diff options
Diffstat (limited to 'share/doc/gccint/Alias-analysis.html')
| -rw-r--r-- | share/doc/gccint/Alias-analysis.html | 190 |
1 files changed, 190 insertions, 0 deletions
diff --git a/share/doc/gccint/Alias-analysis.html b/share/doc/gccint/Alias-analysis.html new file mode 100644 index 0000000..86929b7 --- /dev/null +++ b/share/doc/gccint/Alias-analysis.html @@ -0,0 +1,190 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>GNU Compiler Collection (GCC) Internals: Alias analysis</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Alias analysis"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Alias analysis"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Tree-SSA.html#Tree-SSA" rel="up" title="Tree SSA"> +<link href="Memory-model.html#Memory-model" rel="next" title="Memory model"> +<link href="SSA.html#SSA" rel="previous" title="SSA"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Alias-analysis"></a> +<div class="header"> +<p> +Next: <a href="Memory-model.html#Memory-model" accesskey="n" rel="next">Memory model</a>, Previous: <a href="SSA.html#SSA" accesskey="p" rel="previous">SSA</a>, Up: <a href="Tree-SSA.html#Tree-SSA" accesskey="u" rel="up">Tree SSA</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Alias-analysis-1"></a> +<h3 class="section">13.4 Alias analysis</h3> +<a name="index-alias"></a> +<a name="index-flow_002dsensitive-alias-analysis"></a> +<a name="index-flow_002dinsensitive-alias-analysis"></a> + +<p>Alias analysis in GIMPLE SSA form consists of two pieces. First +the virtual SSA web ties conflicting memory accesses and provides +a SSA use-def chain and SSA immediate-use chains for walking +possibly dependent memory accesses. Second an alias-oracle can +be queried to disambiguate explicit and implicit memory references. +</p> +<ol> +<li> Memory SSA form. + +<p>All statements that may use memory have exactly one accompanied use of +a virtual SSA name that represents the state of memory at the +given point in the IL. +</p> +<p>All statements that may define memory have exactly one accompanied +definition of a virtual SSA name using the previous state of memory +and defining the new state of memory after the given point in the IL. +</p> +<div class="smallexample"> +<pre class="smallexample">int i; +int foo (void) +{ + # .MEM_3 = VDEF <.MEM_2(D)> + i = 1; + # VUSE <.MEM_3> + return i; +} +</pre></div> + +<p>The virtual SSA names in this case are <code>.MEM_2(D)</code> and +<code>.MEM_3</code>. The store to the global variable <code>i</code> +defines <code>.MEM_3</code> invalidating <code>.MEM_2(D)</code>. The +load from <code>i</code> uses that new state <code>.MEM_3</code>. +</p> +<p>The virtual SSA web serves as constraints to SSA optimizers +preventing illegitimate code-motion and optimization. It +also provides a way to walk related memory statements. +</p> +</li><li> Points-to and escape analysis. + +<p>Points-to analysis builds a set of constraints from the GIMPLE +SSA IL representing all pointer operations and facts we do +or do not know about pointers. Solving this set of constraints +yields a conservatively correct solution for each pointer +variable in the program (though we are only interested in +SSA name pointers) as to what it may possibly point to. +</p> +<p>This points-to solution for a given SSA name pointer is stored +in the <code>pt_solution</code> sub-structure of the +<code>SSA_NAME_PTR_INFO</code> record. The following accessor +functions are available: +</p> +<ul> +<li> <code>pt_solution_includes</code> +</li><li> <code>pt_solutions_intersect</code> +</li></ul> + +<p>Points-to analysis also computes the solution for two special +set of pointers, <code>ESCAPED</code> and <code>CALLUSED</code>. Those +represent all memory that has escaped the scope of analysis +or that is used by pure or nested const calls. +</p> +</li><li> Type-based alias analysis + +<p>Type-based alias analysis is frontend dependent though generic +support is provided by the middle-end in <code>alias.cc</code>. TBAA +code is used by both tree optimizers and RTL optimizers. +</p> +<p>Every language that wishes to perform language-specific alias analysis +should define a function that computes, given a <code>tree</code> +node, an alias set for the node. Nodes in different alias sets are not +allowed to alias. For an example, see the C front-end function +<code>c_get_alias_set</code>. +</p> +</li><li> Tree alias-oracle + +<p>The tree alias-oracle provides means to disambiguate two memory +references and memory references against statements. The following +queries are available: +</p> +<ul> +<li> <code>refs_may_alias_p</code> +</li><li> <code>ref_maybe_used_by_stmt_p</code> +</li><li> <code>stmt_may_clobber_ref_p</code> +</li></ul> + +<p>In addition to those two kind of statement walkers are available +walking statements related to a reference ref. +<code>walk_non_aliased_vuses</code> walks over dominating memory defining +statements and calls back if the statement does not clobber ref +providing the non-aliased VUSE. The walk stops at +the first clobbering statement or if asked to. +<code>walk_aliased_vdefs</code> walks over dominating memory defining +statements and calls back on each statement clobbering ref +providing its aliasing VDEF. The walk stops if asked to. +</p> +</li></ol> + + +<hr> +<div class="header"> +<p> +Next: <a href="Memory-model.html#Memory-model" accesskey="n" rel="next">Memory model</a>, Previous: <a href="SSA.html#SSA" accesskey="p" rel="previous">SSA</a>, Up: <a href="Tree-SSA.html#Tree-SSA" accesskey="u" rel="up">Tree SSA</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |