Daniel Bratell writes:
> This is likely to be the one FAQ you've all learned to answer but
> as I was bitten by it, I just wanted to make sure that what I saw
> was expected.
>
> I've used -ffast-math for a slight speedup of floating point
> arithmetics, but I've also used doubles to store integers. Since
> integers (up to a certain size) can be stored without loss of
> precision in a double it's been no problems until I started using
> gcc 4.0 (this is with gcc 4.0.1 on a fedora core 3, x86 system).
>
> Now I see my "exact" integer arithmetic converted to inexact
> fractional double arithmetic. More specifically a division by 7
> (can be exactly represented in a double) is converted to a
> multiplication by 0.14285... (about 1/7) and then rounding errors
> can make my "integers" get another value.
Yes. That's one of the things -ffast-math does.
> Is this to be expected? Should I stop using
> -funsafe-math-optimizations? Somehow I've been tricked into
> thinking that it would be safe to use that flag unless I relied on
> extreme precision or behaviour related to NaN or +-Inf, and that
> normal simple arithmetics should still give the same result.
>
> So there you have it. Now you can kick me. ;-)
>
> Test program below. Expected 1, get 3. Compiled with
> gcc -O2 -ffast-math
>
> ---------------------------------
>
> #include <stdio.h>
>
> int foo(double diff_days)
> {
> if(diff_days/7.0 != (int)(diff_days/7.0))
> return 3;
> return 1;
> }
>
> int main(int argc, char** argv)
> {
> printf("Return value (7.0/7.0) = %d\n", foo(7.0));
> }
>
> --------------------------------
This is due to the most popular gcc bug of all, PR323. See
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323.
gcc implements division by 7 by multiplying by
0.14285714285714284921269268124888185.
The result of this is 0.99999999999999994448884876874217298 (raw
0x3ffefffffffffffffc00).
If you stored this in a variable and used -ffloat-store, the problem
you see wouldn't happen.
// if(diff_days/7.0 != (int)(diff_days/7.0))
if(x != (int)(diff_days/7.0))
$ gcc -O2 -ffast-math p.c -save-temps -g -ffloat-store
$ ./a.out
1
Andrew.
