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| <div class="doc_title">LLVM's Analysis and Transform Passes</div> |
| |
| <ol> |
| <li><a href="#intro">Introduction</a></li> |
| <li><a href="#analyses">Analysis Passes</a> |
| <li><a href="#transforms">Transform Passes</a></li> |
| <li><a href="#utilities">Utility Passes</a></li> |
| </ol> |
| |
| <div class="doc_author"> |
| <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a> |
| and Gordon Henriksen</p> |
| </div> |
| |
| <!-- ======================================================================= --> |
| <div class="doc_section"> <a name="intro">Introduction</a> </div> |
| <div class="doc_text"> |
| <p>This document serves as a high level summary of the optimization features |
| that LLVM provides. Optimizations are implemented as Passes that traverse some |
| portion of a program to either collect information or transform the program. |
| The table below divides the passes that LLVM provides into three categories. |
| Analysis passes compute information that other passes can use or for debugging |
| or program visualization purposes. Transform passes can use (or invalidate) |
| the analysis passes. Transform passes all mutate the program in some way. |
| Utility passes provides some utility but don't otherwise fit categorization. |
| For example passes to extract functions to bitcode or write a module to |
| bitcode are neither analysis nor transform passes. |
| <p>The table below provides a quick summary of each pass and links to the more |
| complete pass description later in the document.</p> |
| </div> |
| <div class="doc_text" > |
| <table> |
| <tr><th colspan="2"><b>ANALYSIS PASSES</b></th></tr> |
| <tr><th>Option</th><th>Name</th></tr> |
| <tr><td><a href="#aa-eval">-aa-eval</a></td><td>Exhaustive Alias Analysis Precision Evaluator</td></tr> |
| <tr><td><a href="#anders-aa">-anders-aa</a></td><td>Andersen's Interprocedural Alias Analysis</td></tr> |
| <tr><td><a href="#basicaa">-basicaa</a></td><td>Basic Alias Analysis (default AA impl)</td></tr> |
| <tr><td><a href="#basiccg">-basiccg</a></td><td>Basic CallGraph Construction</td></tr> |
| <tr><td><a href="#basicvn">-basicvn</a></td><td>Basic Value Numbering (default GVN impl)</td></tr> |
| <tr><td><a href="#callgraph">-callgraph</a></td><td>Print a call graph</td></tr> |
| <tr><td><a href="#callscc">-callscc</a></td><td>Print SCCs of the Call Graph</td></tr> |
| <tr><td><a href="#cfgscc">-cfgscc</a></td><td>Print SCCs of each function CFG</td></tr> |
| <tr><td><a href="#codegenprepare">-codegenprepare</a></td><td>Optimize for code generation</td></tr> |
| <tr><td><a href="#count-aa">-count-aa</a></td><td>Count Alias Analysis Query Responses</td></tr> |
| <tr><td><a href="#debug-aa">-debug-aa</a></td><td>AA use debugger</td></tr> |
| <tr><td><a href="#domfrontier">-domfrontier</a></td><td>Dominance Frontier Construction</td></tr> |
| <tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr> |
| <tr><td><a href="#externalfnconstants">-externalfnconstants</a></td><td>Print external fn callsites passed constants</td></tr> |
| <tr><td><a href="#globalsmodref-aa">-globalsmodref-aa</a></td><td>Simple mod/ref analysis for globals</td></tr> |
| <tr><td><a href="#instcount">-instcount</a></td><td>Counts the various types of Instructions</td></tr> |
| <tr><td><a href="#intervals">-intervals</a></td><td>Interval Partition Construction</td></tr> |
| <tr><td><a href="#load-vn">-load-vn</a></td><td>Load Value Numbering</td></tr> |
| <tr><td><a href="#loops">-loops</a></td><td>Natural Loop Construction</td></tr> |
| <tr><td><a href="#memdep">-memdep</a></td><td>Memory Dependence Analysis</td></tr> |
| <tr><td><a href="#no-aa">-no-aa</a></td><td>No Alias Analysis (always returns 'may' alias)</td></tr> |
| <tr><td><a href="#no-profile">-no-profile</a></td><td>No Profile Information</td></tr> |
| <tr><td><a href="#postdomfrontier">-postdomfrontier</a></td><td>Post-Dominance Frontier Construction</td></tr> |
| <tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr> |
| <tr><td><a href="#print">-print</a></td><td>Print function to stderr</td></tr> |
| <tr><td><a href="#print-alias-sets">-print-alias-sets</a></td><td>Alias Set Printer</td></tr> |
| <tr><td><a href="#print-callgraph">-print-callgraph</a></td><td>Print Call Graph to 'dot' file</td></tr> |
| <tr><td><a href="#print-cfg">-print-cfg</a></td><td>Print CFG of function to 'dot' file</td></tr> |
| <tr><td><a href="#print-cfg-only">-print-cfg-only</a></td><td>Print CFG of function to 'dot' file (with no function bodies)</td></tr> |
| <tr><td><a href="#printm">-printm</a></td><td>Print module to stderr</td></tr> |
| <tr><td><a href="#printusedtypes">-printusedtypes</a></td><td>Find Used Types</td></tr> |
| <tr><td><a href="#profile-loader">-profile-loader</a></td><td>Load profile information from llvmprof.out</td></tr> |
| <tr><td><a href="#scalar-evolution">-scalar-evolution</a></td><td>Scalar Evolution Analysis</td></tr> |
| <tr><td><a href="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr> |
| |
| |
| <tr><th colspan="2"><b>TRANSFORM PASSES</b></th></tr> |
| <tr><th>Option</th><th>Name</th></tr> |
| <tr><td><a href="#adce">-adce</a></td><td>Aggressive Dead Code Elimination</td></tr> |
| <tr><td><a href="#argpromotion">-argpromotion</a></td><td>Promote 'by reference' arguments to scalars</td></tr> |
| <tr><td><a href="#block-placement">-block-placement</a></td><td>Profile Guided Basic Block Placement</td></tr> |
| <tr><td><a href="#break-crit-edges">-break-crit-edges</a></td><td>Break critical edges in CFG</td></tr> |
| <tr><td><a href="#cee">-cee</a></td><td>Correlated Expression Elimination</td></tr> |
| <tr><td><a href="#condprop">-condprop</a></td><td>Conditional Propagation</td></tr> |
| <tr><td><a href="#constmerge">-constmerge</a></td><td>Merge Duplicate Global Constants</td></tr> |
| <tr><td><a href="#constprop">-constprop</a></td><td>Simple constant propagation</td></tr> |
| <tr><td><a href="#dce">-dce</a></td><td>Dead Code Elimination</td></tr> |
| <tr><td><a href="#deadargelim">-deadargelim</a></td><td>Dead Argument Elimination</td></tr> |
| <tr><td><a href="#deadtypeelim">-deadtypeelim</a></td><td>Dead Type Elimination</td></tr> |
| <tr><td><a href="#die">-die</a></td><td>Dead Instruction Elimination</td></tr> |
| <tr><td><a href="#dse">-dse</a></td><td>Dead Store Elimination</td></tr> |
| <tr><td><a href="#gcse">-gcse</a></td><td>Global Common Subexpression Elimination</td></tr> |
| <tr><td><a href="#globaldce">-globaldce</a></td><td>Dead Global Elimination</td></tr> |
| <tr><td><a href="#globalopt">-globalopt</a></td><td>Global Variable Optimizer</td></tr> |
| <tr><td><a href="#gvn">-gvn</a></td><td>Global Value Numbering</td></tr> |
| <tr><td><a href="#gvnpre">-gvnpre</a></td><td>Global Value Numbering/Partial Redundancy Elimination</td></tr> |
| <tr><td><a href="#indmemrem">-indmemrem</a></td><td>Indirect Malloc and Free Removal</td></tr> |
| <tr><td><a href="#indvars">-indvars</a></td><td>Canonicalize Induction Variables</td></tr> |
| <tr><td><a href="#inline">-inline</a></td><td>Function Integration/Inlining</td></tr> |
| <tr><td><a href="#insert-block-profiling">-insert-block-profiling</a></td><td>Insert instrumentation for block profiling</td></tr> |
| <tr><td><a href="#insert-edge-profiling">-insert-edge-profiling</a></td><td>Insert instrumentation for edge profiling</td></tr> |
| <tr><td><a href="#insert-function-profiling">-insert-function-profiling</a></td><td>Insert instrumentation for function profiling</td></tr> |
| <tr><td><a href="#insert-null-profiling-rs">-insert-null-profiling-rs</a></td><td>Measure profiling framework overhead</td></tr> |
| <tr><td><a href="#insert-rs-profiling-framework">-insert-rs-profiling-framework</a></td><td>Insert random sampling instrumentation framework</td></tr> |
| <tr><td><a href="#instcombine">-instcombine</a></td><td>Combine redundant instructions</td></tr> |
| <tr><td><a href="#internalize">-internalize</a></td><td>Internalize Global Symbols</td></tr> |
| <tr><td><a href="#ipconstprop">-ipconstprop</a></td><td>Interprocedural constant propagation</td></tr> |
| <tr><td><a href="#ipsccp">-ipsccp</a></td><td>Interprocedural Sparse Conditional Constant Propagation</td></tr> |
| <tr><td><a href="#lcssa">-lcssa</a></td><td>Loop-Closed SSA Form Pass</td></tr> |
| <tr><td><a href="#licm">-licm</a></td><td>Loop Invariant Code Motion</td></tr> |
| <tr><td><a href="#loop-extract">-loop-extract</a></td><td>Extract loops into new functions</td></tr> |
