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Date: Fri, 26 Aug 2011 12:52:00 -0000
Message-ID: <CAK1BsWpuYaPPoetu96e=2H8NesLZrSFwGnZGow88vph-Ay+4kA@mail.gmail.com>
Subject: linking time eroor with -fast-math -O0
From: Vladimir Yakovlev <vbyakovl23@gmail.com>
To: gcc@gcc.gnu.org
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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