[InstCombine] insert a new shuffle before its uses (PR26015)

[spatel]

Summary

Although this solves the test case in PR26015:
https://llvm.org/bugs/show_bug.cgi?id=26015

...I suspect this is not the best solution. I think we want to insert the new shuffle just ahead of the earliest ExtractElementInst that we're replacing, but I don't know how that should be implemented. Is there a way to compare/record the position of each extract that we encounter in the loop?

[• tstellarAMD]

I can't comment on the correctness of the implementation, but it fixes the regression reported in PR26015. Thanks for the quick fix.

lib/Transforms/InstCombine/InstCombineVectorOps.cpp

for (unsigned i = 0; i < NumExtElts; ++i)
     ExtendMask.push_back(ConstantInt::get(IntType, i));
   for (unsigned i = NumExtElts; i < NumInsElts; ++i)
     ExtendMask.push_back(UndefValue::get(IntType));
 
   Value *ExtVecOp = ExtElt->getVectorOperand();
   auto *WideVec = new ShuffleVectorInst(ExtVecOp, UndefValue::get(ExtVecType),
                                         ConstantVector::get(ExtendMask));
 
+  // Insert the new shuffle after the vector operand of the extract is defined
+  // or at the start of the basic block, so any subsequent extracts can use it.
+  if (auto *ExtVecOpInst = dyn_cast<Instruction>(ExtVecOp))
+    WideVec->insertAfter(ExtVecOpInst);
+  else
+    IC.InsertNewInstWith(WideVec, *ExtElt->getParent()->getFirstInsertionPt());
+
   // Replace all extracts from the original narrow vector with extracts from
   // the new wide vector.
-  WideVec->insertBefore(ExtElt);
   for (User *U : ExtVecOp->users()) {
     if (ExtractElementInst *OldExt = dyn_cast<ExtractElementInst>(U)) {
       auto *NewExt = ExtractElementInst::Create(WideVec, OldExt->getOperand(1));
       NewExt->insertAfter(WideVec);
       IC.ReplaceInstUsesWith(*OldExt, NewExt);
     }
   }
 }

test/Transforms/InstCombine/insert-extract-shuffle.ll

 ; CHECK-NEXT: %[[WIDEVEC:.*]] = shufflevector <2 x float> %ext, <2 x float> undef, <8 x i32> <i32 0, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef>
 ; CHECK-NEXT: shufflevector <8 x float> %ins, <8 x float> %[[WIDEVEC]], <8 x i32> <i32 0, i32 1, i32 8, i32 3, i32 4, i32 5, i32 6, i32 7>
 ; CHECK-NEXT: ret <8 x float> %i1
   %e1 = extractelement <2 x float> %ext, i32 0
   %i1 = insertelement <8 x float> %ins, float %e1, i32 2
   ret <8 x float> %i1
 }
 
+; PR26015: https://llvm.org/bugs/show_bug.cgi?id=26015
+define <8 x i16> @pr26015(<4 x i16> %t0) {
+; CHECK-LABEL: @pr26015(
+; CHECK-NEXT:  %[[WIDEVEC:.*]] = shufflevector <4 x i16> %t0, <4 x i16> undef, <8 x i32> <i32 undef, i32 undef, i32 undef, i32 3, i32 undef, i32 undef, i32 undef, i32 undef>
+; CHECK-NEXT:  %[[EXT:.*]] = extractelement <4 x i16> %t0, i32 2
+; CHECK-NEXT:  %t2 = insertelement <8 x i16> <i16 0, i16 0, i16 0, i16 undef, i16 0, i16 0, i16 undef, i16 undef>, i16 %[[EXT]], i32 3
+; CHECK-NEXT:  %t3 = insertelement <8 x i16> %t2, i16 0, i32 6
+; CHECK-NEXT:  %t5 = shufflevector <8 x i16> %t3, <8 x i16> %[[WIDEVEC]], <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 11>
+; CHECK-NEXT:  ret <8 x i16> %t5
+  %t1 = extractelement <4 x i16> %t0, i32 2
+  %t2 = insertelement <8 x i16> zeroinitializer, i16 %t1, i32 3
+  %t3 = insertelement <8 x i16> %t2, i16 0, i32 6
+  %t4 = extractelement <4 x i16> %t0, i32 3
+  %t5 = insertelement <8 x i16> %t3, i16 %t4, i32 7
+  ret <8 x i16> %t5
+}
+