| <tr><td><a href="#loop-extract-single">-loop-extract-single</a></td><td>Extract at most one loop into a new function</td></tr> |
| <tr><td><a href="#loop-index-split">-loop-index-split</a></td><td>Index Split Loops</td></tr> |
| <tr><td><a href="#loop-reduce">-loop-reduce</a></td><td>Loop Strength Reduction</td></tr> |
| <tr><td><a href="#loop-rotate">-loop-rotate</a></td><td>Rotate Loops</td></tr> |
| <tr><td><a href="#loop-unroll">-loop-unroll</a></td><td>Unroll loops</td></tr> |
| <tr><td><a href="#loop-unswitch">-loop-unswitch</a></td><td>Unswitch loops</td></tr> |
| <tr><td><a href="#loopsimplify">-loopsimplify</a></td><td>Canonicalize natural loops</td></tr> |
| <tr><td><a href="#lower-packed">-lower-packed</a></td><td>lowers packed operations to operations on smaller packed datatypes</td></tr> |
| <tr><td><a href="#lowerallocs">-lowerallocs</a></td><td>Lower allocations from instructions to calls</td></tr> |
| <tr><td><a href="#lowergc">-lowergc</a></td><td>Lower GC intrinsics, for GCless code generators</td></tr> |
| <tr><td><a href="#lowerinvoke">-lowerinvoke</a></td><td>Lower invoke and unwind, for unwindless code generators</td></tr> |
| <tr><td><a href="#lowerselect">-lowerselect</a></td><td>Lower select instructions to branches</td></tr> |
| <tr><td><a href="#lowersetjmp">-lowersetjmp</a></td><td>Lower Set Jump</td></tr> |
| <tr><td><a href="#lowerswitch">-lowerswitch</a></td><td>Lower SwitchInst's to branches</td></tr> |
| <tr><td><a href="#mem2reg">-mem2reg</a></td><td>Promote Memory to Register</td></tr> |
| <tr><td><a href="#mergereturn">-mergereturn</a></td><td>Unify function exit nodes</td></tr> |
| <tr><td><a href="#predsimplify">-predsimplify</a></td><td>Predicate Simplifier</td></tr> |
| <tr><td><a href="#prune-eh">-prune-eh</a></td><td>Remove unused exception handling info</td></tr> |
| <tr><td><a href="#raiseallocs">-raiseallocs</a></td><td>Raise allocations from calls to instructions</td></tr> |
| <tr><td><a href="#reassociate">-reassociate</a></td><td>Reassociate expressions</td></tr> |
| <tr><td><a href="#reg2mem">-reg2mem</a></td><td>Demote all values to stack slots</td></tr> |
| <tr><td><a href="#scalarrepl">-scalarrepl</a></td><td>Scalar Replacement of Aggregates</td></tr> |
| <tr><td><a href="#sccp">-sccp</a></td><td>Sparse Conditional Constant Propagation</td></tr> |
| <tr><td><a href="#simplify-libcalls">-simplify-libcalls</a></td><td>Simplify well-known library calls</td></tr> |
| <tr><td><a href="#simplifycfg">-simplifycfg</a></td><td>Simplify the CFG</td></tr> |
| <tr><td><a href="#strip">-strip</a></td><td>Strip all symbols from a module</td></tr> |
| <tr><td><a href="#tailcallelim">-tailcallelim</a></td><td>Tail Call Elimination</td></tr> |
| <tr><td><a href="#tailduplicate">-tailduplicate</a></td><td>Tail Duplication</td></tr> |
| |
| |
| <tr><th colspan="2"><b>UTILITY PASSES</b></th></tr> |
| <tr><th>Option</th><th>Name</th></tr> |
| <tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</td></tr> |
| <tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (for bugpoint use)</td></tr> |
| <tr><td><a href="#preverify">-preverify</a></td><td>Preliminary module verification</td></tr> |
| <tr><td><a href="#verify">-verify</a></td><td>Module Verifier</td></tr> |
| <tr><td><a href="#view-cfg">-view-cfg</a></td><td>View CFG of function</td></tr> |
| <tr><td><a href="#view-cfg-only">-view-cfg-only</a></td><td>View CFG of function (with no function bodies)</td></tr> |
| </table> |
| </div> |
| |
| <!-- ======================================================================= --> |
| <div class="doc_section"> <a name="example">Analysis Passes</a></div> |
| <div class="doc_text"> |
| <p>This section describes the LLVM Analysis Passes.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="aa-eval">Exhaustive Alias Analysis Precision Evaluator</a> |
| </div> |
| <div class="doc_text"> |
| <p>This is a simple N^2 alias analysis accuracy evaluator. |
| Basically, for each function in the program, it simply queries to see how the |
| alias analysis implementation answers alias queries between each pair of |
| pointers in the function.</p> |
| |
| <p>This is inspired and adapted from code by: Naveen Neelakantam, Francesco |
| Spadini, and Wojciech Stryjewski.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="anders-aa">Andersen's Interprocedural Alias Analysis</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This is an implementation of Andersen's interprocedural alias |
| analysis |
| </p> |
| |
| <p> |
| In pointer analysis terms, this is a subset-based, flow-insensitive, |
| field-sensitive, and context-insensitive algorithm pointer algorithm. |
| </p> |
| |
| <p> |
| This algorithm is implemented as three stages: |
| </p> |
| |
| <ol> |
| <li>Object identification.</li> |
| <li>Inclusion constraint identification.</li> |
| <li>Offline constraint graph optimization.</li> |
| <li>Inclusion constraint solving.</li> |
| </ol> |
| |
| <p> |
| The object identification stage identifies all of the memory objects in the |
| program, which includes globals, heap allocated objects, and stack allocated |
| objects. |
| </p> |
| |
| <p> |
| The inclusion constraint identification stage finds all inclusion constraints |
| in the program by scanning the program, looking for pointer assignments and |
| other statements that effect the points-to graph. For a statement like |
| <code><var>A</var> = <var>B</var></code>, this statement is processed to |
| indicate that <var>A</var> can point to anything that <var>B</var> can point |
| to. Constraints can handle copies, loads, and stores, and address taking. |
| </p> |
| |
| <p> |
| The offline constraint graph optimization portion includes offline variable |
| substitution algorithms intended to computer pointer and location |
| equivalences. Pointer equivalences are those pointers that will have the |
| same points-to sets, and location equivalences are those variables that |
| always appear together in points-to sets. |
| </p> |
| |
| <p> |
| The inclusion constraint solving phase iteratively propagates the inclusion |
| constraints until a fixed point is reached. This is an O(<var>n</var>³) |
| algorithm. |
| </p> |
| |
| <p> |
| Function constraints are handled as if they were structs with <var>X</var> |
| fields. Thus, an access to argument <var>X</var> of function <var>Y</var> is |
| an access to node index <code>getNode(<var>Y</var>) + <var>X</var></code>. |
| This representation allows handling of indirect calls without any issues. To |
| wit, an indirect call <code><var>Y</var>(<var>a</var>,<var>b</var>)</code> is |
| equivalent to <code>*(<var>Y</var> + 1) = <var>a</var>, *(<var>Y</var> + 2) = |
| <var>b</var></code>. The return node for a function <var>F</var> is always |
| located at <code>getNode(<var>F</var>) + CallReturnPos</code>. The arguments |
| start at <code>getNode(<var>F</var>) + CallArgPos</code>. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="basicaa">Basic Alias Analysis (default AA impl)</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This is the default implementation of the Alias Analysis interface |
| that simply implements a few identities (two different globals cannot alias, |
| etc), but otherwise does no analysis. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="basiccg">Basic CallGraph Construction</a> |
| </div> |
| <div class="doc_text"> |
| <p>Yet to be written.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="basicvn">Basic Value Numbering (default GVN impl)</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This is the default implementation of the <code>ValueNumbering</code> |
| interface. It walks the SSA def-use chains to trivially identify |
| lexically identical expressions. This does not require any ahead of time |
| analysis, so it is a very fast default implementation. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="callgraph">Print a call graph</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass, only available in <code>opt</code>, prints the call graph to |
| standard output in a human-readable form. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="callscc">Print SCCs of the Call Graph</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass, only available in <code>opt</code>, prints the SCCs of the call |
| graph to standard output in a human-readable form. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="cfgscc">Print SCCs of each function CFG</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass, only available in <code>opt</code>, prints the SCCs of each |
