the loads and stores in the - loop of the pointer to use a temporary alloca'd variable. We then use - the mem2reg functionality to construct the appropriate SSA form for the - variable.</li> - </ul> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loop-deletion">-loop-deletion: Delete dead loops</a> -</h3> -<div> - <p> - This file implements the Dead Loop Deletion Pass. This pass is responsible - for eliminating loops with non-infinite computable trip counts that have no - side effects or volatile instructions, and do not contribute to the - computation of the function's return value. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loop-extract">-loop-extract: Extract loops into new functions</a> -</h3> -<div> - <p> - A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to - extract each top-level loop into its own new function. If the loop is the - <em>only</em> loop in a given function, it is not touched. This is a pass most - useful for debugging via bugpoint. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loop-extract-single">-loop-extract-single: Extract at most one loop into a new function</a> -</h3> -<div> - <p> - Similar to <a href="#loop-extract">Extract loops into new functions</a>, - this pass extracts one natural loop from the program into a function if it - can. This is used by bugpoint. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loop-reduce">-loop-reduce: Loop Strength Reduction</a> -</h3> -<div> - <p> - This pass performs a strength reduction on array references inside loops that - have as one or more of their components the loop induction variable. This is - accomplished by creating a new value to hold the initial value of the array - access for the first iteration, and then creating a new GEP instruction in - the loop to increment the value by the appropriate amount. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loop-rotate">-loop-rotate: Rotate Loops</a> -</h3> -<div> - <p>A simple loop rotation transformation.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loop-simplify">-loop-simplify: Canonicalize natural loops</a> -</h3> -<div> - <p> - This pass performs several transformations to transform natural loops into a - simpler form, which makes subsequent analyses and transformations simpler and - more effective. - </p> - - <p> - Loop pre-header insertion guarantees that there is a single, non-critical - entry edge from outside of the loop to the loop header. This simplifies a - number of analyses and transformations, such as LICM. - </p> - - <p> - Loop exit-block insertion guarantees that all exit blocks from the loop - (blocks which are outside of the loop that have predecessors inside of the - loop) only have predecessors from inside of the loop (and are thus dominated - by the loop header). This simplifies transformations such as store-sinking - that are built into LICM. - </p> - - <p> - This pass also guarantees that loops will have exactly one backedge. - </p> - - <p> - Note that the simplifycfg pass will clean up blocks which are split out but - end up being unnecessary, so usage of this pass should not pessimize - generated code. - </p> - - <p> - This pass obviously modifies the CFG, but updates loop information and - dominator information. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loop-unroll">-loop-unroll: Unroll loops</a> -</h3> -<div> - <p> - This pass implements a simple loop unroller. It works best when loops have - been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass, - allowing it to determine the trip counts of loops easily. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loop-unswitch">-loop-unswitch: Unswitch loops</a> -</h3> -<div> - <p> - This pass transforms loops that contain branches on loop-invariant conditions - to have multiple loops. For example, it turns the left into the right code: - </p> - - <pre ->for (...) if (lic) - A for (...) - if (lic) A; B; C - B else - C for (...) - A; C</pre> - - <p> - This can increase the size of the code exponentially (doubling it every time - a loop is unswitched) so we only unswitch if the resultant code will be - smaller than a threshold. - </p> - - <p> - This pass expects LICM to be run before it to hoist invariant conditions out - of the loop, to make the unswitching opportunity obvious. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="loweratomic">-loweratomic: Lower atomic intrinsics to non-atomic form</a> -</h3> -<div> - <p> - This pass lowers atomic intrinsics to non-atomic form for use in a known - non-preemptible environment. - </p> - - <p> - The pass does not verify that the environment is non-preemptible (in - general this would require knowledge of the entire call graph of the - program including any libraries which may not be available in bitcode form); - it simply lowers every atomic intrinsic. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="lowerinvoke">-lowerinvoke: Lower invoke and unwind, for unwindless code generators</a> -</h3> -<div> - <p> - This transformation is designed for use by code generators which do not yet - support stack unwinding. This pass supports two models of exception handling - lowering, the 'cheap' support and the 'expensive' support. - </p> - - <p> - 'Cheap' exception handling support gives the program the ability to execute - any program which does not "throw an exception", by turning 'invoke' - instructions into calls and by turning 'unwind' instructions into calls to - abort(). If the program does dynamically use the unwind instruction, the - program will print a message then abort. - </p> - - <p> - 'Expensive' exception handling support gives the full exception handling - support to the program at the cost of making the 'invoke' instruction - really expensive. It basically inserts setjmp/longjmp calls to emulate the - exception handling as necessary. - </p> - - <p> - Because the 'expensive' support slows down programs a lot, and EH is only - used for a subset of the programs, it must be specifically enabled by the - <tt>-enable-correct-eh-support</tt> option. - </p> - - <p> - Note that after this pass runs the CFG is not entirely accurate (exceptional - control flow edges are not correct anymore) so only very simple things should - be done after the lowerinvoke pass has run (like generation of native code). - This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't - support the invoke instruction yet" lowering pass. