761 lines
34 KiB
Plaintext
761 lines
34 KiB
Plaintext
This is gprof.info, produced by makeinfo version 4.0 from gprof.texi.
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START-INFO-DIR-ENTRY
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* gprof: (gprof). Profiling your program's execution
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END-INFO-DIR-ENTRY
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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 Free Software Foundation,
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Inc.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of
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this manual under the conditions for verbatim copying, provided that
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the entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this
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manual into another language, under the above conditions for modified
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versions.
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File: gprof.info, Node: Line-by-line, Next: Annotated Source, Prev: Call Graph, Up: Output
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Line-by-line Profiling
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======================
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`gprof''s `-l' option causes the program to perform "line-by-line"
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profiling. In this mode, histogram samples are assigned not to
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functions, but to individual lines of source code. The program usually
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must be compiled with a `-g' option, in addition to `-pg', in order to
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generate debugging symbols for tracking source code lines.
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The flat profile is the most useful output table in line-by-line
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mode. The call graph isn't as useful as normal, since the current
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version of `gprof' does not propagate call graph arcs from source code
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lines to the enclosing function. The call graph does, however, show
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each line of code that called each function, along with a count.
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Here is a section of `gprof''s output, without line-by-line
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profiling. Note that `ct_init' accounted for four histogram hits, and
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13327 calls to `init_block'.
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Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self self total
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time seconds seconds calls us/call us/call name
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30.77 0.13 0.04 6335 6.31 6.31 ct_init
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Call graph (explanation follows)
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granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
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index % time self children called name
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0.00 0.00 1/13496 name_too_long
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0.00 0.00 40/13496 deflate
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0.00 0.00 128/13496 deflate_fast
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0.00 0.00 13327/13496 ct_init
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[7] 0.0 0.00 0.00 13496 init_block
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Now let's look at some of `gprof''s output from the same program run,
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this time with line-by-line profiling enabled. Note that `ct_init''s
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four histogram hits are broken down into four lines of source code -
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one hit occurred on each of lines 349, 351, 382 and 385. In the call
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graph, note how `ct_init''s 13327 calls to `init_block' are broken down
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into one call from line 396, 3071 calls from line 384, 3730 calls from
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line 385, and 6525 calls from 387.
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Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self
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time seconds seconds calls name
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7.69 0.10 0.01 ct_init (trees.c:349)
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7.69 0.11 0.01 ct_init (trees.c:351)
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7.69 0.12 0.01 ct_init (trees.c:382)
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7.69 0.13 0.01 ct_init (trees.c:385)
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Call graph (explanation follows)
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granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
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% time self children called name
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0.00 0.00 1/13496 name_too_long (gzip.c:1440)
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0.00 0.00 1/13496 deflate (deflate.c:763)
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0.00 0.00 1/13496 ct_init (trees.c:396)
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0.00 0.00 2/13496 deflate (deflate.c:727)
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0.00 0.00 4/13496 deflate (deflate.c:686)
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0.00 0.00 5/13496 deflate (deflate.c:675)
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0.00 0.00 12/13496 deflate (deflate.c:679)
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0.00 0.00 16/13496 deflate (deflate.c:730)
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0.00 0.00 128/13496 deflate_fast (deflate.c:654)
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0.00 0.00 3071/13496 ct_init (trees.c:384)
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0.00 0.00 3730/13496 ct_init (trees.c:385)
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0.00 0.00 6525/13496 ct_init (trees.c:387)
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[6] 0.0 0.00 0.00 13496 init_block (trees.c:408)
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File: gprof.info, Node: Annotated Source, Prev: Line-by-line, Up: Output
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The Annotated Source Listing
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============================
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`gprof''s `-A' option triggers an annotated source listing, which
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lists the program's source code, each function labeled with the number
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of times it was called. You may also need to specify the `-I' option,
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if `gprof' can't find the source code files.
