| <!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>Using the GNU Compiler Collection (GCC): Gcov and Optimization</title> |
| |
| <meta name="description" content="Using the GNU Compiler Collection (GCC): Gcov and Optimization"> |
| <meta name="keywords" content="Using the GNU Compiler Collection (GCC): Gcov and Optimization"> |
| <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.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="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. |
| </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> |
