3 files changed, 78 insertions, 6 deletions
diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index acbf39447..0b47d3398 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -620,16 +620,33 @@ struct OptimizeInstructions : public WalkerPass<PostWalker<OptimizeInstructions,
             std::swap(iff->ifTrue, iff->ifFalse);
           }
         }
-        if (ExpressionAnalyzer::equal(iff->ifTrue, iff->ifFalse)) {
+        if (iff->condition->type != unreachable && ExpressionAnalyzer::equal(iff->ifTrue, iff->ifFalse)) {
           // sides are identical, fold
-          if (!EffectAnalyzer(getPassOptions(), iff->condition).hasSideEffects()) {
+          // if we can replace the if with one arm, and no side effects in the condition, do that
+          auto needCondition = EffectAnalyzer(getPassOptions(), iff->condition).hasSideEffects();
+          auto typeIsIdentical = iff->ifTrue->type == iff->type;
+          if (typeIsIdentical && !needCondition) {
             return iff->ifTrue;
           } else {
             Builder builder(*getModule());
-            return builder.makeSequence(
-              builder.makeDrop(iff->condition),
-              iff->ifTrue
-            );
+            if (typeIsIdentical) {
+              return builder.makeSequence(
+                builder.makeDrop(iff->condition),
+                iff->ifTrue
+              );
+            } else {
+              // the types diff. as the condition is reachable, that means the if must be
+              // concrete while the arm is not
+              assert(isConcreteWasmType(iff->type) && iff->ifTrue->type == unreachable);
+              // emit a block with a forced type
+              auto* ret = builder.makeBlock();
+              if (needCondition) {
+                ret->list.push_back(builder.makeDrop(iff->condition));
+              }
+              ret->list.push_back(iff->ifTrue);
+              ret->finalize(iff->type);
+              return ret;
+            }
           }
         }
       }
diff --git a/test/passes/optimize-instructions.txt b/test/passes/optimize-instructions.txt
index 6a99ee106..aae0724a0 100644
--- a/test/passes/optimize-instructions.txt
+++ b/test/passes/optimize-instructions.txt
@@ -1913,6 +1913,40 @@
     )
    )
   )
+  (drop
+   (block (result i32)
+    (i32.add
+     (get_local $1)
+     (unreachable)
+    )
+   )
+  )
+  (drop
+   (block (result i32)
+    (drop
+     (tee_local $0
+      (get_local $1)
+     )
+    )
+    (i32.add
+     (get_local $1)
+     (unreachable)
+    )
+   )
+  )
+  (drop
+   (if (result i32)
+    (unreachable)
+    (i32.add
+     (get_local $1)
+     (unreachable)
+    )
+    (i32.add
+     (get_local $1)
+     (unreachable)
+    )
+   )
+  )
  )
  (func $select-parallel (type $4) (param $0 i32) (param $1 i32)
   (drop
diff --git a/test/passes/optimize-instructions.wast b/test/passes/optimize-instructions.wast
index c26aad196..f84be8330 100644
--- a/test/passes/optimize-instructions.wast
+++ b/test/passes/optimize-instructions.wast
@@ -2321,6 +2321,27 @@
         (i32.add (get_local $1) (i32.const 1))
       )
     )
+    (drop
+      (if (result i32)
+        (get_local $0)
+        (i32.add (get_local $1) (unreachable)) ;; folding them would change the type of the if
+        (i32.add (get_local $1) (unreachable))
+      )
+    )
+    (drop
+      (if (result i32)
+        (tee_local $0 (get_local $1)) ;; side effects!
+        (i32.add (get_local $1) (unreachable)) ;; folding them would change the type of the if
+        (i32.add (get_local $1) (unreachable))
+      )
+    )
+    (drop
+      (if (result i32)
+        (unreachable) ;; !!!
+        (i32.add (get_local $1) (unreachable)) ;; folding them would change the type of the if
+        (i32.add (get_local $1) (unreachable))
+      )
+    )
   )
   (func $select-parallel (param $0 i32) (param $1 i32)
     (drop