wasm2js: Avoid emitting non-JS code during opt (#5378)

As noted in #4806, trying to optimize past level 0 can result in
passes emitting non-JS code, which is then unable to be converted during
final output.

This commit creates a new targetJS option in PassOptions, which can
be checked inside each pass where non-JS code might be emitted.

This commit initially adds that logic to OptimizeInstructions, where
this issue was first noticed.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/wasm2js.h b/src/wasm2js.h
index 982184510..db394c36c 100644
--- a/src/wasm2js.h
+++ b/src/wasm2js.h
@@ -173,8 +173,11 @@ public:
     // and store would need to do a check. Given that, we can just ignore
     // implicit traps like those when optimizing. (When not optimizing, it's
     // nice to see codegen that matches wasm more precisely.)
+    // It is also important to prevent the optimizer from adding new things that
+    // require additional lowering, as we could hit a cycle.
     if (options.optimizeLevel > 0) {
       options.ignoreImplicitTraps = true;
+      options.targetJS = true;
     }
   }