fix handling of unreachable br values in merge-blocks

Author: kripken

src/passes/MergeBlocks.cpp

@@ -122,6 +122,11 @@ struct BreakValueDropper : public ControlFlowWalker<BreakValueDropper> {
     if (curr->value && curr->name == origin) {
       Builder builder(*getModule());
       auto* value = curr->value;
+      if (value->type == unreachable) {
+        // the break isn't even reached
+        replaceCurrent(value);
+        return;
+      }
       curr->value = nullptr;
       curr->finalize();
       replaceCurrent(builder.makeSequence(builder.makeDrop(value), curr));
@@ -130,7 +135,8 @@ struct BreakValueDropper : public ControlFlowWalker<BreakValueDropper> {
 
   void visitDrop(Drop* curr) {
     // if we dropped a br_if whose value we removed, then we are now dropping a (block (drop value) (br_if)) with type none, which does not need a drop
-    if (curr->value->type == none) {
+    // likewise, unreachable does not need to be dropped, so we just leave drops of concrete values
+    if (!isConcreteWasmType(curr->value->type)) {
       replaceCurrent(curr->value);
     }
   }

test/passes/merge-blocks.txt

@@ -1,6 +1,7 @@
 (module
  (type $0 (func))
  (type $1 (func (param i32)))
+ (type $2 (func (result i32)))
  (memory $0 0)
  (func $drop-block (type $0)
   (block $block
@@ -75,4 +76,15 @@
    )
   )
  )
+ (func $drop-unreachable-br_if (type $2) (result i32)
+  (block $label$0 (result i32)
+   (block $label$2
+    (drop
+     (br $label$0
+      (i32.const 538976371)
+     )
+    )
+   )
+  )
+ )
 )

test/passes/merge-blocks.wast

@@ -53,5 +53,21 @@
       )
     )
   )
+  (func $drop-unreachable-br_if (result i32)
+    (block $label$0 (result i32)
+     (drop
+      (block $label$2
+       (drop
+        (br_if $label$2
+         (br $label$0
+          (i32.const 538976371)
+         )
+         (i32.const 1918987552)
+        )
+       )
+      )
+     )
+    )
+  )
 )