[PATCH] D132517: [AArch64][DAGCombine] Fix a bug in performBuildVectorCombine where it could produce an invalid EXTRACT_SUBVECTOR

[Usman Nadeem via Phabricator via llvm-commits]

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EXTRACT_SUBVECTOR requires that Idx be a constant multiple of ResultType's
known minimum vector length.

Something like this will produce an invalid extract_subvector:

  t1: v4i16 = .....
  t2: i32 = extract_vector_elt t1, Constant:i64<1>
  t3: i32 = extract_vector_elt t1, Constant:i64<2>
  t4: v2i32 = BUILD_VECTOR t2, t3
  // produces
  t5: v2i32 = extract_subvector t...., Constant:i64<1>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D132517

Files:
  llvm/lib/Target/AArch64/AArch64ISelLowering.cpp


Index: llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
===================================================================
--- llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -16136,6 +16136,7 @@
                                          TargetLowering::DAGCombinerInfo &DCI,
                                          SelectionDAG &DAG) {
   SDLoc DL(N);
+  EVT VT = N->getValueType(0);
 
   // A build vector of two extracted elements is equivalent to an
   // extract subvector where the inner vector is any-extended to the
@@ -16146,10 +16147,17 @@
 
   // For now, only consider the v2i32 case, which arises as a result of
   // legalization.
-  if (N->getValueType(0) != MVT::v2i32)
+  if (VT != MVT::v2i32)
     return SDValue();
 
   SDValue Elt0 = N->getOperand(0), Elt1 = N->getOperand(1);
+  uint64_t Idx = Elt0->getConstantOperandVal(1);
+
+  // EXTRACT_SUBVECTOR requires that Idx be a constant multiple of ResultType's
+  // known minimum vector length.
+  if (Idx % VT.getVectorMinNumElements() != 0)
+    return SDValue();
+
   // Reminder, EXTRACT_VECTOR_ELT has the effect of any-extending to its VT.
   if (Elt0->getOpcode() == ISD::EXTRACT_VECTOR_ELT &&
       Elt1->getOpcode() == ISD::EXTRACT_VECTOR_ELT &&
@@ -16159,7 +16167,7 @@
       // Both EXTRACT_VECTOR_ELT from same vector...
       Elt0->getOperand(0) == Elt1->getOperand(0) &&
       // ... and contiguous. First element's index +1 == second element's index.
-      Elt0->getConstantOperandVal(1) + 1 == Elt1->getConstantOperandVal(1)) {
+      Idx + 1 == Elt1->getConstantOperandVal(1)) {
     SDValue VecToExtend = Elt0->getOperand(0);
     EVT ExtVT = VecToExtend.getValueType().changeVectorElementType(MVT::i32);
     if (!DAG.getTargetLoweringInfo().isTypeLegal(ExtVT))


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