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<title>Internals - GNU gprof</title>
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This file documents the gprof profiler of the GNU system.
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<a name="Internals"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Debugging.html#Debugging">Debugging</a>,
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Previous: <a rel="previous" accesskey="p" href="File-Format.html#File-Format">File Format</a>,
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Up: <a rel="up" accesskey="u" href="Details.html#Details">Details</a>
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</div>
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<h3 class="section">9.3 <code>gprof</code>'s Internal Operation</h3>
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<p>Like most programs, <code>gprof</code> begins by processing its options.
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During this stage, it may building its symspec list
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(<code>sym_ids.c:sym_id_add</code>), if
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options are specified which use symspecs.
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<code>gprof</code> maintains a single linked list of symspecs,
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which will eventually get turned into 12 symbol tables,
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organized into six include/exclude pairs—one
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pair each for the flat profile (INCL_FLAT/EXCL_FLAT),
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the call graph arcs (INCL_ARCS/EXCL_ARCS),
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printing in the call graph (INCL_GRAPH/EXCL_GRAPH),
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timing propagation in the call graph (INCL_TIME/EXCL_TIME),
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the annotated source listing (INCL_ANNO/EXCL_ANNO),
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and the execution count listing (INCL_EXEC/EXCL_EXEC).
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<p>After option processing, <code>gprof</code> finishes
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building the symspec list by adding all the symspecs in
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<code>default_excluded_list</code> to the exclude lists
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EXCL_TIME and EXCL_GRAPH, and if line-by-line profiling is specified,
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EXCL_FLAT as well.
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These default excludes are not added to EXCL_ANNO, EXCL_ARCS, and EXCL_EXEC.
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<p>Next, the BFD library is called to open the object file,
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verify that it is an object file,
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and read its symbol table (<code>core.c:core_init</code>),
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using <code>bfd_canonicalize_symtab</code> after mallocing
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an appropriately sized array of symbols. At this point,
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function mappings are read (if the ‘<samp><span class="samp">--file-ordering</span></samp>’ option
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has been specified), and the core text space is read into
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memory (if the ‘<samp><span class="samp">-c</span></samp>’ option was given).
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<p><code>gprof</code>'s own symbol table, an array of Sym structures,
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is now built.
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This is done in one of two ways, by one of two routines, depending
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on whether line-by-line profiling (‘<samp><span class="samp">-l</span></samp>’ option) has been
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enabled.
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For normal profiling, the BFD canonical symbol table is scanned.
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For line-by-line profiling, every
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text space address is examined, and a new symbol table entry
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gets created every time the line number changes.
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In either case, two passes are made through the symbol
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table—one to count the size of the symbol table required,
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and the other to actually read the symbols. In between the
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two passes, a single array of type <code>Sym</code> is created of
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the appropriate length.
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Finally, <code>symtab.c:symtab_finalize</code>
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is called to sort the symbol table and remove duplicate entries
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(entries with the same memory address).
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<p>The symbol table must be a contiguous array for two reasons.
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First, the <code>qsort</code> library function (which sorts an array)
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will be used to sort the symbol table.
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Also, the symbol lookup routine (<code>symtab.c:sym_lookup</code>),
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which finds symbols
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based on memory address, uses a binary search algorithm
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which requires the symbol table to be a sorted array.
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Function symbols are indicated with an <code>is_func</code> flag.
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Line number symbols have no special flags set.
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Additionally, a symbol can have an <code>is_static</code> flag
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to indicate that it is a local symbol.
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<p>With the symbol table read, the symspecs can now be translated
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into Syms (<code>sym_ids.c:sym_id_parse</code>). Remember that a single
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symspec can match multiple symbols.
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An array of symbol tables
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(<code>syms</code>) is created, each entry of which is a symbol table
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of Syms to be included or excluded from a particular listing.
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The master symbol table and the symspecs are examined by nested
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loops, and every symbol that matches a symspec is inserted
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into the appropriate syms table. This is done twice, once to
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count the size of each required symbol table, and again to build
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the tables, which have been malloced between passes.
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From now on, to determine whether a symbol is on an include
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or exclude symspec list, <code>gprof</code> simply uses its
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standard symbol lookup routine on the appropriate table
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in the <code>syms</code> array.
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<p>Now the profile data file(s) themselves are read
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(<code>gmon_io.c:gmon_out_read</code>),
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first by checking for a new-style ‘<samp><span class="samp">gmon.out</span></samp>’ header,
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then assuming this is an old-style BSD ‘<samp><span class="samp">gmon.out</span></samp>’
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if the magic number test failed.
