Fix powerpc fmax, fmin sNaN handling (bug 20947) [committed]

[Joseph Myers]

On Fri, 16 Dec 2016, Steven Munroe wrote:

> > This patch fixes the powerpc versions of these functions (shared by
> > float and double, 32-bit and 64-bit).  The structure of those versions
> > is that all ordered cases are already handled before anything dealing
> > with the case where the arguments are unordered; thus, this patch
> > causes no change to the code executed in the common case (neither
> > argument a NaN).
> > 
> This patch, as is, will cause a sever performance regression for
> power6/7/8 and should be fixed or reverted before 2.25 freezes.

A general assumption in libm functions is that NaN inputs are an uncommon 
case and performance for NaN inputs is not of much importance; the code 
for NaN inputs should just aim for correctness and simplicity.  I think 
people calling fmax or fmin generally want the maximum or minimum of two 
finite numbers.

As I said, there is no change to the code executed for the normal case of 
non-NaN inputs, so I see no reason for performance in the common case to 
be affected.  Of course performance improvements for the NaN case are 
still welcome (and that sort of small local fix would seem perfectly 
suitable to go in during the freeze if architecture maintainers think it 
appropriate).

If the presence of this code for the NaN case somehow affects the 
performance of non-NaN case, please let me know; it wasn't meant to do so.

-- 
Joseph S. Myers
joseph@codesourcery.com