[VPlan] Use SetVector for VPExternalDefs to prevent non-determinism.

[huihuiz]

Summary

Use SetVector instead of SmallPtrSet for external definitions created for VPlan.
Doing this can help avoid non-determinism caused by iterating over unordered containers.

This bug was found with reverse iteration turning on,
--extra-llvm-cmake-variables="-DLLVM_REVERSE_ITERATION=ON".
Failing LLVM-Unit test VPRecipeTest.dump.

[fhahn]

llvm/lib/Transforms/Vectorize/VPlan.h

1770

Should this be SetVector? I think we still want to avoid adding duplicates.

[huihuiz]

Thank you for the feedback!
Addressed review comments.

[MaskRay]

LGTM.

llvm/lib/Transforms/Vectorize/VPlan.h

class VPlan {
   /// Holds the name of the VPlan, for printing.
   std::string Name;
 
   /// Holds all the external definitions created for this VPlan.
   // TODO: Introduce a specific representation for external definitions in
   // VPlan. External definitions must be immutable and hold a pointer to its
   // underlying IR that will be used to implement its structural comparison
   // (operators '==' and '<').
-  SmallPtrSet<VPValue *, 16> VPExternalDefs;
+  SetVector<VPValue *> VPExternalDefs;
 
   /// Represents the backedge taken count of the original loop, for folding
   /// the tail.
   VPValue *BackedgeTakenCount = nullptr;
 
   /// Holds a mapping between Values and their corresponding VPValue inside
   /// VPlan.
   Value2VPValueTy Value2VPValue;
public:
   bool hasVF(ElementCount VF) { return VFs.count(VF); }
 
   const std::string &getName() const { return Name; }
 
   void setName(const Twine &newName) { Name = newName.str(); }
 
   /// Add \p VPVal to the pool of external definitions if it's not already
   /// in the pool.
-  void addExternalDef(VPValue *VPVal) {
-    VPExternalDefs.insert(VPVal);
-  }
+  void addExternalDef(VPValue *VPVal) { VPExternalDefs.insert(VPVal); }
 
   void addVPValue(Value *V) {
     assert(V && "Trying to add a null Value to VPlan");
     assert(!Value2VPValue.count(V) && "Value already exists in VPlan");
     VPValue *VPV = new VPValue(V);
     Value2VPValue[V] = VPV;
     VPValuesToFree.push_back(VPV);
   }