[dart2wasm] Remove unused default states in br_tables in state machines

When generating the `br_table` for the state machines in `sync*` and
`async` functions, we currently generate a default state (required by
`br_table`) with an `unreachable` instruction.

Instead, remove the default state and make the last state in the
function the default state.

This saves a few instructions per `sync*` and `async` functions.

Issue: #60433
Change-Id: Ie89c3581304a97082628b150cc5604f9e4c2849e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419540
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

pkg/dart2wasm/lib/async.dart

@@ -226,11 +226,16 @@ class AsyncStateMachineCodeGenerator extends StateMachineCodeGenerator {
     // Switch on the target index.
     masterLoop = b.loop(const [], const []);
     labels = List.generate(targets.length, (_) => b.block()).reversed.toList();
-    w.Label defaultLabel = b.block();
+
+    // There should be at least two states: inner and after targets for the
+    // [FunctionNode].
+    assert(labels.length >= 2);
+
+    // Use the last target label as the default `br_table` target.
+    final brTableLabels = labels.sublist(0, labels.length - 1);
+    final brTableDefaultLabel = labels.last;
     b.local_get(targetIndexLocal);
-    b.br_table(labels, defaultLabel);
-    b.end(); // defaultLabel
-    b.unreachable();
+    b.br_table(brTableLabels, brTableDefaultLabel);
 
     // Initial state
     final StateTarget initialTarget = targets.first;

pkg/dart2wasm/lib/sync_star.dart

@@ -195,11 +195,16 @@ class SyncStarStateMachineCodeGenerator extends StateMachineCodeGenerator {
     // Switch on the target index.
     masterLoop = b.loop(const [], const [w.NumType.i32]);
     labels = List.generate(targets.length, (_) => b.block()).reversed.toList();
-    w.Label defaultLabel = b.block();
+
+    // There should be at least two states: inner and after targets for the
+    // [FunctionNode].
+    assert(labels.length >= 2);
+
+    // Use the last target label as the default `br_table` target.
+    final brTableLabels = labels.sublist(0, labels.length - 1);
+    final brTableDefaultLabel = labels.last;
     b.local_get(targetIndexLocal);
-    b.br_table(labels, defaultLabel);
-    b.end(); // defaultLabel
-    b.unreachable();
+    b.br_table(brTableLabels, brTableDefaultLabel);
 
     // Initial state, executed on first [moveNext] on the iterator.
     StateTarget initialTarget = targets.first;