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diff --git a/share/doc/gccint/LCSSA.html b/share/doc/gccint/LCSSA.html new file mode 100644 index 0000000..4f3a3ca --- /dev/null +++ b/share/doc/gccint/LCSSA.html @@ -0,0 +1,127 @@ +<!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: LCSSA</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: LCSSA"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: LCSSA"> +<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="previous" title="Loop manipulation"> +<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="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="previous">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="previous">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> |