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<p>New-style histogram records are read by <code>hist.c:hist_read_rec</code>.
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For the first histogram record, allocate a memory array to hold
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all the bins, and read them in.
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When multiple profile data files (or files with multiple histogram
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records) are read, the memory ranges of each pair of histogram records
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must be either equal, or non-overlapping. For each pair of histogram
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records, the resolution (memory region size divided by the number of
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bins) must be the same. The time unit must be the same for all
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histogram records. If the above containts are met, all histograms
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for the same memory range are merged.
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<p>As each call graph record is read (<code>call_graph.c:cg_read_rec</code>),
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the parent and child addresses
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are matched to symbol table entries, and a call graph arc is
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created by <code>cg_arcs.c:arc_add</code>, unless the arc fails a symspec
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check against INCL_ARCS/EXCL_ARCS. As each arc is added,
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a linked list is maintained of the parent's child arcs, and of the child's
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parent arcs.
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Both the child's call count and the arc's call count are
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incremented by the record's call count.
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<p>Basic-block records are read (<code>basic_blocks.c:bb_read_rec</code>),
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but only if line-by-line profiling has been selected.
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Each basic-block address is matched to a corresponding line
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symbol in the symbol table, and an entry made in the symbol's
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bb_addr and bb_calls arrays. Again, if multiple basic-block
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records are present for the same address, the call counts
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are cumulative.
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<p>A gmon.sum file is dumped, if requested (<code>gmon_io.c:gmon_out_write</code>).
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<p>If histograms were present in the data files, assign them to symbols
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(<code>hist.c:hist_assign_samples</code>) by iterating over all the sample
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bins and assigning them to symbols. Since the symbol table
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is sorted in order of ascending memory addresses, we can
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simple follow along in the symbol table as we make our pass
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over the sample bins.
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This step includes a symspec check against INCL_FLAT/EXCL_FLAT.
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Depending on the histogram
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scale factor, a sample bin may span multiple symbols,
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in which case a fraction of the sample count is allocated
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to each symbol, proportional to the degree of overlap.
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This effect is rare for normal profiling, but overlaps
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are more common during line-by-line profiling, and can
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cause each of two adjacent lines to be credited with half
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a hit, for example.
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<p>If call graph data is present, <code>cg_arcs.c:cg_assemble</code> is called.
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First, if ‘<samp><span class="samp">-c</span></samp>’ was specified, a machine-dependent
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routine (<code>find_call</code>) scans through each symbol's machine code,
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looking for subroutine call instructions, and adding them
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to the call graph with a zero call count.
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A topological sort is performed by depth-first numbering
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all the symbols (<code>cg_dfn.c:cg_dfn</code>), so that
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children are always numbered less than their parents,
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then making a array of pointers into the symbol table and sorting it into
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numerical order, which is reverse topological
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order (children appear before parents).
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Cycles are also detected at this point, all members
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of which are assigned the same topological number.
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Two passes are now made through this sorted array of symbol pointers.
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The first pass, from end to beginning (parents to children),
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computes the fraction of child time to propagate to each parent
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and a print flag.
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The print flag reflects symspec handling of INCL_GRAPH/EXCL_GRAPH,
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with a parent's include or exclude (print or no print) property
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being propagated to its children, unless they themselves explicitly appear
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in INCL_GRAPH or EXCL_GRAPH.
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A second pass, from beginning to end (children to parents) actually
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propagates the timings along the call graph, subject
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to a check against INCL_TIME/EXCL_TIME.
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With the print flag, fractions, and timings now stored in the symbol
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structures, the topological sort array is now discarded, and a
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new array of pointers is assembled, this time sorted by propagated time.
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<p>Finally, print the various outputs the user requested, which is now fairly
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straightforward. The call graph (<code>cg_print.c:cg_print</code>) and
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flat profile (<code>hist.c:hist_print</code>) are regurgitations of values
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already computed. The annotated source listing
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(<code>basic_blocks.c:print_annotated_source</code>) uses basic-block
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information, if present, to label each line of code with call counts,
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otherwise only the function call counts are presented.
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<p>The function ordering code is marginally well documented
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in the source code itself (<code>cg_print.c</code>). Basically,
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the functions with the most use and the most parents are
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placed first, followed by other functions with the most use,
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followed by lower use functions, followed by unused functions
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at the end.
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</body></html>
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