LocalCSE: ignore traps (#4085)

If we replace

A
A
A

with

(local.set A)
(local.get)
(local.get)

then it is ok for A to trap (so long as it does so deterministically), as if
it does trap then the first appearance will do so, and the others not
be reached anyhow.

This helps GC code as often there are repeated struct.gets and such that
may trap.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/passes/LocalCSE.cpp b/src/passes/LocalCSE.cpp
index b55da976c..1ae77d155 100644
--- a/src/passes/LocalCSE.cpp
+++ b/src/passes/LocalCSE.cpp
@@ -422,6 +422,15 @@ struct Checker
       // away repeated apperances if it has any.
       EffectAnalyzer effects(options, getModule()->features, curr);
 
+      // We can ignore traps here, as we replace a repeating expression with a
+      // single appearance of it, a store to a local, and gets in the other
+      // locations, and so if the expression traps then the first appearance -
+      // that we keep around - would trap, and the others are never reached
+      // anyhow. (The other checks we perform here, including invalidation and
+      // determinism, will ensure that either all of the appearances trap, or
+      // none of them.)
+      effects.trap = false;
+
       // We also cannot optimize away something that is intrinsically
       // nondeterministic: even if it has no side effects, if it may return a
       // different result each time, then we cannot optimize away repeats.