You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
158 lines
7.4 KiB
HTML
158 lines
7.4 KiB
HTML
4 years ago
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>Gcov and Optimization (Using the GNU Compiler Collection (GCC))</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Gcov and Optimization (Using the GNU Compiler Collection (GCC))">
|
||
|
|
<meta name="keywords" content="Gcov and Optimization (Using the GNU Compiler Collection (GCC))">
|
||
|
|
<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="Gcov.html#Gcov" rel="up" title="Gcov">
|
||
|
|
<link href="Gcov-Data-Files.html#Gcov-Data-Files" rel="next" title="Gcov Data Files">
|
||
|
|
<link href="Invoking-Gcov.html#Invoking-Gcov" rel="prev" title="Invoking Gcov">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.indentedblock {margin-right: 0em}
|
||
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
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.nolinebreak {white-space: nowrap}
|
||
|
|
span.roman {font-family: initial; font-weight: normal}
|
||
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<a name="Gcov-and-Optimization"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Gcov-Data-Files.html#Gcov-Data-Files" accesskey="n" rel="next">Gcov Data Files</a>, Previous: <a href="Invoking-Gcov.html#Invoking-Gcov" accesskey="p" rel="prev">Invoking Gcov</a>, Up: <a href="Gcov.html#Gcov" accesskey="u" rel="up">Gcov</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="Using-gcov-with-GCC-Optimization"></a>
|
||
|
|
<h3 class="section">10.3 Using <code>gcov</code> with GCC Optimization</h3>
|
||
|
|
|
||
|
|
<p>If you plan to use <code>gcov</code> to help optimize your code, you must
|
||
|
|
first compile your program with two special GCC options:
|
||
|
|
‘<samp>-fprofile-arcs -ftest-coverage</samp>’. Aside from that, you can use any
|
||
|
|
other GCC options; but if you want to prove that every single line
|
||
|
|
in your program was executed, you should not compile with optimization
|
||
|
|
at the same time. On some machines the optimizer can eliminate some
|
||
|
|
simple code lines by combining them with other lines. For example, code
|
||
|
|
like this:
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample">if (a != b)
|
||
|
|
c = 1;
|
||
|
|
else
|
||
|
|
c = 0;
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>can be compiled into one instruction on some machines. In this case,
|
||
|
|
there is no way for <code>gcov</code> to calculate separate execution counts
|
||
|
|
for each line because there isn’t separate code for each line. Hence
|
||
|
|
the <code>gcov</code> output looks like this if you compiled the program with
|
||
|
|
optimization:
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample"> 100: 12:if (a != b)
|
||
|
|
100: 13: c = 1;
|
||
|
|
100: 14:else
|
||
|
|
100: 15: c = 0;
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>The output shows that this block of code, combined by optimization,
|
||
|
|
executed 100 times. In one sense this result is correct, because there
|
||
|
|
was only one instruction representing all four of these lines. However,
|
||
|
|
the output does not indicate how many times the result was 0 and how
|
||
|
|
many times the result was 1.
|
||
|
|
</p>
|
||
|
|
<p>Inlineable functions can create unexpected line counts. Line counts are
|
||
|
|
shown for the source code of the inlineable function, but what is shown
|
||
|
|
depends on where the function is inlined, or if it is not inlined at all.
|
||
|
|
</p>
|
||
|
|
<p>If the function is not inlined, the compiler must emit an out of line
|
||
|
|
copy of the function, in any object file that needs it. If
|
||
|
|
<samp>fileA.o</samp> and <samp>fileB.o</samp> both contain out of line bodies of a
|
||
|
|
particular inlineable function, they will also both contain coverage
|
||
|
|
counts for that function. When <samp>fileA.o</samp> and <samp>fileB.o</samp> are
|
||
|
|
linked together, the linker will, on many systems, select one of those
|
||
|
|
out of line bodies for all calls to that function, and remove or ignore
|
||
|
|
the other. Unfortunately, it will not remove the coverage counters for
|
||
|
|
the unused function body. Hence when instrumented, all but one use of
|
||
|
|
that function will show zero counts.
|
||
|
|
</p>
|
||
|
|
<p>If the function is inlined in several places, the block structure in
|
||
|
|
each location might not be the same. For instance, a condition might
|
||
|
|
now be calculable at compile time in some instances. Because the
|
||
|
|
coverage of all the uses of the inline function will be shown for the
|
||
|
|
same source lines, the line counts themselves might seem inconsistent.
|
||
|
|
</p>
|
||
|
|
<p>Long-running applications can use the <code>__gcov_reset</code> and <code>__gcov_dump</code>
|
||
|
|
facilities to restrict profile collection to the program region of
|
||
|
|
interest. Calling <code>__gcov_reset(void)</code> will clear all profile counters
|
||
|
|
to zero, and calling <code>__gcov_dump(void)</code> will cause the profile information
|
||
|
|
collected at that point to be dumped to <samp>.gcda</samp> output files.
|
||
|
|
Instrumented applications use a static destructor with priority 99
|
||
|
|
to invoke the <code>__gcov_dump</code> function. Thus <code>__gcov_dump</code>
|
||
|
|
is executed after all user defined static destructors,
|
||
|
|
as well as handlers registered with <code>atexit</code>.
|
||
|
|
If an executable loads a dynamic shared object via dlopen functionality,
|
||
|
|
<samp>-Wl,--dynamic-list-data</samp> is needed to dump all profile data.
|
||
|
|
</p>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Gcov-Data-Files.html#Gcov-Data-Files" accesskey="n" rel="next">Gcov Data Files</a>, Previous: <a href="Invoking-Gcov.html#Invoking-Gcov" accesskey="p" rel="prev">Invoking Gcov</a>, Up: <a href="Gcov.html#Gcov" accesskey="u" rel="up">Gcov</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>
|