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="lowerswitch">-lowerswitch: Lower SwitchInst's to branches</a> -</h3> -<div> - <p> - Rewrites <tt>switch</tt> instructions with a sequence of branches, which - allows targets to get away with not implementing the switch instruction until - it is convenient. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="mem2reg">-mem2reg: Promote Memory to Register</a> -</h3> -<div> - <p> - This file promotes memory references to be register references. It promotes - <tt>alloca</tt> instructions which only have <tt>load</tt>s and - <tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator - frontiers to place <tt>phi</tt> nodes, then traversing the function in - depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as - appropriate. This is just the standard SSA construction algorithm to construct - "pruned" SSA form. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="memcpyopt">-memcpyopt: MemCpy Optimization</a> -</h3> -<div> - <p> - This pass performs various transformations related to eliminating memcpy - calls, or transforming sets of stores into memset's. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="mergefunc">-mergefunc: Merge Functions</a> -</h3> -<div> - <p>This pass looks for equivalent functions that are mergable and folds them. - - A hash is computed from the function, based on its type and number of - basic blocks. - - Once all hashes are computed, we perform an expensive equality comparison - on each function pair. This takes n^2/2 comparisons per bucket, so it's - important that the hash function be high quality. The equality comparison - iterates through each instruction in each basic block. - - When a match is found the functions are folded. If both functions are - overridable, we move the functionality into a new internal function and - leave two overridable thunks to it. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="mergereturn">-mergereturn: Unify function exit nodes</a> -</h3> -<div> - <p> - Ensure that functions have at most one <tt>ret</tt> instruction in them. - Additionally, it keeps track of which node is the new exit node of the CFG. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="partial-inliner">-partial-inliner: Partial Inliner</a> -</h3> -<div> - <p>This pass performs partial inlining, typically by inlining an if - statement that surrounds the body of the function. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="prune-eh">-prune-eh: Remove unused exception handling info</a> -</h3> -<div> - <p> - This file implements a simple interprocedural pass which walks the call-graph, - turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and - only if the callee cannot throw an exception. It implements this as a - bottom-up traversal of the call-graph. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="reassociate">-reassociate: Reassociate expressions</a> -</h3> -<div> - <p> - This pass reassociates commutative expressions in an order that is designed - to promote better constant propagation, GCSE, LICM, PRE, etc. - </p> - - <p> - For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5) - </p> - - <p> - In the implementation of this algorithm, constants are assigned rank = 0, - function arguments are rank = 1, and other values are assigned ranks - corresponding to the reverse post order traversal of current function - (starting at 2), which effectively gives values in deep loops higher rank - than values not in loops. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="reg2mem">-reg2mem: Demote all values to stack slots</a> -</h3> -<div> - <p> - This file demotes all registers to memory references. It is intended to be - the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to - <tt>load</tt> instructions, the only values live across basic blocks are - <tt>alloca</tt> instructions and <tt>load</tt> instructions before - <tt>phi</tt> nodes. It is intended that this should make CFG hacking much - easier. To make later hacking easier, the entry block is split into two, such - that all introduced <tt>alloca</tt> instructions (and nothing else) are in the - entry block. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="scalarrepl">-scalarrepl: Scalar Replacement of Aggregates (DT)</a> -</h3> -<div> - <p> - The well-known scalar replacement of aggregates transformation. This - transform breaks up <tt>alloca</tt> instructions of aggregate type (structure - or array) into individual <tt>alloca</tt> instructions for each member if - possible. Then, if possible, it transforms the individual <tt>alloca</tt> - instructions into nice clean scalar SSA form. - </p> - - <p> - This combines a simple scalar replacement of aggregates algorithm with the <a - href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact, - especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>, - then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to - promote works well. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="sccp">-sccp: Sparse Conditional Constant Propagation</a> -</h3> -<div> - <p> - Sparse conditional constant propagation and merging, which can be summarized - as: - </p> - - <ol> - <li>Assumes values are constant unless proven otherwise</li> - <li>Assumes BasicBlocks are dead unless proven otherwise</li> - <li>Proves values to be constant, and replaces them with constants</li> - <li>Proves conditional branches to be unconditional</li> - </ol> - - <p> - Note that this pass has a habit of making definitions be dead. It is a good - idea to to run a DCE pass sometime after running this pass. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="simplify-libcalls">-simplify-libcalls: Simplify well-known library calls</a> -</h3> -<div> - <p> - Applies a variety of small optimizations for calls to specific well-known - function calls (e.g. runtime library functions). For example, a call - <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be - transformed into simply <tt>return 3</tt>. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="simplifycfg">-simplifycfg: Simplify the CFG</a> -</h3> -<div> - <p> - Performs dead code elimination and basic block merging. Specifically: - </p> - - <ol> - <li>Removes basic blocks with no predecessors.</li> - <li>Merges a basic block into its predecessor if there is only one and the - predecessor only has one successor.</li> - <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li> - <li>Eliminates a basic block that only contains an unconditional - branch.</li> - </ol> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="sink">-sink: Code sinking</a> -</h3> -<div> - <p>This pass moves instructions into successor blocks, when possible, so that - they aren't executed on paths where their results aren't needed. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="strip">-strip: Strip all symbols from a module</a> -</h3> -<div> - <p> - performs code stripping. this transformation can delete: - </p> - - <ol> - <li>names for virtual registers</li> - <li>symbols for internal globals and functions</li> - <li>debug information</li> - </ol> - - <p> - note that this transformation makes code much less readable, so it should - only be used in situations where the <tt>strip</tt> utility would be used, - such as reducing code size or making it harder to reverse engineer code. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="strip-dead-debug-info">-strip-dead-debug-info: Strip debug info for unused symbols</a> -</h3> -<div> - <p> - performs code stripping. this transformation can delete: - </p> - - <ol> - <li>names for virtual registers</li> - <li>symbols for internal globals and functions</li> - <li>debug information</li> - </ol> - - <p> - note that this transformation makes code much less readable, so it should - only be used in situations where the <tt>strip</tt> utility would be used, - such as reducing code size or making it harder to reverse engineer code. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="strip-dead-prototypes">-strip-dead-prototypes: Strip Unused Function Prototypes</a> -</h3> -<div> - <p> - This pass loops over all of the functions in the input module, looking for - dead declarations and removes them. Dead declarations are declarations of - functions for which no implementation is available (i.e., declarations for - unused library functions). - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="strip-debug-declare">-strip-debug-declare: Strip all llvm.dbg.declare intrinsics</a> -</h3> -<div> - <p>This pass implements code stripping. Specifically, it can delete:</p> - <ul> - <li>names for virtual registers</li> - <li>symbols for internal globals and functions</li> - <li>debug information</li> - </ul> - <p> - Note that this transformation makes code much less readable, so it should - only be used in situations where the 'strip' utility would be used, such as - reducing code size or making it harder to reverse engineer code. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="strip-nondebug">-strip-nondebug: Strip all symbols, except dbg symbols, from a module</a> -</h3> -<div> - <p>This pass implements code stripping. Specifically, it can delete:</p> - <ul> - <li>names for virtual registers</li> - <li>symbols for internal globals and functions</li> - <li>debug information</li> - </ul> - <p> - Note that this transformation makes code much less readable, so it should - only be used in situations where the 'strip' utility would be used, such as - reducing code size or making it harder to reverse engineer code. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="tailcallelim">-tailcallelim: Tail Call Elimination</a> -</h3> -<div> - <p> - This file transforms calls of the current function (self recursion) followed - by a return instruction with a branch to the entry of the function, creating - a loop. This pass also implements the following extensions to the basic - algorithm: - </p> - - <ul> - <li>Trivial instructions between the call and return do not prevent the - transformation from taking place, though currently the analysis cannot - support moving any really useful instructions (only dead ones). - <li>This pass transforms functions that are prevented from being tail - recursive by an associative expression to use an accumulator variable, - thus compiling the typical naive factorial or <tt>fib</tt> implementation - into efficient code. - <li>TRE is performed if the function returns void, if the return - returns the result returned by the call, or if the function returns a - run-time constant on all exits from the function. It is possible, though - unlikely, that the return returns something else (like constant 0), and - can still be TRE'd. It can be TRE'd if <em>all other</em> return - instructions in the function return the exact same value. - <li>If it can prove that callees do not access theier caller stack frame, - they are marked as eligible for tail call elimination (by the code - generator). - </ul> -</div> - -<!