<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1988-2015 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 5.2, 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="prev" 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="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> &nbsp; [<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">15.7 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 &ndash; 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 &ndash; <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 &ndash;
<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 &ndash; 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>
<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="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> &nbsp; [<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>