[SOLVED] Error with -mno-sse flag and gettimeofday() in C

Error with -mno-sse flag and gettimeofday() in C

A simple C program which uses gettimeofday() works fine when compiled without any flags ( gcc-4.5.1) but doesn't give output when compiled with the flag -mno-sse.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    struct timeval s,e;
    float time;
    int i;
    gettimeofday(&s, NULL);
    for( i=0; i< 10000; i++);
    gettimeofday(&e, NULL);
    time = e.tv_sec - s.tv_sec + e.tv_usec - s.tv_usec;
    printf("%f\n", time);
    return 0;
}

I have CFLAGS=-march=native -mtune=native Could someone explain why this happens? The program returns a correct value normally, but prints "0" when compiled with -mno-sse enabled.

Solution

The flag -mno-sse causes floating point arguments to be passed on the stack, whereas the usual x86_64 ABI specifies that they should be passed via SSE registers.

Since printf() in your C library was compiled without -mno-sse, it is expecting floating point arguments to be passed in accordance with the ABI. This is why your code fails. It has nothing to do with gettimeofday().

If you wish to use printf() from your code compiled with -mno-sse and pass it floating point arguments, you will need to recompile your C library with that option and link against that version.