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Compiling with `gcc ... -g -pg -a' augments your program with
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basic-block counting code, in addition to function counting code. This
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enables `gprof' to determine how many times each line of code was
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executed. For example, consider the following function, taken from
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gzip, with line numbers added:
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1 ulg updcrc(s, n)
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2 uch *s;
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3 unsigned n;
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4 {
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5 register ulg c;
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6
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7 static ulg crc = (ulg)0xffffffffL;
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8
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9 if (s == NULL) {
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10 c = 0xffffffffL;
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11 } else {
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12 c = crc;
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13 if (n) do {
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14 c = crc_32_tab[...];
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15 } while (--n);
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16 }
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17 crc = c;
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18 return c ^ 0xffffffffL;
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19 }
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`updcrc' has at least five basic-blocks. One is the function
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itself. The `if' statement on line 9 generates two more basic-blocks,
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one for each branch of the `if'. A fourth basic-block results from the
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`if' on line 13, and the contents of the `do' loop form the fifth
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basic-block. The compiler may also generate additional basic-blocks to
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handle various special cases.
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A program augmented for basic-block counting can be analyzed with
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`gprof -l -A'. I also suggest use of the `-x' option, which ensures
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that each line of code is labeled at least once. Here is `updcrc''s
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annotated source listing for a sample `gzip' run:
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ulg updcrc(s, n)
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uch *s;
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unsigned n;
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2 ->{
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register ulg c;
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static ulg crc = (ulg)0xffffffffL;
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2 -> if (s == NULL) {
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1 -> c = 0xffffffffL;
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1 -> } else {
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1 -> c = crc;
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1 -> if (n) do {
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26312 -> c = crc_32_tab[...];
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26312,1,26311 -> } while (--n);
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}
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2 -> crc = c;
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2 -> return c ^ 0xffffffffL;
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2 ->}
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In this example, the function was called twice, passing once through
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each branch of the `if' statement. The body of the `do' loop was
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executed a total of 26312 times. Note how the `while' statement is
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annotated. It began execution 26312 times, once for each iteration
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through the loop. One of those times (the last time) it exited, while
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it branched back to the beginning of the loop 26311 times.
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File: gprof.info, Node: Inaccuracy, Next: How do I?, Prev: Output, Up: Top
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Inaccuracy of `gprof' Output
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****************************
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* Menu:
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* Sampling Error:: Statistical margins of error
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* Assumptions:: Estimating children times
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File: gprof.info, Node: Sampling Error, Next: Assumptions, Up: Inaccuracy
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Statistical Sampling Error
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==========================
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The run-time figures that `gprof' gives you are based on a sampling
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process, so they are subject to statistical inaccuracy. If a function
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runs only a small amount of time, so that on the average the sampling
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process ought to catch that function in the act only once, there is a
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pretty good chance it will actually find that function zero times, or
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twice.
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By contrast, the number-of-calls and basic-block figures are derived
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by counting, not sampling. They are completely accurate and will not
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vary from run to run if your program is deterministic.
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The "sampling period" that is printed at the beginning of the flat
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profile says how often samples are taken. The rule of thumb is that a
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run-time figure is accurate if it is considerably bigger than the
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sampling period.
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The actual amount of error can be predicted. For N samples, the
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_expected_ error is the square-root of N. For example, if the sampling
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period is 0.01 seconds and `foo''s run-time is 1 second, N is 100
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samples (1 second/0.01 seconds), sqrt(N) is 10 samples, so the expected
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error in `foo''s run-time is 0.1 seconds (10*0.01 seconds), or ten
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percent of the observed value. Again, if the sampling period is 0.01
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seconds and `bar''s run-time is 100 seconds, N is 10000 samples,
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sqrt(N) is 100 samples, so the expected error in `bar''s run-time is 1
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second, or one percent of the observed value. It is likely to vary
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this much _on the average_ from one profiling run to the next.
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(_Sometimes_ it will vary more.)
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This does not mean that a small run-time figure is devoid of
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information. If the program's _total_ run-time is large, a small
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run-time for one function does tell you that that function used an
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insignificant fraction of the whole program's time. Usually this means
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it is not worth optimizing.
