Fix powerpc32 lround, lroundf spurious exceptions (bug 19134) [committed]

[Joseph Myers]

On Wed, 14 Oct 2015, Matt Turner wrote:

> On Wed, Oct 14, 2015 at 2:15 PM, Joseph Myers <joseph@codesourcery.com> wrote:
> > Committed.  Any optimizations of this code anyone may wish to make are of
> > course welcome.
> 
> The code seems to do the (|x| + 0.5) and round towards zero trick,
> which it notes does not work for an input of nextafter(0.5, 0.0)
> (i.e., 0x1.fffffffffffffp-2):
> 
> > It is necessary to detect when x is (+-)0x1.fffffffffffffp-2 because adding +-0.5 in this case will cause an erroneous shift, carry and round.  We simply return 0 if 0.5 > x > -0.5.
> 
> and so it handles 0.5 > x > -0.5 specially. Can it not simply add
> 0x1.fffffffffffffp-2 instead of 0.5 and remove the special case?

No, that wouldn't work for any argument of the form x.5 in 
round-toward-zero mode.

-- 
Joseph S. Myers
joseph@codesourcery.com