GCC

# GCC Profiling

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## Overview

To use profiling, the program must be compiled and linked with the -qg profiling option:

We will use an called profiling_test.c (full code can be found at https://github.com/gbmhunter/BlogAssets/tree/master/Programming/ProfilingGprof):

#include <stdio.h>

int fibonacci(int n) {
if(n == 0)
return 0;
else if(n == 1)
return 1;
else
return(fibonacci(n-1) + fibonacci(n-2));
}

int loop100M() {
int val = 0;
for(int i = 0; i < 100000000; i++) {
if(i % 10 == 0)
val++;
else if(i % 3)
val--;
}
return val;
}

int main (void) {
printf("Fibonacci value = %u\n", fibonacci(40));
printf("Loop value = %u\n", loop100M());

return 0;
}


We will then compile it with the command:

$gcc -pg profiling_test.c -o profiling_test  This creates what is called an instrumented executable. It contains additional code which records the time spent in each function. When run, the program will produce a file gmon.out in the same directory as it is run. You can pass your program to gprof to display the profiling results: $ gprof ./profiling_test
Flat profile:

Each sample counts as 0.01 seconds.
%   cumulative   self              self     total
time   seconds   seconds    calls  ms/call  ms/call  name
60.96      0.68     0.68        1   676.63   676.63  fibonacci
31.84      1.03     0.35        1   353.47   353.47  loop100M
8.19      1.12     0.09                             frame_dummy
...


You can see above that approximately 60% of the time was spent calculating the Fibonacci sequence, while 30% was spent looping 100 million times. If this was a real life scenario, you could now start to optimise your code!

If you find text hard to analyze, see the gprof2dot section below on how to create a visualization of the above results.

The above command will write the profiling results to the terminal. Instead, if you wish to write the results to a file, use the following command:

$gprof profiling_test > profiling_results.txt  Clean Exiting gmon.out is only written to if your C/C++ program exits cleanly, that is, it either calls exit() or returns from main(). Here is the relevant info from the gprof manual: The profiled program must call "exit"(2) or return normally for the profiling information to be saved in the gmon.out file. Your program doesn’t count as a clean exit if it is running in a Linux terminal and Ctrl-C is pressed! However, there is a way to fix this, by catching the Ctrl-C signal and writing to the file before exiting… #include <dlfcn.h> #include <stdio.h> #include <unistd.h> void SigIntHandler(int sig) { fprintf(stderr, "Exiting on SIGUSR1\n"); void (*_mcleanup)(void); _mcleanup = (void (*)(void)) dlsym(RTLD_DEFAULT, "_mcleanup"); if (_mcleanup == NULL) fprintf(stderr, "Unable to find gprof exit hook\n"); else _mcleanup(); _exit(0); } int main() { signal(SIGINT, SigIntHandler); ... code that does not return here }  ## gprof2dot gprof2dot is a tool that can create a visualization of the gprof output. TO install gprof2dot: $ pip install gprof2dot


To install graphviz (which is needed if you are going to make “dot” graphs like below):

$sudo apt install graphviz  To create a dot graph image: $ gprof2dot ./profiling.txt | dot -Tpng -o profiling.png


This created the below image for the example code above: