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<div class="doc_title">LLVM's Analysis and Transform Passes</div>
<ol>
</ol>
<div class="doc_author">
- <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
+ <p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
+ and Gordon Henriksen</p>
</div>
<!-- ======================================================================= -->
<p>This document serves as a high level summary of the optimization features
that LLVM provides. Optimizations are implemented as Passes that traverse some
portion of a program to either collect information or transform the program.
- THe table below divides the passes that LLVM provides into three categories.
+ 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 ome utility but don't otherwise fit categorization.
- For example passes to extract functions to bytecode or write a module to
- bytecode are neither analysis nor transform passes.
+ 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="3"><b>ANALYSIS PASSES</b></th></tr>
-<tr><th>Option</th><th>Name</th><th>Directory</th></tr>
+<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="#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="#domset">-domset</a></td><td>Dominator Set Construction</td></tr>
<tr><td><a href="#domtree">-domtree</a></td><td>Dominator Tree Construction</td></tr>
-<tr><td><a href="#etforest">-etforest</a></td><td>ET Forest 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="#idom">-idom</a></td><td>Immediate Dominators Construction</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="#postdomset">-postdomset</a></td><td>Post-Dominator Set Construction</td></tr>
<tr><td><a href="#postdomtree">-postdomtree</a></td><td>Post-Dominator Tree Construction</td></tr>
-<tr><td><a href="#postetforest">-postetforest</a></td><td>Post-ET-Forest Construction</td></tr>
-<tr><td><a href="#postidom">-postidom</a></td><td>Immediate Post-Dominators 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="#targetdata">-targetdata</a></td><td>Target Data Layout</td></tr>
-<tr><th colspan="3"><b>TRANSFORM PASSES</b></th></tr>
-<tr><th>Option</th><th>Name</th><th>Directory</th></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="#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="#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="#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-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>Lower Packed Operations</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</td></tr>
-<tr><td><a href="#lowerselect">-lowerselect</a></td><td>Lower Selects To Branches</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="#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 Values to Memory</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="#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="3"><b>UTILITY PASSES</b></th></tr>
-<tr><th>Option</th><th>Name</th><th>Directory</th></tr>
-<tr><td><a href="#deadarghaX0r">-deadarghaX0r</a></td><td>Dead Argument Hacking (BUGPOINT ONLY)</td></tr>
-<tr><td><a href="#extract-blocks">-extract-blocks</a></td><td>Extract Basic Blocks From Module (BUGPOINT ONLY)</td></tr>
-<tr><td><a href="#emitbytecode">-emitbytecode</a></td><td>Bytecode Writer</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>
<a name="aa-eval">Exhaustive Alias Analysis Precision Evaluator</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="anders-aa">Andersen's Interprocedural Alias Analysis</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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</a>
+ <a name="basicaa">Basic Alias Analysis (default AA impl)</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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 Call Graph Construction</a>
+ <a name="basiccg">Basic CallGraph Construction</a>
</div>
<div class="doc_text">
<p>Yet to be written.</p>
<!-------------------------------------------------------------------------- -->
<div class="doc_subsection">
- <a name="basicvn">Basic Value Numbering</a>
+ <a name="basicvn">Basic Value Numbering (default GVN impl)</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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 Call Graph</a>
+ <a name="callgraph">Print a call graph</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ This pass, only available in <code>opt</code>, prints the call graph to
+ standard output in a human-readable form.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="callscc">Print SCCs of the Call Graph</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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 Control Flow Graph</a>
+ <a name="cfgscc">Print SCCs of each function CFG</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="count-aa">Count Alias Analysis Query Responses</a>
+ <a name="codegenprepare">Optimize for code generation</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="debug-aa">Alias Analysis Usage Debugger</a>
+ <a name="count-aa">Count Alias Analysis Query Responses</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="domfrontier">Dominance Frontier Construction</a>
+ <a name="debug-aa">AA use debugger</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="domset">Dominator Set Construction</a>
+ <a name="domfrontier">Dominance Frontier Construction</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ This pass is a simple dominator construction algorithm for finding forward
+ dominator frontiers.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="domtree">Dominator Tree Construction</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
-</div>
-
-<!-------------------------------------------------------------------------- -->
-<div class="doc_subsection">
- <a name="etforest">ET Forest Construction</a>
-</div>
-<div class="doc_text">
- <p>Yet to be written.</p>
-</div>
-
-<!-------------------------------------------------------------------------- -->
-<div class="doc_subsection">
- <a name="basicvn">Basic Value Numbering</a>
-</div>
-<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ This pass is a simple dominator construction algorithm for finding forward
+ dominators.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="externalfnconstants">Print external fn callsites passed constants</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="globalsmodref-aa">Simple mod/ref analysis for globals</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
-</div>
-
