[InstCombine] Pull shuffles out of binops with splatted ops (#137948)

Given a binary op on splatted vector and a splatted constant,
InstCombine will normally pull the shuffle out in
`InstCombinerImpl::foldVectorBinop`:

```llvm
define <4 x i32> @f(i32 %x) {
  %x.insert = insertelement <4 x i32> poison, i32 %x, i64 0
  %x.splat = shufflevector <4 x i32> %x.insert, <4 x i32> poison, <4 x i32> zeroinitializer
  %res = add <4 x i32> %x.splat, splat (i32 42)
  ret <4 x i32> %res
}
```

```llvm
define <4 x i32> @f(i32 %x) {
  %x.insert = insertelement <4 x i32> poison, i32 %x, i64 0
  %1 = add <4 x i32> %x.insert, <i32 42, i32 poison, i32 poison, i32 poison>
  %res = shufflevector <4 x i32> %1, <4 x i32> poison, <4 x i32> zeroinitializer
  ret <4 x i32> %res
}
```

However, this currently only operates on fixed length vectors. Splats of
scalable vectors don't currently have their shuffle pulled out, e.g:

```llvm
define <vscale x 4 x i32> @f(i32 %x) {
  %x.insert = insertelement <vscale x 4 x i32> poison, i32 %x, i64 0
  %x.splat = shufflevector <vscale x 4 x i32> %x.insert, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
  %res = add <vscale x 4 x i32> %x.splat, splat (i32 42)
  ret <vscale x 4 x i32> %res
}
```

Having this canonical form with the shuffle pulled out is important as
VectorCombine relies on it in order to scalarize binary ops in
`scalarizeBinopOrCmp`, which would prevent the need for #137786. This
also brings it in line for scalable binary ops with two non-constant
operands: https://godbolt.org/z/M9f7ebzca

This adds a combine just after the fixed-length version, but restricted
to splats at index 0 so that it also handles the scalable case:

So the whilst the existing combine looks like: `Op(shuffle(V1, Mask), C)
-> shuffle(Op(V1, NewC), Mask)`

This patch adds: `Op(shuffle(V1, 0), (splat C)) -> shuffle(Op(V1, (splat
C)), 0)`

I think this could be generalized to other splat indexes that aren't
zero, but I think it would be dead code since only fixed-length vectors
can have non-zero shuffle indices, which would be covered by the
existing combine.

Author: lukel97

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2271,6 +2271,27 @@ Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
     }
   }
 
+  // Similar to the combine above, but handles the case for scalable vectors
+  // where both shuffle(V1, 0) and C are splats.
+  //
+  // Op(shuffle(V1, 0), (splat C)) -> shuffle(Op(V1, (splat C)), 0)
+  if (isa<ScalableVectorType>(Inst.getType()) &&
+      match(&Inst, m_c_BinOp(m_OneUse(m_Shuffle(m_Value(V1), m_Poison(),
+                                                m_ZeroMask())),
+                             m_ImmConstant(C)))) {
+    if (Constant *Splat = C->getSplatValue()) {
+      bool ConstOp1 = isa<Constant>(RHS);
+      VectorType *V1Ty = cast<VectorType>(V1->getType());
+      Constant *NewC = ConstantVector::getSplat(V1Ty->getElementCount(), Splat);
+
+      Value *NewLHS = ConstOp1 ? V1 : NewC;
+      Value *NewRHS = ConstOp1 ? NewC : V1;
+      VectorType *VTy = cast<VectorType>(Inst.getType());
+      SmallVector<int> Mask(VTy->getElementCount().getKnownMinValue(), 0);
+      return createBinOpShuffle(NewLHS, NewRHS, Mask);
+    }
+  }
+
   // Try to reassociate to sink a splat shuffle after a binary operation.
   if (Inst.isAssociative() && Inst.isCommutative()) {
     // Canonicalize shuffle operand as LHS.

