gprof
's `-A' option triggers an annotated source listing,
which lists the program's source code, each function labeled with the
number of times it was called. You may also need to specify the
`-I' option, if gprof
can't find the source code files.
Compiling with `gcc ... -g -pg -a' augments your program
with basic-block counting code, in addition to function counting code.
This enables gprof
to determine how many times each line
of code was exeucted.
For example, consider the following function, taken from gzip,
with line numbers added:
1 ulg updcrc(s, n) 2 uch *s; 3 unsigned n; 4 { 5 register ulg c; 6 7 static ulg crc = (ulg)0xffffffffL; 8 9 if (s == NULL) { 10 c = 0xffffffffL; 11 } else { 12 c = crc; 13 if (n) do { 14 c = crc_32_tab[...]; 15 } while (--n); 16 } 17 crc = c; 18 return c ^ 0xffffffffL; 19 }
updcrc
has at least five basic-blocks.
One is the function itself. The
if
statement on line 9 generates two more basic-blocks, one
for each branch of the if
. A fourth basic-block results from
the if
on line 13, and the contents of the do
loop form
the fifth basic-block. The compiler may also generate additional
basic-blocks to handle various special cases.
A program augmented for basic-block counting can be analyzed with
gprof -l -A
. I also suggest use of the `-x' option,
which ensures that each line of code is labeled at least once.
Here is updcrc
's
annotated source listing for a sample gzip
run:
ulg updcrc(s, n) uch *s; unsigned n; 2 ->{ register ulg c; static ulg crc = (ulg)0xffffffffL; 2 -> if (s == NULL) { 1 -> c = 0xffffffffL; 1 -> } else { 1 -> c = crc; 1 -> if (n) do { 26312 -> c = crc_32_tab[...]; 26312,1,26311 -> } while (--n); } 2 -> crc = c; 2 -> return c ^ 0xffffffffL; 2 ->}
In this example, the function was called twice, passing once through
each branch of the if
statement. The body of the do
loop was executed a total of 26312 times. Note how the while
statement is annotated. It began execution 26312 times, once for
each iteration through the loop. One of those times (the last time)
it exited, while it branched back to the beginning of the loop 26311 times.
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