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One way to get more accuracy is to give your program more (but
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similar) input data so it will take longer. Another way is to combine
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the data from several runs, using the `-s' option of `gprof'. Here is
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how:
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1. Run your program once.
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2. Issue the command `mv gmon.out gmon.sum'.
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3. Run your program again, the same as before.
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4. Merge the new data in `gmon.out' into `gmon.sum' with this command:
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gprof -s EXECUTABLE-FILE gmon.out gmon.sum
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5. Repeat the last two steps as often as you wish.
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6. Analyze the cumulative data using this command:
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gprof EXECUTABLE-FILE gmon.sum > OUTPUT-FILE
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File: gprof.info, Node: Assumptions, Prev: Sampling Error, Up: Inaccuracy
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Estimating `children' Times
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===========================
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Some of the figures in the call graph are estimates--for example, the
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`children' time values and all the the time figures in caller and
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subroutine lines.
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There is no direct information about these measurements in the
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profile data itself. Instead, `gprof' estimates them by making an
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assumption about your program that might or might not be true.
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The assumption made is that the average time spent in each call to
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any function `foo' is not correlated with who called `foo'. If `foo'
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used 5 seconds in all, and 2/5 of the calls to `foo' came from `a',
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then `foo' contributes 2 seconds to `a''s `children' time, by
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assumption.
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This assumption is usually true enough, but for some programs it is
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far from true. Suppose that `foo' returns very quickly when its
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argument is zero; suppose that `a' always passes zero as an argument,
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while other callers of `foo' pass other arguments. In this program,
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all the time spent in `foo' is in the calls from callers other than `a'.
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But `gprof' has no way of knowing this; it will blindly and incorrectly
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charge 2 seconds of time in `foo' to the children of `a'.
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We hope some day to put more complete data into `gmon.out', so that
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this assumption is no longer needed, if we can figure out how. For the
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nonce, the estimated figures are usually more useful than misleading.
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File: gprof.info, Node: How do I?, Next: Incompatibilities, Prev: Inaccuracy, Up: Top
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Answers to Common Questions
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***************************
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How do I find which lines in my program were executed the most times?
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Compile your program with basic-block counting enabled, run it,
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then use the following pipeline:
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gprof -l -C OBJFILE | sort -k 3 -n -r
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This listing will show you the lines in your code executed most
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often, but not necessarily those that consumed the most time.
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How do I find which lines in my program called a particular function?
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Use `gprof -l' and lookup the function in the call graph. The
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callers will be broken down by function and line number.
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How do I analyze a program that runs for less than a second?
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Try using a shell script like this one:
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for i in `seq 1 100`; do
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fastprog
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mv gmon.out gmon.out.$i
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done
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gprof -s fastprog gmon.out.*
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gprof fastprog gmon.sum
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If your program is completely deterministic, all the call counts
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will be simple multiples of 100 (i.e. a function called once in
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each run will appear with a call count of 100).
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File: gprof.info, Node: Incompatibilities, Next: Details, Prev: How do I?, Up: Top
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Incompatibilities with Unix `gprof'
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***********************************
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GNU `gprof' and Berkeley Unix `gprof' use the same data file
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`gmon.out', and provide essentially the same information. But there
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are a few differences.
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* GNU `gprof' uses a new, generalized file format with support for
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basic-block execution counts and non-realtime histograms. A magic
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cookie and version number allows `gprof' to easily identify new
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style files. Old BSD-style files can still be read. *Note File
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Format::.
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* For a recursive function, Unix `gprof' lists the function as a
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parent and as a child, with a `calls' field that lists the number
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of recursive calls. GNU `gprof' omits these lines and puts the
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number of recursive calls in the primary line.
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* When a function is suppressed from the call graph with `-e', GNU
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`gprof' still lists it as a subroutine of functions that call it.
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* GNU `gprof' accepts the `-k' with its argument in the form
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`from/to', instead of `from to'.
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* In the annotated source listing, if there are multiple basic
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blocks on the same line, GNU `gprof' prints all of their counts,
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separated by commas.
