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1931 lines
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<html>
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<title>LLVM's Analysis and Transform Passes</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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<!--
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If Passes.html is up to date, the following "one-liner" should print
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an empty diff.
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egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \
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-e '^ <a name=".*">.*</a>$' < Passes.html >html; \
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perl >help <<'EOT' && diff -u help html; rm -f help html
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open HTML, "<Passes.html" or die "open: Passes.html: $!\n";
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while (<HTML>) {
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m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next;
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$order{$1} = sprintf("%03d", 1 + int %order);
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open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n";
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while (<HELP>) {
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m:^ -([^ ]+) +- (.*)$: or next;
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my $o = $order{$1};
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$o = "000" unless defined $o;
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push @x, "$o<tr><td><a href=\"#$1\">-$1</a></td><td>$2</td></tr>\n";
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push @y, "$o <a name=\"$1\">$2</a>\n";
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@x = map { s/^\d\d\d//; $_ } sort @x;
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@y = map { s/^\d\d\d//; $_ } sort @y;
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print @x, @y;
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EOT
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This (real) one-liner can also be helpful when converting comments to HTML:
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perl -e '$/ = 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'
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-->
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<div class="doc_title">LLVM's Analysis and Transform Passes</div>
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<ol>
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<li><a href="#intro">Introduction</a></li>
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<li><a href="#analyses">Analysis Passes</a>
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<li><a href="#transforms">Transform Passes</a></li>
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<li><a href="#utilities">Utility Passes</a></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
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and Gordon Henriksen</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_section"> <a name="intro">Introduction</a> </div>
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<div class="doc_text">
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<p>This document serves as a high level summary of the optimization features
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that LLVM provides. Optimizations are implemented as Passes that traverse some
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portion of a program to either collect information or transform the program.
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The table below divides the passes that LLVM provides into three categories.
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Analysis passes compute information that other passes can use or for debugging
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or program visualization purposes. Transform passes can use (or invalidate)
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the analysis passes. Transform passes all mutate the program in some way.
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Utility passes provides some utility but don't otherwise fit categorization.
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For example passes to extract functions to bitcode or write a module to
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bitcode are neither analysis nor transform passes.
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<p>The table below provides a quick summary of each pass and links to the more
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complete pass description later in the document.</p>
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</div>
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<div class="doc_text" >
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<table>
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<tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr>
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<tr><th>Option</th><th>Name</th></tr>
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<tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr>
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<tr><td><a href="#anders-aa">-anders-aa</a></td><td>Andersen's Interprocedural Alias Analysis</td></tr>
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<tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (default AA impl)</td></tr>
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<tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr>
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<tr><td><a href="#basicvn">-basicvn</a></td><td>Basic Value Numbering (default GVN impl)</td></tr>
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<tr><td><a href="#callgraph">-callgraph</a></td><td>Print a call graph</td></tr>
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<tr><td><a href="#callscc">-callscc</a></td><td>Print SCCs of the Call Graph</td></tr>
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<tr><td><a href="#cfgscc">-cfgscc</a></td><td>Print SCCs of each function CFG</td></tr>
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<tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr>
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<tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
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<tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
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<tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
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<tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
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<tr><td><a href="#externalfnconstants">-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
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<tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr>
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<tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr>
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<tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr>
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<tr><td><a href="#load-vn">-load-vn</a></td><td>Load Value Numbering</td></tr>
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<tr><td><a href="#loops">-loops</a></td><td>Natural Loop Construction</td></tr>
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<tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
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<tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
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<tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
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<tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr>
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<tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
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<tr><td><a href="#print">-print</a></td><td>Print function to stderr</td></tr>
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<tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
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<tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
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<tr><td><a href="#print-cfg">-print-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
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<tr><td><a href="#print-cfg-only">-print-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr>
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<tr><td><a href="#printm">-printm</a></td><td>Print module to stderr</td></tr>
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<tr><td><a href="#printusedtypes">-printusedtypes</a></td><td>Find Used Types</td></tr>
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<tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr>
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<tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr>
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<tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
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<tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr>
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<tr><th>Option</th><th>Name</th></tr>
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<tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr>
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<tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr>
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<tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr>
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<tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr>
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<tr><td><a href="#cee">-cee</a></td><td>Correlated Expression Elimination</td></tr>
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<tr><td><a href="#condprop">-condprop</a></td><td>Conditional Propagation</td></tr>
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<tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr>
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<tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr>
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<tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr>
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<tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr>
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<tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr>
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<tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr>
