[PATCH] D108725: [AArch64][GlobalISel] Implement custom legalization for s32/s64 G_FCOPYSIGN

[Amara Emerson via Phabricator via llvm-commits]

aemerson accepted this revision.
aemerson added inline comments.
This revision is now accepted and ready to land.


================
Comment at: llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp:1393
+  auto Zero = MIRBuilder.buildConstant(DstTy, 0);
+  auto Ins1 = MIRBuilder.buildInsertVectorElement(
+      VecTy, Undef, MI.getOperand(1).getReg(), Zero);
----------------
paquette wrote:
> aemerson wrote:
> > What if we use G_MERGE instead? Do we get an INSERT_SUBREG?
> Not quite.
> 
> We get the following for s32 with G_MERGE_VALUES:
> 
> ```
> legalize_s32:
> 	adrp	x8, .LCPI0_0
> 	mov	v0.s[1], v0.s[0]
> 	mov	v1.s[1], v0.s[0]
> 	ldr	q2, [x8, :lo12:.LCPI0_0]
> 	mov	v0.s[2], v0.s[0]
> 	mov	v0.s[3], v0.s[0]
> 	mov	v1.s[2], v0.s[0]
> 	mov	v1.s[3], v0.s[0]
> 	bit	v0.16b, v1.16b, v2.16b
> 	ret
> ```
> 
> Meanwhile, with SDAG we get
> 
> ```
> 	movi	v2.4s, #128, lsl #24 ; We should emit the constant like this, but we don't have that optimization
> 	bit	v0.16b, v1.16b, v2.16b
> 	ret
> ```
> 
> We can probably change the selector code to recognize the pattern though. Using G_INSERT_VECTOR_ELT is only slightly better.
Ok, let's go with the insert for now.


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