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* The blurbs, field widths, and output formats are different. GNU
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`gprof' prints blurbs after the tables, so that you can see the
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tables without skipping the blurbs.
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File: gprof.info, Node: Details, Prev: Incompatibilities, Up: Top
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Details of Profiling
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********************
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* Menu:
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* Implementation:: How a program collects profiling information
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* File Format:: Format of `gmon.out' files
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* Internals:: `gprof''s internal operation
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* Debugging:: Using `gprof''s `-d' option
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File: gprof.info, Node: Implementation, Next: File Format, Up: Details
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Implementation of Profiling
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===========================
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Profiling works by changing how every function in your program is
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compiled so that when it is called, it will stash away some information
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about where it was called from. From this, the profiler can figure out
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what function called it, and can count how many times it was called.
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This change is made by the compiler when your program is compiled with
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the `-pg' option, which causes every function to call `mcount' (or
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`_mcount', or `__mcount', depending on the OS and compiler) as one of
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its first operations.
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The `mcount' routine, included in the profiling library, is
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responsible for recording in an in-memory call graph table both its
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parent routine (the child) and its parent's parent. This is typically
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done by examining the stack frame to find both the address of the
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child, and the return address in the original parent. Since this is a
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very machine-dependent operation, `mcount' itself is typically a short
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assembly-language stub routine that extracts the required information,
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and then calls `__mcount_internal' (a normal C function) with two
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arguments - `frompc' and `selfpc'. `__mcount_internal' is responsible
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for maintaining the in-memory call graph, which records `frompc',
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`selfpc', and the number of times each of these call arcs was traversed.
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GCC Version 2 provides a magical function
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(`__builtin_return_address'), which allows a generic `mcount' function
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to extract the required information from the stack frame. However, on
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some architectures, most notably the SPARC, using this builtin can be
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very computationally expensive, and an assembly language version of
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`mcount' is used for performance reasons.
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Number-of-calls information for library routines is collected by
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using a special version of the C library. The programs in it are the
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same as in the usual C library, but they were compiled with `-pg'. If
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you link your program with `gcc ... -pg', it automatically uses the
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profiling version of the library.
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Profiling also involves watching your program as it runs, and
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keeping a histogram of where the program counter happens to be every
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now and then. Typically the program counter is looked at around 100
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times per second of run time, but the exact frequency may vary from
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system to system.
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This is done is one of two ways. Most UNIX-like operating systems
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provide a `profil()' system call, which registers a memory array with
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the kernel, along with a scale factor that determines how the program's
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address space maps into the array. Typical scaling values cause every
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2 to 8 bytes of address space to map into a single array slot. On
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every tick of the system clock (assuming the profiled program is
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running), the value of the program counter is examined and the
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corresponding slot in the memory array is incremented. Since this is
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done in the kernel, which had to interrupt the process anyway to handle
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the clock interrupt, very little additional system overhead is required.
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However, some operating systems, most notably Linux 2.0 (and
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earlier), do not provide a `profil()' system call. On such a system,
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arrangements are made for the kernel to periodically deliver a signal
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to the process (typically via `setitimer()'), which then performs the
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same operation of examining the program counter and incrementing a slot
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in the memory array. Since this method requires a signal to be
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delivered to user space every time a sample is taken, it uses
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considerably more overhead than kernel-based profiling. Also, due to
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the added delay required to deliver the signal, this method is less
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accurate as well.
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A special startup routine allocates memory for the histogram and
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either calls `profil()' or sets up a clock signal handler. This
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routine (`monstartup') can be invoked in several ways. On Linux
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systems, a special profiling startup file `gcrt0.o', which invokes
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`monstartup' before `main', is used instead of the default `crt0.o'.
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Use of this special startup file is one of the effects of using `gcc
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... -pg' to link. On SPARC systems, no special startup files are used.
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Rather, the `mcount' routine, when it is invoked for the first time
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(typically when `main' is called), calls `monstartup'.
