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From: "bisqwit at iki dot fi" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/31485] C complex numbers, amd64 SSE, missed
 optimization opportunity
Date: Tue, 21 Apr 2020 08:34:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D31485

--- Comment #20 from Joel Yliluoma <bisqwit at iki dot fi> ---
(In reply to Jakub Jelinek from comment #16)
> (In reply to Joel Yliluoma from comment #15)
> > (In reply to Richard Biener from comment #14)
> > > I also think llvms code generation is bogus since it appears the ABI
> > > does not guarantee zeroed upper elements of the xmm0 argument
> > > which means they could contain sNaNs:
> >=20
> > Why would it matter that the unused portions of the register contain Na=
Ns?
>=20
> Because it could then raise exceptions that shouldn't be raised?

Which exceptions would be generated by data in an unused portion of a regis=
ter?
Does for example =E2=80=9Caddps=E2=80=9D generate an exception if one or tw=
o of the operands
contains NaNs? Which instructions would generate exceptions?

I can only think of divps, when dividing by a zero, but it does not seem th=
at
even LLVM compiles the two-element vector division into divps.

If the register is passed as a parameter to a library function, they would =
not
make judgments based on the values of the unused portions of the registers.=