diff --git a/llvm/lib/Support/IntEqClasses.cpp b/llvm/lib/Support/IntEqClasses.cpp
--- a/llvm/lib/Support/IntEqClasses.cpp
+++ b/llvm/lib/Support/IntEqClasses.cpp
@@ -47,6 +47,9 @@
       eca = EC[a];
     }
 
+  // Make sure that eca is actually the leader of the equivalence class (this
+  // can happen if a and b were already in the same equivalence class).
+  eca = findLeader(eca);
   return eca;
 }
 
diff --git a/llvm/unittests/ADT/IntEqClassesTest.cpp b/llvm/unittests/ADT/IntEqClassesTest.cpp
--- a/llvm/unittests/ADT/IntEqClassesTest.cpp
+++ b/llvm/unittests/ADT/IntEqClassesTest.cpp
@@ -103,4 +103,17 @@
   EXPECT_EQ(0u, ec.findLeader(9));
 }
 
+TEST(IntEqClasses, JoinReturnsLeader) {
+  IntEqClasses ec(10);
+  EXPECT_EQ(1u, ec.join(1, 2));
+  EXPECT_EQ(1u, ec.join(1, 3));
+  EXPECT_EQ(0u, ec.join(0, 1));
+  EXPECT_EQ(0u, ec.findLeader(1));
+  // At this point, EC[2] = EC[3] = 1, but EC[1] = 0. Be sure that trying to
+  // join EC[2] and EC[3] returns the correct leader, 0, and not their common
+  // node 1.
+  EXPECT_EQ(0u, ec.join(2, 3));
+  EXPECT_EQ(0u, ec.findLeader(2));
+}
+
 } // end anonymous namespace