Re: Compiler optimization and floating point operations

On Thu, 24 Oct 2013 15:51:59 +0200, Jean-Marc Bourguet wrote:


> George Neuner <gneuner2@comcast.net> writes:
>
>> [I think you mean they're not associative. I don't know any situations
>> where a+b != b+a, but lots where a+(b+c) != (a+b)+c -John]
>
> You forgot about NaN ;-) (but I agree associative was probably intended).
>
> Yours,
> --
> Jean-Marc [How are NaNs not commutative? -John]


I believe Jean-Marc is referring to something like the following


#include <stdio.h>
int
main(void)
{
      float a, b;
      a = 0.;
      b = a / a;
      if (a + b != b + a) printf("fubar\n");
      return (0);
}


Can an optimizing compiler eliminate the if-statement?


--
steve
[If it does sufficient dataflow and constant substitutions so it can tell
that a+b will always be a NaN, I don't see why not. -John]

Return to the comp.compilers page.
Search the comp.compilers archives again.