[codegen] Compile copysign more optimally

[lattner]

Bugzilla Link	702
Resolution	FIXED
Resolved on	Feb 22, 2010 12:55
Version	1.0
OS	All

Extended Description

It would be nice to add:

1. an llvm intrinsic (llvm.copysign.f32/f64), that represents copysign[f] when negative zeros
   and SNANs are not important (basically -ffast-math), the CFE could generate this.
2. a selectiondag node to represent the same condition, also when UnsafeFPMath is enabled.

Then the legalizer can turn copysign into either 1) stores to mem followed by bitfield stuff and a load,
or 2) fabs/fneg/setcc/select if the target has them.

With this, the PPC backend would codegen copysign as:

copysign(x, y) {
fabs xPos, x
fnabs xNeg, x
fsel x, y, xPos, xNeg
return x;
}

which is nice and fast. X86/SSE has a similar lowering, but it would be custom lowered.

This is important for codes that use the FORTRAN copysign intrinsic or the C99 copysign functions
heavily.

-Chris

[lattner]

we have codegen support for copysign now.

-Chris