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