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<tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr>
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<tr><td><a href="#gcse">-gcse</a></td><td>Global Common Subexpression Elimination</td></tr>
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<tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr>
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<tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr>
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<tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr>
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<tr><td><a href="#gvnpre">-gvnpre</a></td><td>Global Value Numbering/Partial Redundancy Elimination</td></tr>
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<tr><td><a href="#indmemrem">-indmemrem</a></td><td>Indirect Malloc and Free Removal</td></tr>
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<tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr>
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<tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr>
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<tr><td><a href="#insert-block-profiling">-insert-block-profiling</a></td><td>Insert instrumentation for block profiling</td></tr>
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<tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr>
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<tr><td><a href="#insert-function-profiling">-insert-function-profiling</a></td><td>Insert instrumentation for function profiling</td></tr>
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<tr><td><a href="#insert-null-profiling-rs">-insert-null-profiling-rs</a></td><td>Measure profiling framework overhead</td></tr>
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<tr><td><a href="#insert-rs-profiling-framework">-insert-rs-profiling-framework</a></td><td>Insert random sampling instrumentation framework</td></tr>
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<tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr>
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<tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr>
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<tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr>
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<tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr>
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<tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr>
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<tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr>
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<tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr>
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<tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr>
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<tr><td><a href="#loop-index-split">-loop-index-split</a></td><td>Index Split Loops</td></tr>
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<tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr>
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<tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr>
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<tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr>
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<tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr>
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<tr><td><a href="#loopsimplify">-loopsimplify</a></td><td>Canonicalize natural loops</td></tr>
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<tr><td><a href="#lower-packed">-lower-packed</a></td><td>lowers packed operations to operations on smaller packed datatypes</td></tr>
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<tr><td><a href="#lowerallocs">-lowerallocs</a></td><td>Lower allocations from instructions to calls</td></tr>
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<tr><td><a href="#lowergc">-lowergc</a></td><td>Lower GC intrinsics, for GCless code generators</td></tr>
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<tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr>
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<tr><td><a href="#lowerselect">-lowerselect</a></td><td>Lower select instructions to branches</td></tr>
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<tr><td><a href="#lowersetjmp">-lowersetjmp</a></td><td>Lower Set Jump</td></tr>
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<tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr>
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<tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr>
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<tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr>
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<tr><td><a href="#predsimplify">-predsimplify</a></td><td>Predicate Simplifier</td></tr>
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<tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr>
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<tr><td><a href="#raiseallocs">-raiseallocs</a></td><td>Raise allocations from calls to instructions</td></tr>
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<tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr>
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<tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr>
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<tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates</td></tr>
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<tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr>
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<tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr>
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<tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr>
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<tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr>
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<tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr>
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<tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr>
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<tr><th colspan="2"><b>UTILITY PASSES</b></th></tr>
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<tr><th>Option</th><th>Name</th></tr>
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<tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr>
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<tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr>
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<tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr>
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<tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr>
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<tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr>
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<tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr>
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</table>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_section"> <a name="example">Analysis Passes</a></div>
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<div class="doc_text">
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<p>This section describes the LLVM Analysis Passes.</p>
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</div>
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<!-------------------------------------------------------------------------- -->
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<div class="doc_subsection">
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<a name="aa-eval">Exhaustive Alias Analysis Precision Evaluator</a>
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</div>
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<div class="doc_text">
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<p>This is a simple N^2 alias analysis accuracy evaluator.
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Basically, for each function in the program, it simply queries to see how the
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alias analysis implementation answers alias queries between each pair of
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pointers in the function.</p>
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<p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco
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Spadini, and Wojciech Stryjewski.</p>
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</div>
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<!-------------------------------------------------------------------------- -->
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<div class="doc_subsection">
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<a name="anders-aa">Andersen's Interprocedural Alias Analysis</a>
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</div>
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<div class="doc_text">
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<p>
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This is an implementation of Andersen's interprocedural alias
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analysis
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</p>
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<p>
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In pointer analysis terms, this is a subset-based, flow-insensitive,
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field-sensitive, and context-insensitive algorithm pointer algorithm.
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</p>
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<p>
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This algorithm is implemented as three stages:
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</p>
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<ol>
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<li>Object identification.</li>
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<li>Inclusion constraint identification.</li>
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<li>Offline constraint graph optimization.</li>
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<li>Inclusion constraint solving.</li>
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</ol>
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<p>
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The object identification stage identifies all of the memory objects in the
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program, which includes globals, heap allocated objects, and stack allocated
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objects.