-<!-------------------------------------------------------------------------- -->
-<div class="doc_subsection">
- <a name="idom">Immediate Dominators Construction</a>
-</div>
-<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="instcount">Counts the various types of Instructions</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ This pass collects the count of all instructions and reports them
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="intervals">Interval Partition Construction</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="load-vn">Load Value Numbering</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="loops">Natural Loop Construction</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="no-aa">No Alias Analysis (always returns 'may' alias)</a>
+ <a name="memdep">Memory Dependence Analysis</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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-profile">No Profile Information</a>
+ <a name="no-aa">No Alias Analysis (always returns 'may' alias)</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="postdomfrontier">Post-Dominance Frontier Construction</a>
+ <a name="no-profile">No Profile Information</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ The default "no profile" implementation of the abstract
+ <code>ProfileInfo</code> interface.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<div class="doc_subsection">
- <a name="postdomset">Post-Dominator Set Construction</a>
+ <a name="postdomfrontier">Post-Dominance Frontier Construction</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ This pass is a simple post-dominator construction algorithm for finding
+ post-dominator frontiers.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="postdomtree">Post-Dominator Tree Construction</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
-</div>
-
-<!-------------------------------------------------------------------------- -->
-<div class="doc_subsection">
- <a name="postetforest">Post-ET-Forest Construction</a>
-</div>
-<div class="doc_text">
- <p>Yet to be written.</p>
-</div>
-
-<!-------------------------------------------------------------------------- -->
-<div class="doc_subsection">
- <a name="postidom">Immediate Post-Dominators Construction</a>
-</div>
-<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ This pass is a simple post-dominator construction algorithm for finding
+ post-dominators.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="print">Print function to stderr</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="print-callgraph">Print Call Graph to 'dot' file</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="print-cfg">Print CFG of function to 'dot' file</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="print-cfg-only">Print CFG of function to 'dot' file (with no function bodies)</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="printm">Print module to stderr</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ This pass simply prints out the entire module when it is executed.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="printusedtypes">Find Used Types</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="profile-loader">Load profile information from llvmprof.out</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ A concrete implementation of profiling information that loads the information
+ from a profile dump file.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="scalar-evolution">Scalar Evolution Analysis</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="targetdata">Target Data Layout</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-- ======================================================================= -->
<a name="argpromotion">Promote 'by reference' arguments to scalars</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="block-placement">Profile Guided Basic Block Placement</a>
</div>
<div class="doc_text">
- <p>This pass implements 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>
- <p>The algorithm implemented here is basically "Algo1" from "Profile Guided
- Code Positioning" by Pettis and Hansen, except that it uses basic block
- counts instead of edge counts. This could be improved in many ways, but is
- very simple for now.</p>
- <p>Basically we "place" the entry block, then loop over all successors in a
- DFO, placing the most frequently executed successor until we run out of
- blocks. Did we mention that this was <b>extremely</b> simplistic? This is
- also much slower than it could be. When it becomes important, this pass
- will be rewritten to use a better algorithm, and then we can worry about
- efficiency.</p>
+ <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>
+ <a name="break-crit-edges">Break critical edges in CFG</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<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:
- <pre>
- if (i == 7)
- ... 4*i; // constant propagation
+ 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></p>
+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
<a name="constmerge">Merge Duplicate Global Constants</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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">Constant Propagation</a>
+ <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:
- <pre>add i32 1, 2</pre><br/>
- becomes
- <pre>i32 3</pre></p>
+ 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>
<a name="dce">Dead Code Elimination</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="deadargelim">Dead Argument Elimination</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="deadtypeelim">Dead Type Elimination</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="die">Dead Instruction Elimination</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ Dead instruction elimination performs a single pass over the function,
+ removing instructions that are obviously dead.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="dse">Dead Store Elimination</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ A trivial dead store elimination that only considers basic-block local
