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

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)

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D144621

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp b/llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
index 1b9740a..0b51382 100644
--- a/llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
+++ b/llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
@@ -1630,6 +1630,10 @@ AssignmentTrackingLowering::joinLocMap(const LocMap &A, const LocMap &B) {
   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) {
@@ -1721,6 +1725,10 @@ AssignmentTrackingLowering::joinAssignmentMap(const AssignmentMap &A,
   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) {