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</p>
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<p>
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The inclusion constraint identification stage finds all inclusion constraints
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in the program by scanning the program, looking for pointer assignments and
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other statements that effect the points-to graph. For a statement like
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<code><var>A</var> = <var>B</var></code>, this statement is processed to
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indicate that <var>A</var> can point to anything that <var>B</var> can point
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to. Constraints can handle copies, loads, and stores, and address taking.
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</p>
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<p>
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The offline constraint graph optimization portion includes offline variable
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substitution algorithms intended to computer pointer and location
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equivalences. Pointer equivalences are those pointers that will have the
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same points-to sets, and location equivalences are those variables that
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always appear together in points-to sets.
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</p>
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<p>
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The inclusion constraint solving phase iteratively propagates the inclusion
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constraints until a fixed point is reached. This is an O(<var>n</var>³)
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algorithm.
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</p>
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<p>
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Function constraints are handled as if they were structs with <var>X</var>
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fields. Thus, an access to argument <var>X</var> of function <var>Y</var> is
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an access to node index <code>getNode(<var>Y</var>) + <var>X</var></code>.
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This representation allows handling of indirect calls without any issues. To
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wit, an indirect call <code><var>Y</var>(<var>a</var>,<var>b</var>)</code> is
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equivalent to <code>*(<var>Y</var> + 1) = <var>a</var>, *(<var>Y</var> + 2) =
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<var>b</var></code>. The return node for a function <var>F</var> is always
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located at <code>getNode(<var>F</var>) + CallReturnPos</code>. The arguments
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start at <code>getNode(<var>F</var>) + CallArgPos</code>.
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</p>
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</div>
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<!-------------------------------------------------------------------------- -->
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<div class="doc_subsection">
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<a name="basicaa">Basic Alias Analysis (default AA impl)</a>
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</div>
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<div class="doc_text">
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<p>
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This is the default implementation of the Alias Analysis interface
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that simply implements a few identities (two different globals cannot alias,
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etc), but otherwise does no analysis.
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</p>
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</div>
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<!-------------------------------------------------------------------------- -->
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<div class="doc_subsection">
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<a name="basiccg">Basic CallGraph Construction</a>
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</div>
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<div class="doc_text">
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<p>Yet to be written.</p>
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</div>
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<!-------------------------------------------------------------------------- -->
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<div class="doc_subsection">
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<a name="basicvn">Basic Value Numbering (default GVN impl)</a>
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</div>
|
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<div class="doc_text">
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<p>
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This is the default implementation of the <code>ValueNumbering</code>
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interface. It walks the SSA def-use chains to trivially identify
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lexically identical expressions. This does not require any ahead of time
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analysis, so it is a very fast default implementation.
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</p>
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</div>
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<!-------------------------------------------------------------------------- -->
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<div class="doc_subsection">
|
|
<a name="callgraph">Print a call graph</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass, only available in <code>opt</code>, prints the call graph to
|
|
standard output in a human-readable form.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="callscc">Print SCCs of the Call Graph</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass, only available in <code>opt</code>, prints the SCCs of the call
|
|
graph to standard output in a human-readable form.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="cfgscc">Print SCCs of each function CFG</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass, only available in <code>opt</code>, prints the SCCs of each
|
|
function CFG to standard output in a human-readable form.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="codegenprepare">Optimize for code generation</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
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.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="count-aa">Count Alias Analysis Query Responses</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="debug-aa">AA use debugger</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="domfrontier">Dominance Frontier Construction</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass is a simple dominator construction algorithm for finding forward
|
|
dominator frontiers.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="domtree">Dominator Tree Construction</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass is a simple dominator construction algorithm for finding forward
|
|
dominators.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="externalfnconstants">Print external fn callsites passed constants</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="globalsmodref-aa">Simple mod/ref analysis for globals</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="instcount">Counts the various types of Instructions</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass collects the count of all instructions and reports them
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="intervals">Interval Partition Construction</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="load-vn">Load Value Numbering</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass value numbers load and call instructions. To do this, it finds
|
|
lexically identical load instructions, and uses alias analysis to determine
|
|
which loads are guaranteed to produce the same value. To value number call
|
|
instructions, it looks for calls to functions that do not write to memory
|
|
which do not have intervening instructions that clobber the memory that is
|
|
read from.
