Activate -mrecip with -ffast-math?

[Bradley Lucier]

On Jun 18, 2007, at 2:27 PM, Bradley Lucier wrote:

>> But even if sqrt is corrected for 0.0 * inf, there would still be  
>> a lot of problems with the combinations of NR-enhanced rsqrt and  
>> rcp. Consider for example:
>>
>> 1.0/sqrt(a/b) alias rsqrt(a/b)
>>
>> Having a=0, b != 0, the result is inf.
>
> As already stated, -ffast-math turns on -ffinite-math-only, which  
> allows the compiler to assume that a result of inf cannot happen,  
> so gcc is allowed to ignore this possiblity.  Producing NaN instead  
> of inf seems to be allowed.

Let me restate this.

If -ffinite-math-only is specified, then producing NaN instead of inf  
should be allowed.

If -fno-finite-math-only is specified, then the generated code should  
"do the right thing" if an argument or result is inf or NaN.

In any case, I would consider it an error if the argument is finite,  
the result is supposed to be finite, and inf or NaN is produced.

Brad