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Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]11<!--
12
13If Passes.html is up to date, the following "one-liner" should print
14an empty diff.
15
16egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \
17 -e '^ <a name=".*">.*</a>$' < Passes.html >html; \
18perl >help <<'EOT' && diff -u help html; rm -f help html
19open HTML, "<Passes.html" or die "open: Passes.html: $!\n";
20while (<HTML>) {
21 m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next;
22 $order{$1} = sprintf("%03d", 1 + int %order);
23}
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]24open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n";
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]25while (<HELP>) {
26 m:^ -([^ ]+) +- (.*)$: or next;
27 my $o = $order{$1};
28 $o = "000" unless defined $o;
29 push @x, "$o<tr><td><a href=\"#$1\">-$1</a></td><td>$2</td></tr>\n";
30 push @y, "$o <a name=\"$1\">$2</a>\n";
31}
32@x = map { s/^\d\d\d//; $_ } sort @x;
33@y = map { s/^\d\d\d//; $_ } sort @y;
34print @x, @y;
35EOT
36
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]37This (real) one-liner can also be helpful when converting comments to HTML:
38
39perl -e '$/ = 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'
40
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]41 -->
42
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]43<div class="doc_title">LLVM's Analysis and Transform Passes</div>
44
45<ol>
46 <li><a href="#intro">Introduction</a></li>
47 <li><a href="#analyses">Analysis Passes</a>
48 <li><a href="#transforms">Transform Passes</a></li>
49 <li><a href="#utilities">Utility Passes</a></li>
50</ol>
51
52<div class="doc_author">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]53 <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
54 and Gordon Henriksen</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]55</div>
56
57<!-- ======================================================================= -->
58<div class="doc_section"> <a name="intro">Introduction</a> </div>
59<div class="doc_text">
60 <p>This document serves as a high level summary of the optimization features
61 that LLVM provides. Optimizations are implemented as Passes that traverse some
62 portion of a program to either collect information or transform the program.
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]63 The table below divides the passes that LLVM provides into three categories.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]64 Analysis passes compute information that other passes can use or for debugging
65 or program visualization purposes. Transform passes can use (or invalidate)
66 the analysis passes. Transform passes all mutate the program in some way.
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]67 Utility passes provides some utility but don't otherwise fit categorization.
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]68 For example passes to extract functions to bitcode or write a module to
69 bitcode are neither analysis nor transform passes.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]70 <p>The table below provides a quick summary of each pass and links to the more
71 complete pass description later in the document.</p>
72</div>
73<div class="doc_text" >
74<table>
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]75<tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr>
76<tr><th>Option</th><th>Name</th></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]77<tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]78<tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (default AA impl)</td></tr>
79<tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]80<tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]81<tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr>
82<tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr>
83<tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]84<tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]85<tr><td><a href="#dot-callgraph">-dot-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr>
86<tr><td><a href="#dot-cfg">-dot-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr>
87<tr><td><a href="#dot-cfg-only">-dot-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]88<tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]89<tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr>
90<tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]91<tr><td><a href="#loops">-loops</a></td><td>Natural Loop Construction</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]92<tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]93<tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr>
94<tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr>
95<tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]96<tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]97<tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr>
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]98<tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print a call graph</td></tr>
99<tr><td><a href="#print-callgraph-sccs">-print-callgraph-sccs</a></td><td>Print SCCs of the Call Graph</td></tr>
100<tr><td><a href="#print-cfg-sccs">-print-cfg-sccs</a></td><td>Print SCCs of each function CFG</td></tr>
101<tr><td><a href="#print-externalfnconstants">-print-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr>
102<tr><td><a href="#print-function">-print-function</a></td><td>Print function to stderr</td></tr>
103<tr><td><a href="#print-module">-print-module</a></td><td>Print module to stderr</td></tr>
104<tr><td><a href="#print-used-types">-print-used-types</a></td><td>Find Used Types</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]105<tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr>
106<tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr>
107<tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
108
109
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]110<tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr>
111<tr><th>Option</th><th>Name</th></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]112<tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr>
113<tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr>
114<tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]115<tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]116<tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Prepare a function for code generation </td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]117<tr><td><a href="#condprop">-condprop</a></td><td>Conditional Propagation</td></tr>
118<tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr>
119<tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr>
120<tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr>
121<tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr>
122<tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr>
123<tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr>
124<tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]125<tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr>
126<tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr>
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]127<tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]128<tr><td><a href="#indmemrem">-indmemrem</a></td><td>Indirect Malloc and Free Removal</td></tr>
129<tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr>
130<tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr>
131<tr><td><a href="#insert-block-profiling">-insert-block-profiling</a></td><td>Insert instrumentation for block profiling</td></tr>
132<tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr>
133<tr><td><a href="#insert-function-profiling">-insert-function-profiling</a></td><td>Insert instrumentation for function profiling</td></tr>
134<tr><td><a href="#insert-null-profiling-rs">-insert-null-profiling-rs</a></td><td>Measure profiling framework overhead</td></tr>
135<tr><td><a href="#insert-rs-profiling-framework">-insert-rs-profiling-framework</a></td><td>Insert random sampling instrumentation framework</td></tr>
136<tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr>
137<tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr>
138<tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr>
139<tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]140<tr><td><a href="#jump-threading">-jump-threading</a></td><td>Thread control through conditional blocks </td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]141<tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr>
142<tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]143<tr><td><a href="#loop-deletion">-loop-deletion</a></td><td>Dead Loop Deletion Pass </td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]144<tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr>
145<tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]146<tr><td><a href="#loop-index-split">-loop-index-split</a></td><td>Index Split Loops</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]147<tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]148<tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr>
149<tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr>
150<tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr>
151<tr><td><a href="#loopsimplify">-loopsimplify</a></td><td>Canonicalize natural loops</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]152<tr><td><a href="#lowerallocs">-lowerallocs</a></td><td>Lower allocations from instructions to calls</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]153<tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]154<tr><td><a href="#lowersetjmp">-lowersetjmp</a></td><td>Lower Set Jump</td></tr>
155<tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr>
156<tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]157<tr><td><a href="#memcpyopt">-memcpyopt</a></td><td>Optimize use of memcpy and friends</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]158<tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]159<tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]160<tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr>
161<tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]162<tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates</td></tr>
163<tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr>
164<tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr>
165<tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr>
166<tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]167<tr><td><a href="#strip-dead-prototypes">-strip-dead-prototypes</a></td><td>Remove unused function declarations</td></tr>
168<tr><td><a href="#sretpromotion">-sretpromotion</a></td><td>Promote sret arguments</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]169<tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr>
170<tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]171
172
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]173<tr><th colspan="2"><b>UTILITY PASSES</b></th></tr>
174<tr><th>Option</th><th>Name</th></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]175<tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr>
176<tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr>
Gordon Henriksen90a52142007-11-05 02:05:35 +0000[diff] [blame]177<tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]178<tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]179<tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr>
180<tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]181</table>
182</div>
183
184<!-- ======================================================================= -->
185<div class="doc_section"> <a name="example">Analysis Passes</a></div>
186<div class="doc_text">
187 <p>This section describes the LLVM Analysis Passes.</p>
188</div>
189
190<!-------------------------------------------------------------------------- -->
191<div class="doc_subsection">
192 <a name="aa-eval">Exhaustive Alias Analysis Precision Evaluator</a>
193</div>
194<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]195 <p>This is a simple N^2 alias analysis accuracy evaluator.
196 Basically, for each function in the program, it simply queries to see how the
197 alias analysis implementation answers alias queries between each pair of
198 pointers in the function.</p>
199
200 <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco
201 Spadini, and Wojciech Stryjewski.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]202</div>
203
204<!-------------------------------------------------------------------------- -->
205<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]206 <a name="basicaa">Basic Alias Analysis (default AA impl)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]207</div>
208<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]209 <p>
210 This is the default implementation of the Alias Analysis interface
211 that simply implements a few identities (two different globals cannot alias,
212 etc), but otherwise does no analysis.
