(gprof.info) File Format

 
 Profiling Data File Format
 ==========================
 
    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 `gprof' file.  Furthermore, it does not provide a version number,
 thus rendering changes to the file format almost impossible.  GNU
 `gprof' uses a new file format that provides these features.  For
 backward compatibility, GNU `gprof' 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.
 
    The new file format is defined in header file `gmon_out.h'.  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 host on which the
 profile was collected.  GNU `gprof' adapts automatically to the
 byte-order in use.
 
    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, GNU `gprof' 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.
 
 Histogram Records
 -----------------
 
    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.
 
    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.
 
 Call-Graph Records
 ------------------
 
    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.
 
 Basic-Block Execution Count Records
 -----------------------------------
 
    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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