[llvm] [RISCV][ISel] Fold FSGNJX idioms (PR #100718)

[Craig Topper via llvm-commits]

topperc wrote:

> > > Any reason this can't be done by e.g. instcombine?
> > 
> > 
> > What IR could InstCombine generate?
> 
> We have a llvm.copysign, so wouldn't this just be (copysign X, Y)? Or is there some corner case here I'm missing?

fmul X, (fcopysign 1.0, Y) is multiply X by 1.0 if Y is positive, multiply X by -1.0 if Y is negative. This has the effect of toggling X's sign bit if Y is negative. So its not a copy.

https://github.com/llvm/llvm-project/pull/100718