213 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]214</div>
215
216<!-------------------------------------------------------------------------- -->
217<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]218 <a name="basiccg">Basic CallGraph Construction</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]219</div>
220<div class="doc_text">
221 <p>Yet to be written.</p>
222</div>
223
224<!-------------------------------------------------------------------------- -->
225<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]226 <a name="codegenprepare">Optimize for code generation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]227</div>
228<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]229 <p>
230 This pass munges the code in the input function to better prepare it for
231 SelectionDAG-based code generation. This works around limitations in it's
232 basic-block-at-a-time approach. It should eventually be removed.
233 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]234</div>
235
236<!-------------------------------------------------------------------------- -->
237<div class="doc_subsection">
238 <a name="count-aa">Count Alias Analysis Query Responses</a>
239</div>
240<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]241 <p>
242 A pass which can be used to count how many alias queries
243 are being made and how the alias analysis implementation being used responds.
244 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]245</div>
246
247<!-------------------------------------------------------------------------- -->
248<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]249 <a name="debug-aa">AA use debugger</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]250</div>
251<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]252 <p>
253 This simple pass checks alias analysis users to ensure that if they
254 create a new value, they do not query AA without informing it of the value.
255 It acts as a shim over any other AA pass you want.
256 </p>
257
258 <p>
259 Yes keeping track of every value in the program is expensive, but this is
260 a debugging pass.
261 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]262</div>
263
264<!-------------------------------------------------------------------------- -->
265<div class="doc_subsection">
266 <a name="domfrontier">Dominance Frontier Construction</a>
267</div>
268<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]269 <p>
270 This pass is a simple dominator construction algorithm for finding forward
271 dominator frontiers.
272 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]273</div>
274
275<!-------------------------------------------------------------------------- -->
276<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]277 <a name="domtree">Dominator Tree Construction</a>
278</div>
279<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]280 <p>
281 This pass is a simple dominator construction algorithm for finding forward
282 dominators.
283 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]284</div>
285
286<!-------------------------------------------------------------------------- -->
287<div class="doc_subsection">
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]288 <a name="dot-callgraph">Print Call Graph to 'dot' file</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]289</div>
290<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]291 <p>
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]292 This pass, only available in <code>opt</code>, prints the call graph into a
293 <code>.dot</code> graph. This graph can then be processed with the "dot" tool
294 to convert it to postscript or some other suitable format.
295 </p>
296</div>
297
298<!-------------------------------------------------------------------------- -->
299<div class="doc_subsection">
300 <a name="dot-cfg">Print CFG of function to 'dot' file</a>
301</div>
302<div class="doc_text">
303 <p>
304 This pass, only available in <code>opt</code>, prints the control flow graph
305 into a <code>.dot</code> graph. This graph can then be processed with the
306 "dot" tool to convert it to postscript or some other suitable format.
307 </p>
308</div>
309
310<!-------------------------------------------------------------------------- -->
311<div class="doc_subsection">
312 <a name="dot-cfg-only">Print CFG of function to 'dot' file (with no function bodies)</a>
313</div>
314<div class="doc_text">
315 <p>
316 This pass, only available in <code>opt</code>, prints the control flow graph
317 into a <code>.dot</code> graph, omitting the function bodies. This graph can
318 then be processed with the "dot" tool to convert it to postscript or some
319 other suitable format.
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]320 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]321</div>
322
323<!-------------------------------------------------------------------------- -->
324<div class="doc_subsection">
325 <a name="globalsmodref-aa">Simple mod/ref analysis for globals</a>
326</div>
327<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]328 <p>
329 This simple pass provides alias and mod/ref information for global values
330 that do not have their address taken, and keeps track of whether functions
331 read or write memory (are "pure"). For this simple (but very common) case,
332 we can provide pretty accurate and useful information.
333 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]334</div>
335
336<!-------------------------------------------------------------------------- -->
337<div class="doc_subsection">
338 <a name="instcount">Counts the various types of Instructions</a>
339</div>
340<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]341 <p>
342 This pass collects the count of all instructions and reports them
343 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]344</div>
345
346<!-------------------------------------------------------------------------- -->
347<div class="doc_subsection">
348 <a name="intervals">Interval Partition Construction</a>
349</div>
350<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]351 <p>
352 This analysis calculates and represents the interval partition of a function,
353 or a preexisting interval partition.
354 </p>
355
356 <p>
357 In this way, the interval partition may be used to reduce a flow graph down
358 to its degenerate single node interval partition (unless it is irreducible).
359 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]360</div>
361
362<!-------------------------------------------------------------------------- -->
363<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]364 <a name="loops">Natural Loop Construction</a>
365</div>
366<div class="doc_text">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]367 <p>
368 This analysis is used to identify natural loops and determine the loop depth
369 of various nodes of the CFG. Note that the loops identified may actually be
370 several natural loops that share the same header node... not just a single
371 natural loop.
372 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]373</div>
374
375<!-------------------------------------------------------------------------- -->
376<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]377 <a name="memdep">Memory Dependence Analysis</a>
378</div>
379<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]380 <p>
381 An analysis that determines, for a given memory operation, what preceding
382 memory operations it depends on. It builds on alias analysis information, and
383 tries to provide a lazy, caching interface to a common kind of alias
384 information query.
385 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]386</div>
387
388<!-------------------------------------------------------------------------- -->
389<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]390 <a name="no-aa">No Alias Analysis (always returns 'may' alias)</a>
391</div>
392<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]393 <p>
394 Always returns "I don't know" for alias queries. NoAA is unlike other alias
395 analysis implementations, in that it does not chain to a previous analysis. As
396 such it doesn't follow many of the rules that other alias analyses must.
397 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]398</div>
399
400<!-------------------------------------------------------------------------- -->
401<div class="doc_subsection">
402 <a name="no-profile">No Profile Information</a>
403</div>
404<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]405 <p>
406 The default "no profile" implementation of the abstract
407 <code>ProfileInfo</code> interface.
408 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]409</div>
410
411<!-------------------------------------------------------------------------- -->
412<div class="doc_subsection">
413 <a name="postdomfrontier">Post-Dominance Frontier Construction</a>
414</div>
415<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]416 <p>
417 This pass is a simple post-dominator construction algorithm for finding
418 post-dominator frontiers.
419 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]420</div>
421
422<!-------------------------------------------------------------------------- -->
423<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]424 <a name="postdomtree">Post-Dominator Tree Construction</a>
425</div>
426<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]427 <p>
428 This pass is a simple post-dominator construction algorithm for finding
429 post-dominators.
430 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]431</div>
432
433<!-------------------------------------------------------------------------- -->
434<div class="doc_subsection">
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]435 <a name="print-alias-sets">Alias Set Printer</a>
436</div>
437<div class="doc_text">
438 <p>Yet to be written.</p>
439</div>
440
441<!-------------------------------------------------------------------------- -->
442<div class="doc_subsection">
443 <a name="print-callgraph">Print a call graph</a>
444</div>
445<div class="doc_text">
446 <p>
447 This pass, only available in <code>opt</code>, prints the call graph to
448 standard output in a human-readable form.
449 </p>
450</div>
451
452<!-------------------------------------------------------------------------- -->
453<div class="doc_subsection">
454 <a name="print-callgraph-sccs">Print SCCs of the Call Graph</a>
455</div>
456<div class="doc_text">
457 <p>
458 This pass, only available in <code>opt</code>, prints the SCCs of the call
459 graph to standard output in a human-readable form.
