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diff --git a/share/doc/gccint/Number-of-iterations.html b/share/doc/gccint/Number-of-iterations.html new file mode 100644 index 0000000..ff43d70 --- /dev/null +++ b/share/doc/gccint/Number-of-iterations.html @@ -0,0 +1,164 @@ +<!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). 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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: Number of iterations</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Number of iterations"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Number of iterations"> +<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="Dependency-analysis.html#Dependency-analysis" rel="next" title="Dependency analysis"> +<link href="loop_002div.html#loop_002div" rel="previous" title="loop-iv"> +<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="Number-of-iterations"></a> +<div class="header"> +<p> +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="previous">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> +</div> +<hr> +<a name="Number-of-iterations-analysis"></a> +<h3 class="section">16.7 Number of iterations analysis</h3> +<a name="index-Number-of-iterations-analysis"></a> + +<p>Both on GIMPLE and on RTL, there are functions available to determine +the number of iterations of a loop, with a similar interface. The +number of iterations of a loop in GCC is defined as the number of +executions of the loop latch. In many cases, it is not possible to +determine the number of iterations unconditionally – the determined +number is correct only if some assumptions are satisfied. The analysis +tries to verify these conditions using the information contained in the +program; if it fails, the conditions are returned together with the +result. The following information and conditions are provided by the +analysis: +</p> +<ul> +<li> <code>assumptions</code>: If this condition is false, the rest of +the information is invalid. +</li><li> <code>noloop_assumptions</code> on RTL, <code>may_be_zero</code> on GIMPLE: If +this condition is true, the loop exits in the first iteration. +</li><li> <code>infinite</code>: If this condition is true, the loop is infinite. +This condition is only available on RTL. On GIMPLE, conditions for +finiteness of the loop are included in <code>assumptions</code>. +</li><li> <code>niter_expr</code> on RTL, <code>niter</code> on GIMPLE: The expression +that gives number of iterations. The number of iterations is defined as +the number of executions of the loop latch. +</li></ul> + +<p>Both on GIMPLE and on RTL, it necessary for the induction variable +analysis framework to be initialized (SCEV on GIMPLE, loop-iv on RTL). +On GIMPLE, the results are stored to <code>struct tree_niter_desc</code> +structure. Number of iterations before the loop is exited through a +given exit can be determined using <code>number_of_iterations_exit</code> +function. On RTL, the results are returned in <code>struct niter_desc</code> +structure. The corresponding function is named +<code>check_simple_exit</code>. There are also functions that pass through +all the exits of a loop and try to find one with easy to determine +number of iterations – <code>find_loop_niter</code> on GIMPLE and +<code>find_simple_exit</code> on RTL. Finally, there are functions that +provide the same information, but additionally cache it, so that +repeated calls to number of iterations are not so costly – +<code>number_of_latch_executions</code> on GIMPLE and <code>get_simple_loop_desc</code> +on RTL. +</p> +<p>Note that some of these functions may behave slightly differently than +others – some of them return only the expression for the number of +iterations, and fail if there are some assumptions. The function +<code>number_of_latch_executions</code> works only for single-exit loops. +The function <code>number_of_cond_exit_executions</code> can be used to +determine number of executions of the exit condition of a single-exit +loop (i.e., the <code>number_of_latch_executions</code> increased by one). +</p> +<p>On GIMPLE, below constraint flags affect semantics of some APIs of number +of iterations analyzer: +</p> +<ul> +<li> <code>LOOP_C_INFINITE</code>: If this constraint flag is set, the loop +is known to be infinite. APIs like <code>number_of_iterations_exit</code> can +return false directly without doing any analysis. +</li><li> <code>LOOP_C_FINITE</code>: If this constraint flag is set, the loop is +known to be finite, in other words, loop’s number of iterations can be +computed with <code>assumptions</code> be true. +</li></ul> + +<p>Generally, the constraint flags are set/cleared by consumers which are +loop optimizers. It’s also the consumers’ responsibility to set/clear +constraints correctly. Failing to do that might result in hard to track +down bugs in scev/niter consumers. One typical use case is vectorizer: +it drives number of iterations analyzer by setting <code>LOOP_C_FINITE</code> +and vectorizes possibly infinite loop by versioning loop with analysis +result. In return, constraints set by consumers can also help number of +iterations analyzer in following optimizers. For example, <code>niter</code> +of a loop versioned under <code>assumptions</code> is valid unconditionally. +</p> +<p>Other constraints may be added in the future, for example, a constraint +indicating that loops’ latch must roll thus <code>may_be_zero</code> would be +false unconditionally. +</p> +<hr> +<div class="header"> +<p> +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="previous">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> +</div> + + + +</body> +</html> |