|
|
</p>
|
|
|
|
<p>
|
|
This pass builds off of another value numbering pass to implement value
|
|
numbering for non-load and non-call instructions. It uses Alias Analysis so
|
|
that it can disambiguate the load instructions. The more powerful these base
|
|
analyses are, the more powerful the resultant value numbering will be.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loops">Natural Loop Construction</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="memdep">Memory Dependence Analysis</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="no-aa">No Alias Analysis (always returns 'may' alias)</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="no-profile">No Profile Information</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
The default "no profile" implementation of the abstract
|
|
<code>ProfileInfo</code> interface.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="postdomfrontier">Post-Dominance Frontier Construction</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass is a simple post-dominator construction algorithm for finding
|
|
post-dominator frontiers.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="postdomtree">Post-Dominator Tree Construction</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass is a simple post-dominator construction algorithm for finding
|
|
post-dominators.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="print">Print function to stderr</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="print-alias-sets">Alias Set Printer</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>Yet to be written.</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="print-callgraph">Print Call Graph to 'dot' file</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="print-cfg">Print CFG of function to 'dot' file</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="print-cfg-only">Print CFG of function to 'dot' file (with no function bodies)</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="printm">Print module to stderr</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass simply prints out the entire module when it is executed.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="printusedtypes">Find Used Types</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="profile-loader">Load profile information from llvmprof.out</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
A concrete implementation of profiling information that loads the information
|
|
from a profile dump file.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="scalar-evolution">Scalar Evolution Analysis</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="targetdata">Target Data Layout</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>Provides other passes access to information on how the size and alignment
|
|
required by the the target ABI for various data types.</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_section"> <a name="transform">Transform Passes</a></div>
|
|
<div class="doc_text">
|
|
<p>This section describes the LLVM Transform Passes.</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="adce">Aggressive Dead Code Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="argpromotion">Promote 'by reference' arguments to scalars</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="block-placement">Profile Guided Basic Block Placement</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="break-crit-edges">Break critical edges in CFG</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="cee">Correlated Expression Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>Correlated Expression Elimination propagates information from conditional
|
|
branches to blocks dominated by destinations of the branch. It propagates
|
|
information from the condition check itself into the body of the branch,
|
|
allowing transformations like these for example:</p>
|
|
|
|
<blockquote><pre>
|
|
if (i == 7)
|
|
... 4*i; // constant propagation
|
|
|
|
M = i+1; N = j+1;
|
|
if (i == j)
|
|
X = M-N; // = M-M == 0;
|
|
</pre></blockquote>
|
|
|
|
<p>This is called Correlated Expression Elimination because we eliminate or
|
|
simplify expressions that are correlated with the direction of a branch. In
|
|
this way we use static information to give us some information about the
|
|
dynamic value of a variable.</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="condprop">Conditional Propagation</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>This pass propagates information about conditional expressions through the
|
|
program, allowing it to eliminate conditional branches in some cases.</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="constmerge">Merge Duplicate Global Constants</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="constprop">Simple constant propagation</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="dce">Dead Code Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="deadargelim">Dead Argument Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="deadtypeelim">Dead Type Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="die">Dead Instruction Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Dead instruction elimination performs a single pass over the function,
|
|
removing instructions that are obviously dead.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="dse">Dead Store Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
A trivial dead store elimination that only considers basic-block local
|
|
redundant stores.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="gcse">Global Common Subexpression Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass is designed to be a very quick global transformation that
|
|
eliminates global common subexpressions from a function. It does this by
|
|
using an existing value numbering implementation to identify the common
|
|
subexpressions, eliminating them when possible.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="globaldce">Dead Global Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="globalopt">Global Variable Optimizer</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="gvn">Global Value Numbering</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass performs global value numbering to eliminate fully redundant
|
|
instructions. It also performs simple dead load elimination.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="gvnpre">Global Value Numbering/Partial Redundancy Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass performs a hybrid of global value numbering and partial redundancy
|
|
elimination, known as GVN-PRE. It performs partial redundancy elimination on
|
|
values, rather than lexical expressions, allowing a more comprehensive view
|
|
the optimization. It replaces redundant values with uses of earlier
|
|
occurences of the same value. While this is beneficial in that it eliminates
|
|
unneeded computation, it also increases register pressure by creating large
|
|
live ranges, and should be used with caution on platforms that are very
|
|
sensitive to register pressure.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="indmemrem">Indirect Malloc and Free Removal</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass finds places where memory allocation functions may escape into
|
|
indirect land. Some transforms are much easier (aka possible) only if free
|
|
or malloc are not called indirectly.