460 </p>
461</div>
462
463<!-------------------------------------------------------------------------- -->
464<div class="doc_subsection">
465 <a name="print-cfg-sccs">Print SCCs of each function CFG</a>
466</div>
467<div class="doc_text">
468 <p>
469 This pass, only available in <code>opt</code>, prints the SCCs of each
470 function CFG to standard output in a human-readable form.
471 </p>
472</div>
473
474<!-------------------------------------------------------------------------- -->
475<div class="doc_subsection">
476 <a name="print-externalfnconstants">Print external fn callsites passed constants</a>
477</div>
478<div class="doc_text">
479 <p>
480 This pass, only available in <code>opt</code>, prints out call sites to
481 external functions that are called with constant arguments. This can be
482 useful when looking for standard library functions we should constant fold
483 or handle in alias analyses.
484 </p>
485</div>
486
487<!-------------------------------------------------------------------------- -->
488<div class="doc_subsection">
489 <a name="print-function">Print function to stderr</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]490</div>
491<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]492 <p>
493 The <code>PrintFunctionPass</code> class is designed to be pipelined with
494 other <code>FunctionPass</code>es, and prints out the functions of the module
495 as they are processed.
496 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]497</div>
498
499<!-------------------------------------------------------------------------- -->
500<div class="doc_subsection">
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]501 <a name="print-module">Print module to stderr</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]502</div>
503<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]504 <p>
505 This pass simply prints out the entire module when it is executed.
506 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]507</div>
508
509<!-------------------------------------------------------------------------- -->
510<div class="doc_subsection">
Duncan Sands3ee8fc92008-09-23 12:47:39 +0000[diff] [blame]511 <a name="print-used-types">Find Used Types</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]512</div>
513<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]514 <p>
515 This pass is used to seek out all of the types in use by the program. Note
516 that this analysis explicitly does not include types only used by the symbol
517 table.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]518</div>
519
520<!-------------------------------------------------------------------------- -->
521<div class="doc_subsection">
522 <a name="profile-loader">Load profile information from llvmprof.out</a>
523</div>
524<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]525 <p>
526 A concrete implementation of profiling information that loads the information
527 from a profile dump file.
528 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]529</div>
530
531<!-------------------------------------------------------------------------- -->
532<div class="doc_subsection">
533 <a name="scalar-evolution">Scalar Evolution Analysis</a>
534</div>
535<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]536 <p>
537 The <code>ScalarEvolution</code> analysis can be used to analyze and
538 catagorize scalar expressions in loops. It specializes in recognizing general
539 induction variables, representing them with the abstract and opaque
540 <code>SCEV</code> class. Given this analysis, trip counts of loops and other
541 important properties can be obtained.
542 </p>
543
544 <p>
545 This analysis is primarily useful for induction variable substitution and
546 strength reduction.
547 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]548</div>
549
550<!-------------------------------------------------------------------------- -->
551<div class="doc_subsection">
552 <a name="targetdata">Target Data Layout</a>
553</div>
554<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]555 <p>Provides other passes access to information on how the size and alignment
556 required by the the target ABI for various data types.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]557</div>
558
559<!-- ======================================================================= -->
560<div class="doc_section"> <a name="transform">Transform Passes</a></div>
561<div class="doc_text">
562 <p>This section describes the LLVM Transform Passes.</p>
563</div>
564
565<!-------------------------------------------------------------------------- -->
566<div class="doc_subsection">
567 <a name="adce">Aggressive Dead Code Elimination</a>
568</div>
569<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]570 <p>ADCE aggressively tries to eliminate code. This pass is similar to
571 <a href="#dce">DCE</a> but it assumes that values are dead until proven
572 otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to
573 the liveness of values.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]574</div>
575
576<!-------------------------------------------------------------------------- -->
577<div class="doc_subsection">
578 <a name="argpromotion">Promote 'by reference' arguments to scalars</a>
579</div>
580<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]581 <p>
582 This pass promotes "by reference" arguments to be "by value" arguments. In
583 practice, this means looking for internal functions that have pointer
584 arguments. If it can prove, through the use of alias analysis, that an
585 argument is *only* loaded, then it can pass the value into the function
586 instead of the address of the value. This can cause recursive simplification
587 of code and lead to the elimination of allocas (especially in C++ template
588 code like the STL).
589 </p>
590
591 <p>
592 This pass also handles aggregate arguments that are passed into a function,
593 scalarizing them if the elements of the aggregate are only loaded. Note that
594 it refuses to scalarize aggregates which would require passing in more than
595 three operands to the function, because passing thousands of operands for a
596 large array or structure is unprofitable!
597 </p>
598
599 <p>
600 Note that this transformation could also be done for arguments that are only
601 stored to (returning the value instead), but does not currently. This case
602 would be best handled when and if LLVM starts supporting multiple return
603 values from functions.
604 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]605</div>
606
607<!-------------------------------------------------------------------------- -->
608<div class="doc_subsection">
609 <a name="block-placement">Profile Guided Basic Block Placement</a>
610</div>
611<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]612 <p>This pass is a very simple profile guided basic block placement algorithm.
613 The idea is to put frequently executed blocks together at the start of the
614 function and hopefully increase the number of fall-through conditional
615 branches. If there is no profile information for a particular function, this
616 pass basically orders blocks in depth-first order.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]617</div>
618
619<!-------------------------------------------------------------------------- -->
620<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]621 <a name="break-crit-edges">Break critical edges in CFG</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]622</div>
623<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]624 <p>
625 Break all of the critical edges in the CFG by inserting a dummy basic block.
626 It may be "required" by passes that cannot deal with critical edges. This
627 transformation obviously invalidates the CFG, but can update forward dominator
628 (set, immediate dominators, tree, and frontier) information.
629 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]630</div>
631
632<!-------------------------------------------------------------------------- -->
633<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]634 <a name="codegenprepare">Prepare a function for code generation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]635</div>
636<div class="doc_text">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]637 This pass munges the code in the input function to better prepare it for
638 SelectionDAG-based code generation. This works around limitations in it's
639 basic-block-at-a-time approach. It should eventually be removed.
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]640</div>
641
642<!-------------------------------------------------------------------------- -->
643<div class="doc_subsection">
644 <a name="condprop">Conditional Propagation</a>
645</div>
646<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]647 <p>This pass propagates information about conditional expressions through the
648 program, allowing it to eliminate conditional branches in some cases.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]649</div>
650
651<!-------------------------------------------------------------------------- -->
652<div class="doc_subsection">
653 <a name="constmerge">Merge Duplicate Global Constants</a>
654</div>
655<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]656 <p>
657 Merges duplicate global constants together into a single constant that is
658 shared. This is useful because some passes (ie TraceValues) insert a lot of
659 string constants into the program, regardless of whether or not an existing
660 string is available.
661 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]662</div>
663
664<!-------------------------------------------------------------------------- -->
665<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]666 <a name="constprop">Simple constant propagation</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]667</div>
668<div class="doc_text">
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]669 <p>This file implements constant propagation and merging. It looks for
670 instructions involving only constant operands and replaces them with a
Gordon Henriksenddaa61d2007-10-25 08:58:56 +0000[diff] [blame]671 constant value instead of an instruction. For example:</p>
672 <blockquote><pre>add i32 1, 2</pre></blockquote>
673 <p>becomes</p>
674 <blockquote><pre>i32 3</pre></blockquote>
Reid Spenceraf4af3a2007-03-27 02:49:31 +0000[diff] [blame]675 <p>NOTE: this pass has a habit of making definitions be dead. It is a good
676 idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass
677 sometime after running this pass.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]678</div>
679
680<!-------------------------------------------------------------------------- -->
681<div class="doc_subsection">
682 <a name="dce">Dead Code Elimination</a>
683</div>
684<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]685 <p>
686 Dead code elimination is similar to <a href="#die">dead instruction
687 elimination</a>, but it rechecks instructions that were used by removed
688 instructions to see if they are newly dead.
