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118 lines
5.7 KiB
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118 lines
5.7 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Gcov and Optimization - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Gcov.html#Gcov" title="Gcov">
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<link rel="prev" href="Invoking-Gcov.html#Invoking-Gcov" title="Invoking Gcov">
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<link rel="next" href="Gcov-Data-Files.html#Gcov-Data-Files" title="Gcov Data Files">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 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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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Gcov-and-Optimization"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Gcov-Data-Files.html#Gcov-Data-Files">Gcov Data Files</a>,
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Previous: <a rel="previous" accesskey="p" href="Invoking-Gcov.html#Invoking-Gcov">Invoking Gcov</a>,
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Up: <a rel="up" accesskey="u" href="Gcov.html#Gcov">Gcov</a>
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<hr>
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</div>
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<h3 class="section">10.3 Using <samp><span class="command">gcov</span></samp> with GCC Optimization</h3>
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<p>If you plan to use <samp><span class="command">gcov</span></samp> to help optimize your code, you must
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first compile your program with two special GCC options:
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‘<samp><span class="samp">-fprofile-arcs -ftest-coverage</span></samp>’. Aside from that, you can use any
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other GCC options; but if you want to prove that every single line
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in your program was executed, you should not compile with optimization
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at the same time. On some machines the optimizer can eliminate some
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simple code lines by combining them with other lines. For example, code
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like this:
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<pre class="smallexample"> if (a != b)
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c = 1;
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else
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c = 0;
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</pre>
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<p class="noindent">can be compiled into one instruction on some machines. In this case,
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there is no way for <samp><span class="command">gcov</span></samp> to calculate separate execution counts
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for each line because there isn't separate code for each line. Hence
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the <samp><span class="command">gcov</span></samp> output looks like this if you compiled the program with
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optimization:
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<pre class="smallexample"> 100: 12:if (a != b)
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100: 13: c = 1;
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100: 14:else
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100: 15: c = 0;
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</pre>
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<p>The output shows that this block of code, combined by optimization,
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executed 100 times. In one sense this result is correct, because there
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was only one instruction representing all four of these lines. However,
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the output does not indicate how many times the result was 0 and how
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many times the result was 1.
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<p>Inlineable functions can create unexpected line counts. Line counts are
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shown for the source code of the inlineable function, but what is shown
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depends on where the function is inlined, or if it is not inlined at all.
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<p>If the function is not inlined, the compiler must emit an out of line
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copy of the function, in any object file that needs it. If
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<samp><span class="file">fileA.o</span></samp> and <samp><span class="file">fileB.o</span></samp> both contain out of line bodies of a
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particular inlineable function, they will also both contain coverage
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counts for that function. When <samp><span class="file">fileA.o</span></samp> and <samp><span class="file">fileB.o</span></samp> are
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linked together, the linker will, on many systems, select one of those
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out of line bodies for all calls to that function, and remove or ignore
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the other. Unfortunately, it will not remove the coverage counters for
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the unused function body. Hence when instrumented, all but one use of
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that function will show zero counts.
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<p>If the function is inlined in several places, the block structure in
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each location might not be the same. For instance, a condition might
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now be calculable at compile time in some instances. Because the
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coverage of all the uses of the inline function will be shown for the
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same source lines, the line counts themselves might seem inconsistent.
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<p>Long-running applications can use the <code>_gcov_reset</code> and <code>_gcov_dump</code>
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facilities to restrict profile collection to the program region of
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interest. Calling <code>_gcov_reset(void)</code> will clear all profile counters
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to zero, and calling <code>_gcov_dump(void)</code> will cause the profile information
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collected at that point to be dumped to <samp><span class="file">.gcda</span></samp> output files.
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<!-- man end -->
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</body></html>
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