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Re: [PATCH] tile: create new math-tests.h header
On Fri, 18 Nov 2016, Chris Metcalf wrote:
> On 11/18/2016 1:11 PM, Joseph Myers wrote:
> > On Fri, 18 Nov 2016, Chris Metcalf wrote:
> >
> > > We clearly should also tell libgcc not to bother preserving nan
> > > payloads given that the hardware doesn't do it in any case; I'll pass
> > > that along to the compiler team, but I'll also commit my original
> > > patch since it seems to match the hardware behavior.
> > FWIW, I wouldn't expect any significant performance difference between
> > copying a payload and setting a payload to the canonical one (which is
> > what you get if !_FP_KEEPNANFRACP).
>
> My assumption was that we should tweak it for consistency, not for
> performance. Is your sense that we might as well keep the NaN
> payloads in libgcc even if we don't with inline float operations?
Consistency with hardware is logical (I think the reason for such hooks in
soft-fp is to allows its use in Linux kernel floating-point emulation to
behave the same as the hardware being emulated). Note however that when
soft-fp creates a default NaN it also gives it a default sign - if that's
what you want for consistency with hardware, you'll need a new
math-tests.h hook for these canonicalize tests to disregard the sign of
the result as well as its payload.
--
Joseph S. Myers
joseph@codesourcery.com