Re: [PATCH] tile: create new math-tests.h header

[Chris Metcalf <cmetcalf at mellanox dot com>]

On 11/18/2016 4:18 PM, Joseph Myers wrote:
> 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.

Good point, but, the hardware does preserve the sign of NaN's, so
just disabling the payload testing is sufficient for the tests to pass.

I think we may just leave the payload copying enabled in soft-fp
unless you think it's a bad choice; it seems harmless and arguably
might give us better results in some circumstances.

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com