[llvm-commits] [llvm] r43337 - /llvm/trunk/docs/Passes.html
Gordon Henriksen
gordonhenriksen at mac.com
Thu Oct 25 03:18:31 PDT 2007
Author: gordon
Date: Thu Oct 25 05:18:27 2007
New Revision: 43337
URL: http://llvm.org/viewvc/llvm-project?rev=43337&view=rev
Log:
Fleshing out docs/Passes.html for some analyses.
Modified:
llvm/trunk/docs/Passes.html
Modified: llvm/trunk/docs/Passes.html
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/Passes.html?rev=43337&r1=43336&r2=43337&view=diff
==============================================================================
--- llvm/trunk/docs/Passes.html (original)
+++ llvm/trunk/docs/Passes.html Thu Oct 25 05:18:27 2007
@@ -34,6 +34,10 @@
print @x, @y;
EOT
+This (real) one-liner can also be helpful when converting comments to HTML:
+
+perl -e '$/ = undef; for (split(/\n/, <>)) { s:^ *///? ?::; print " <p>\n" if !$on && $_ =~ /\S/; print " </p>\n" if $on && $_ =~ /^\s*$/; print " $_\n"; $on = ($_ =~ /\S/); } print " </p>\n" if $on'
+
-->
<div class="doc_title">LLVM's Analysis and Transform Passes</div>
@@ -46,7 +50,8 @@
</ol>
<div class="doc_author">
- <p>Written by <a href="mailto:rspencer at x10sys.com">Reid Spencer</a></p>
+ <p>Written by <a href="mailto:rspencer at x10sys.com">Reid Spencer</a>
+ and Gordon Henriksen</p>
</div>
<!-- ======================================================================= -->
@@ -84,7 +89,6 @@
<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="#gvn">-gvn</a></td><td>Global Value Numbering</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>
@@ -124,6 +128,7 @@
<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>
@@ -192,7 +197,13 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -200,7 +211,67 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -208,7 +279,11 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -224,7 +299,12 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -232,7 +312,11 @@
<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 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>
<!-------------------------------------------------------------------------- -->
@@ -240,7 +324,10 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -248,7 +335,10 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -256,7 +346,11 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -264,7 +358,10 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -272,7 +369,16 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -280,7 +386,10 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -288,7 +397,10 @@
<a name="domtree">Dominator Tree 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
+ dominators.
+ </p>
</div>
<!-------------------------------------------------------------------------- -->
@@ -296,7 +408,12 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -304,15 +421,12 @@
<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="gvn">Global Value Numbering</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>
<!-------------------------------------------------------------------------- -->
@@ -320,7 +434,9 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -328,7 +444,15 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -336,7 +460,21 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -344,7 +482,12 @@
<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>
<!-------------------------------------------------------------------------- -->
@@ -506,6 +649,7 @@
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
@@ -646,10 +790,30 @@
<!-------------------------------------------------------------------------- -->
<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>Yet to be written.</p>
+ <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>
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