linking time eroor with -fast-math -O0

[Vladimir Yakovlev]

Hi,

Following test fails in linking if compiled with ffast-math and O0,
but it compiled successfully with ffast-math and O2. Also no problem
if -lm is added.

$ cat t.c
#include <stdio.h>

float foo(float x)
{
   float y = 0;
   while (x > 0.00000001) {
     y += x*x*x*x*x*x*x*x*x*x*x*x*x;
     x = x/2;
   }
   return y;
}

int main (int argc, char    *argv[])
{
  float y = atoi(argv[1]);
  printf("%f\n", foo(y));
  return 0;
}


$ gcc  -ffast-math -O0   t.c
/tmp/cccA1sUB.o: In function `foo':
t.c:(.text+0x2c): undefined reference to `powf'
collect2: error: ld returned 1 exit status
$ gcc  -ffast-math -O2   t.c
$ ./a.out 5
1220852096.000000


FE with -ffast-math replaced x*x*...*x with __builtin_powf. Later with
-O2 this call is replaced back into multiplications in sincos phase.
The stability with -O0 is because sincos phase doesn't work on -O0.

I think we must avoid doing this optimization in FE and turn off
-ffast-math if -O0 is used. Your opinion.

Thanks,
Vladimir