[Wasm Exceptions] Fix StackIR writing of nested delegates (#3599)

We were missing a pop of catchIndexStack at a Delegate. It ends the scope,
so it should do that, like TryEnd does.

Found by emscripten-core/emscripten#13485 on -O2.

2 files changed, 7 insertions, 0 deletions

diff --git a/src/wasm-stack.h b/src/wasm-stack.h
index 7f94573fa..5af9adbce 100644
--- a/src/wasm-stack.h
+++ b/src/wasm-stack.h
@@ -350,6 +350,8 @@ template<typename SubType> void BinaryenIRWriter<SubType>::visitTry(Try* curr) {
   }
   if (curr->isDelegate()) {
     emitDelegate(curr);
+    // Note that when we emit a delegate we do not need to also emit a scope
+    // ending, as the delegate ends the scope.
   } else {
     emitScopeEnd(curr);
   }
diff --git a/src/wasm/wasm-stack.cpp b/src/wasm/wasm-stack.cpp
index b6c0392ca..4c4e4e9d1 100644
--- a/src/wasm/wasm-stack.cpp
+++ b/src/wasm/wasm-stack.cpp
@@ -1925,6 +1925,9 @@ void BinaryInstWriter::emitCatchAll(Try* curr) {
 }
 
 void BinaryInstWriter::emitDelegate(Try* curr) {
+  // The delegate ends the scope in effect, and pops the try's name. Note that
+  // the getBreakIndex is intentionally after that pop, as the delegate cannot
+  // target its own try.
   assert(!breakStack.empty());
   breakStack.pop_back();
   o << int8_t(BinaryConsts::Delegate)
@@ -2333,6 +2336,8 @@ void StackIRToBinaryWriter::write() {
       }
       case StackInst::Delegate: {
         writer.emitDelegate(inst->origin->cast<Try>());
+        // Delegates end the try, like a TryEnd.
+        catchIndexStack.pop_back();
         break;
       }
       default: