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<!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:[email protected]">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