[Exceptions] Optimize in CodePushing even with exceptions thrown (#5028)

We had some concerns about this not working in the past, but thinking about it
now, I believe it is safe to do. Specifically, a throw is either like a break or a return -
either it jumps out to an outer scope (like a break) or it jumps out of the function
(like a return), and both breaks and returns have already been handled here.

This change has some nice effects on J2Wasm output, where there are quite a
lot of throws, which we can now optimize around.

1 files changed, 11 insertions, 1 deletions

diff --git a/src/ir/effects.h b/src/ir/effects.h
index d3ae9ba20..ad3ab235d 100644
--- a/src/ir/effects.h
+++ b/src/ir/effects.h
@@ -881,9 +881,19 @@ public:
     return effects;
   }
 
-  void ignoreBranches() {
+  // Ignores all forms of control flow transfers: breaks, returns, and
+  // exceptions. (Note that traps are not considered relevant here - a trap does
+  // not just transfer control flow, but can be seen as halting the entire
+  // program.)
+  //
+  // This function matches transfersControlFlow(), that is, after calling this
+  // method transfersControlFlow() will always return false.
+  void ignoreControlFlowTransfers() {
     branchesOut = false;
     breakTargets.clear();
+    throws_ = false;
+    delegateTargets.clear();
+    assert(!transfersControlFlow());
   }
 
 private: