Re: [www-patch] bugs.html rewrite, part 2: revamp floating point section

[Bradley Lucier <lucier at math dot purdue dot edu>]

Re:

> +<p>On some architectures calculations inside the floating point unit 
> (FPU)
> +are done with higher precision than memory operations like storing 
> floating
> +point numbers (e.g., a <code>double</code> has 80 bits in the FPU on 
> x86,
> +but only 64 bits in memory). Thus, writing data to memory involves 
> rounding!
> +That's why the following C program might print <code>"Uh 
> oh!"</code>.</p>
> +<blockquote><pre>
> +int main()
> +{
> +  double          x=sqrt(2.0);
> +  volatile double y=x;
> +  if (x!=y) printf("Uh oh!\n");
> +  return 0;
> +}
> +</pre></blockquote>
> +
> +<p>Again, this is not a bug in the compiler, but a result of rounding
> +(in conjunction with the excess precision of the FPU).</p>

I would claim that this *is* a bug in the compiler, but it's one that's 
extremely unlikely to be fixed in the foreseeable future, and we've 
rehashed this all before.

Why not try

> +<blockquote><pre>
> +int main()
> +{
> +  volatile double y=sqrt(2.0);
> +  if (sqrt(2.0)!=y) printf("Uh oh!\n");
> +  return 0;
> +}
> +</pre></blockquote>

Then I think we would both agree that if the programmer didn't expect 
"Uh oh!" to be printed, then their expectations need to be realigned on 
the x86.

Brad