|
|
</p>
|
|
|
|
<p>
|
|
Thus find places where the address of memory functions are taken and construct
|
|
bounce functions with direct calls of those functions.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="indvars">Canonicalize Induction Variables</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="inline">Function Integration/Inlining</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Bottom-up inlining of functions into callees.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="insert-block-profiling">Insert instrumentation for block profiling</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass instruments the specified program with counters for basic block
|
|
profiling, which counts the number of times each basic block executes. This
|
|
is the most basic form of profiling, which can tell which blocks are hot, but
|
|
cannot reliably detect hot paths through the CFG.
|
|
</p>
|
|
|
|
<p>
|
|
Note that this implementation is very naïve. Control equivalent regions of
|
|
the CFG should not require duplicate counters, but it does put duplicate
|
|
counters in.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="insert-edge-profiling">Insert instrumentation for edge profiling</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="insert-function-profiling">Insert instrumentation for function profiling</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass instruments the specified program with counters for function
|
|
profiling, which counts the number of times each function is called.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="insert-null-profiling-rs">Measure profiling framework overhead</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
The basic profiler that does nothing. It is the default profiler and thus
|
|
terminates <code>RSProfiler</code> chains. It is useful for measuring
|
|
framework overhead.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="insert-rs-profiling-framework">Insert random sampling instrumentation framework</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
The second stage of the random-sampling instrumentation framework, duplicates
|
|
all instructions in a function, ignoring the profiling code, then connects the
|
|
two versions together at the entry and at backedges. At each connection point
|
|
a choice is made as to whether to jump to the profiled code (take a sample) or
|
|
execute the unprofiled code.
|
|
</p>
|
|
|
|
<p>
|
|
After this pass, it is highly recommended to run<a href="#mem2reg">mem2reg</a>
|
|
and <a href="#adce">adce</a>. <a href="#instcombine">instcombine</a>,
|
|
<a href="#load-vn">load-vn</a>, <a href="#gdce">gdce</a>, and
|
|
<a href="#dse">dse</a> also are good to run afterwards.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="instcombine">Combine redundant instructions</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="internalize">Internalize Global Symbols</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="ipconstprop">Interprocedural constant propagation</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="ipsccp">Interprocedural Sparse Conditional Constant Propagation</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
|
|
Propagation</a>.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="lcssa">Loop-Closed SSA Form Pass</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="licm">Loop Invariant Code Motion</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loop-extract">Extract loops into new functions</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loop-extract-single">Extract at most one loop into a new function</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loop-index-split">Index Split Loops</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass divides loop's iteration range by spliting loop such that each
|
|
individual loop is executed efficiently.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loop-reduce">Loop Strength Reduction</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loop-rotate">Rotate Loops</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>A simple loop rotation transformation.</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loop-unroll">Unroll loops</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loop-unswitch">Unswitch loops</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="loopsimplify">Canonicalize natural loops</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="lower-packed">lowers packed operations to operations on smaller packed datatypes</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Lowers operations on vector datatypes into operations on more primitive vector
|
|
datatypes, and finally to scalar operations.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="lowerallocs">Lower allocations from instructions to calls</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Turn <tt>malloc</tt> and <tt>free</tt> instructions into <tt>@malloc</tt> and
|
|
<tt>@free</tt> calls.
|
|
</p>
|
|
|
|
<p>
|
|
This is a target-dependent tranformation because it depends on the size of
|
|
data types and alignment constraints.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="lowergc">Lower GC intrinsics, for GCless code generators</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This file implements lowering for the <tt>llvm.gc*</tt> intrinsics for targets
|
|
that do not natively support them (which includes the C backend). Note that
|
|
the code generated is not as efficient as it would be for targets that
|
|
natively support the GC intrinsics, but it is useful for getting new targets
|
|
up-and-running quickly.
|
|
</p>
|
|
|
|
<p>
|
|
This pass implements the code transformation described in this paper:
|
|
</p>
|
|
|
|
<blockquote><p>
|
|
"Accurate Garbage Collection in an Uncooperative Environment"
|
|
Fergus Henderson, ISMM, 2002
|
|
</p></blockquote>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="lowerinvoke">Lower invoke and unwind, for unwindless code generators</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="lowerselect">Lower select instructions to branches</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Lowers select instructions into conditional branches for targets that do not
|
|
have conditional moves or that have not implemented the select instruction
|
|
yet.
