!fixup address latest comments, thanks!

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -168,7 +168,7 @@ class VPLane {
   static VPLane getFirstLane() { return VPLane(0, VPLane::Kind::First); }
 
   static VPLane getLaneFromEnd(const ElementCount &VF, unsigned Offset) {
-    assert(Offset <= VF.getKnownMinValue() &&
+    assert(Offset > 0 && Offset <= VF.getKnownMinValue() &&
            "trying to extract with invalid offset");
     unsigned LaneOffset = VF.getKnownMinValue() - Offset;
     Kind LaneKind;
@@ -1188,9 +1188,10 @@ class VPInstruction : public VPRecipeWithIRFlags {
     BranchOnCount,
     BranchOnCond,
     ComputeReductionResult,
-    // Takes the VPValue to extract from as first operand and the lane to
-    // extract from as second operand. The second operand must be a constant and
-    // <= VF when extracting from a vector or <= UF when extracting from a
+    // Takes the VPValue to extract from as first operand and the lane or part
+    // to extract as second operand, counting from the end starting with 1 for
+    // last. The second operand must be a positive constant and <= VF when
+    // extracting from a vector or <= UF when extracting from an unrolled
     // scalar.
     ExtractFromEnd,
     LogicalAnd, // Non-poison propagating logical And.
@@ -1230,10 +1231,6 @@ class VPInstruction : public VPRecipeWithIRFlags {
   /// value for lane \p Lane.
   Value *generatePerLane(VPTransformState &State, const VPIteration &Lane);
 
-  /// Returns true if this VPInstruction converts a vector value to a scalar,
-  /// e.g. by performing a reduction or extracting a lane.
-  bool isVectorToScalar() const;
-
 #if !defined(NDEBUG)
   /// Return true if the VPInstruction is a floating point math operation, i.e.
   /// has fast-math flags.
@@ -1342,6 +1339,10 @@ class VPInstruction : public VPRecipeWithIRFlags {
     };
     llvm_unreachable("switch should return");
   }
+
+  /// Returns true if this VPInstruction produces a scalar value from a vector,
+  /// e.g. by performing a reduction or extracting a lane.
+  bool isVectorToScalar() const;
 };
 
 /// VPWidenRecipe is a recipe for producing a copy of vector type its
@@ -3672,8 +3673,7 @@ inline bool isUniformAfterVectorization(VPValue *VPV) {
   if (auto *GEP = dyn_cast<VPWidenGEPRecipe>(Def))
     return all_of(GEP->operands(), isUniformAfterVectorization);
   if (auto *VPI = dyn_cast<VPInstruction>(Def))
-    return VPI->getOpcode() == VPInstruction::ComputeReductionResult ||
-           VPI->getOpcode() == VPInstruction::ExtractFromEnd;
+    return VPI->isVectorToScalar();
   return false;
 }
 } // end namespace vputils

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -565,7 +565,7 @@ Value *VPInstruction::generatePerPart(VPTransformState &State, unsigned Part) {
 
     auto *CI = cast<ConstantInt>(getOperand(1)->getLiveInIRValue());
     unsigned Offset = CI->getZExtValue();
-
+    assert(Offset > 0 && "Offset from end must be positive");
     Value *Res;
     if (State.VF.isVector()) {
       assert(Offset <= State.VF.getKnownMinValue() &&