handle merging blocks with items after an unreachable, that if merged would be invalid. stop on the unreachable, it is easier and better

3 files changed, 25 insertions, 9 deletions

diff --git a/src/passes/MergeBlocks.cpp b/src/passes/MergeBlocks.cpp
index 3627a5b0e..1ef34117e 100644
--- a/src/passes/MergeBlocks.cpp
+++ b/src/passes/MergeBlocks.cpp
@@ -214,21 +214,18 @@ static void optimizeBlock(Block* curr, Module* module, PassOptions& passOptions)
       }
       if (!child) continue;
       if (child->name.is()) continue; // named blocks can have breaks to them (and certainly do, if we ran RemoveUnusedNames and RemoveUnusedBrs)
-      if (child->type == unreachable) {
-        // an unreachable block can have a concrete final element (which is never reached)
-        if (!child->list.empty()) {
-          if (isConcreteWasmType(child->list.back()->type)) {
-            // just remove it
-            child->list.pop_back();
-          }
-        }
-      }
       ExpressionList merged(module->allocator);
       for (size_t j = 0; j < i; j++) {
         merged.push_back(curr->list[j]);
       }
       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]);
diff --git a/test/passes/remove-unused-names_merge-blocks.txt b/test/passes/remove-unused-names_merge-blocks.txt
index ff0d8b912..81386b10a 100644
--- a/test/passes/remove-unused-names_merge-blocks.txt
+++ b/test/passes/remove-unused-names_merge-blocks.txt
@@ -906,4 +906,9 @@
    )
   )
  )
+ (func $merging-with-unreachable-in-middle (type $4) (result i32)
+  (return
+   (i32.const 21536)
+  )
+ )
 )
diff --git a/test/passes/remove-unused-names_merge-blocks.wast b/test/passes/remove-unused-names_merge-blocks.wast
index 2911203d1..bf1dc1fda 100644
--- a/test/passes/remove-unused-names_merge-blocks.wast
+++ b/test/passes/remove-unused-names_merge-blocks.wast
@@ -1068,4 +1068,18 @@
    )
   )
  )
+ (func $merging-with-unreachable-in-middle (result i32)
+  (block $label$1 (result i32)
+   (block
+    (return
+     (i32.const 21536)
+    )
+    (block $label$15
+     (br $label$15)
+    )
+    (i32.const 19299)
+   )
+  )
+ )
 )
+