Dear list,
Consider the following small program:
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
double a;
a = strtod(argv[0], NULL);
printf("%f\n", floor(a));
return(0);
}
When compiling this with a -march that supports the roundsd instruction,
the floor() call seems to only be compiled to such an instruction if
-funsafe-math-optimizations is specified.
Why is this? I notice the glibc's floor() implementation (for
SSE4.1-enabled processors) consists of only this instruction, so barring a
bug in glibc, that would seem to imply to me the roundsd is IEEE-compliant
and safe. Why does GCC consider it unsafe?
For reference, these are the complete compile commands I'm using to test:
gcc -O -march=haswell -c -S -o test.s test.c
vs.
gcc -O -funsafe-math-optimizations -march=haswell -c -S -o test.s test.c
--
Fredrik Tolf