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If the compiler's `-a' option was used, basic-block counting is also
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enabled. Each object file is then compiled with a static array of
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counts, initially zero. In the executable code, every time a new
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basic-block begins (i.e. when an `if' statement appears), an extra
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instruction is inserted to increment the corresponding count in the
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array. At compile time, a paired array was constructed that recorded
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the starting address of each basic-block. Taken together, the two
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arrays record the starting address of every basic-block, along with the
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number of times it was executed.
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The profiling library also includes a function (`mcleanup') which is
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typically registered using `atexit()' to be called as the program
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exits, and is responsible for writing the file `gmon.out'. Profiling
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is turned off, various headers are output, and the histogram is
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written, followed by the call-graph arcs and the basic-block counts.
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The output from `gprof' gives no indication of parts of your program
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that are limited by I/O or swapping bandwidth. This is because samples
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of the program counter are taken at fixed intervals of the program's
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run time. Therefore, the time measurements in `gprof' output say
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nothing about time that your program was not running. For example, a
|
||
part of the program that creates so much data that it cannot all fit in
|
||
physical memory at once may run very slowly due to thrashing, but
|
||
`gprof' will say it uses little time. On the other hand, sampling by
|
||
run time has the advantage that the amount of load due to other users
|
||
won't directly affect the output you get.
|
||
|
||
|
||
File: gprof.info, Node: File Format, Next: Internals, Prev: Implementation, Up: Details
|
||
|
||
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.
|
||
|
||
|
||
File: gprof.info, Node: Internals, Next: Debugging, Prev: File Format, Up: Details
|
||
|
||
`gprof''s Internal Operation
|
||
============================
|
||
|
||
Like most programs, `gprof' begins by processing its options.
|
||
During this stage, it may building its symspec list
|
||
(`sym_ids.c:sym_id_add'), if options are specified which use symspecs.
|
||
`gprof' maintains a single linked list of symspecs, which will
|
||
eventually get turned into 12 symbol tables, organized into six
|
||
include/exclude pairs - one pair each for the flat profile
|
||
(INCL_FLAT/EXCL_FLAT), the call graph arcs (INCL_ARCS/EXCL_ARCS),
|
||
printing in the call graph (INCL_GRAPH/EXCL_GRAPH), timing propagation
|
||
in the call graph (INCL_TIME/EXCL_TIME), the annotated source listing
|
||
(INCL_ANNO/EXCL_ANNO), and the execution count listing
|
||
(INCL_EXEC/EXCL_EXEC).
|
||
|
||
After option processing, `gprof' finishes building the symspec list
|
||
by adding all the symspecs in `default_excluded_list' to the exclude
|
||
lists EXCL_TIME and EXCL_GRAPH, and if line-by-line profiling is
|
||
specified, EXCL_FLAT as well. These default excludes are not added to
|
||
EXCL_ANNO, EXCL_ARCS, and EXCL_EXEC.
|
||
|
||
Next, the BFD library is called to open the object file, verify that
|
||
it is an object file, and read its symbol table (`core.c:core_init'),
|
||
using `bfd_canonicalize_symtab' after mallocing an appropriately sized
|
||
array of symbols. At this point, function mappings are read (if the
|
||
`--file-ordering' option has been specified), and the core text space
|
||
is read into memory (if the `-c' option was given).
|
||
|
||
`gprof''s own symbol table, an array of Sym structures, is now built.
|
||
This is done in one of two ways, by one of two routines, depending on
|
||
whether line-by-line profiling (`-l' option) has been enabled. For
|
||
normal profiling, the BFD canonical symbol table is scanned. For
|
||
line-by-line profiling, every text space address is examined, and a new
|
||
symbol table entry gets created every time the line number changes. In
|
||
either case, two passes are made through the symbol table - one to
|
||
count the size of the symbol table required, and the other to actually
|
||
read the symbols. In between the two passes, a single array of type
|
||
`Sym' is created of the appropriate length. Finally,
|
||
`symtab.c:symtab_finalize' is called to sort the symbol table and
|
||
remove duplicate entries (entries with the same memory address).
