1 files changed, 19 insertions, 2 deletions
diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 255ff4cbd..7cdc048f7 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -380,16 +380,32 @@ struct OptimizeInstructions : public WalkerPass<PostWalker<OptimizeInstructions,
         //     Y
         //     X
         //   )
+        // Note that this reorders X and Y, so we need to be careful about that.
         if (auto* sub = binary->left->dynCast<Binary>()) {
           if (sub->op == SubInt32) {
             if (auto* subZero = sub->left->dynCast<Const>()) {
               if (subZero->value.geti32() == 0) {
-                sub->left = binary->right;
-                return sub;
+                if (EffectAnalyzer::canReorder(getPassOptions(), sub->right, binary->right)) {
+                  sub->left = binary->right;
+                  return sub;
+                }
               }
             }
           }
         }
+        // The flip case is even easier, as no reordering occurs:
+        //   (i32.add
+        //     Y
+        //     (i32.sub
+        //       (i32.const 0)
+        //       X
+        //     )
+        //   )
+        // =>
+        //   (i32.sub
+        //     Y
+        //     X
+        //   )
         if (auto* sub = binary->right->dynCast<Binary>()) {
           if (sub->op == SubInt32) {
             if (auto* subZero = sub->left->dynCast<Const>()) {
@@ -761,6 +777,7 @@ private:
           constant += value * mul;
           constants.push_back(c);
         }
+        return;
       } else if (auto* binary = curr->dynCast<Binary>()) {
         if (binary->op == AddInt32) {
           seek(binary->left, mul);