|
|
</p>
|
|
|
|
<p>
|
|
Note that this pass could be improved. In particular it turns every select
|
|
instruction into a new conditional branch, even though some common cases have
|
|
select instructions on the same predicate next to each other. It would be
|
|
better to use the same branch for the whole group of selects.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="lowersetjmp">Lower Set Jump</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Lowers <tt>setjmp</tt> and <tt>longjmp</tt> to use the LLVM invoke and unwind
|
|
instructions as necessary.
|
|
</p>
|
|
|
|
<p>
|
|
Lowering of <tt>longjmp</tt> is fairly trivial. We replace the call with a
|
|
call to the LLVM library function <tt>__llvm_sjljeh_throw_longjmp()</tt>.
|
|
This unwinds the stack for us calling all of the destructors for
|
|
objects allocated on the stack.
|
|
</p>
|
|
|
|
<p>
|
|
At a <tt>setjmp</tt> call, the basic block is split and the <tt>setjmp</tt>
|
|
removed. The calls in a function that have a <tt>setjmp</tt> are converted to
|
|
invoke where the except part checks to see if it's a <tt>longjmp</tt>
|
|
exception and, if so, if it's handled in the function. If it is, then it gets
|
|
the value returned by the <tt>longjmp</tt> and goes to where the basic block
|
|
was split. <tt>invoke</tt> instructions are handled in a similar fashion with
|
|
the original except block being executed if it isn't a <tt>longjmp</tt>
|
|
except that is handled by that function.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="lowerswitch">Lower SwitchInst's to branches</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="mem2reg">Promote Memory to Register</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="mergereturn">Unify function exit nodes</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="predsimplify">Predicate Simplifier</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Path-sensitive optimizer. In a branch where <tt>x == y</tt>, replace uses of
|
|
<tt>x</tt> with <tt>y</tt>. Permits further optimization, such as the
|
|
elimination of the unreachable call:
|
|
</p>
|
|
|
|
<blockquote><pre
|
|
>void test(int *p, int *q)
|
|
{
|
|
if (p != q)
|
|
return;
|
|
|
|
if (*p != *q)
|
|
foo(); // unreachable
|
|
}</pre></blockquote>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="prune-eh">Remove unused exception handling info</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="raiseallocs">Raise allocations from calls to instructions</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Converts <tt>@malloc</tt> and <tt>@free</tt> calls to <tt>malloc</tt> and
|
|
<tt>free</tt> instructions.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="reassociate">Reassociate expressions</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="reg2mem">Demote all values to stack slots</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This file demotes all registers to memory references. It is intented to be
|
|
the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to
|
|
<tt>load</tt> instructions, the only values live accross 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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="scalarrepl">Scalar Replacement of Aggregates</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="sccp">Sparse Conditional Constant Propagation</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="simplify-libcalls">Simplify well-known library calls</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="simplifycfg">Simplify the CFG</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="strip">Strip all symbols from a module</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="tailcallelim">Tail Call Elimination</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="tailduplicate">Tail Duplication</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass performs a limited form of tail duplication, intended to simplify
|
|
CFGs by removing some unconditional branches. This pass is necessary to
|
|
straighten out loops created by the C front-end, but also is capable of
|
|
making other code nicer. After this pass is run, the CFG simplify pass
|
|
should be run to clean up the mess.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_section"> <a name="transform">Utility Passes</a></div>
|
|
<div class="doc_text">
|
|
<p>This section describes the LLVM Utility Passes.</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="deadarghaX0r">Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="extract-blocks">Extract Basic Blocks From Module (for bugpoint use)</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
This pass is used by bugpoint to extract all blocks from the module into their
|
|
own functions.</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="preverify">Preliminary module verification</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="verify">Module Verifier</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<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 should be illegal to put a label into any other type (like a
|
|
structure) or to return one. [except constant arrays!]</li>
|
|
<li>Only phi nodes can be self referential: <tt>%x = add int %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 a 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>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="view-cfg">View CFG of function</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Displays the control flow graph using the GraphViz tool.
|
|
</p>
|
|
</div>
|
|
|
|
<!-------------------------------------------------------------------------- -->
|
|
<div class="doc_subsection">
|
|
<a name="view-cfg-only">View CFG of function (with no function bodies)</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>
|
|
Displays the control flow graph using the GraphViz tool, but omitting function
|
|
bodies.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" 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>
|