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Re: Fix sysdeps/ieee754 pow handling of sNaN arguments (bug 20916) [committed]
On Wed, 14 Dec 2016, Andreas Krebbel wrote:
> The problem with the load and test instruction is that it is not
> consistent to doing a load and a compare separately. Our floating
> point loads do not quiet a NaN.
>
> While the IEEE standard leaves it open to quiet NaNs with loads it is
> probably supposed to be the same for all loads.
>
> The plan is to disable the use of the FPR result of a load-and-test
> instruction in the compiler. We would leave the Glibc tests failing
> since this appears to be a real finding to me.
>
> Does this sound reasonable?
It sounds reasonable, given the quality-of-implementation goal to allow
signaling NaNs to be copied without quieting. If using that FPR result is
useful for optimization, and harmless in the absence of signaling NaNs, of
course you could restrict the disabling to the -fsignaling-nans case (and
arrange for pow to be built with that option in glibc for s390).
--
Joseph S. Myers
joseph@codesourcery.com