| function CFG to standard output in a human-readable form. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="codegenprepare">Optimize for code generation</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass munges the code in the input function to better prepare it for |
| SelectionDAG-based code generation. This works around limitations in it's |
| basic-block-at-a-time approach. It should eventually be removed. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="count-aa">Count Alias Analysis Query Responses</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| A pass which can be used to count how many alias queries |
| are being made and how the alias analysis implementation being used responds. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="debug-aa">AA use debugger</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This simple pass checks alias analysis users to ensure that if they |
| create a new value, they do not query AA without informing it of the value. |
| It acts as a shim over any other AA pass you want. |
| </p> |
| |
| <p> |
| Yes keeping track of every value in the program is expensive, but this is |
| a debugging pass. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="domfrontier">Dominance Frontier Construction</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass is a simple dominator construction algorithm for finding forward |
| dominator frontiers. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="domtree">Dominator Tree Construction</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass is a simple dominator construction algorithm for finding forward |
| dominators. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="externalfnconstants">Print external fn callsites passed constants</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass, only available in <code>opt</code>, prints out call sites to |
| external functions that are called with constant arguments. This can be |
| useful when looking for standard library functions we should constant fold |
| or handle in alias analyses. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="globalsmodref-aa">Simple mod/ref analysis for globals</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This simple pass provides alias and mod/ref information for global values |
| that do not have their address taken, and keeps track of whether functions |
| read or write memory (are "pure"). For this simple (but very common) case, |
| we can provide pretty accurate and useful information. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="instcount">Counts the various types of Instructions</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass collects the count of all instructions and reports them |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="intervals">Interval Partition Construction</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This analysis calculates and represents the interval partition of a function, |
| or a preexisting interval partition. |
| </p> |
| |
| <p> |
| In this way, the interval partition may be used to reduce a flow graph down |
| to its degenerate single node interval partition (unless it is irreducible). |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="load-vn">Load Value Numbering</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass value numbers load and call instructions. To do this, it finds |
| lexically identical load instructions, and uses alias analysis to determine |
| which loads are guaranteed to produce the same value. To value number call |
| instructions, it looks for calls to functions that do not write to memory |
| which do not have intervening instructions that clobber the memory that is |
| read from. |
| </p> |
| |
| <p> |
| This pass builds off of another value numbering pass to implement value |
| numbering for non-load and non-call instructions. It uses Alias Analysis so |
| that it can disambiguate the load instructions. The more powerful these base |
| analyses are, the more powerful the resultant value numbering will be. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loops">Natural Loop Construction</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This analysis is used to identify natural loops and determine the loop depth |
| of various nodes of the CFG. Note that the loops identified may actually be |
| several natural loops that share the same header node... not just a single |
| natural loop. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="memdep">Memory Dependence Analysis</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| An analysis that determines, for a given memory operation, what preceding |
| memory operations it depends on. It builds on alias analysis information, and |
| tries to provide a lazy, caching interface to a common kind of alias |
| information query. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="no-aa">No Alias Analysis (always returns 'may' alias)</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Always returns "I don't know" for alias queries. NoAA is unlike other alias |
| analysis implementations, in that it does not chain to a previous analysis. As |
| such it doesn't follow many of the rules that other alias analyses must. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="no-profile">No Profile Information</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| The default "no profile" implementation of the abstract |
| <code>ProfileInfo</code> interface. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="postdomfrontier">Post-Dominance Frontier Construction</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass is a simple post-dominator construction algorithm for finding |
| post-dominator frontiers. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="postdomtree">Post-Dominator Tree Construction</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass is a simple post-dominator construction algorithm for finding |
| post-dominators. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="print">Print function to stderr</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| The <code>PrintFunctionPass</code> class is designed to be pipelined with |
| other <code>FunctionPass</code>es, and prints out the functions of the module |
| as they are processed. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="print-alias-sets">Alias Set Printer</a> |
| </div> |
| <div class="doc_text"> |
| <p>Yet to be written.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="print-callgraph">Print Call Graph to 'dot' file</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass, only available in <code>opt</code>, prints the call graph into a |
| <code>.dot</code> graph. This graph can then be processed with the "dot" tool |
| to convert it to postscript or some other suitable format. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="print-cfg">Print CFG of function to 'dot' file</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass, only available in <code>opt</code>, prints the control flow graph |
| into a <code>.dot</code> graph. This graph can then be processed with the |
| "dot" tool to convert it to postscript or some other suitable format. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="print-cfg-only">Print CFG of function to 'dot' file (with no function bodies)</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass, only available in <code>opt</code>, prints the control flow graph |
| into a <code>.dot</code> graph, omitting the function bodies. This graph can |
| then be processed with the "dot" tool to convert it to postscript or some |
| other suitable format. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="printm">Print module to stderr</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass simply prints out the entire module when it is executed. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="printusedtypes">Find Used Types</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass is used to seek out all of the types in use by the program. Note |
| that this analysis explicitly does not include types only used by the symbol |
| table. |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="profile-loader">Load profile information from llvmprof.out</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| A concrete implementation of profiling information that loads the information |
| from a profile dump file. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="scalar-evolution">Scalar Evolution Analysis</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| The <code>ScalarEvolution</code> analysis can be used to analyze and |