|
||
|
||
The symbol table must be a contiguous array for two reasons. First,
|
||
the `qsort' library function (which sorts an array) will be used to
|
||
sort the symbol table. Also, the symbol lookup routine
|
||
(`symtab.c:sym_lookup'), which finds symbols based on memory address,
|
||
uses a binary search algorithm which requires the symbol table to be a
|
||
sorted array. Function symbols are indicated with an `is_func' flag.
|
||
Line number symbols have no special flags set. Additionally, a symbol
|
||
can have an `is_static' flag to indicate that it is a local symbol.
|
||
|
||
With the symbol table read, the symspecs can now be translated into
|
||
Syms (`sym_ids.c:sym_id_parse'). Remember that a single symspec can
|
||
match multiple symbols. An array of symbol tables (`syms') is created,
|
||
each entry of which is a symbol table of Syms to be included or
|
||
excluded from a particular listing. The master symbol table and the
|
||
symspecs are examined by nested loops, and every symbol that matches a
|
||
symspec is inserted into the appropriate syms table. This is done
|
||
twice, once to count the size of each required symbol table, and again
|
||
to build the tables, which have been malloced between passes. From now
|
||
on, to determine whether a symbol is on an include or exclude symspec
|
||
list, `gprof' simply uses its standard symbol lookup routine on the
|
||
appropriate table in the `syms' array.
|
||
|
||
Now the profile data file(s) themselves are read
|
||
(`gmon_io.c:gmon_out_read'), first by checking for a new-style
|
||
`gmon.out' header, then assuming this is an old-style BSD `gmon.out' if
|
||
the magic number test failed.
|
||
|
||
New-style histogram records are read by `hist.c:hist_read_rec'. For
|
||
the first histogram record, allocate a memory array to hold all the
|
||
bins, and read them in. When multiple profile data files (or files
|
||
with multiple histogram records) are read, the starting address, ending
|
||
address, number of bins and sampling rate must match between the
|
||
various histograms, or a fatal error will result. If everything
|
||
matches, just sum the additional histograms into the existing in-memory
|
||
array.
|
||
|
||
As each call graph record is read (`call_graph.c:cg_read_rec'), the
|
||
parent and child addresses are matched to symbol table entries, and a
|
||
call graph arc is created by `cg_arcs.c:arc_add', unless the arc fails
|
||
a symspec check against INCL_ARCS/EXCL_ARCS. As each arc is added, a
|
||
linked list is maintained of the parent's child arcs, and of the child's
|
||
parent arcs. Both the child's call count and the arc's call count are
|
||
incremented by the record's call count.
|
||
|
||
Basic-block records are read (`basic_blocks.c:bb_read_rec'), but
|
||
only if line-by-line profiling has been selected. Each basic-block
|
||
address is matched to a corresponding line symbol in the symbol table,
|
||
and an entry made in the symbol's bb_addr and bb_calls arrays. Again,
|
||
if multiple basic-block records are present for the same address, the
|
||
call counts are cumulative.
|
||
|
||
A gmon.sum file is dumped, if requested (`gmon_io.c:gmon_out_write').
|
||
|
||
If histograms were present in the data files, assign them to symbols
|
||
(`hist.c:hist_assign_samples') by iterating over all the sample bins
|
||
and assigning them to symbols. Since the symbol table is sorted in
|
||
order of ascending memory addresses, we can simple follow along in the
|
||
symbol table as we make our pass over the sample bins. This step
|
||
includes a symspec check against INCL_FLAT/EXCL_FLAT. Depending on the
|
||
histogram scale factor, a sample bin may span multiple symbols, in
|
||
which case a fraction of the sample count is allocated to each symbol,
|
||
proportional to the degree of overlap. This effect is rare for normal
|
||
profiling, but overlaps are more common during line-by-line profiling,
|
||
and can cause each of two adjacent lines to be credited with half a
|
||
hit, for example.
|
||
|
||
If call graph data is present, `cg_arcs.c:cg_assemble' is called.
