properly handle merging of blocks with concrete unreachable elements in the middle

Author: kripken

src/passes/MergeBlocks.cpp

@@ -220,16 +220,20 @@ static void optimizeBlock(Block* curr, Module* module, PassOptions& passOptions)
       }
       for (auto item : child->list) {
         merged.push_back(item);
-        if (item->type == unreachable) {
-          // we don't need anything else from the child,
-          // and it is always valid to end on an unreachable,
-          // so stop there (otherwise, we need to be careful)
-          break;
-        }
       }
       for (size_t j = i + 1; j < curr->list.size(); j++) {
         merged.push_back(curr->list[j]);
       }
+      // if we merged a concrete element in the middle, drop it
+      if (!merged.empty()) {
+        auto* last = merged.back();
+        for (auto*& item : merged) {
+          if (item != last && isConcreteWasmType(item->type)) {
+            Builder builder(*module);
+            item = builder.makeDrop(item);
+          }
+        }
+      }
       curr->list.swap(merged);
       more = true;
       changed = true;

test/passes/remove-unused-names_merge-blocks.txt

@@ -854,6 +854,9 @@
  )
  (func $concrete_finale_in_unreachable (type $5) (result f64)
   (unreachable)
+  (drop
+   (f64.const 6.322092475576799e-96)
+  )
   (f64.const -1)
  )
  (func $dont-move-unreachable (type $3)
@@ -910,5 +913,19 @@
   (return
    (i32.const 21536)
   )
+  (block $label$15
+   (br $label$15)
+  )
+  (i32.const 19299)
+ )
+ (func $remove-br-after-unreachable (type $3)
+  (block $label$9
+   (drop
+    (block
+     (return)
+     (br $label$9)
+    )
+   )
+  )
  )
 )

test/passes/remove-unused-names_merge-blocks.wast

@@ -1081,5 +1081,17 @@
    )
   )
  )
+ (func $remove-br-after-unreachable
+  (block $label$9
+   (drop
+    (block
+     (block
+      (return)
+      (br $label$9) ;; removing this leads to the block becoming unreachable
+     )
+    )
+   )
+  )
+ )
 )