Vacuum trivial trys (#5046)

A try whose body throws, and does nothing else, and the try catches that
exception, can be removed.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/passes/Vacuum.cpp b/src/passes/Vacuum.cpp
index 6adab04e9..208e973da 100644
--- a/src/passes/Vacuum.cpp
+++ b/src/passes/Vacuum.cpp
@@ -365,6 +365,19 @@ struct Vacuum : public WalkerPass<ExpressionStackWalker<Vacuum>> {
       for (auto* catchBody : curr->catchBodies) {
         typeUpdater.noteRecursiveRemoval(catchBody);
       }
+      return;
+    }
+
+    // The try's body does throw. However, throwing may be the only thing it
+    // does, and if the try has a catch-all, then the entire try including
+    // children may have no effects. Note that this situation can only happen
+    // if we do have a catch-all, so avoid wasted work by checking that first.
+    // Also, we can't do this if a result is returned, so check the type.
+    if (curr->type == Type::none && curr->hasCatchAll() &&
+        !EffectAnalyzer(getPassOptions(), *getModule(), curr)
+           .hasUnremovableSideEffects()) {
+      typeUpdater.noteRecursiveRemoval(curr);
+      ExpressionManipulator::nop(curr);
     }
   }