689 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]690</div>
691
692<!-------------------------------------------------------------------------- -->
693<div class="doc_subsection">
694 <a name="deadargelim">Dead Argument Elimination</a>
695</div>
696<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]697 <p>
698 This pass deletes dead arguments from internal functions. Dead argument
699 elimination removes arguments which are directly dead, as well as arguments
700 only passed into function calls as dead arguments of other functions. This
701 pass also deletes dead arguments in a similar way.
702 </p>
703
704 <p>
705 This pass is often useful as a cleanup pass to run after aggressive
706 interprocedural passes, which add possibly-dead arguments.
707 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]708</div>
709
710<!-------------------------------------------------------------------------- -->
711<div class="doc_subsection">
712 <a name="deadtypeelim">Dead Type Elimination</a>
713</div>
714<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]715 <p>
716 This pass is used to cleanup the output of GCC. It eliminate names for types
717 that are unused in the entire translation unit, using the <a
718 href="#findusedtypes">find used types</a> pass.
719 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]720</div>
721
722<!-------------------------------------------------------------------------- -->
723<div class="doc_subsection">
724 <a name="die">Dead Instruction Elimination</a>
725</div>
726<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]727 <p>
728 Dead instruction elimination performs a single pass over the function,
729 removing instructions that are obviously dead.
730 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]731</div>
732
733<!-------------------------------------------------------------------------- -->
734<div class="doc_subsection">
735 <a name="dse">Dead Store Elimination</a>
736</div>
737<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]738 <p>
739 A trivial dead store elimination that only considers basic-block local
740 redundant stores.
741 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]742</div>
743
744<!-------------------------------------------------------------------------- -->
745<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]746 <a name="globaldce">Dead Global Elimination</a>
747</div>
748<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]749 <p>
750 This transform is designed to eliminate unreachable internal globals from the
751 program. It uses an aggressive algorithm, searching out globals that are
752 known to be alive. After it finds all of the globals which are needed, it
753 deletes whatever is left over. This allows it to delete recursive chunks of
754 the program which are unreachable.
755 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]756</div>
757
758<!-------------------------------------------------------------------------- -->
759<div class="doc_subsection">
760 <a name="globalopt">Global Variable Optimizer</a>
761</div>
762<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]763 <p>
764 This pass transforms simple global variables that never have their address
765 taken. If obviously true, it marks read/write globals as constant, deletes
766 variables only stored to, etc.
767 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]768</div>
769
770<!-------------------------------------------------------------------------- -->
771<div class="doc_subsection">
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]772 <a name="gvn">Global Value Numbering</a>
773</div>
774<div class="doc_text">
775 <p>
Chris Lattner60f03402009-10-10 18:40:48 +0000[diff] [blame]776 This pass performs global value numbering to eliminate fully and partially
777 redundant instructions. It also performs redundant load elimination.
Matthijs Kooijman845f5242008-06-05 07:55:49 +0000[diff] [blame]778 </p>
Gordon Henriksen0e15dc22007-10-25 10:18:27 +0000[diff] [blame]779</div>
780
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]781
782<!-------------------------------------------------------------------------- -->
783<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]784 <a name="indmemrem">Indirect Malloc and Free Removal</a>
785</div>
786<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]787 <p>
788 This pass finds places where memory allocation functions may escape into
789 indirect land. Some transforms are much easier (aka possible) only if free
790 or malloc are not called indirectly.
791 </p>
792
793 <p>
794 Thus find places where the address of memory functions are taken and construct
795 bounce functions with direct calls of those functions.
796 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]797</div>
798
799<!-------------------------------------------------------------------------- -->
800<div class="doc_subsection">
801 <a name="indvars">Canonicalize Induction Variables</a>
802</div>
803<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]804 <p>
805 This transformation analyzes and transforms the induction variables (and
806 computations derived from them) into simpler forms suitable for subsequent
807 analysis and transformation.
808 </p>
809
810 <p>
811 This transformation makes the following changes to each loop with an
812 identifiable induction variable:
813 </p>
814
815 <ol>
816 <li>All loops are transformed to have a <em>single</em> canonical
817 induction variable which starts at zero and steps by one.</li>
818 <li>The canonical induction variable is guaranteed to be the first PHI node
819 in the loop header block.</li>
820 <li>Any pointer arithmetic recurrences are raised to use array
821 subscripts.</li>
822 </ol>
823
824 <p>
825 If the trip count of a loop is computable, this pass also makes the following
826 changes:
827 </p>
828
829 <ol>
830 <li>The exit condition for the loop is canonicalized to compare the
831 induction value against the exit value. This turns loops like:
832 <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote>
833 into
834 <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li>
835 <li>Any use outside of the loop of an expression derived from the indvar
836 is changed to compute the derived value outside of the loop, eliminating
837 the dependence on the exit value of the induction variable. If the only
838 purpose of the loop is to compute the exit value of some derived
839 expression, this transformation will make the loop dead.</li>
Gordon Henriksene626bbe2007-11-04 16:17:00 +0000[diff] [blame]840 </ol>
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]841
842 <p>
843 This transformation should be followed by strength reduction after all of the
844 desired loop transformations have been performed. Additionally, on targets
845 where it is profitable, the loop could be transformed to count down to zero
846 (the "do loop" optimization).
847 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]848</div>
849
850<!-------------------------------------------------------------------------- -->
851<div class="doc_subsection">
852 <a name="inline">Function Integration/Inlining</a>
853</div>
854<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]855 <p>
856 Bottom-up inlining of functions into callees.
857 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]858</div>
859
860<!-------------------------------------------------------------------------- -->
861<div class="doc_subsection">
862 <a name="insert-block-profiling">Insert instrumentation for block profiling</a>
863</div>
864<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]865 <p>
866 This pass instruments the specified program with counters for basic block
867 profiling, which counts the number of times each basic block executes. This
868 is the most basic form of profiling, which can tell which blocks are hot, but
869 cannot reliably detect hot paths through the CFG.
870 </p>
871
872 <p>
873 Note that this implementation is very naïve. Control equivalent regions of
874 the CFG should not require duplicate counters, but it does put duplicate
875 counters in.
876 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]877</div>
878
879<!-------------------------------------------------------------------------- -->
880<div class="doc_subsection">
881 <a name="insert-edge-profiling">Insert instrumentation for edge profiling</a>
882</div>
883<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]884 <p>
885 This pass instruments the specified program with counters for edge profiling.
886 Edge profiling can give a reasonable approximation of the hot paths through a
887 program, and is used for a wide variety of program transformations.
888 </p>
889
890 <p>
891 Note that this implementation is very naïve. It inserts a counter for
892 <em>every</em> edge in the program, instead of using control flow information
893 to prune the number of counters inserted.
894 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]895</div>
896
897<!-------------------------------------------------------------------------- -->
898<div class="doc_subsection">
899 <a name="insert-function-profiling">Insert instrumentation for function profiling</a>
900</div>
901<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]902 <p>
903 This pass instruments the specified program with counters for function
904 profiling, which counts the number of times each function is called.
905 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]906</div>
907
908<!-------------------------------------------------------------------------- -->
909<div class="doc_subsection">
910 <a name="insert-null-profiling-rs">Measure profiling framework overhead</a>
911</div>
912<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]913 <p>
914 The basic profiler that does nothing. It is the default profiler and thus
915 terminates <code>RSProfiler</code> chains. It is useful for measuring
916 framework overhead.
917 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]918</div>
919
920<!-------------------------------------------------------------------------- -->
921<div class="doc_subsection">
922 <a name="insert-rs-profiling-framework">Insert random sampling instrumentation framework</a>
923</div>
924<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]925 <p>
926 The second stage of the random-sampling instrumentation framework, duplicates
927 all instructions in a function, ignoring the profiling code, then connects the
928 two versions together at the entry and at backedges. At each connection point
929 a choice is made as to whether to jump to the profiled code (take a sample) or
930 execute the unprofiled code.
931 </p>
932
933 <p>
934 After this pass, it is highly recommended to run<a href="#mem2reg">mem2reg</a>
935 and <a href="#adce">adce</a>. <a href="#instcombine">instcombine</a>,
936 <a href="#load-vn">load-vn</a>, <a href="#gdce">gdce</a>, and
937 <a href="#dse">dse</a> also are good to run afterwards.
938 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]939</div>
940
941<!-------------------------------------------------------------------------- -->
942<div class="doc_subsection">
943 <a name="instcombine">Combine redundant instructions</a>
944</div>
945<div class="doc_text">
Gordon Henriksen55cbec32007-10-26 03:03:51 +0000[diff] [blame]946 <p>
947 Combine instructions to form fewer, simple
948 instructions. This pass does not modify the CFG This pass is where algebraic
949 simplification happens.
950 </p>
951
952 <p>
953 This pass combines things like:
954 </p>
955
956<blockquote><pre
957>%Y = add i32 %X, 1
958%Z = add i32 %Y, 1</pre></blockquote>
959
960 <p>
961 into:
962 </p>
963
964<blockquote><pre
965>%Z = add i32 %X, 2</pre></blockquote>
966
967 <p>
968 This is a simple worklist driven algorithm.
969 </p>
970
971 <p>
972 This pass guarantees that the following canonicalizations are performed on
973 the program:
974 </p>
975
976 <ul>
977 <li>If a binary operator has a constant operand, it is moved to the right-
978 hand side.</li>
979 <li>Bitwise operators with constant operands are always grouped so that
980 shifts are performed first, then <code>or</code>s, then
981 <code>and</code>s, then <code>xor</code>s.</li>
982 <li>Compare instructions are converted from <code>&lt;</code>,
983 <code>&gt;</code>, <code>≤</code>, or <code>≥</code> to
984 <code>=</code> or <code>≠</code> if possible.</li>
985 <li>All <code>cmp</code> instructions on boolean values are replaced with
986 logical operations.</li>
987 <li><code>add <var>X</var>, <var>X</var></code> is represented as
988 <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li>
989 <li>Multiplies with a constant power-of-two argument are transformed into
990 shifts.</li>
991 <li>… etc.</li>
992 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]993</div>
994
995<!-------------------------------------------------------------------------- -->
996<div class="doc_subsection">
997 <a name="internalize">Internalize Global Symbols</a>
998</div>
999<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1000 <p>
1001 This pass loops over all of the functions in the input module, looking for a
1002 main function. If a main function is found, all other functions and all
1003 global variables with initializers are marked as internal.
1004 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1005</div>
1006
1007<!-------------------------------------------------------------------------- -->
1008<div class="doc_subsection">
1009 <a name="ipconstprop">Interprocedural constant propagation</a>
1010</div>
1011<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1012 <p>
1013 This pass implements an <em>extremely</em> simple interprocedural constant
1014 propagation pass. It could certainly be improved in many different ways,
1015 like using a worklist. This pass makes arguments dead, but does not remove
1016 them. The existing dead argument elimination pass should be run after this
1017 to clean up the mess.
1018 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1019</div>
1020
1021<!-------------------------------------------------------------------------- -->
1022<div class="doc_subsection">
1023 <a name="ipsccp">Interprocedural Sparse Conditional Constant Propagation</a>
1024</div>
1025<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1026 <p>
1027 An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
1028 Propagation</a>.
1029 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1030</div>
1031
1032<!-------------------------------------------------------------------------- -->
1033<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]1034 <a name="jump-threading">Thread control through conditional blocks</a>
1035</div>
1036<div class="doc_text">
1037 <p>
1038 Jump threading tries to find distinct threads of control flow running through
1039 a basic block. This pass looks at blocks that have multiple predecessors and
1040 multiple successors. If one or more of the predecessors of the block can be
1041 proven to always cause a jump to one of the successors, we forward the edge
1042 from the predecessor to the successor by duplicating the contents of this
1043 block.
1044 </p>
1045 <p>
1046 An example of when this can occur is code like this:
1047 </p>
1048
1049 <pre
1050>if () { ...
1051 X = 4;
1052}
1053if (X &lt; 3) {</pre>
1054
1055 <p>
1056 In this case, the unconditional branch at the end of the first if can be
1057 revectored to the false side of the second if.
1058 </p>
1059</div>
1060
1061<!-------------------------------------------------------------------------- -->
1062<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1063 <a name="lcssa">Loop-Closed SSA Form Pass</a>
1064</div>
1065<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1066 <p>
1067 This pass transforms loops by placing phi nodes at the end of the loops for
1068 all values that are live across the loop boundary. For example, it turns
1069 the left into the right code:
1070 </p>
1071
1072 <pre
1073>for (...) for (...)
1074 if (c) if (c)
1075 X1 = ... X1 = ...
1076 else else
1077 X2 = ... X2 = ...
1078 X3 = phi(X1, X2) X3 = phi(X1, X2)
1079... = X3 + 4 X4 = phi(X3)
1080 ... = X4 + 4</pre>
1081
1082 <p>
1083 This is still valid LLVM; the extra phi nodes are purely redundant, and will
1084 be trivially eliminated by <code>InstCombine</code>. The major benefit of
1085 this transformation is that it makes many other loop optimizations, such as
1086 LoopUnswitching, simpler.
1087 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1088</div>
1089
1090<!-------------------------------------------------------------------------- -->
1091<div class="doc_subsection">
1092 <a name="licm">Loop Invariant Code Motion</a>
1093</div>
1094<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1095 <p>
1096 This pass performs loop invariant code motion, attempting to remove as much
1097 code from the body of a loop as possible. It does this by either hoisting
1098 code into the preheader block, or by sinking code to the exit blocks if it is
1099 safe. This pass also promotes must-aliased memory locations in the loop to
1100 live in registers, thus hoisting and sinking "invariant" loads and stores.
1101 </p>
1102
1103 <p>
1104 This pass uses alias analysis for two purposes:
1105 </p>
1106
1107 <ul>
1108 <li>Moving loop invariant loads and calls out of loops. If we can determine
1109 that a load or call inside of a loop never aliases anything stored to,
1110 we can hoist it or sink it like any other instruction.</li>
1111 <li>Scalar Promotion of Memory - If there is a store instruction inside of
1112 the loop, we try to move the store to happen AFTER the loop instead of
1113 inside of the loop. This can only happen if a few conditions are true:
1114 <ul>
1115 <li>The pointer stored through is loop invariant.</li>
1116 <li>There are no stores or loads in the loop which <em>may</em> alias
1117 the pointer. There are no calls in the loop which mod/ref the
1118 pointer.</li>
1119 </ul>
1120 If these conditions are true, we can promote the loads and stores in the
1121 loop of the pointer to use a temporary alloca'd variable. We then use
1122 the mem2reg functionality to construct the appropriate SSA form for the
1123 variable.</li>
1124 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1125</div>
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]1126<!-------------------------------------------------------------------------- -->
1127<div class="doc_subsection">
1128 <a name="loop-deletion">Dead Loop Deletion Pass</a>
1129</div>
1130<div class="doc_text">
1131 <p>
1132 This file implements the Dead Loop Deletion Pass. This pass is responsible
1133 for eliminating loops with non-infinite computable trip counts that have no
1134 side effects or volatile instructions, and do not contribute to the
1135 computation of the function's return value.
