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| <h3 class="section">5.3 Line-by-line Profiling</h3> |
| |
| <p><code>gprof</code>'s ‘<samp><span class="samp">-l</span></samp>’ option causes the program to perform |
| <dfn>line-by-line</dfn> profiling. In this mode, histogram |
| samples are assigned not to functions, but to individual |
| lines of source code. This only works with programs compiled with |
| older versions of the <code>gcc</code> compiler. Newer versions of <code>gcc</code> |
| use a different program - <code>gcov</code> - to display line-by-line |
| profiling information. |
| |
| <p>With the older versions of <code>gcc</code> the program usually has to be |
| compiled with a ‘<samp><span class="samp">-g</span></samp>’ option, in addition to ‘<samp><span class="samp">-pg</span></samp>’, in order |
| to generate debugging symbols for tracking source code lines. |
| Note, in much older versions of <code>gcc</code> the program had to be |
| compiled with the ‘<samp><span class="samp">-a</span></samp>’ command line option as well. |
| |
| <p>The flat profile is the most useful output table |
| in line-by-line mode. |
| The call graph isn't as useful as normal, since |
| the current version of <code>gprof</code> does not propagate |
| call graph arcs from source code lines to the enclosing function. |
| The call graph does, however, show each line of code |
| that called each function, along with a count. |
| |
| <p>Here is a section of <code>gprof</code>'s output, without line-by-line profiling. |
| Note that <code>ct_init</code> accounted for four histogram hits, and |
| 13327 calls to <code>init_block</code>. |
| |
| <pre class="smallexample"> Flat profile: |
| |
| Each sample counts as 0.01 seconds. |
| % cumulative self self total |
| time seconds seconds calls us/call us/call name |
| 30.77 0.13 0.04 6335 6.31 6.31 ct_init |
| |
| |
| Call graph (explanation follows) |
| |
| |
| granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds |
| |
| index % time self children called name |
| |
| 0.00 0.00 1/13496 name_too_long |
| 0.00 0.00 40/13496 deflate |
| 0.00 0.00 128/13496 deflate_fast |
| 0.00 0.00 13327/13496 ct_init |
| [7] 0.0 0.00 0.00 13496 init_block |
| |
| </pre> |
| <p>Now let's look at some of <code>gprof</code>'s output from the same program run, |
| this time with line-by-line profiling enabled. Note that <code>ct_init</code>'s |
| four histogram hits are broken down into four lines of source code—one hit |
| occurred on each of lines 349, 351, 382 and 385. In the call graph, |
| note how |
| <code>ct_init</code>'s 13327 calls to <code>init_block</code> are broken down |
| into one call from line 396, 3071 calls from line 384, 3730 calls |
| from line 385, and 6525 calls from 387. |
| |
| <pre class="smallexample"> Flat profile: |
| |
| Each sample counts as 0.01 seconds. |
| % cumulative self |
| time seconds seconds calls name |
| 7.69 0.10 0.01 ct_init (trees.c:349) |
| 7.69 0.11 0.01 ct_init (trees.c:351) |
| 7.69 0.12 0.01 ct_init (trees.c:382) |
| 7.69 0.13 0.01 ct_init (trees.c:385) |
| |
| |
| Call graph (explanation follows) |
| |
| |
| granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds |
| |
| % time self children called name |
| |
| 0.00 0.00 1/13496 name_too_long (gzip.c:1440) |
| 0.00 0.00 1/13496 deflate (deflate.c:763) |
| 0.00 0.00 1/13496 ct_init (trees.c:396) |
| 0.00 0.00 2/13496 deflate (deflate.c:727) |
| 0.00 0.00 4/13496 deflate (deflate.c:686) |
| 0.00 0.00 5/13496 deflate (deflate.c:675) |
| 0.00 0.00 12/13496 deflate (deflate.c:679) |
| 0.00 0.00 16/13496 deflate (deflate.c:730) |
| 0.00 0.00 128/13496 deflate_fast (deflate.c:654) |
| 0.00 0.00 3071/13496 ct_init (trees.c:384) |
| 0.00 0.00 3730/13496 ct_init (trees.c:385) |
| 0.00 0.00 6525/13496 ct_init (trees.c:387) |
| [6] 0.0 0.00 0.00 13496 init_block (trees.c:408) |
| |
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