Collecting LLVM Edge Profiling with llvm-prof

Question

I'm using these commands to compile the code below in order to collect edge/blocks profiling in trunk-llvm:

clang -emit-llvm -c sort.c -o sort.bc
opt -insert-edge-profiling sort.bc -o sort_prof.bc
clang sort_prof.bc -lprofile_rt -L/llvms/lib -o sort_prof

then I run the program and display the profiling information using llvm-prof sort_prof.bc, and the result is:

===-------------------------------------------------------------------------===
Function execution frequencies:

 ##   Frequency
  1. 4.3e+05/708539 main
  2. 2.8e+05/708539 quickSort

  NOTE: 2 functions were never executed!
.....

My question is regarding the execution frequencies. Does make any sense main executing 4.3e+05 times? Why so? The code I'm compiling is below.

###################### sort.c ########################
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

const int MAX = 1000000;

void swap(int* a, int* b) {
  int tmp;
  tmp = *a;
  *a = *b;
  *b = tmp;
}

int partition(int vec[], int left, int right) {
  int i, j;

  i = left;
  for (j = left + 1; j <= right; ++j) {
    if (vec[j] < vec[left]) {
      ++i;
      swap(&vec[i], &vec[j]);
    }
  }
  swap(&vec[left], &vec[i]);

  return i;
}

void quickSort(int vec[], int left, int right) {
  int r;

  if (right > left) {
    r = partition(vec, left, right);
    quickSort(vec, left, r - 1);
    quickSort(vec, r + 1, right);
  }
}

int main(void) {

        int vet[MAX], i=0;

        srand(time(NULL));

        for (i=0; i<MAX; i++) {
                vet[i] = rand() % 654321;
        }

        quickSort(vet, 0, MAX-1);

        for (i=0; i<MAX; i++) {
                if ((rand() % 7) > 2) {
                        printf("Num$[%d] = %d\n", i, vet[i]);
                }
                else if ((rand() % 4) > 2) {
                        printf("Num@[%d] = %d\n", i, vet[i]);
                }
                else if ((rand() % 2) > 1) {
                        printf("Num#[%d] = %d\n", i, vet[i]);
                }
        }

        return 0;
}

Answer 1

The problem was that I was passing to llvm-prof the bitcode file with instrumentation, the correct is to use the original file (without instrumentation):

llvm-prof sort.bc

another problem related to llvm-prof is that it's rounding the function/block execution frequency due to scientific notation. I've submitted a patch to llvm for correcting that.

Another tip is llvm-prof per default shows only the top 20 most executed basic blocks and it doesn't provide the user any means to change that. I've submitted another patch that add a command line parameter enabling the user to set how many basic blocks he/she wants in the output.