[Assignment Tracking][NFC] Avoid doing some work when maps have same keys

[Orlando]

Summary

Where the new checks have been added, SymmetricDifference - still being built - contains entries for variables present in A and not in B.
If SymmetricDifference is empty at this point it means the variables (map keys) in A are a subset of those in B, so if A and B are the same size then we know they're identical.

This reduces the number of instructions retired building some of the CTMark projects in a ReleaseLTO-g configuration (geomean change -0.05% with the best improvement being -0.24% for tramp3d-v4)

[StephenTozer]

Small suggestion below, but LGTM with or without it.

llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

1644

I think this (and the check below) could be alternatively implemented as if (SymmetricDifference.empty() && A.size() == B.size()) return Join;, since the only thing left in the function outside of this block is the asserts which look to trivially always hold if this block isn't executed - but just a suggestion, if you disagree then no issues merging.

[Orlando]

llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

1644

Done! Thanks.

llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

if (R == B.end()) {
       continue;
     }
     // There is an entry for Var in both, join it.
     Join[Var] = joinKind(Loc, R->second);
   }
   unsigned IntersectSize = Join.size();
   (void)IntersectSize;
 
+  // Check if A and B contain the same variables.
+  if (SymmetricDifference.empty() && A.size() == B.size())
+    return Join;
+
   // Add the elements in B with variables that are not in A into
   // SymmetricDifference.
   for (const auto &Pair : B) {
     VariableID Var = Pair.first;
     if (A.count(Var) == 0)
       SymmetricDifference.push_back(Var);
   }
 
   // Add SymmetricDifference elements to Join and return the result.
   for (const auto &Var : SymmetricDifference)
     Join.insert({Var, LocKind::None});
 
   assert(Join.size() == (IntersectSize + SymmetricDifference.size()));
   assert(Join.size() >= A.size() && Join.size() >= B.size());
   return Join;
 }
if (R == B.end()) {
       continue;
     }
     // There is an entry for Var in both, join it.
     Join[Var] = joinAssignment(AV, R->second);
   }
   unsigned IntersectSize = Join.size();
   (void)IntersectSize;
 
+  // Check if A and B contain the same variables.
+  if (SymmetricDifference.empty() && A.size() == B.size())
+    return Join;
+
   // Add the elements in B with variables that are not in A into
   // SymmetricDifference.
   for (const auto &Pair : B) {
     VariableID Var = Pair.first;
     if (A.count(Var) == 0)
       SymmetricDifference.push_back(Var);
   }