-- ======================================================================= --> -<h2><a name="utilities">Utility Passes</a></h2> -<div> - <p>This section describes the LLVM Utility Passes.</p> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="deadarghaX0r">-deadarghaX0r: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a> -</h3> -<div> - <p> - Same as dead argument elimination, but deletes arguments to functions which - are external. This is only for use by <a - href="Bugpoint.html">bugpoint</a>.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="extract-blocks">-extract-blocks: Extract Basic Blocks From Module (for bugpoint use)</a> -</h3> -<div> - <p> - This pass is used by bugpoint to extract all blocks from the module into their - own functions.</p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="instnamer">-instnamer: Assign names to anonymous instructions</a> -</h3> -<div> - <p>This is a little utility pass that gives instructions names, this is mostly - useful when diffing the effect of an optimization because deleting an - unnamed instruction can change all other instruction numbering, making the - diff very noisy. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="preverify">-preverify: Preliminary module verification</a> -</h3> -<div> - <p> - Ensures that the module is in the form required by the <a - href="#verifier">Module Verifier</a> pass. - </p> - - <p> - Running the verifier runs this pass automatically, so there should be no need - to use it directly. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="verify">-verify: Module Verifier</a> -</h3> -<div> - <p> - Verifies an LLVM IR code. This is useful to run after an optimization which is - undergoing testing. Note that <tt>llvm-as</tt> verifies its input before - emitting bitcode, and also that malformed bitcode is likely to make LLVM - crash. All language front-ends are therefore encouraged to verify their output - before performing optimizing transformations. - </p> - - <ul> - <li>Both of a binary operator's parameters are of the same type.</li> - <li>Verify that the indices of mem access instructions match other - operands.</li> - <li>Verify that arithmetic and other things are only performed on - first-class types. Verify that shifts and logicals only happen on - integrals f.e.</li> - <li>All of the constants in a switch statement are of the correct type.</li> - <li>The code is in valid SSA form.</li> - <li>It is illegal to put a label into any other type (like a structure) or - to return one.</li> - <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is - invalid.</li> - <li>PHI nodes must have an entry for each predecessor, with no extras.</li> - <li>PHI nodes must be the first thing in a basic block, all grouped - together.</li> - <li>PHI nodes must have at least one entry.</li> - <li>All basic blocks should only end with terminator insts, not contain - them.</li> - <li>The entry node to a function must not have predecessors.</li> - <li>All Instructions must be embedded into a basic block.</li> - <li>Functions cannot take a void-typed parameter.</li> - <li>Verify that a function's argument list agrees with its declared - type.</li> - <li>It is illegal to specify a name for a void value.</li> - <li>It is illegal to have an internal global value with no initializer.</li> - <li>It is illegal to have a ret instruction that returns a value that does - not agree with the function return value type.</li> - <li>Function call argument types match the function prototype.</li> - <li>All other things that are tested by asserts spread about the code.</li> - </ul> - - <p> - Note that this does not provide full security verification (like Java), but - instead just tries to ensure that code is well-formed. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="view-cfg">-view-cfg: View CFG of function</a> -</h3> -<div> - <p> - Displays the control flow graph using the GraphViz tool. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="view-cfg-only">-view-cfg-only: View CFG of function (with no function bodies)</a> -</h3> -<div> - <p> - Displays the control flow graph using the GraphViz tool, but omitting function - bodies. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="view-dom">-view-dom: View dominance tree of function</a> -</h3> -<div> - <p> - Displays the dominator tree using the GraphViz tool. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="view-dom-only">-view-dom-only: View dominance tree of function (with no function bodies)</a> -</h3> -<div> - <p> - Displays the dominator tree using the GraphViz tool, but omitting function - bodies. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="view-postdom">-view-postdom: View postdominance tree of function</a> -</h3> -<div> - <p> - Displays the post dominator tree using the GraphViz tool. - </p> -</div> - -<!-------------------------------------------------------------------------- --> -<h3> - <a name="view-postdom-only">-view-postdom-only: View postdominance tree of function (with no function bodies)</a> -</h3> -<div> - <p> - Displays the post dominator tree using the GraphViz tool, but omitting - function bodies. - </p> -</div> - -</div> - -<!-- *********************************************************************** --> - -<hr> -<address> - <a href="http://jigsaw.w3.org/css-validator/check/referer"><img - src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> - <a href="http://validator.w3.org/check/referer"><img - src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> - - <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br> - <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> - Last modified: $Date$ -</address> - -</body> -</html> |