+ redundant stores.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="gcse">Global Common Subexpression Elimination</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="globaldce">Dead Global Elimination</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="globalopt">Global Variable Optimizer</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="indmemrem">Indirect Malloc and Free Removal</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="indvars">Canonicalize Induction Variables</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="inline">Function Integration/Inlining</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ Bottom-up inlining of functions into callees.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="insert-block-profiling">Insert instrumentation for block profiling</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="insert-edge-profiling">Insert instrumentation for edge profiling</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="insert-function-profiling">Insert instrumentation for function profiling</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ This pass instruments the specified program with counters for function
+ profiling, which counts the number of times each function is called.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="insert-null-profiling-rs">Measure profiling framework overhead</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="insert-rs-profiling-framework">Insert random sampling instrumentation framework</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="instcombine">Combine redundant instructions</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="internalize">Internalize Global Symbols</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="ipconstprop">Interprocedural constant propagation</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="ipsccp">Interprocedural Sparse Conditional Constant Propagation</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>
+ An interprocedural variant of <a href="#sccp">Sparse Conditional Constant
+ Propagation</a>.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
<a name="lcssa">Loop-Closed SSA Form Pass</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="licm">Loop Invariant Code Motion</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="loop-extract">Extract loops into new functions</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="loop-extract-single">Extract at most one loop into a new function</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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-reduce">Loop Strength Reduction</a>
+ <a name="loop-index-split">Index Split Loops</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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-unroll">Unroll Loops</a>
+ <a name="loop-reduce">Loop Strength Reduction</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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-unswitch">Unswitch Loops</a>
+ <a name="loop-rotate">Rotate Loops</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <p>A simple loop rotation transformation.</p>
</div>
<!-------------------------------------------------------------------------- -->
<div class="doc_subsection">
- <a name="loopsimplify">Canonicalize Natural Loops</a>
+ <a name="loop-unroll">Unroll loops</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="lower-packed">Lower Packed Operations</a>
+ <a name="loop-unswitch">Unswitch loops</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="lowerallocs">Lower allocations from instructions to calls</a>
+ <a name="loopsimplify">Canonicalize natural loops</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="lowergc">Lower GC intrinsics, for GCless code generators</a>
+ <a name="lower-packed">lowers packed operations to operations on smaller packed datatypes</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="lowerinvoke">Lower Invoke and Unwind</a>
+ <a name="lowerallocs">Lower allocations from instructions to calls</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="lowerselect">Lower Selects To Branches</a>
+ <a name="lowergc">Lower GC intrinsics, for GCless code generators</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="lowersetjmp">Lower Set Jump</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="lowerswitch">Lower SwitchInst's to branches</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="mem2reg">Promote Memory to Register</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
+ <a name="mergereturn">Unify function exit nodes</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="predsimplify">Predicate Simplifier</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="prune-eh">Remove unused exception handling info</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="raiseallocs">Raise allocations from calls to instructions</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
+ <a name="reassociate">Reassociate expressions</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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 Values to Memory</a>
+ <a name="reg2mem">Demote all values to stack slots</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="scalarrepl">Scalar Replacement of Aggregates</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="sccp">Sparse Conditional Constant Propagation</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="simplify-libcalls">Simplify well-known library calls</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="simplifycfg">Simplify the CFG</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="strip">Strip all symbols from a module</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="tailcallelim">Tail Call Elimination</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="tailduplicate">Tail Duplication</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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_subsection">
- <a name="deadarghaX0r">Dead Argument Hacking (BUGPOINT ONLY)</a>
+ <a name="deadarghaX0r">Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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 (BUGPOINT ONLY)</a>
+ <a name="extract-blocks">Extract Basic Blocks From Module (for bugpoint use)</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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="emitbytecode">Bytecode Writer</a>
+ <a name="preverify">Preliminary module verification</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-------------------------------------------------------------------------- -->
<a name="verify">Module Verifier</a>
</div>
<div class="doc_text">
- <p>Yet to be written.</p>
+ <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>
<!-- *********************************************************************** -->
projects / oota-llvm.git / blobdiff