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<title>GNU gprof: Annotated Source</title>
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<a name="Annotated-Source"></a>
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<div class="header">
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<p>
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Previous: <a href="Line_002dby_002dline.html#Line_002dby_002dline" accesskey="p" rel="previous">Line-by-line</a>, Up: <a href="Output.html#Output" accesskey="u" rel="up">Output</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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<a name="The-Annotated-Source-Listing"></a>
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<h3 class="section">5.4 The Annotated Source Listing</h3>
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<p><code>gprof</code>’s ‘<samp>-A</samp>’ option triggers an annotated source listing,
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which lists the program’s source code, each function labeled with the
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number of times it was called. You may also need to specify the
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‘<samp>-I</samp>’ option, if <code>gprof</code> can’t find the source code files.
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</p>
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<p>With older versions of <code>gcc</code> compiling with ‘<samp>gcc … -g
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-pg -a</samp>’ augments your program with basic-block counting code, in
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addition to function counting code. This enables <code>gprof</code> to
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determine how many times each line of code was executed. With newer
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versions of <code>gcc</code> support for displaying basic-block counts is
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provided by the <code>gcov</code> program.
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</p>
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<p>For example, consider the following function, taken from gzip,
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with line numbers added:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> 1 ulg updcrc(s, n)
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2 uch *s;
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3 unsigned n;
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4 {
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5 register ulg c;
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6
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7 static ulg crc = (ulg)0xffffffffL;
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8
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9 if (s == NULL) {
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10 c = 0xffffffffL;
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11 } else {
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12 c = crc;
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13 if (n) do {
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14 c = crc_32_tab[...];
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15 } while (--n);
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16 }
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17 crc = c;
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18 return c ^ 0xffffffffL;
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19 }
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</pre></div>
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<p><code>updcrc</code> has at least five basic-blocks.
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One is the function itself. The
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<code>if</code> statement on line 9 generates two more basic-blocks, one
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for each branch of the <code>if</code>. A fourth basic-block results from
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the <code>if</code> on line 13, and the contents of the <code>do</code> loop form
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the fifth basic-block. The compiler may also generate additional
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basic-blocks to handle various special cases.
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</p>
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<p>A program augmented for basic-block counting can be analyzed with
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‘<samp>gprof -l -A</samp>’.
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The ‘<samp>-x</samp>’ option is also helpful,
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to ensure that each line of code is labeled at least once.
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Here is <code>updcrc</code>’s
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annotated source listing for a sample <code>gzip</code> run:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> ulg updcrc(s, n)
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uch *s;
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unsigned n;
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2 ->{
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register ulg c;
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static ulg crc = (ulg)0xffffffffL;
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2 -> if (s == NULL) {
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1 -> c = 0xffffffffL;
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1 -> } else {
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1 -> c = crc;
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1 -> if (n) do {
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26312 -> c = crc_32_tab[...];
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26312,1,26311 -> } while (--n);
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}
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2 -> crc = c;
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2 -> return c ^ 0xffffffffL;
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2 ->}
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</pre></div>
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<p>In this example, the function was called twice, passing once through
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each branch of the <code>if</code> statement. The body of the <code>do</code>
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loop was executed a total of 26312 times. Note how the <code>while</code>
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statement is annotated. It began execution 26312 times, once for
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each iteration through the loop. One of those times (the last time)
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it exited, while it branched back to the beginning of the loop 26311 times.
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</p>
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