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164 lines
8.4 KiB
HTML
164 lines
8.4 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Number of iterations (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Number of iterations (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Number of iterations (GNU Compiler Collection (GCC) Internals)">
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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="Dependency-analysis.html#Dependency-analysis" rel="next" title="Dependency analysis">
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<link href="loop_002div.html#loop_002div" rel="prev" title="loop-iv">
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</head>
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<body lang="en">
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<a name="Number-of-iterations"></a>
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<div class="header">
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<p>
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Next: <a href="Dependency-analysis.html#Dependency-analysis" accesskey="n" rel="next">Dependency analysis</a>, Previous: <a href="loop_002div.html#loop_002div" accesskey="p" rel="prev">loop-iv</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="Number-of-iterations-analysis"></a>
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<h3 class="section">16.7 Number of iterations analysis</h3>
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<a name="index-Number-of-iterations-analysis"></a>
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<p>Both on GIMPLE and on RTL, there are functions available to determine
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the number of iterations of a loop, with a similar interface. The
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number of iterations of a loop in GCC is defined as the number of
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executions of the loop latch. In many cases, it is not possible to
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determine the number of iterations unconditionally – the determined
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number is correct only if some assumptions are satisfied. The analysis
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tries to verify these conditions using the information contained in the
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program; if it fails, the conditions are returned together with the
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result. The following information and conditions are provided by the
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analysis:
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</p>
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<ul>
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<li> <code>assumptions</code>: If this condition is false, the rest of
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the information is invalid.
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</li><li> <code>noloop_assumptions</code> on RTL, <code>may_be_zero</code> on GIMPLE: If
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this condition is true, the loop exits in the first iteration.
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</li><li> <code>infinite</code>: If this condition is true, the loop is infinite.
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This condition is only available on RTL. On GIMPLE, conditions for
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finiteness of the loop are included in <code>assumptions</code>.
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</li><li> <code>niter_expr</code> on RTL, <code>niter</code> on GIMPLE: The expression
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that gives number of iterations. The number of iterations is defined as
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the number of executions of the loop latch.
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</li></ul>
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<p>Both on GIMPLE and on RTL, it necessary for the induction variable
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analysis framework to be initialized (SCEV on GIMPLE, loop-iv on RTL).
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On GIMPLE, the results are stored to <code>struct tree_niter_desc</code>
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structure. Number of iterations before the loop is exited through a
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given exit can be determined using <code>number_of_iterations_exit</code>
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function. On RTL, the results are returned in <code>struct niter_desc</code>
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structure. The corresponding function is named
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<code>check_simple_exit</code>. There are also functions that pass through
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all the exits of a loop and try to find one with easy to determine
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number of iterations – <code>find_loop_niter</code> on GIMPLE and
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<code>find_simple_exit</code> on RTL. Finally, there are functions that
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provide the same information, but additionally cache it, so that
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repeated calls to number of iterations are not so costly –
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<code>number_of_latch_executions</code> on GIMPLE and <code>get_simple_loop_desc</code>
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on RTL.
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</p>
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<p>Note that some of these functions may behave slightly differently than
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others – some of them return only the expression for the number of
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iterations, and fail if there are some assumptions. The function
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<code>number_of_latch_executions</code> works only for single-exit loops.
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The function <code>number_of_cond_exit_executions</code> can be used to
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determine number of executions of the exit condition of a single-exit
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loop (i.e., the <code>number_of_latch_executions</code> increased by one).
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</p>
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<p>On GIMPLE, below constraint flags affect semantics of some APIs of number
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of iterations analyzer:
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</p>
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<ul>
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<li> <code>LOOP_C_INFINITE</code>: If this constraint flag is set, the loop
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is known to be infinite. APIs like <code>number_of_iterations_exit</code> can
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return false directly without doing any analysis.
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</li><li> <code>LOOP_C_FINITE</code>: If this constraint flag is set, the loop is
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known to be finite, in other words, loop’s number of iterations can be
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computed with <code>assumptions</code> be true.
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</li></ul>
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<p>Generally, the constraint flags are set/cleared by consumers which are
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loop optimizers. It’s also the consumers’ responsibility to set/clear
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constraints correctly. Failing to do that might result in hard to track
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down bugs in scev/niter consumers. One typical use case is vectorizer:
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it drives number of iterations analyzer by setting <code>LOOP_C_FINITE</code>
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and vectorizes possibly infinite loop by versioning loop with analysis
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result. In return, constraints set by consumers can also help number of
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iterations analyzer in following optimizers. For example, <code>niter</code>
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of a loop versioned under <code>assumptions</code> is valid unconditionally.
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</p>
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<p>Other constraints may be added in the future, for example, a constraint
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indicating that loops’ latch must roll thus <code>may_be_zero</code> would be
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false unconditionally.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Dependency-analysis.html#Dependency-analysis" accesskey="n" rel="next">Dependency analysis</a>, Previous: <a href="loop_002div.html#loop_002div" accesskey="p" rel="prev">loop-iv</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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