fabs(X) * fabs(Y) --> fabs(X * Y)
fabs(X) / fabs(Y) --> fabs(X / Y)
If both operands of fmul/fdiv are positive, then the result must be positive.
There's a NAN corner-case that prevents removing the more specific fold just above this one:
fabs(X) * fabs(X) -> X * X
That fold works even with NAN because the sign-bit result of the multiply is not specified if X is NAN.
We can't remove that and use the more general fold that is proposed here because once we convert to this:
fabs (X * X)
...it is not legal to simplify the 'fabs' out of that expression when X is NAN. That's because fabs() guarantees that the sign-bit is always cleared - even for NAN values.
So this patch has the potential to lose information, but it seems unlikely if we do the more specific fold ahead of this one.
[Side note: Alive2 doesn't appear to support 'llvm.fabs' yet - that would be a nice enhancement.]