diff --git a/mlir/lib/Transforms/Utils/RegionUtils.cpp b/mlir/lib/Transforms/Utils/RegionUtils.cpp
--- a/mlir/lib/Transforms/Utils/RegionUtils.cpp
+++ b/mlir/lib/Transforms/Utils/RegionUtils.cpp
@@ -464,10 +464,6 @@
   /// A set of operand+index pairs that correspond to operands that need to be
   /// replaced by arguments when the cluster gets merged.
   std::set<std::pair<int, int>> operandsToMerge;
-
-  /// A map of operations with external uses to a replacement within the leader
-  /// block.
-  DenseMap<Operation *, Operation *> opsToReplace;
 };
 } // end anonymous namespace
 
@@ -480,7 +476,6 @@
 
   // A set of operands that mismatch between the leader and the new block.
   SmallVector<std::pair<int, int>, 8> mismatchedOperands;
-  SmallVector<std::pair<Operation *, Operation *>, 2> newOpsToReplace;
   auto lhsIt = leaderBlock->begin(), lhsE = leaderBlock->end();
   auto rhsIt = blockData.block->begin(), rhsE = blockData.block->end();
   for (int opI = 0; lhsIt != lhsE && rhsIt != rhsE; ++lhsIt, ++rhsIt, ++opI) {
@@ -519,9 +514,16 @@
         return failure();
     }
 
-    // If the rhs has external uses, it will need to be replaced.
-    if (rhsIt->isUsedOutsideOfBlock(mergeBlock))
-      newOpsToReplace.emplace_back(&*rhsIt, &*lhsIt);
+    // If the lhs or rhs has external uses, the blocks cannot be merged as the
+    // merged version of this operation will not be either the lhs or rhs
+    // alone (thus semantically incorrect), but some mix dependening on which
+    // block preceeded this.
+    // TODO allow merging of operations when one block does not dominate the
+    // other
+    if (rhsIt->isUsedOutsideOfBlock(mergeBlock) ||
+        lhsIt->isUsedOutsideOfBlock(leaderBlock)) {
+      return failure();
+    }
   }
   // Make sure that the block sizes are equivalent.
   if (lhsIt != lhsE || rhsIt != rhsE)
@@ -529,7 +531,6 @@
 
   // If we get here, the blocks are equivalent and can be merged.
   operandsToMerge.insert(mismatchedOperands.begin(), mismatchedOperands.end());
-  opsToReplace.insert(newOpsToReplace.begin(), newOpsToReplace.end());
   blocksToMerge.insert(blockData.block);
   return success();
 }
@@ -561,10 +562,6 @@
         !llvm::all_of(blocksToMerge, ableToUpdatePredOperands))
       return failure();
 
-    // Replace any necessary operations.
-    for (std::pair<Operation *, Operation *> &it : opsToReplace)
-      it.first->replaceAllUsesWith(it.second);
-
     // Collect the iterators for each of the blocks to merge. We will walk all
     // of the iterators at once to avoid operand index invalidation.
     SmallVector<Block::iterator, 2> blockIterators;
diff --git a/mlir/test/Transforms/canonicalize-block-merge.mlir b/mlir/test/Transforms/canonicalize-block-merge.mlir
--- a/mlir/test/Transforms/canonicalize-block-merge.mlir
+++ b/mlir/test/Transforms/canonicalize-block-merge.mlir
@@ -174,26 +174,24 @@
   return
 }
 
-// Check that properly handles back edges and the case where a value from one
-// block is used in another.
+// Check that properly handles back edges.
 
 // CHECK-LABEL: func @mismatch_loop(
 // CHECK-SAME: %[[ARG:.*]]: i1, %[[ARG2:.*]]: i1
 func @mismatch_loop(%cond : i1, %cond2 : i1) {
+  // CHECK-NEXT: %[[LOOP_CARRY:.*]] = "foo.op"
   // CHECK: cond_br %{{.*}}, ^bb1(%[[ARG2]] : i1), ^bb2
 
+  %cond3 = "foo.op"() : () -> (i1)
   cond_br %cond, ^bb2, ^bb3
 
 ^bb1:
   // CHECK: ^bb1(%[[ARG3:.*]]: i1):
-  // CHECK-NEXT: %[[LOOP_CARRY:.*]] = "foo.op"
   // CHECK-NEXT: cond_br %[[ARG3]], ^bb1(%[[LOOP_CARRY]] : i1), ^bb2
 
-  %ignored = "foo.op"() : () -> (i1)
   cond_br %cond3, ^bb1, ^bb3
 
 ^bb2:
-  %cond3 = "foo.op"() : () -> (i1)
   cond_br %cond2, ^bb1, ^bb3
 
 ^bb3:
@@ -224,3 +222,32 @@
   store %true, %arg2[] : memref<i1>
   br ^bb1
 }
+
+// Check that it is illegal to merge blocks containing an operand
+// with an external user. Incorrectly performing the optimization
+// anyways will result in print(merged, merged) rather than
+// distinct operands.
+func private @print(%arg0: i32, %arg1: i32)
+// CHECK-LABEL: @nomerge
+func @nomerge(%arg0: i32, %i: i32) {
+  %c1_i32 = constant 1 : i32
+  %icmp = cmpi "slt", %i, %arg0 : i32
+  cond_br %icmp, ^bb2, ^bb3
+
+^bb2:  // pred: ^bb1
+  %ip1 = addi %i, %c1_i32 : i32
+  br ^bb4(%ip1 : i32)
+
+^bb7:  // pred: ^bb5
+  %jp1 = addi %j, %c1_i32 : i32
+  br ^bb4(%jp1 : i32)
+
+^bb4(%j: i32):  // 2 preds: ^bb2, ^bb7
+  %jcmp = cmpi "slt", %j, %arg0 : i32
+// CHECK-NOT:  call @print(%[[arg1:.+]], %[[arg1]])
+  call @print(%j, %ip1) : (i32, i32) -> ()
+  cond_br %jcmp, ^bb7, ^bb3
+
+^bb3:  // pred: ^bb1
+  return
+}