<html lang="en">
<head>
<title>Number of iterations - GNU Compiler Collection (GCC) Internals</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="GNU Compiler Collection (GCC) Internals">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" title="Loop Analysis and Representation">
<link rel="prev" href="loop_002div.html#loop_002div" title="loop-iv">
<link rel="next" href="Dependency-analysis.html#Dependency-analysis" title="Dependency analysis">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
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.-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
pre.display { font-family:inherit }
pre.format { font-family:inherit }
pre.smalldisplay { font-family:inherit; font-size:smaller }
pre.smallformat { font-family:inherit; font-size:smaller }
pre.smallexample { font-size:smaller }
pre.smalllisp { font-size:smaller }
span.sc { font-variant:small-caps }
span.roman { font-family:serif; font-weight:normal; }
span.sansserif { font-family:sans-serif; font-weight:normal; }
--></style>
</head>
<body>
<div class="node">
<a name="Number-of-iterations"></a>
<p>
Next: <a rel="next" accesskey="n" href="Dependency-analysis.html#Dependency-analysis">Dependency analysis</a>,
Previous: <a rel="previous" accesskey="p" href="loop_002div.html#loop_002div">loop-iv</a>,
Up: <a rel="up" accesskey="u" href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation">Loop Analysis and Representation</a>
<hr>
</div>
<h3 class="section">15.7 Number of iterations analysis</h3>
<p><a name="index-Number-of-iterations-analysis-3238"></a>
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:
<ul>
<li><code>assumptions</code>: If this condition is false, the rest of
the information is invalid.
<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><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><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.
</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>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).
</body></html>