[InstCombine] Explicitly canonicalize splat shuffles to use poison RHS

This is usually handled by demanded elements simplification. However,
as that is not supported for scalable vectors, also handle it
explicitly here.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -2774,6 +2774,11 @@ Instruction *InstCombinerImpl::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
   if (Instruction *I = simplifyBinOpSplats(SVI))
     return I;
 
+  // Canonicalize splat shuffle to use poison RHS. Handle this explicitly in
+  // order to support scalable vectors.
+  if (match(SVI.getShuffleMask(), m_ZeroMask()) && !isa<PoisonValue>(RHS))
+    return replaceOperand(SVI, 1, PoisonValue::get(RHS->getType()));
+
   if (isa<ScalableVectorType>(LHS->getType()))
     return nullptr;
 

llvm/test/Transforms/InstCombine/vec_shuffle.ll

@@ -67,7 +67,7 @@ define float @test6(<4 x float> %X) {
 
 define float @testvscale6(<vscale x 4 x float> %X) {
 ; CHECK-LABEL: @testvscale6(
-; CHECK-NEXT:    [[T2:%.*]] = shufflevector <vscale x 4 x float> [[X:%.*]], <vscale x 4 x float> undef, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    [[T2:%.*]] = shufflevector <vscale x 4 x float> [[X:%.*]], <vscale x 4 x float> poison, <vscale x 4 x i32> zeroinitializer
 ; CHECK-NEXT:    [[R:%.*]] = extractelement <vscale x 4 x float> [[T2]], i64 0
 ; CHECK-NEXT:    ret float [[R]]
 ;

llvm/test/Transforms/InstCombine/vscale_extractelement.ll

@@ -56,7 +56,7 @@ define i8 @extractelement_bitcast_useless_insert(<vscale x 2 x i32> %a, i32 %x)
 define i32 @extractelement_shuffle_maybe_out_of_range(i32 %v) {
 ; CHECK-LABEL: @extractelement_shuffle_maybe_out_of_range(
 ; CHECK-NEXT:    [[IN:%.*]] = insertelement <vscale x 4 x i32> undef, i32 [[V:%.*]], i64 0
-; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[IN]], <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[IN]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
 ; CHECK-NEXT:    [[R:%.*]] = extractelement <vscale x 4 x i32> [[SPLAT]], i64 4
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
@@ -69,7 +69,7 @@ define i32 @extractelement_shuffle_maybe_out_of_range(i32 %v) {
 define i32 @extractelement_shuffle_invalid_index(i32 %v) {
 ; CHECK-LABEL: @extractelement_shuffle_invalid_index(
 ; CHECK-NEXT:    [[IN:%.*]] = insertelement <vscale x 4 x i32> undef, i32 [[V:%.*]], i64 0
-; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[IN]], <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[IN]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
 ; CHECK-NEXT:    [[R:%.*]] = extractelement <vscale x 4 x i32> [[SPLAT]], i64 4294967295
 ; CHECK-NEXT:    ret i32 [[R]]
 ;

llvm/test/Transforms/InstCombine/vscale_insertelement.ll

@@ -89,7 +89,7 @@ define <vscale x 4 x float> @insertelement_sequene_may_not_be_splat(float %x) {
 define void @ossfuzz_27416(i32 %v, ptr %p) {
 ; CHECK-LABEL: @ossfuzz_27416(
 ; CHECK-NEXT:    [[IN:%.*]] = insertelement <vscale x 4 x i32> undef, i32 [[V:%.*]], i64 0
-; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[IN]], <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[IN]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
 ; CHECK-NEXT:    [[I1:%.*]] = insertelement <vscale x 4 x i32> [[SPLAT]], i32 undef, i64 128
 ; CHECK-NEXT:    store <vscale x 4 x i32> [[I1]], ptr [[P:%.*]], align 16
 ; CHECK-NEXT:    ret void