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Date: Wed, 27 Feb 2019 19:00:38 +0300 (MSK)
From: Alexander Monakov <>
Subject: Re: FE Exception triggered by comparison

On Thu, 28 Feb 2019, Damian McGuckin wrote:

> I tried the code below, I was just shocked.
> 	#include	<math.h>
> 	#include	<stdio.h>
> 	main()
> 	{
> 		double x = 5.0;
> 		x -= x, x /= x;
> 		printf("what %s\n", isnan(x) ? "yes" : "no!");
> 		return(0);
> 	}
> Looking at the assembler, there is a subroutine call to __isnan. Awful!

Hm, no, for x86 with GCC you should not see that: the compiler knows how
to expand isnan efficiently.  Are you perhaps on OS X and the 'gcc' command
actually invokes Clang/LLVM?  If not, can you show output of 'gcc -v',
command-line flags you used, and the assembly you're seeing?


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