[InstCombine] allow lengthening of insertelement to eliminate shuffles

As noted in post-commit comments for D52548, the limitation on 
increasing vector length can be applied by opcode.
As a first step, this patch only allows insertelement to be
widened because that has no logical downsides for IR and has 
little risk of pessimizing codegen.

This may cause PR39132 to go into hiding during a full compile,
but that bug is not fixed.

llvm-svn: 343406

2 files changed, 12 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index a87e323fd4b..c391034dc00 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -918,6 +918,13 @@ static bool canEvaluateShuffled(Value *V, ArrayRef<int> Mask,
     case Instruction::FPTrunc:
     case Instruction::FPExt:
     case Instruction::GetElementPtr: {
+      // Bail out if we would create longer vector ops. We could allow creating
+      // longer vector ops, but that may result in more expensive codegen. We
+      // would also need to limit the transform to avoid undefined behavior for
+      // integer div/rem.
+      Type *ITy = I->getType();
+      if (ITy->isVectorTy() && Mask.size() > ITy->getVectorNumElements())
+        return false;
       for (Value *Operand : I->operands()) {
         if (!canEvaluateShuffled(Operand, Mask, Depth - 1))
           return false;
@@ -1464,8 +1471,7 @@ Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
     if (isRHSID) return replaceInstUsesWith(SVI, RHS);
   }
 
-  if (isa<UndefValue>(RHS) && !SVI.increasesLength() &&
-      canEvaluateShuffled(LHS, Mask)) {
+  if (isa<UndefValue>(RHS) && canEvaluateShuffled(LHS, Mask)) {
     Value *V = evaluateInDifferentElementOrder(LHS, Mask);
     return replaceInstUsesWith(SVI, V);
   }
diff --git a/llvm/test/Transforms/InstCombine/vec_shuffle.ll b/llvm/test/Transforms/InstCombine/vec_shuffle.ll
index b3f2ca54be9..7a5ff3628d2 100644
--- a/llvm/test/Transforms/InstCombine/vec_shuffle.ll
+++ b/llvm/test/Transforms/InstCombine/vec_shuffle.ll
@@ -218,14 +218,13 @@ define <3 x i32> @div_wider(i32 %y, i32 %z) {
   ret <3 x i32> %ext
 }
 
-; TODO: Increasing length of insertelements (no math ops) is a good canonicalization.
+; Increasing length of insertelements (no math ops) is a good canonicalization.
 
 define <3 x i8> @fold_inselts_with_widening_shuffle(i8 %x, i8 %y) {
 ; CHECK-LABEL: @fold_inselts_with_widening_shuffle(
-; CHECK-NEXT:    [[INS0:%.*]] = insertelement <2 x i8> undef, i8 [[X:%.*]], i32 0
-; CHECK-NEXT:    [[INS1:%.*]] = insertelement <2 x i8> [[INS0]], i8 [[Y:%.*]], i32 1
-; CHECK-NEXT:    [[WIDEN:%.*]] = shufflevector <2 x i8> [[INS1]], <2 x i8> undef, <3 x i32> <i32 0, i32 1, i32 undef>
-; CHECK-NEXT:    ret <3 x i8> [[WIDEN]]
+; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <3 x i8> undef, i8 [[X:%.*]], i32 0
+; CHECK-NEXT:    [[TMP2:%.*]] = insertelement <3 x i8> [[TMP1]], i8 [[Y:%.*]], i32 1
+; CHECK-NEXT:    ret <3 x i8> [[TMP2]]
 ;
   %ins0 = insertelement <2 x i8> undef, i8 %x, i32 0
   %ins1 = insertelement <2 x i8> %ins0, i8 %y, i32 1