| catagorize scalar expressions in loops. It specializes in recognizing general |
| induction variables, representing them with the abstract and opaque |
| <code>SCEV</code> class. Given this analysis, trip counts of loops and other |
| important properties can be obtained. |
| </p> |
| |
| <p> |
| This analysis is primarily useful for induction variable substitution and |
| strength reduction. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="targetdata">Target Data Layout</a> |
| </div> |
| <div class="doc_text"> |
| <p>Provides other passes access to information on how the size and alignment |
| required by the the target ABI for various data types.</p> |
| </div> |
| |
| <!-- ======================================================================= --> |
| <div class="doc_section"> <a name="transform">Transform Passes</a></div> |
| <div class="doc_text"> |
| <p>This section describes the LLVM Transform Passes.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="adce">Aggressive Dead Code Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p>ADCE aggressively tries to eliminate code. This pass is similar to |
| <a href="#dce">DCE</a> but it assumes that values are dead until proven |
| otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to |
| the liveness of values.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="argpromotion">Promote 'by reference' arguments to scalars</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass promotes "by reference" arguments to be "by value" arguments. In |
| practice, this means looking for internal functions that have pointer |
| arguments. If it can prove, through the use of alias analysis, that an |
| argument is *only* loaded, then it can pass the value into the function |
| instead of the address of the value. This can cause recursive simplification |
| of code and lead to the elimination of allocas (especially in C++ template |
| code like the STL). |
| </p> |
| |
| <p> |
| This pass also handles aggregate arguments that are passed into a function, |
| scalarizing them if the elements of the aggregate are only loaded. Note that |
| it refuses to scalarize aggregates which would require passing in more than |
| three operands to the function, because passing thousands of operands for a |
| large array or structure is unprofitable! |
| </p> |
| |
| <p> |
| Note that this transformation could also be done for arguments that are only |
| stored to (returning the value instead), but does not currently. This case |
| would be best handled when and if LLVM starts supporting multiple return |
| values from functions. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="block-placement">Profile Guided Basic Block Placement</a> |
| </div> |
| <div class="doc_text"> |
| <p>This pass is a very simple profile guided basic block placement algorithm. |
| The idea is to put frequently executed blocks together at the start of the |
| function and hopefully increase the number of fall-through conditional |
| branches. If there is no profile information for a particular function, this |
| pass basically orders blocks in depth-first order.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="break-crit-edges">Break critical edges in CFG</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Break all of the critical edges in the CFG by inserting a dummy basic block. |
| It may be "required" by passes that cannot deal with critical edges. This |
| transformation obviously invalidates the CFG, but can update forward dominator |
| (set, immediate dominators, tree, and frontier) information. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="cee">Correlated Expression Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p>Correlated Expression Elimination propagates information from conditional |
| branches to blocks dominated by destinations of the branch. It propagates |
| information from the condition check itself into the body of the branch, |
| allowing transformations like these for example:</p> |
| |
| <blockquote><pre> |
| if (i == 7) |
| ... 4*i; // constant propagation |
| |
| M = i+1; N = j+1; |
| if (i == j) |
| X = M-N; // = M-M == 0; |
| </pre></blockquote> |
| |
| <p>This is called Correlated Expression Elimination because we eliminate or |
| simplify expressions that are correlated with the direction of a branch. In |
| this way we use static information to give us some information about the |
| dynamic value of a variable.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="condprop">Conditional Propagation</a> |
| </div> |
| <div class="doc_text"> |
| <p>This pass propagates information about conditional expressions through the |
| program, allowing it to eliminate conditional branches in some cases.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="constmerge">Merge Duplicate Global Constants</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Merges duplicate global constants together into a single constant that is |
| shared. This is useful because some passes (ie TraceValues) insert a lot of |
| string constants into the program, regardless of whether or not an existing |
| string is available. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="constprop">Simple constant propagation</a> |
| </div> |
| <div class="doc_text"> |
| <p>This file implements constant propagation and merging. It looks for |
| instructions involving only constant operands and replaces them with a |
| constant value instead of an instruction. For example:</p> |
| <blockquote><pre>add i32 1, 2</pre></blockquote> |
| <p>becomes</p> |
| <blockquote><pre>i32 3</pre></blockquote> |
| <p>NOTE: this pass has a habit of making definitions be dead. It is a good |
| idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass |
| sometime after running this pass.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="dce">Dead Code Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Dead code elimination is similar to <a href="#die">dead instruction |
| elimination</a>, but it rechecks instructions that were used by removed |
| instructions to see if they are newly dead. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="deadargelim">Dead Argument Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass deletes dead arguments from internal functions. Dead argument |
| elimination removes arguments which are directly dead, as well as arguments |
| only passed into function calls as dead arguments of other functions. This |
| pass also deletes dead arguments in a similar way. |
| </p> |
| |
| <p> |
| This pass is often useful as a cleanup pass to run after aggressive |
| interprocedural passes, which add possibly-dead arguments. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="deadtypeelim">Dead Type Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass is used to cleanup the output of GCC. It eliminate names for types |
| that are unused in the entire translation unit, using the <a |
| href="#findusedtypes">find used types</a> pass. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="die">Dead Instruction Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Dead instruction elimination performs a single pass over the function, |
| removing instructions that are obviously dead. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="dse">Dead Store Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| A trivial dead store elimination that only considers basic-block local |
| redundant stores. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="gcse">Global Common Subexpression Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass is designed to be a very quick global transformation that |
| eliminates global common subexpressions from a function. It does this by |
| using an existing value numbering implementation to identify the common |
| subexpressions, eliminating them when possible. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="globaldce">Dead Global Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This transform is designed to eliminate unreachable internal globals from the |
| program. It uses an aggressive algorithm, searching out globals that are |
| known to be alive. After it finds all of the globals which are needed, it |
| deletes whatever is left over. This allows it to delete recursive chunks of |
| the program which are unreachable. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="globalopt">Global Variable Optimizer</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass transforms simple global variables that never have their address |