1136 </p>
1137</div>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1138
1139<!-------------------------------------------------------------------------- -->
1140<div class="doc_subsection">
1141 <a name="loop-extract">Extract loops into new functions</a>
1142</div>
1143<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1144 <p>
1145 A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to
1146 extract each top-level loop into its own new function. If the loop is the
1147 <em>only</em> loop in a given function, it is not touched. This is a pass most
1148 useful for debugging via bugpoint.
1149 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1150</div>
1151
1152<!-------------------------------------------------------------------------- -->
1153<div class="doc_subsection">
1154 <a name="loop-extract-single">Extract at most one loop into a new function</a>
1155</div>
1156<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1157 <p>
1158 Similar to <a href="#loop-extract">Extract loops into new functions</a>,
1159 this pass extracts one natural loop from the program into a function if it
1160 can. This is used by bugpoint.
1161 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1162</div>
1163
1164<!-------------------------------------------------------------------------- -->
1165<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1166 <a name="loop-index-split">Index Split Loops</a>
1167</div>
1168<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1169 <p>
1170 This pass divides loop's iteration range by spliting loop such that each
1171 individual loop is executed efficiently.
1172 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1173</div>
1174
1175<!-------------------------------------------------------------------------- -->
1176<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1177 <a name="loop-reduce">Loop Strength Reduction</a>
1178</div>
1179<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1180 <p>
1181 This pass performs a strength reduction on array references inside loops that
1182 have as one or more of their components the loop induction variable. This is
1183 accomplished by creating a new value to hold the initial value of the array
1184 access for the first iteration, and then creating a new GEP instruction in
1185 the loop to increment the value by the appropriate amount.
1186 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1187</div>
1188
1189<!-------------------------------------------------------------------------- -->
1190<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1191 <a name="loop-rotate">Rotate Loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1192</div>
1193<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1194 <p>A simple loop rotation transformation.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1195</div>
1196
1197<!-------------------------------------------------------------------------- -->
1198<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1199 <a name="loop-unroll">Unroll loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1200</div>
1201<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1202 <p>
1203 This pass implements a simple loop unroller. It works best when loops have
1204 been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass,
1205 allowing it to determine the trip counts of loops easily.
1206 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1207</div>
1208
1209<!-------------------------------------------------------------------------- -->
1210<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1211 <a name="loop-unswitch">Unswitch loops</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1212</div>
1213<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1214 <p>
1215 This pass transforms loops that contain branches on loop-invariant conditions
1216 to have multiple loops. For example, it turns the left into the right code:
1217 </p>
1218
1219 <pre
1220>for (...) if (lic)
1221 A for (...)
1222 if (lic) A; B; C
1223 B else
1224 C for (...)
1225 A; C</pre>
1226
1227 <p>
1228 This can increase the size of the code exponentially (doubling it every time
1229 a loop is unswitched) so we only unswitch if the resultant code will be
1230 smaller than a threshold.
1231 </p>
1232
1233 <p>
1234 This pass expects LICM to be run before it to hoist invariant conditions out
1235 of the loop, to make the unswitching opportunity obvious.
1236 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1237</div>
1238
1239<!-------------------------------------------------------------------------- -->
1240<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1241 <a name="loopsimplify">Canonicalize natural loops</a>
1242</div>
1243<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1244 <p>
1245 This pass performs several transformations to transform natural loops into a
1246 simpler form, which makes subsequent analyses and transformations simpler and
1247 more effective.
1248 </p>
1249
1250 <p>
1251 Loop pre-header insertion guarantees that there is a single, non-critical
1252 entry edge from outside of the loop to the loop header. This simplifies a
1253 number of analyses and transformations, such as LICM.
1254 </p>
1255
1256 <p>
1257 Loop exit-block insertion guarantees that all exit blocks from the loop
1258 (blocks which are outside of the loop that have predecessors inside of the
1259 loop) only have predecessors from inside of the loop (and are thus dominated
1260 by the loop header). This simplifies transformations such as store-sinking
1261 that are built into LICM.
1262 </p>
1263
1264 <p>
1265 This pass also guarantees that loops will have exactly one backedge.
1266 </p>
1267
1268 <p>
1269 Note that the simplifycfg pass will clean up blocks which are split out but
1270 end up being unnecessary, so usage of this pass should not pessimize
1271 generated code.
1272 </p>
1273
1274 <p>
1275 This pass obviously modifies the CFG, but updates loop information and
1276 dominator information.
1277 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1278</div>
1279
1280<!-------------------------------------------------------------------------- -->
1281<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1282 <a name="lowerallocs">Lower allocations from instructions to calls</a>
1283</div>
1284<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1285 <p>
1286 Turn <tt>malloc</tt> and <tt>free</tt> instructions into <tt>@malloc</tt> and
1287 <tt>@free</tt> calls.
1288 </p>
1289
1290 <p>
1291 This is a target-dependent tranformation because it depends on the size of
1292 data types and alignment constraints.
1293 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1294</div>
1295
1296<!-------------------------------------------------------------------------- -->
1297<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1298 <a name="lowerinvoke">Lower invoke and unwind, for unwindless code generators</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1299</div>
1300<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1301 <p>
1302 This transformation is designed for use by code generators which do not yet
1303 support stack unwinding. This pass supports two models of exception handling
1304 lowering, the 'cheap' support and the 'expensive' support.
1305 </p>
1306
1307 <p>
1308 'Cheap' exception handling support gives the program the ability to execute
1309 any program which does not "throw an exception", by turning 'invoke'
1310 instructions into calls and by turning 'unwind' instructions into calls to
1311 abort(). If the program does dynamically use the unwind instruction, the
1312 program will print a message then abort.
1313 </p>
1314
1315 <p>
1316 'Expensive' exception handling support gives the full exception handling
1317 support to the program at the cost of making the 'invoke' instruction
1318 really expensive. It basically inserts setjmp/longjmp calls to emulate the
1319 exception handling as necessary.
1320 </p>
1321
1322 <p>
1323 Because the 'expensive' support slows down programs a lot, and EH is only
1324 used for a subset of the programs, it must be specifically enabled by the
1325 <tt>-enable-correct-eh-support</tt> option.
1326 </p>
1327
1328 <p>
1329 Note that after this pass runs the CFG is not entirely accurate (exceptional
1330 control flow edges are not correct anymore) so only very simple things should
1331 be done after the lowerinvoke pass has run (like generation of native code).
1332 This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't
1333 support the invoke instruction yet" lowering pass.
1334 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1335</div>
1336
1337<!-------------------------------------------------------------------------- -->
1338<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1339 <a name="lowersetjmp">Lower Set Jump</a>
1340</div>
1341<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1342 <p>
1343 Lowers <tt>setjmp</tt> and <tt>longjmp</tt> to use the LLVM invoke and unwind
1344 instructions as necessary.
1345 </p>
1346
1347 <p>
1348 Lowering of <tt>longjmp</tt> is fairly trivial. We replace the call with a
1349 call to the LLVM library function <tt>__llvm_sjljeh_throw_longjmp()</tt>.
1350 This unwinds the stack for us calling all of the destructors for
1351 objects allocated on the stack.
1352 </p>
1353
1354 <p>
1355 At a <tt>setjmp</tt> call, the basic block is split and the <tt>setjmp</tt>
1356 removed. The calls in a function that have a <tt>setjmp</tt> are converted to
1357 invoke where the except part checks to see if it's a <tt>longjmp</tt>
1358 exception and, if so, if it's handled in the function. If it is, then it gets
1359 the value returned by the <tt>longjmp</tt> and goes to where the basic block
1360 was split. <tt>invoke</tt> instructions are handled in a similar fashion with
1361 the original except block being executed if it isn't a <tt>longjmp</tt>
1362 except that is handled by that function.
