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| <title>GNU gprof: File Format</title> |
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| <a name="File-Format"></a> |
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| Next: <a href="Internals.html#Internals" accesskey="n" rel="next">Internals</a>, Previous: <a href="Implementation.html#Implementation" accesskey="p" rel="prev">Implementation</a>, Up: <a href="Details.html#Details" accesskey="u" rel="up">Details</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p> |
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| <a name="Profiling-Data-File-Format"></a> |
| <h3 class="section">9.2 Profiling Data File Format</h3> |
| |
| <p>The old BSD-derived file format used for profile data does not contain a |
| magic cookie that allows to check whether a data file really is a |
| <code>gprof</code> file. Furthermore, it does not provide a version number, thus |
| rendering changes to the file format almost impossible. <small>GNU</small> <code>gprof</code> |
| uses a new file format that provides these features. For backward |
| compatibility, <small>GNU</small> <code>gprof</code> continues to support the old BSD-derived |
| format, but not all features are supported with it. For example, |
| basic-block execution counts cannot be accommodated by the old file |
| format. |
| </p> |
| <p>The new file format is defined in header file <samp>gmon_out.h</samp>. It |
| consists of a header containing the magic cookie and a version number, |
| as well as some spare bytes available for future extensions. All data |
| in a profile data file is in the native format of the target for which |
| the profile was collected. <small>GNU</small> <code>gprof</code> adapts automatically |
| to the byte-order in use. |
| </p> |
| <p>In the new file format, the header is followed by a sequence of |
| records. Currently, there are three different record types: histogram |
| records, call-graph arc records, and basic-block execution count |
| records. Each file can contain any number of each record type. When |
| reading a file, <small>GNU</small> <code>gprof</code> will ensure records of the same type are |
| compatible with each other and compute the union of all records. For |
| example, for basic-block execution counts, the union is simply the sum |
| of all execution counts for each basic-block. |
| </p> |
| <a name="Histogram-Records"></a> |
| <h4 class="subsection">9.2.1 Histogram Records</h4> |
| |
| <p>Histogram records consist of a header that is followed by an array of |
| bins. The header contains the text-segment range that the histogram |
| spans, the size of the histogram in bytes (unlike in the old BSD |
| format, this does not include the size of the header), the rate of the |
| profiling clock, and the physical dimension that the bin counts |
| represent after being scaled by the profiling clock rate. The |
| physical dimension is specified in two parts: a long name of up to 15 |
| characters and a single character abbreviation. For example, a |
| histogram representing real-time would specify the long name as |
| “seconds” and the abbreviation as “s”. This feature is useful for |
| architectures that support performance monitor hardware (which, |
| fortunately, is becoming increasingly common). For example, under DEC |
| OSF/1, the “uprofile” command can be used to produce a histogram of, |
| say, instruction cache misses. In this case, the dimension in the |
| histogram header could be set to “i-cache misses” and the abbreviation |
| could be set to “1” (because it is simply a count, not a physical |
| dimension). Also, the profiling rate would have to be set to 1 in |
| this case. |
| </p> |
| <p>Histogram bins are 16-bit numbers and each bin represent an equal |
| amount of text-space. For example, if the text-segment is one |
| thousand bytes long and if there are ten bins in the histogram, each |
| bin represents one hundred bytes. |
| </p> |
| |
| <a name="Call_002dGraph-Records"></a> |
| <h4 class="subsection">9.2.2 Call-Graph Records</h4> |
| |
| <p>Call-graph records have a format that is identical to the one used in |
| the BSD-derived file format. It consists of an arc in the call graph |
| and a count indicating the number of times the arc was traversed |
| during program execution. Arcs are specified by a pair of addresses: |
| the first must be within caller’s function and the second must be |
| within the callee’s function. When performing profiling at the |
| function level, these addresses can point anywhere within the |
| respective function. However, when profiling at the line-level, it is |
| better if the addresses are as close to the call-site/entry-point as |
| possible. This will ensure that the line-level call-graph is able to |
| identify exactly which line of source code performed calls to a |
| function. |
| </p> |
| <a name="Basic_002dBlock-Execution-Count-Records"></a> |
| <h4 class="subsection">9.2.3 Basic-Block Execution Count Records</h4> |
| |
| <p>Basic-block execution count records consist of a header followed by a |
| sequence of address/count pairs. The header simply specifies the |
| length of the sequence. In an address/count pair, the address |
| identifies a basic-block and the count specifies the number of times |
| that basic-block was executed. Any address within the basic-address can |
| be used. |
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