Re: [PATCH] rs6000: Canonicalize copysign (x, -1) back to -abs (x) in the backend [PR112606]

[Xi Ruoyao <xry111@xry111.site>]

On Sat, 2023-11-25 at 11:17 +0100, Jakub Jelinek wrote:
> The middle-end has been changed quite recently to canonicalize
> -abs (x) to copysign (x, -1) rather than the other way around.
> While I agree with that at GIMPLE level, since it matches the GIMPLE
> goal of as few operations as possible for a canonical form (-abs (x)
> is 2 GIMPLE statements, copysign (x, -1) is just one), I must say
> I don't really like that being done on RTL as well (or at least
> not canonicalizing (COPYSIGN x, negative) back to (NEG (ABS x))),
> because on most targets most of floating point constants need to be loaded
> from memory, there are a few exceptions but -1 is often not one of them.

On LoongArch fneg+fabs is even slower than loading a -1 from mem then do
copysign for some micro-architectural reason I don't know.  (FWIW on
LoongArch with LSX, the fastest way may be directly setting the sign bit
with LSX vbitseti instruction - it will also set the sign bits for
"junk" elements in the high bits of the vector register but there is no
harm.)

Can we make a target hook to control this?

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University