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136 lines
6.4 KiB
HTML
136 lines
6.4 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Loop manipulation (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Loop manipulation (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Loop manipulation (GNU Compiler Collection (GCC) Internals)">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" rel="up" title="Loop Analysis and Representation">
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<link href="LCSSA.html#LCSSA" rel="next" title="LCSSA">
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<link href="Loop-querying.html#Loop-querying" rel="prev" title="Loop querying">
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<style type="text/css">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Loop-manipulation"></a>
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<div class="header">
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<p>
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Next: <a href="LCSSA.html#LCSSA" accesskey="n" rel="next">LCSSA</a>, Previous: <a href="Loop-querying.html#Loop-querying" accesskey="p" rel="prev">Loop querying</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>
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</div>
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<hr>
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<a name="Loop-manipulation-1"></a>
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<h3 class="section">16.3 Loop manipulation</h3>
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<a name="index-Loop-manipulation"></a>
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<p>The loops tree can be manipulated using the following functions:
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</p>
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<ul>
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<li> <code>flow_loop_tree_node_add</code>: Adds a node to the tree.
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</li><li> <code>flow_loop_tree_node_remove</code>: Removes a node from the tree.
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</li><li> <code>add_bb_to_loop</code>: Adds a basic block to a loop.
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</li><li> <code>remove_bb_from_loops</code>: Removes a basic block from loops.
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</li></ul>
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<p>Most low-level CFG functions update loops automatically. The following
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functions handle some more complicated cases of CFG manipulations:
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</p>
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<ul>
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<li> <code>remove_path</code>: Removes an edge and all blocks it dominates.
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</li><li> <code>split_loop_exit_edge</code>: Splits exit edge of the loop,
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ensuring that PHI node arguments remain in the loop (this ensures that
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loop-closed SSA form is preserved). Only useful on GIMPLE.
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</li></ul>
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<p>Finally, there are some higher-level loop transformations implemented.
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While some of them are written so that they should work on non-innermost
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loops, they are mostly untested in that case, and at the moment, they
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are only reliable for the innermost loops:
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</p>
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<ul>
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<li> <code>create_iv</code>: Creates a new induction variable. Only works on
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GIMPLE. <code>standard_iv_increment_position</code> can be used to find a
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suitable place for the iv increment.
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</li><li> <code>duplicate_loop_to_header_edge</code>,
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<code>tree_duplicate_loop_to_header_edge</code>: These functions (on RTL and
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on GIMPLE) duplicate the body of the loop prescribed number of times on
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one of the edges entering loop header, thus performing either loop
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unrolling or loop peeling. <code>can_duplicate_loop_p</code>
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(<code>can_unroll_loop_p</code> on GIMPLE) must be true for the duplicated
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loop.
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</li><li> <code>loop_version</code>: This function creates a copy of a loop, and
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a branch before them that selects one of them depending on the
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prescribed condition. This is useful for optimizations that need to
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verify some assumptions in runtime (one of the copies of the loop is
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usually left unchanged, while the other one is transformed in some way).
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</li><li> <code>tree_unroll_loop</code>: Unrolls the loop, including peeling the
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extra iterations to make the number of iterations divisible by unroll
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factor, updating the exit condition, and removing the exits that now
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cannot be taken. Works only on GIMPLE.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="LCSSA.html#LCSSA" accesskey="n" rel="next">LCSSA</a>, Previous: <a href="Loop-querying.html#Loop-querying" accesskey="p" rel="prev">Loop querying</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>
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</div>
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</body>
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</html>
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