| taken. If obviously true, it marks read/write globals as constant, deletes |
| variables only stored to, etc. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="gvn">Global Value Numbering</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass performs global value numbering to eliminate fully redundant |
| instructions. It also performs simple dead load elimination. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="gvnpre">Global Value Numbering/Partial Redundancy Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass performs a hybrid of global value numbering and partial redundancy |
| elimination, known as GVN-PRE. It performs partial redundancy elimination on |
| values, rather than lexical expressions, allowing a more comprehensive view |
| the optimization. It replaces redundant values with uses of earlier |
| occurences of the same value. While this is beneficial in that it eliminates |
| unneeded computation, it also increases register pressure by creating large |
| live ranges, and should be used with caution on platforms that are very |
| sensitive to register pressure. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="indmemrem">Indirect Malloc and Free Removal</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass finds places where memory allocation functions may escape into |
| indirect land. Some transforms are much easier (aka possible) only if free |
| or malloc are not called indirectly. |
| </p> |
| |
| <p> |
| Thus find places where the address of memory functions are taken and construct |
| bounce functions with direct calls of those functions. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="indvars">Canonicalize Induction Variables</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This transformation analyzes and transforms the induction variables (and |
| computations derived from them) into simpler forms suitable for subsequent |
| analysis and transformation. |
| </p> |
| |
| <p> |
| This transformation makes the following changes to each loop with an |
| identifiable induction variable: |
| </p> |
| |
| <ol> |
| <li>All loops are transformed to have a <em>single</em> canonical |
| induction variable which starts at zero and steps by one.</li> |
| <li>The canonical induction variable is guaranteed to be the first PHI node |
| in the loop header block.</li> |
| <li>Any pointer arithmetic recurrences are raised to use array |
| subscripts.</li> |
| </ol> |
| |
| <p> |
| If the trip count of a loop is computable, this pass also makes the following |
| changes: |
| </p> |
| |
| <ol> |
| <li>The exit condition for the loop is canonicalized to compare the |
| induction value against the exit value. This turns loops like: |
| <blockquote><pre>for (i = 7; i*i < 1000; ++i)</pre></blockquote> |
| into |
| <blockquote><pre>for (i = 0; i != 25; ++i)</pre></blockquote></li> |
| <li>Any use outside of the loop of an expression derived from the indvar |
| is changed to compute the derived value outside of the loop, eliminating |
| the dependence on the exit value of the induction variable. If the only |
| purpose of the loop is to compute the exit value of some derived |
| expression, this transformation will make the loop dead.</li> |
| </ol> |
| |
| <p> |
| This transformation should be followed by strength reduction after all of the |
| desired loop transformations have been performed. Additionally, on targets |
| where it is profitable, the loop could be transformed to count down to zero |
| (the "do loop" optimization). |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="inline">Function Integration/Inlining</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Bottom-up inlining of functions into callees. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="insert-block-profiling">Insert instrumentation for block profiling</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass instruments the specified program with counters for basic block |
| profiling, which counts the number of times each basic block executes. This |
| is the most basic form of profiling, which can tell which blocks are hot, but |
| cannot reliably detect hot paths through the CFG. |
| </p> |
| |
| <p> |
| Note that this implementation is very naïve. Control equivalent regions of |
| the CFG should not require duplicate counters, but it does put duplicate |
| counters in. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="insert-edge-profiling">Insert instrumentation for edge profiling</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass instruments the specified program with counters for edge profiling. |
| Edge profiling can give a reasonable approximation of the hot paths through a |
| program, and is used for a wide variety of program transformations. |
| </p> |
| |
| <p> |
| Note that this implementation is very naïve. It inserts a counter for |
| <em>every</em> edge in the program, instead of using control flow information |
| to prune the number of counters inserted. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="insert-function-profiling">Insert instrumentation for function profiling</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass instruments the specified program with counters for function |
| profiling, which counts the number of times each function is called. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="insert-null-profiling-rs">Measure profiling framework overhead</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| The basic profiler that does nothing. It is the default profiler and thus |
| terminates <code>RSProfiler</code> chains. It is useful for measuring |
| framework overhead. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="insert-rs-profiling-framework">Insert random sampling instrumentation framework</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| The second stage of the random-sampling instrumentation framework, duplicates |
| all instructions in a function, ignoring the profiling code, then connects the |
| two versions together at the entry and at backedges. At each connection point |
| a choice is made as to whether to jump to the profiled code (take a sample) or |
| execute the unprofiled code. |
| </p> |
| |
| <p> |
| After this pass, it is highly recommended to run<a href="#mem2reg">mem2reg</a> |
| and <a href="#adce">adce</a>. <a href="#instcombine">instcombine</a>, |
| <a href="#load-vn">load-vn</a>, <a href="#gdce">gdce</a>, and |
| <a href="#dse">dse</a> also are good to run afterwards. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="instcombine">Combine redundant instructions</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Combine instructions to form fewer, simple |
| instructions. This pass does not modify the CFG This pass is where algebraic |
| simplification happens. |
| </p> |
| |
| <p> |
| This pass combines things like: |
| </p> |
| |
| <blockquote><pre |
| >%Y = add i32 %X, 1 |
| %Z = add i32 %Y, 1</pre></blockquote> |
| |
| <p> |
| into: |
| </p> |
| |
| <blockquote><pre |
| >%Z = add i32 %X, 2</pre></blockquote> |
| |
| <p> |
| This is a simple worklist driven algorithm. |
| </p> |
| |
| <p> |
| This pass guarantees that the following canonicalizations are performed on |
| the program: |
| </p> |
| |
| <ul> |
| <li>If a binary operator has a constant operand, it is moved to the right- |
| hand side.</li> |
| <li>Bitwise operators with constant operands are always grouped so that |
| shifts are performed first, then <code>or</code>s, then |
| <code>and</code>s, then <code>xor</code>s.</li> |
| <li>Compare instructions are converted from <code><</code>, |
| <code>></code>, <code>≤</code>, or <code>≥</code> to |
| <code>=</code> or <code>≠</code> if possible.</li> |
| <li>All <code>cmp</code> instructions on boolean values are replaced with |
| logical operations.</li> |
| <li><code>add <var>X</var>, <var>X</var></code> is represented as |
| <code>mul <var>X</var>, 2</code> ⇒ <code>shl <var>X</var>, 1</code></li> |
| <li>Multiplies with a constant power-of-two argument are transformed into |
| shifts.</li> |
| <li>… etc.</li> |
| </ul> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="internalize">Internalize Global Symbols</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass loops over all of the functions in the input module, looking for a |
| main function. If a main function is found, all other functions and all |
| global variables with initializers are marked as internal. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="ipconstprop">Interprocedural constant propagation</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass implements an <em>extremely</em> simple interprocedural constant |