llvm/test/Transforms/InstCombine/getelementptr.ll

@@ -282,8 +282,8 @@ define <2 x i1> @test13_fixed_scalable(i64 %X, ptr %P, <2 x i64> %y) nounwind {
 define <vscale x 2 x i1> @test13_scalable_scalable(i64 %X, ptr %P, <vscale x 2 x i64> %y) nounwind {
 ; CHECK-LABEL: @test13_scalable_scalable(
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <vscale x 2 x i64> poison, i64 [[X:%.*]], i64 0
-; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <vscale x 2 x i64> [[DOTSPLATINSERT]], <vscale x 2 x i64> poison, <vscale x 2 x i32> zeroinitializer
-; CHECK-NEXT:    [[A_IDX:%.*]] = shl nsw <vscale x 2 x i64> [[DOTSPLAT]], splat (i64 3)
+; CHECK-NEXT:    [[TMP3:%.*]] = shl nsw <vscale x 2 x i64> [[DOTSPLATINSERT]], splat (i64 3)
+; CHECK-NEXT:    [[A_IDX:%.*]] = shufflevector <vscale x 2 x i64> [[TMP3]], <vscale x 2 x i64> poison, <vscale x 2 x i32> zeroinitializer
 ; CHECK-NEXT:    [[TMP1:%.*]] = call i64 @llvm.vscale.i64()
 ; CHECK-NEXT:    [[TMP2:%.*]] = shl i64 [[TMP1]], 4
 ; CHECK-NEXT:    [[DOTSPLATINSERT1:%.*]] = insertelement <vscale x 2 x i64> poison, i64 [[TMP2]], i64 0

llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll

@@ -1789,3 +1789,39 @@ define <4 x i32> @PR46872(<4 x i32> %x) {
   ret <4 x i32> %a
 }
 
+define <vscale x 4 x i32> @scalable_splat_binop_constant_rhs(<vscale x 4 x i32> %x) {
+; CHECK-LABEL: @scalable_splat_binop_constant_rhs(
+; CHECK-NEXT:    [[R1:%.*]] = add <vscale x 4 x i32> [[R:%.*]], splat (i32 42)
+; CHECK-NEXT:    [[R2:%.*]] = shufflevector <vscale x 4 x i32> [[R1]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <vscale x 4 x i32> [[R2]]
+;
+
+  %splatx = shufflevector <vscale x 4 x i32> %x, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+  %r = add <vscale x 4 x i32> %splatx, splat (i32 42)
+  ret <vscale x 4 x i32> %r
+}
+
+define <vscale x 4 x float> @scalable_splat_binop_constant_lhs(<vscale x 4 x float> %x) {
+; CHECK-LABEL: @scalable_splat_binop_constant_lhs(
+; CHECK-NEXT:    [[R1:%.*]] = fadd <vscale x 4 x float> [[R:%.*]], splat (float 4.200000e+01)
+; CHECK-NEXT:    [[R2:%.*]] = shufflevector <vscale x 4 x float> [[R1]], <vscale x 4 x float> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <vscale x 4 x float> [[R2]]
+;
+
+  %splatx = shufflevector <vscale x 4 x float> %x, <vscale x 4 x float> poison, <vscale x 4 x i32> zeroinitializer
+  %r = fadd <vscale x 4 x float> splat (float 42.0), %splatx
+  ret <vscale x 4 x float> %r
+}
+
+; Negative test - shouldn't pull shuffle out as it udiv isn't safe to speculate.
+define <vscale x 4 x i32> @scalable_splat_binop_constant_ub(<vscale x 4 x i32> %x) {
+; CHECK-LABEL: @scalable_splat_binop_constant_ub(
+; CHECK-NEXT:    [[SPLATX:%.*]] = shufflevector <vscale x 4 x i32> [[X:%.*]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    [[R:%.*]] = udiv <vscale x 4 x i32> splat (i32 42), [[SPLATX]]
+; CHECK-NEXT:    ret <vscale x 4 x i32> [[R]]
+;
+
+  %splatx = shufflevector <vscale x 4 x i32> %x, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+  %r = udiv <vscale x 4 x i32> splat (i32 42), %splatx
+  ret <vscale x 4 x i32> %r
+}