1363 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1364</div>
1365
1366<!-------------------------------------------------------------------------- -->
1367<div class="doc_subsection">
1368 <a name="lowerswitch">Lower SwitchInst's to branches</a>
1369</div>
1370<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1371 <p>
1372 Rewrites <tt>switch</tt> instructions with a sequence of branches, which
1373 allows targets to get away with not implementing the switch instruction until
1374 it is convenient.
1375 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1376</div>
1377
1378<!-------------------------------------------------------------------------- -->
1379<div class="doc_subsection">
1380 <a name="mem2reg">Promote Memory to Register</a>
1381</div>
1382<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1383 <p>
1384 This file promotes memory references to be register references. It promotes
1385 <tt>alloca</tt> instructions which only have <tt>load</tt>s and
1386 <tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator
1387 frontiers to place <tt>phi</tt> nodes, then traversing the function in
1388 depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as
1389 appropriate. This is just the standard SSA construction algorithm to construct
1390 "pruned" SSA form.
1391 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1392</div>
1393
1394<!-------------------------------------------------------------------------- -->
1395<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]1396 <a name="memcpyopt">Optimize use of memcpy and friend</a>
1397</div>
1398<div class="doc_text">
1399 <p>
1400 This pass performs various transformations related to eliminating memcpy
1401 calls, or transforming sets of stores into memset's.
1402 </p>
1403</div>
1404
1405<!-------------------------------------------------------------------------- -->
1406<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1407 <a name="mergereturn">Unify function exit nodes</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1408</div>
1409<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1410 <p>
1411 Ensure that functions have at most one <tt>ret</tt> instruction in them.
1412 Additionally, it keeps track of which node is the new exit node of the CFG.
1413 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1414</div>
1415
1416<!-------------------------------------------------------------------------- -->
1417<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1418 <a name="prune-eh">Remove unused exception handling info</a>
1419</div>
1420<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1421 <p>
1422 This file implements a simple interprocedural pass which walks the call-graph,
1423 turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and
1424 only if the callee cannot throw an exception. It implements this as a
1425 bottom-up traversal of the call-graph.
1426 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1427</div>
1428
1429<!-------------------------------------------------------------------------- -->
1430<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1431 <a name="reassociate">Reassociate expressions</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1432</div>
1433<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1434 <p>
1435 This pass reassociates commutative expressions in an order that is designed
1436 to promote better constant propagation, GCSE, LICM, PRE, etc.
1437 </p>
1438
1439 <p>
1440 For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5)
1441 </p>
1442
1443 <p>
1444 In the implementation of this algorithm, constants are assigned rank = 0,
1445 function arguments are rank = 1, and other values are assigned ranks
1446 corresponding to the reverse post order traversal of current function
1447 (starting at 2), which effectively gives values in deep loops higher rank
1448 than values not in loops.
1449 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1450</div>
1451
1452<!-------------------------------------------------------------------------- -->
1453<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1454 <a name="reg2mem">Demote all values to stack slots</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1455</div>
1456<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1457 <p>
1458 This file demotes all registers to memory references. It is intented to be
1459 the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]1460 <tt>load</tt> instructions, the only values live across basic blocks are
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1461 <tt>alloca</tt> instructions and <tt>load</tt> instructions before
1462 <tt>phi</tt> nodes. It is intended that this should make CFG hacking much
1463 easier. To make later hacking easier, the entry block is split into two, such
1464 that all introduced <tt>alloca</tt> instructions (and nothing else) are in the
1465 entry block.
1466 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1467</div>
1468
1469<!-------------------------------------------------------------------------- -->
1470<div class="doc_subsection">
1471 <a name="scalarrepl">Scalar Replacement of Aggregates</a>
1472</div>
1473<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1474 <p>
1475 The well-known scalar replacement of aggregates transformation. This
1476 transform breaks up <tt>alloca</tt> instructions of aggregate type (structure
1477 or array) into individual <tt>alloca</tt> instructions for each member if
1478 possible. Then, if possible, it transforms the individual <tt>alloca</tt>
1479 instructions into nice clean scalar SSA form.
1480 </p>
1481
1482 <p>
1483 This combines a simple scalar replacement of aggregates algorithm with the <a
1484 href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact,
1485 especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>,
1486 then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to
1487 promote works well.
1488 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1489</div>
1490
1491<!-------------------------------------------------------------------------- -->
1492<div class="doc_subsection">
1493 <a name="sccp">Sparse Conditional Constant Propagation</a>
1494</div>
1495<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1496 <p>
1497 Sparse conditional constant propagation and merging, which can be summarized
1498 as:
1499 </p>
1500
1501 <ol>
1502 <li>Assumes values are constant unless proven otherwise</li>
1503 <li>Assumes BasicBlocks are dead unless proven otherwise</li>
1504 <li>Proves values to be constant, and replaces them with constants</li>
1505 <li>Proves conditional branches to be unconditional</li>
1506 </ol>
1507
1508 <p>
1509 Note that this pass has a habit of making definitions be dead. It is a good
1510 idea to to run a DCE pass sometime after running this pass.
1511 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1512</div>
1513
1514<!-------------------------------------------------------------------------- -->
1515<div class="doc_subsection">
1516 <a name="simplify-libcalls">Simplify well-known library calls</a>
1517</div>
1518<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1519 <p>
1520 Applies a variety of small optimizations for calls to specific well-known
1521 function calls (e.g. runtime library functions). For example, a call
1522 <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be
1523 transformed into simply <tt>return 3</tt>.
1524 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1525</div>
1526
1527<!-------------------------------------------------------------------------- -->
1528<div class="doc_subsection">
1529 <a name="simplifycfg">Simplify the CFG</a>
1530</div>
1531<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1532 <p>
1533 Performs dead code elimination and basic block merging. Specifically:
1534 </p>
1535
1536 <ol>
1537 <li>Removes basic blocks with no predecessors.</li>
1538 <li>Merges a basic block into its predecessor if there is only one and the
1539 predecessor only has one successor.</li>
1540 <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li>
1541 <li>Eliminates a basic block that only contains an unconditional
1542 branch.</li>
1543 </ol>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1544</div>
1545
1546<!-------------------------------------------------------------------------- -->
1547<div class="doc_subsection">
1548 <a name="strip">Strip all symbols from a module</a>
1549</div>
1550<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1551 <p>
1552 Performs code stripping. This transformation can delete:
1553 </p>
1554
1555 <ol>
1556 <li>names for virtual registers</li>
1557 <li>symbols for internal globals and functions</li>
1558 <li>debug information</li>
1559 </ol>
1560
1561 <p>
1562 Note that this transformation makes code much less readable, so it should
1563 only be used in situations where the <tt>strip</tt> utility would be used,
1564 such as reducing code size or making it harder to reverse engineer code.
1565 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1566</div>
1567
1568<!-------------------------------------------------------------------------- -->
1569<div class="doc_subsection">
Gordon Henriksena8a118b2008-05-08 17:46:35 +0000[diff] [blame]1570 <a name="strip-dead-prototypes">Remove unused function declarations</a>
1571</div>
1572<div class="doc_text">
1573 <p>
1574 This pass loops over all of the functions in the input module, looking for
1575 dead declarations and removes them. Dead declarations are declarations of
1576 functions for which no implementation is available (i.e., declarations for
1577 unused library functions).
1578 </p>
1579</div>
1580
1581<!-------------------------------------------------------------------------- -->
1582<div class="doc_subsection">
1583 <a name="sretpromotion">Promote sret arguments</a>
1584</div>
1585<div class="doc_text">
1586 <p>
1587 This pass finds functions that return a struct (using a pointer to the struct
1588 as the first argument of the function, marked with the '<tt>sret</tt>' attribute) and
1589 replaces them with a new function that simply returns each of the elements of
1590 that struct (using multiple return values).