| propagation pass. It could certainly be improved in many different ways, |
| like using a worklist. This pass makes arguments dead, but does not remove |
| them. The existing dead argument elimination pass should be run after this |
| to clean up the mess. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="ipsccp">Interprocedural Sparse Conditional Constant Propagation</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| An interprocedural variant of <a href="#sccp">Sparse Conditional Constant |
| Propagation</a>. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="lcssa">Loop-Closed SSA Form Pass</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass transforms loops by placing phi nodes at the end of the loops for |
| all values that are live across the loop boundary. For example, it turns |
| the left into the right code: |
| </p> |
| |
| <pre |
| >for (...) for (...) |
| if (c) if (c) |
| X1 = ... X1 = ... |
| else else |
| X2 = ... X2 = ... |
| X3 = phi(X1, X2) X3 = phi(X1, X2) |
| ... = X3 + 4 X4 = phi(X3) |
| ... = X4 + 4</pre> |
| |
| <p> |
| This is still valid LLVM; the extra phi nodes are purely redundant, and will |
| be trivially eliminated by <code>InstCombine</code>. The major benefit of |
| this transformation is that it makes many other loop optimizations, such as |
| LoopUnswitching, simpler. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="licm">Loop Invariant Code Motion</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass performs loop invariant code motion, attempting to remove as much |
| code from the body of a loop as possible. It does this by either hoisting |
| code into the preheader block, or by sinking code to the exit blocks if it is |
| safe. This pass also promotes must-aliased memory locations in the loop to |
| live in registers, thus hoisting and sinking "invariant" loads and stores. |
| </p> |
| |
| <p> |
| This pass uses alias analysis for two purposes: |
| </p> |
| |
| <ul> |
| <li>Moving loop invariant loads and calls out of loops. If we can determine |
| that a load or call inside of a loop never aliases anything stored to, |
| we can hoist it or sink it like any other instruction.</li> |
| <li>Scalar Promotion of Memory - If there is a store instruction inside of |
| the loop, we try to move the store to happen AFTER the loop instead of |
| inside of the loop. This can only happen if a few conditions are true: |
| <ul> |
| <li>The pointer stored through is loop invariant.</li> |
| <li>There are no stores or loads in the loop which <em>may</em> alias |
| the pointer. There are no calls in the loop which mod/ref the |
| pointer.</li> |
| </ul> |
| If these conditions are true, we can promote the loads and stores in the |
| loop of the pointer to use a temporary alloca'd variable. We then use |
| the mem2reg functionality to construct the appropriate SSA form for the |
| variable.</li> |
| </ul> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loop-extract">Extract loops into new functions</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| A pass wrapper around the <code>ExtractLoop()</code> scalar transformation to |
| extract each top-level loop into its own new function. If the loop is the |
| <em>only</em> loop in a given function, it is not touched. This is a pass most |
| useful for debugging via bugpoint. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loop-extract-single">Extract at most one loop into a new function</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Similar to <a href="#loop-extract">Extract loops into new functions</a>, |
| this pass extracts one natural loop from the program into a function if it |
| can. This is used by bugpoint. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loop-index-split">Index Split Loops</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass divides loop's iteration range by spliting loop such that each |
| individual loop is executed efficiently. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loop-reduce">Loop Strength Reduction</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass performs a strength reduction on array references inside loops that |
| have as one or more of their components the loop induction variable. This is |
| accomplished by creating a new value to hold the initial value of the array |
| access for the first iteration, and then creating a new GEP instruction in |
| the loop to increment the value by the appropriate amount. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loop-rotate">Rotate Loops</a> |
| </div> |
| <div class="doc_text"> |
| <p>A simple loop rotation transformation.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loop-unroll">Unroll loops</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass implements a simple loop unroller. It works best when loops have |
| been canonicalized by the <a href="#indvars"><tt>-indvars</tt></a> pass, |
| allowing it to determine the trip counts of loops easily. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loop-unswitch">Unswitch loops</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass transforms loops that contain branches on loop-invariant conditions |
| to have multiple loops. For example, it turns the left into the right code: |
| </p> |
| |
| <pre |
| >for (...) if (lic) |
| A for (...) |
| if (lic) A; B; C |
| B else |
| C for (...) |
| A; C</pre> |
| |
| <p> |
| This can increase the size of the code exponentially (doubling it every time |
| a loop is unswitched) so we only unswitch if the resultant code will be |
| smaller than a threshold. |
| </p> |
| |
| <p> |
| This pass expects LICM to be run before it to hoist invariant conditions out |
| of the loop, to make the unswitching opportunity obvious. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="loopsimplify">Canonicalize natural loops</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass performs several transformations to transform natural loops into a |
| simpler form, which makes subsequent analyses and transformations simpler and |
| more effective. |
| </p> |
| |
| <p> |
| Loop pre-header insertion guarantees that there is a single, non-critical |
| entry edge from outside of the loop to the loop header. This simplifies a |
| number of analyses and transformations, such as LICM. |
| </p> |
| |
| <p> |
| Loop exit-block insertion guarantees that all exit blocks from the loop |
| (blocks which are outside of the loop that have predecessors inside of the |
| loop) only have predecessors from inside of the loop (and are thus dominated |
| by the loop header). This simplifies transformations such as store-sinking |
| that are built into LICM. |
| </p> |
| |
| <p> |
| This pass also guarantees that loops will have exactly one backedge. |
| </p> |
| |
| <p> |
| Note that the simplifycfg pass will clean up blocks which are split out but |
| end up being unnecessary, so usage of this pass should not pessimize |
| generated code. |
| </p> |
| |
| <p> |
| This pass obviously modifies the CFG, but updates loop information and |
| dominator information. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="lower-packed">lowers packed operations to operations on smaller packed datatypes</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Lowers operations on vector datatypes into operations on more primitive vector |
| datatypes, and finally to scalar operations. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="lowerallocs">Lower allocations from instructions to calls</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Turn <tt>malloc</tt> and <tt>free</tt> instructions into <tt>@malloc</tt> and |
| <tt>@free</tt> calls. |
| </p> |
| |
| <p> |
| This is a target-dependent tranformation because it depends on the size of |
| data types and alignment constraints. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="lowergc">Lower GC intrinsics, for GCless code generators</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This file implements lowering for the <tt>llvm.gc*</tt> intrinsics for targets |
| that do not natively support them (which includes the C backend). Note that |
| the code generated is not as efficient as it would be for targets that |
| natively support the GC intrinsics, but it is useful for getting new targets |
| up-and-running quickly. |
| </p> |
| |
| <p> |
| This pass implements the code transformation described in this paper: |
| </p> |
| |
| <blockquote><p> |
| "Accurate Garbage Collection in an Uncooperative Environment" |
| Fergus Henderson, ISMM, 2002 |
| </p></blockquote> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="lowerinvoke">Lower invoke and unwind, for unwindless code generators</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This transformation is designed for use by code generators which do not yet |
| support stack unwinding. This pass supports two models of exception handling |
| lowering, the 'cheap' support and the 'expensive' support. |
| </p> |
| |
| <p> |
| 'Cheap' exception handling support gives the program the ability to execute |
| any program which does not "throw an exception", by turning 'invoke' |
| instructions into calls and by turning 'unwind' instructions into calls to |
| abort(). If the program does dynamically use the unwind instruction, the |
| program will print a message then abort. |
| </p> |
| |
| <p> |
| 'Expensive' exception handling support gives the full exception handling |
| support to the program at the cost of making the 'invoke' instruction |
| really expensive. It basically inserts setjmp/longjmp calls to emulate the |
| exception handling as necessary. |
| </p> |
| |
| <p> |
| Because the 'expensive' support slows down programs a lot, and EH is only |
| used for a subset of the programs, it must be specifically enabled by the |
| <tt>-enable-correct-eh-support</tt> option. |
| </p> |
| |
| <p> |
| Note that after this pass runs the CFG is not entirely accurate (exceptional |
| control flow edges are not correct anymore) so only very simple things should |
| be done after the lowerinvoke pass has run (like generation of native code). |
| This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't |
| support the invoke instruction yet" lowering pass. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="lowerselect">Lower select instructions to branches</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Lowers select instructions into conditional branches for targets that do not |
| have conditional moves or that have not implemented the select instruction |
| yet. |
| </p> |
| |
| <p> |
| Note that this pass could be improved. In particular it turns every select |
| instruction into a new conditional branch, even though some common cases have |
| select instructions on the same predicate next to each other. It would be |
| better to use the same branch for the whole group of selects. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="lowersetjmp">Lower Set Jump</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Lowers <tt>setjmp</tt> and <tt>longjmp</tt> to use the LLVM invoke and unwind |
| instructions as necessary. |
| </p> |
| |
| <p> |
| Lowering of <tt>longjmp</tt> is fairly trivial. We replace the call with a |
| call to the LLVM library function <tt>__llvm_sjljeh_throw_longjmp()</tt>. |
| This unwinds the stack for us calling all of the destructors for |
| objects allocated on the stack. |
| </p> |
| |
| <p> |
| At a <tt>setjmp</tt> call, the basic block is split and the <tt>setjmp</tt> |
| removed. The calls in a function that have a <tt>setjmp</tt> are converted to |
| invoke where the except part checks to see if it's a <tt>longjmp</tt> |
| exception and, if so, if it's handled in the function. If it is, then it gets |
| the value returned by the <tt>longjmp</tt> and goes to where the basic block |
| was split. <tt>invoke</tt> instructions are handled in a similar fashion with |
| the original except block being executed if it isn't a <tt>longjmp</tt> |
| except that is handled by that function. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="lowerswitch">Lower SwitchInst's to branches</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Rewrites <tt>switch</tt> instructions with a sequence of branches, which |
| allows targets to get away with not implementing the switch instruction until |
| it is convenient. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="mem2reg">Promote Memory to Register</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This file promotes memory references to be register references. It promotes |
| <tt>alloca</tt> instructions which only have <tt>load</tt>s and |
| <tt>store</tt>s as uses. An <tt>alloca</tt> is transformed by using dominator |
| frontiers to place <tt>phi</tt> nodes, then traversing the function in |
| depth-first order to rewrite <tt>load</tt>s and <tt>store</tt>s as |
| appropriate. This is just the standard SSA construction algorithm to construct |
| "pruned" SSA form. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="mergereturn">Unify function exit nodes</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Ensure that functions have at most one <tt>ret</tt> instruction in them. |
| Additionally, it keeps track of which node is the new exit node of the CFG. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="predsimplify">Predicate Simplifier</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Path-sensitive optimizer. In a branch where <tt>x == y</tt>, replace uses of |
| <tt>x</tt> with <tt>y</tt>. Permits further optimization, such as the |
| elimination of the unreachable call: |
| </p> |
| |
| <blockquote><pre |
| >void test(int *p, int *q) |
| { |
| if (p != q) |
| return; |
| |
| if (*p != *q) |
| foo(); // unreachable |
| }</pre></blockquote> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="prune-eh">Remove unused exception handling info</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This file implements a simple interprocedural pass which walks the call-graph, |
| turning <tt>invoke</tt> instructions into <tt>call</tt> instructions if and |
| only if the callee cannot throw an exception. It implements this as a |
| bottom-up traversal of the call-graph. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="raiseallocs">Raise allocations from calls to instructions</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Converts <tt>@malloc</tt> and <tt>@free</tt> calls to <tt>malloc</tt> and |
| <tt>free</tt> instructions. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="reassociate">Reassociate expressions</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass reassociates commutative expressions in an order that is designed |
| to promote better constant propagation, GCSE, LICM, PRE, etc. |
| </p> |
| |
| <p> |
| For example: 4 + (<var>x</var> + 5) ⇒ <var>x</var> + (4 + 5) |
| </p> |
| |
| <p> |
| In the implementation of this algorithm, constants are assigned rank = 0, |
| function arguments are rank = 1, and other values are assigned ranks |
| corresponding to the reverse post order traversal of current function |
| (starting at 2), which effectively gives values in deep loops higher rank |
| than values not in loops. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="reg2mem">Demote all values to stack slots</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This file demotes all registers to memory references. It is intented to be |
| the inverse of <a href="#mem2reg"><tt>-mem2reg</tt></a>. By converting to |
| <tt>load</tt> instructions, the only values live accross basic blocks are |
| <tt>alloca</tt> instructions and <tt>load</tt> instructions before |
| <tt>phi</tt> nodes. It is intended that this should make CFG hacking much |
| easier. To make later hacking easier, the entry block is split into two, such |
| that all introduced <tt>alloca</tt> instructions (and nothing else) are in the |
| entry block. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="scalarrepl">Scalar Replacement of Aggregates</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| The well-known scalar replacement of aggregates transformation. This |
| transform breaks up <tt>alloca</tt> instructions of aggregate type (structure |
| or array) into individual <tt>alloca</tt> instructions for each member if |
| possible. Then, if possible, it transforms the individual <tt>alloca</tt> |
| instructions into nice clean scalar SSA form. |
| </p> |
| |
| <p> |
| This combines a simple scalar replacement of aggregates algorithm with the <a |
| href="#mem2reg"><tt>mem2reg</tt></a> algorithm because often interact, |
| especially for C++ programs. As such, iterating between <tt>scalarrepl</tt>, |
| then <a href="#mem2reg"><tt>mem2reg</tt></a> until we run out of things to |
| promote works well. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="sccp">Sparse Conditional Constant Propagation</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Sparse conditional constant propagation and merging, which can be summarized |
| as: |
| </p> |
