Re: Compiler optimization and floating point operations

From	"Steven G. Kargl" <sgk@troutmask.apl.washington.edu>
Newsgroups	comp.compilers
Subject	Re: Compiler optimization and floating point operations
Date	2013-10-24 16:38 +0000
Organization	A noiseless patient Spider
Message-ID	<13-10-032@comp.compilers> (permalink)
References	<13-10-026@comp.compilers> <13-10-029@comp.compilers>

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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]

Thread

Compiler optimization and floating point operations Abid <abidmuslim@gmail.com> - 2013-10-23 13:47 -0700
  Re: Compiler optimization and floating point operations Cameron McInally <cameron.mcinally@nyu.edu> - 2013-10-23 22:55 -0400
  Re: Compiler optimization and floating point operations glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-10-24 02:56 +0000
  Re: Compiler optimization and floating point operations George Neuner <gneuner2@comcast.net> - 2013-10-24 07:41 -0400
    Re: Compiler optimization and floating point operations Jean-Marc Bourguet <jm@bourguet.org> - 2013-10-24 15:51 +0200
    Re: Compiler optimization and floating point operations "Steven G. Kargl" <sgk@troutmask.apl.washington.edu> - 2013-10-24 16:38 +0000
      Re: Compiler optimization and floating point operations christian.bau@cbau.wanadoo.co.uk - 2013-10-28 15:54 -0700
    Re: Compiler optimization and floating point operations George Neuner <gneuner2@comcast.net> - 2013-10-24 15:51 -0400
    Re: Compiler optimization and floating point operations glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-10-24 20:21 +0000
    Re: Compiler optimization and floating point operations christian.bau@cbau.wanadoo.co.uk - 2013-10-28 15:50 -0700
      Re: Compiler optimization and floating point operations Cameron McInally <cameron.mcinally@nyu.edu> - 2013-10-28 20:41 -0400

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