1591 </p>
1592
1593 <p>
1594 This pass works under a number of conditions:
1595 </p>
1596
1597 <ul>
1598 <li>The returned struct must not contain other structs</li>
1599 <li>The returned struct must only be used to load values from</li>
1600 <li>The placeholder struct passed in is the result of an <tt>alloca</tt></li>
1601 </ul>
1602</div>
1603
1604<!-------------------------------------------------------------------------- -->
1605<div class="doc_subsection">
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1606 <a name="tailcallelim">Tail Call Elimination</a>
1607</div>
1608<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1609 <p>
1610 This file transforms calls of the current function (self recursion) followed
1611 by a return instruction with a branch to the entry of the function, creating
1612 a loop. This pass also implements the following extensions to the basic
1613 algorithm:
1614 </p>
1615
1616 <ul>
1617 <li>Trivial instructions between the call and return do not prevent the
1618 transformation from taking place, though currently the analysis cannot
1619 support moving any really useful instructions (only dead ones).
1620 <li>This pass transforms functions that are prevented from being tail
1621 recursive by an associative expression to use an accumulator variable,
1622 thus compiling the typical naive factorial or <tt>fib</tt> implementation
1623 into efficient code.
1624 <li>TRE is performed if the function returns void, if the return
1625 returns the result returned by the call, or if the function returns a
1626 run-time constant on all exits from the function. It is possible, though
1627 unlikely, that the return returns something else (like constant 0), and
1628 can still be TRE'd. It can be TRE'd if <em>all other</em> return
1629 instructions in the function return the exact same value.
1630 <li>If it can prove that callees do not access theier caller stack frame,
1631 they are marked as eligible for tail call elimination (by the code
1632 generator).
1633 </ul>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1634</div>
1635
1636<!-------------------------------------------------------------------------- -->
1637<div class="doc_subsection">
1638 <a name="tailduplicate">Tail Duplication</a>
1639</div>
1640<div class="doc_text">
Gordon Henriksenc86b6772007-11-04 16:15:04 +0000[diff] [blame]1641 <p>
1642 This pass performs a limited form of tail duplication, intended to simplify
1643 CFGs by removing some unconditional branches. This pass is necessary to
1644 straighten out loops created by the C front-end, but also is capable of
1645 making other code nicer. After this pass is run, the CFG simplify pass
1646 should be run to clean up the mess.
1647 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1648</div>
1649
1650<!-- ======================================================================= -->
1651<div class="doc_section"> <a name="transform">Utility Passes</a></div>
1652<div class="doc_text">
1653 <p>This section describes the LLVM Utility Passes.</p>
1654</div>
1655
1656<!-------------------------------------------------------------------------- -->
1657<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1658 <a name="deadarghaX0r">Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1659</div>
1660<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1661 <p>
1662 Same as dead argument elimination, but deletes arguments to functions which
1663 are external. This is only for use by <a
1664 href="Bugpoint.html">bugpoint</a>.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1665</div>
1666
1667<!-------------------------------------------------------------------------- -->
1668<div class="doc_subsection">
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1669 <a name="extract-blocks">Extract Basic Blocks From Module (for bugpoint use)</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1670</div>
1671<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1672 <p>
1673 This pass is used by bugpoint to extract all blocks from the module into their
1674 own functions.</p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1675</div>
1676
1677<!-------------------------------------------------------------------------- -->
1678<div class="doc_subsection">
Gordon Henriksen90a52142007-11-05 02:05:35 +0000[diff] [blame]1679 <a name="preverify">Preliminary module verification</a>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1680</div>
1681<div class="doc_text">
Gordon Henriksen90a52142007-11-05 02:05:35 +0000[diff] [blame]1682 <p>
1683 Ensures that the module is in the form required by the <a
1684 href="#verifier">Module Verifier</a> pass.
1685 </p>
1686
1687 <p>
1688 Running the verifier runs this pass automatically, so there should be no need
1689 to use it directly.
1690 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1691</div>
1692
1693<!-------------------------------------------------------------------------- -->
1694<div class="doc_subsection">
1695 <a name="verify">Module Verifier</a>
1696</div>
1697<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1698 <p>
1699 Verifies an LLVM IR code. This is useful to run after an optimization which is
1700 undergoing testing. Note that <tt>llvm-as</tt> verifies its input before
1701 emitting bitcode, and also that malformed bitcode is likely to make LLVM
1702 crash. All language front-ends are therefore encouraged to verify their output
1703 before performing optimizing transformations.
1704 </p>
1705
Gordon Henriksen23a8ce52007-11-04 18:14:08 +0000[diff] [blame]1706 <ul>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1707 <li>Both of a binary operator's parameters are of the same type.</li>
1708 <li>Verify that the indices of mem access instructions match other
1709 operands.</li>
1710 <li>Verify that arithmetic and other things are only performed on
1711 first-class types. Verify that shifts and logicals only happen on
1712 integrals f.e.</li>
1713 <li>All of the constants in a switch statement are of the correct type.</li>
1714 <li>The code is in valid SSA form.</li>
Chris Lattner46b3abc2009-10-28 04:47:06 +0000[diff] [blame]1715 <li>It is illegal to put a label into any other type (like a structure) or
1716 to return one.</li>
Nick Lewycky0c78ac12008-03-28 06:46:51 +0000[diff] [blame]1717 <li>Only phi nodes can be self referential: <tt>%x = add i32 %x, %x</tt> is
Gordon Henriksen873390e2007-11-04 18:17:58 +0000[diff] [blame]1718 invalid.</li>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1719 <li>PHI nodes must have an entry for each predecessor, with no extras.</li>
1720 <li>PHI nodes must be the first thing in a basic block, all grouped
1721 together.</li>
1722 <li>PHI nodes must have at least one entry.</li>
1723 <li>All basic blocks should only end with terminator insts, not contain
1724 them.</li>
1725 <li>The entry node to a function must not have predecessors.</li>
1726 <li>All Instructions must be embedded into a basic block.</li>
1727 <li>Functions cannot take a void-typed parameter.</li>
1728 <li>Verify that a function's argument list agrees with its declared
1729 type.</li>
1730 <li>It is illegal to specify a name for a void value.</li>
1731 <li>It is illegal to have a internal global value with no initializer.</li>
1732 <li>It is illegal to have a ret instruction that returns a value that does
1733 not agree with the function return value type.</li>
1734 <li>Function call argument types match the function prototype.</li>
1735 <li>All other things that are tested by asserts spread about the code.</li>
Gordon Henriksen23a8ce52007-11-04 18:14:08 +0000[diff] [blame]1736 </ul>
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1737
1738 <p>
1739 Note that this does not provide full security verification (like Java), but
1740 instead just tries to ensure that code is well-formed.
1741 </p>
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1742</div>
1743
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1744<!-------------------------------------------------------------------------- -->
1745<div class="doc_subsection">
1746 <a name="view-cfg">View CFG of function</a>
1747</div>
1748<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1749 <p>
1750 Displays the control flow graph using the GraphViz tool.
1751 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1752</div>
1753
1754<!-------------------------------------------------------------------------- -->
1755<div class="doc_subsection">
1756 <a name="view-cfg-only">View CFG of function (with no function bodies)</a>
1757</div>
1758<div class="doc_text">
Gordon Henriksen75ff18e2007-11-04 18:10:18 +0000[diff] [blame]1759 <p>
1760 Displays the control flow graph using the GraphViz tool, but omitting function
1761 bodies.
1762 </p>
Gordon Henriksen1f5cce02007-10-25 08:46:12 +0000[diff] [blame]1763</div>
1764
Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1765<!-- *********************************************************************** -->
1766
1767<hr>
1768<address>
1769 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Reid Spencerd9aac122007-03-26 09:32:31 +0000[diff] [blame]1773
1774 <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
1775 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
1776 Last modified: $Date$
1777</address>
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1780</html>