| |
| <ol> |
| <li>Assumes values are constant unless proven otherwise</li> |
| <li>Assumes BasicBlocks are dead unless proven otherwise</li> |
| <li>Proves values to be constant, and replaces them with constants</li> |
| <li>Proves conditional branches to be unconditional</li> |
| </ol> |
| |
| <p> |
| Note that this pass has a habit of making definitions be dead. It is a good |
| idea to to run a DCE pass sometime after running this pass. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="simplify-libcalls">Simplify well-known library calls</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Applies a variety of small optimizations for calls to specific well-known |
| function calls (e.g. runtime library functions). For example, a call |
| <tt>exit(3)</tt> that occurs within the <tt>main()</tt> function can be |
| transformed into simply <tt>return 3</tt>. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="simplifycfg">Simplify the CFG</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Performs dead code elimination and basic block merging. Specifically: |
| </p> |
| |
| <ol> |
| <li>Removes basic blocks with no predecessors.</li> |
| <li>Merges a basic block into its predecessor if there is only one and the |
| predecessor only has one successor.</li> |
| <li>Eliminates PHI nodes for basic blocks with a single predecessor.</li> |
| <li>Eliminates a basic block that only contains an unconditional |
| branch.</li> |
| </ol> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="strip">Strip all symbols from a module</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Performs code stripping. This transformation can delete: |
| </p> |
| |
| <ol> |
| <li>names for virtual registers</li> |
| <li>symbols for internal globals and functions</li> |
| <li>debug information</li> |
| </ol> |
| |
| <p> |
| Note that this transformation makes code much less readable, so it should |
| only be used in situations where the <tt>strip</tt> utility would be used, |
| such as reducing code size or making it harder to reverse engineer code. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="tailcallelim">Tail Call Elimination</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This file transforms calls of the current function (self recursion) followed |
| by a return instruction with a branch to the entry of the function, creating |
| a loop. This pass also implements the following extensions to the basic |
| algorithm: |
| </p> |
| |
| <ul> |
| <li>Trivial instructions between the call and return do not prevent the |
| transformation from taking place, though currently the analysis cannot |
| support moving any really useful instructions (only dead ones). |
| <li>This pass transforms functions that are prevented from being tail |
| recursive by an associative expression to use an accumulator variable, |
| thus compiling the typical naive factorial or <tt>fib</tt> implementation |
| into efficient code. |
| <li>TRE is performed if the function returns void, if the return |
| returns the result returned by the call, or if the function returns a |
| run-time constant on all exits from the function. It is possible, though |
| unlikely, that the return returns something else (like constant 0), and |
| can still be TRE'd. It can be TRE'd if <em>all other</em> return |
| instructions in the function return the exact same value. |
| <li>If it can prove that callees do not access theier caller stack frame, |
| they are marked as eligible for tail call elimination (by the code |
| generator). |
| </ul> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="tailduplicate">Tail Duplication</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass performs a limited form of tail duplication, intended to simplify |
| CFGs by removing some unconditional branches. This pass is necessary to |
| straighten out loops created by the C front-end, but also is capable of |
| making other code nicer. After this pass is run, the CFG simplify pass |
| should be run to clean up the mess. |
| </p> |
| </div> |
| |
| <!-- ======================================================================= --> |
| <div class="doc_section"> <a name="transform">Utility Passes</a></div> |
| <div class="doc_text"> |
| <p>This section describes the LLVM Utility Passes.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="deadarghaX0r">Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Same as dead argument elimination, but deletes arguments to functions which |
| are external. This is only for use by <a |
| href="Bugpoint.html">bugpoint</a>.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="extract-blocks">Extract Basic Blocks From Module (for bugpoint use)</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| This pass is used by bugpoint to extract all blocks from the module into their |
| own functions.</p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="preverify">Preliminary module verification</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Ensures that the module is in the form required by the <a |
| href="#verifier">Module Verifier</a> pass. |
| </p> |
| |
| <p> |
| Running the verifier runs this pass automatically, so there should be no need |
| to use it directly. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="verify">Module Verifier</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Verifies an LLVM IR code. This is useful to run after an optimization which is |
| undergoing testing. Note that <tt>llvm-as</tt> verifies its input before |
| emitting bitcode, and also that malformed bitcode is likely to make LLVM |
| crash. All language front-ends are therefore encouraged to verify their output |
| before performing optimizing transformations. |
| </p> |
| |
| <ul> |
| <li>Both of a binary operator's parameters are of the same type.</li> |
| <li>Verify that the indices of mem access instructions match other |
| operands.</li> |
| <li>Verify that arithmetic and other things are only performed on |
| first-class types. Verify that shifts and logicals only happen on |
| integrals f.e.</li> |
| <li>All of the constants in a switch statement are of the correct type.</li> |
| <li>The code is in valid SSA form.</li> |
| <li>It should be illegal to put a label into any other type (like a |
| structure) or to return one. [except constant arrays!]</li> |
| <li>Only phi nodes can be self referential: <tt>%x = add int %x, %x</tt> is |
| invalid.</li> |
| <li>PHI nodes must have an entry for each predecessor, with no extras.</li> |
| <li>PHI nodes must be the first thing in a basic block, all grouped |
| together.</li> |
| <li>PHI nodes must have at least one entry.</li> |
| <li>All basic blocks should only end with terminator insts, not contain |
| them.</li> |
| <li>The entry node to a function must not have predecessors.</li> |
| <li>All Instructions must be embedded into a basic block.</li> |
| <li>Functions cannot take a void-typed parameter.</li> |
| <li>Verify that a function's argument list agrees with its declared |
| type.</li> |
| <li>It is illegal to specify a name for a void value.</li> |
| <li>It is illegal to have a internal global value with no initializer.</li> |
| <li>It is illegal to have a ret instruction that returns a value that does |
| not agree with the function return value type.</li> |
| <li>Function call argument types match the function prototype.</li> |
| <li>All other things that are tested by asserts spread about the code.</li> |
| </ul> |
| |
| <p> |
| Note that this does not provide full security verification (like Java), but |
| instead just tries to ensure that code is well-formed. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="view-cfg">View CFG of function</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Displays the control flow graph using the GraphViz tool. |
| </p> |
| </div> |
| |
| <!-------------------------------------------------------------------------- --> |
| <div class="doc_subsection"> |
| <a name="view-cfg-only">View CFG of function (with no function bodies)</a> |
| </div> |
| <div class="doc_text"> |
| <p> |
| Displays the control flow graph using the GraphViz tool, but omitting function |
| bodies. |
| </p> |
| </div> |
| |
| <!-- *********************************************************************** --> |
| |
| <hr> |
| <address> |
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| |
| <a href="mailto:rspencer@x10sys.com">Reid Spencer</a><br> |
| <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br> |
| Last modified: $Date$ |
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