diff --git a/llvm/lib/Analysis/IRSimilarityIdentifier.cpp b/llvm/lib/Analysis/IRSimilarityIdentifier.cpp
--- a/llvm/lib/Analysis/IRSimilarityIdentifier.cpp
+++ b/llvm/lib/Analysis/IRSimilarityIdentifier.cpp
@@ -526,19 +526,13 @@
   for (Value *V : SourceOperands) {
     ArgVal = SourceValueToNumberMapping.find(V)->second;
 
+    // Instead of finding a current mapping, we attempt to insert a set.
     std::tie(ValueMappingIt, WasInserted) = CurrentSrcTgtNumberMapping.insert(
         std::make_pair(ArgVal, TargetValueNumbers));
 
-    // Instead of finding a current mapping, we inserted a set.  This means a
-    // mapping did not exist for the source Instruction operand, it has no
-    // current constraints we need to check.
-    if (WasInserted)
-      continue;
-
-    // If a mapping already exists for the source operand to the values in the
-    // other IRSimilarityCandidate we need to iterate over the items in other
-    // IRSimilarityCandidate's Instruction to determine whether there is a valid
-    // mapping of Value to Value.
+    // We need to iterate over the items in other IRSimilarityCandidate's
+    // Instruction to determine whether there is a valid mapping of
+    // Value to Value.
     DenseSet<unsigned> NewSet;
     for (unsigned &Curr : ValueMappingIt->second)
       // If we can find the value in the mapping, we add it to the new set.
@@ -558,7 +552,6 @@
     if (ValueMappingIt->second.size() != 1)
       continue;
 
-
     unsigned ValToRemove = *ValueMappingIt->second.begin();
     // When there is only one item left in the mapping for and operand, remove
     // the value from the other operands.  If it results in there being no
diff --git a/llvm/unittests/Analysis/IRSimilarityIdentifierTest.cpp b/llvm/unittests/Analysis/IRSimilarityIdentifierTest.cpp
--- a/llvm/unittests/Analysis/IRSimilarityIdentifierTest.cpp
+++ b/llvm/unittests/Analysis/IRSimilarityIdentifierTest.cpp
@@ -2619,6 +2619,29 @@
   }
 }
 
+// This test ensures that when the first instruction in a sequence is
+// a commutative instruction with the same value (mcomm_inst_same_val), but the
+// corresponding instruction (comm_inst_diff_val) is not, we mark the regions
+// and not similar.
+TEST(IRSimilarityIdentifier, CommutativeSameValueFirstMisMatch) {
+  StringRef ModuleString = R"(
+                          define void @v_1_0(i64 %v_33) {
+                            entry:
+                              %comm_inst_same_val = mul i64 undef, undef
+                              %add = add i64 %comm_inst_same_val, %v_33
+                              %comm_inst_diff_val = mul i64 0, undef
+                              %mul.i = add i64 %comm_inst_diff_val, %comm_inst_diff_val
+                              unreachable
+                            })";
+  LLVMContext Context;
+  std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleString);
+
+  std::vector<std::vector<IRSimilarityCandidate>> SimilarityCandidates;
+  getSimilarities(*M, SimilarityCandidates);
+
+  ASSERT_TRUE(SimilarityCandidates.size() == 0);
+}
+
 // This test makes sure that intrinsic functions that are marked commutative
 // are still treated as non-commutative since they are function calls.
 TEST(IRSimilarityIdentifier, IntrinsicCommutative) {