|
||
First, if `-c' was specified, a machine-dependent routine (`find_call')
|
||
scans through each symbol's machine code, looking for subroutine call
|
||
instructions, and adding them to the call graph with a zero call count.
|
||
A topological sort is performed by depth-first numbering all the
|
||
symbols (`cg_dfn.c:cg_dfn'), so that children are always numbered less
|
||
than their parents, then making a array of pointers into the symbol
|
||
table and sorting it into numerical order, which is reverse topological
|
||
order (children appear before parents). Cycles are also detected at
|
||
this point, all members of which are assigned the same topological
|
||
number. Two passes are now made through this sorted array of symbol
|
||
pointers. The first pass, from end to beginning (parents to children),
|
||
computes the fraction of child time to propagate to each parent and a
|
||
print flag. The print flag reflects symspec handling of
|
||
INCL_GRAPH/EXCL_GRAPH, with a parent's include or exclude (print or no
|
||
print) property being propagated to its children, unless they
|
||
themselves explicitly appear in INCL_GRAPH or EXCL_GRAPH. A second
|
||
pass, from beginning to end (children to parents) actually propagates
|
||
the timings along the call graph, subject to a check against
|
||
INCL_TIME/EXCL_TIME. With the print flag, fractions, and timings now
|
||
stored in the symbol structures, the topological sort array is now
|
||
discarded, and a new array of pointers is assembled, this time sorted
|
||
by propagated time.
|
||
|
||
Finally, print the various outputs the user requested, which is now
|
||
fairly straightforward. The call graph (`cg_print.c:cg_print') and
|
||
flat profile (`hist.c:hist_print') are regurgitations of values already
|
||
computed. The annotated source listing
|
||
(`basic_blocks.c:print_annotated_source') uses basic-block information,
|
||
if present, to label each line of code with call counts, otherwise only
|
||
the function call counts are presented.
|
||
|
||
The function ordering code is marginally well documented in the
|
||
source code itself (`cg_print.c'). Basically, the functions with the
|
||
most use and the most parents are placed first, followed by other
|
||
functions with the most use, followed by lower use functions, followed
|
||
by unused functions at the end.
|
||
|
||
|
||
File: gprof.info, Node: Debugging, Prev: Internals, Up: Details
|
||
|
||
Debugging `gprof'
|
||
-----------------
|
||
|
||
If `gprof' was compiled with debugging enabled, the `-d' option
|
||
triggers debugging output (to stdout) which can be helpful in
|
||
understanding its operation. The debugging number specified is
|
||
interpreted as a sum of the following options:
|
||
|
||
2 - Topological sort
|
||
Monitor depth-first numbering of symbols during call graph analysis
|
||
|
||
4 - Cycles
|
||
Shows symbols as they are identified as cycle heads
|
||
|
||
16 - Tallying
|
||
As the call graph arcs are read, show each arc and how the total
|
||
calls to each function are tallied
|
||
|
||
32 - Call graph arc sorting
|
||
Details sorting individual parents/children within each call graph
|
||
entry
|
||
|
||
64 - Reading histogram and call graph records
|
||
Shows address ranges of histograms as they are read, and each call
|
||
graph arc
|
||
|
||
128 - Symbol table
|
||
Reading, classifying, and sorting the symbol table from the object
|
||
file. For line-by-line profiling (`-l' option), also shows line
|
||
numbers being assigned to memory addresses.
|
||
|
||
256 - Static call graph
|
||
Trace operation of `-c' option
|
||
|
||
512 - Symbol table and arc table lookups
|
||
Detail operation of lookup routines
|
||
|
||
1024 - Call graph propagation
|
||
Shows how function times are propagated along the call graph
|
||
|
||
2048 - Basic-blocks
|
||
Shows basic-block records as they are read from profile data (only
|
||
meaningful with `-l' option)
|
||
|
||
4096 - Symspecs
|
||
Shows symspec-to-symbol pattern matching operation
|
||
|
||
8192 - Annotate source
|
||
Tracks operation of `-A' option
|
||
|
||
|