5.4 The Annotated Source Listing

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.

With older versions of gcc 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 executed. With newer versions of gcc support for displaying basic-block counts is provided by the gcov program.

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’. The ‘-x’ option is also helpful, to ensure 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.