[LLVMdev] Signed NaNs in APFloat arithmetic

[Stephen Canon]

> On Aug 8, 2014, at 2:59 PM, Sanjay Patel <spatel at rotateright.com> wrote:
> 
> FYI, I was looking at the SSE/AVX codegen here:
> http://llvm.org/bugs/show_bug.cgi?id=20578
> 
> If LLVM starts caring about FP exceptions, even this won't be possible. Is there a way of doing an IEEE-754 fneg in C/C++? Ie, there's no fneg() in libm, so any C method we choose could cause an exception, and that's not allowed by the IEEE definition of fneg.

Huh? XOR[PS|PD] is an IEEE-754 negate( ) on x86; it does not raise any FP exceptions.

In C, it’s not specified at present whether –x corresponds to negate( ), but the general belief (among IEEE-754 committee members) seems to be that it should, and historically clang has mostly tried to behave that way.

On a platform where –x does not correspond to the IEEE-754 negate( ) operation, but the FP representation is known, you would type-pun to integer (via union or memcpy), xor the signbit, and type-pun back to FP to get the result.  The only platform I know of where even this isn’t possible is i386 under an ABI that returns on the x87 stack, where loading a single- or double- sNaN necessarily signals invalid.  Thankfully, the last such machine is now … what, 15 years old?

– Steve
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