125 lines
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125 lines
6.3 KiB
HTML
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<title>File Format - GNU gprof</title>
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<!--
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This file documents the gprof profiler of the GNU system.
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Copyright (C) 1988, 92, 97, 98, 99, 2000, 2001, 2003, 2007 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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<p>
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<a name="File-Format"></a>Next: <a rel="next" accesskey="n" href="Internals.html#Internals">Internals</a>,
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Previous: <a rel="previous" accesskey="p" href="Implementation.html#Implementation">Implementation</a>,
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Up: <a rel="up" accesskey="u" href="Details.html#Details">Details</a>
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<hr><br>
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</div>
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<h3 class="section">9.2 Profiling Data File Format</h3>
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<p>The old BSD-derived file format used for profile data does not contain a
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magic cookie that allows to check whether a data file really is a
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<code>gprof</code> file. Furthermore, it does not provide a version number, thus
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rendering changes to the file format almost impossible. <span class="sc">gnu</span> <code>gprof</code>
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uses a new file format that provides these features. For backward
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compatibility, <span class="sc">gnu</span> <code>gprof</code> continues to support the old BSD-derived
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format, but not all features are supported with it. For example,
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basic-block execution counts cannot be accommodated by the old file
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format.
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<p>The new file format is defined in header file <span class="file">gmon_out.h</span>. It
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consists of a header containing the magic cookie and a version number,
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as well as some spare bytes available for future extensions. All data
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in a profile data file is in the native format of the target for which
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the profile was collected. <span class="sc">gnu</span> <code>gprof</code> adapts automatically
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to the byte-order in use.
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<p>In the new file format, the header is followed by a sequence of
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records. Currently, there are three different record types: histogram
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records, call-graph arc records, and basic-block execution count
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records. Each file can contain any number of each record type. When
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reading a file, <span class="sc">gnu</span> <code>gprof</code> will ensure records of the same type are
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compatible with each other and compute the union of all records. For
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example, for basic-block execution counts, the union is simply the sum
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of all execution counts for each basic-block.
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<h4 class="subsection">9.2.1 Histogram Records</h4>
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<p>Histogram records consist of a header that is followed by an array of
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bins. The header contains the text-segment range that the histogram
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spans, the size of the histogram in bytes (unlike in the old BSD
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format, this does not include the size of the header), the rate of the
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profiling clock, and the physical dimension that the bin counts
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represent after being scaled by the profiling clock rate. The
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physical dimension is specified in two parts: a long name of up to 15
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characters and a single character abbreviation. For example, a
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histogram representing real-time would specify the long name as
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“seconds” and the abbreviation as “s”. This feature is useful for
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architectures that support performance monitor hardware (which,
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fortunately, is becoming increasingly common). For example, under DEC
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OSF/1, the “uprofile” command can be used to produce a histogram of,
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say, instruction cache misses. In this case, the dimension in the
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histogram header could be set to “i-cache misses” and the abbreviation
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could be set to “1” (because it is simply a count, not a physical
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dimension). Also, the profiling rate would have to be set to 1 in
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this case.
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<p>Histogram bins are 16-bit numbers and each bin represent an equal
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amount of text-space. For example, if the text-segment is one
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thousand bytes long and if there are ten bins in the histogram, each
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bin represents one hundred bytes.
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<h4 class="subsection">9.2.2 Call-Graph Records</h4>
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<p>Call-graph records have a format that is identical to the one used in
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the BSD-derived file format. It consists of an arc in the call graph
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and a count indicating the number of times the arc was traversed
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during program execution. Arcs are specified by a pair of addresses:
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the first must be within caller's function and the second must be
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within the callee's function. When performing profiling at the
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function level, these addresses can point anywhere within the
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respective function. However, when profiling at the line-level, it is
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better if the addresses are as close to the call-site/entry-point as
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possible. This will ensure that the line-level call-graph is able to
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identify exactly which line of source code performed calls to a
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function.
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<h4 class="subsection">9.2.3 Basic-Block Execution Count Records</h4>
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<p>Basic-block execution count records consist of a header followed by a
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sequence of address/count pairs. The header simply specifies the
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length of the sequence. In an address/count pair, the address
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identifies a basic-block and the count specifies the number of times
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that basic-block was executed. Any address within the basic-address can
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be used.
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</body></html>
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