You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
127 lines
6.1 KiB
HTML
127 lines
6.1 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1988-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>LCSSA (GNU Compiler Collection (GCC) Internals)</title>
|
|
|
|
<meta name="description" content="LCSSA (GNU Compiler Collection (GCC) Internals)">
|
|
<meta name="keywords" content="LCSSA (GNU Compiler Collection (GCC) Internals)">
|
|
<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="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" rel="up" title="Loop Analysis and Representation">
|
|
<link href="Scalar-evolutions.html#Scalar-evolutions" rel="next" title="Scalar evolutions">
|
|
<link href="Loop-manipulation.html#Loop-manipulation" rel="prev" title="Loop manipulation">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
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.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="LCSSA"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Scalar-evolutions.html#Scalar-evolutions" accesskey="n" rel="next">Scalar evolutions</a>, Previous: <a href="Loop-manipulation.html#Loop-manipulation" accesskey="p" rel="prev">Loop manipulation</a>, Up: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="u" rel="up">Loop Analysis and Representation</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="Loop_002dclosed-SSA-form"></a>
|
|
<h3 class="section">16.4 Loop-closed SSA form</h3>
|
|
<a name="index-LCSSA"></a>
|
|
<a name="index-Loop_002dclosed-SSA-form"></a>
|
|
|
|
<p>Throughout the loop optimizations on tree level, one extra condition is
|
|
enforced on the SSA form: No SSA name is used outside of the loop in
|
|
that it is defined. The SSA form satisfying this condition is called
|
|
“loop-closed SSA form” – LCSSA. To enforce LCSSA, PHI nodes must be
|
|
created at the exits of the loops for the SSA names that are used
|
|
outside of them. Only the real operands (not virtual SSA names) are
|
|
held in LCSSA, in order to save memory.
|
|
</p>
|
|
<p>There are various benefits of LCSSA:
|
|
</p>
|
|
<ul>
|
|
<li> Many optimizations (value range analysis, final value
|
|
replacement) are interested in the values that are defined in the loop
|
|
and used outside of it, i.e., exactly those for that we create new PHI
|
|
nodes.
|
|
</li><li> In induction variable analysis, it is not necessary to specify the
|
|
loop in that the analysis should be performed – the scalar evolution
|
|
analysis always returns the results with respect to the loop in that the
|
|
SSA name is defined.
|
|
</li><li> It makes updating of SSA form during loop transformations simpler.
|
|
Without LCSSA, operations like loop unrolling may force creation of PHI
|
|
nodes arbitrarily far from the loop, while in LCSSA, the SSA form can be
|
|
updated locally. However, since we only keep real operands in LCSSA, we
|
|
cannot use this advantage (we could have local updating of real
|
|
operands, but it is not much more efficient than to use generic SSA form
|
|
updating for it as well; the amount of changes to SSA is the same).
|
|
</li></ul>
|
|
|
|
<p>However, it also means LCSSA must be updated. This is usually
|
|
straightforward, unless you create a new value in loop and use it
|
|
outside, or unless you manipulate loop exit edges (functions are
|
|
provided to make these manipulations simple).
|
|
<code>rewrite_into_loop_closed_ssa</code> is used to rewrite SSA form to
|
|
LCSSA, and <code>verify_loop_closed_ssa</code> to check that the invariant of
|
|
LCSSA is preserved.
|
|
</p>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Scalar-evolutions.html#Scalar-evolutions" accesskey="n" rel="next">Scalar evolutions</a>, Previous: <a href="Loop-manipulation.html#Loop-manipulation" accesskey="p" rel="prev">Loop manipulation</a>, Up: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="u" rel="up">Loop Analysis and Representation</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>
|