[Wasm GC] Fix RSE on non-nullable locals (#3943)

RedundantSetElimination checks if a local already has the default
value when we assign the default to it. For a non-nullable local, however,
there is no initial value - it cannot be used before it is assigned to. So
we just need to skip such locals, and not assume they contain a default
value we can compare against (we would assert on trying to create a
"zero" for such a non-nullable type).

Fixes #3942

2 files changed, 22 insertions, 2 deletions

diff --git a/src/passes/RedundantSetElimination.cpp b/src/passes/RedundantSetElimination.cpp
index 8b0914ecf..ccf6e6ac2 100644
--- a/src/passes/RedundantSetElimination.cpp
+++ b/src/passes/RedundantSetElimination.cpp
@@ -184,14 +184,19 @@ struct RedundantSetElimination
       if (block.get() == entry) {
         // params are complex values we can't optimize; vars are zeros
         for (Index i = 0; i < numLocals; i++) {
+          auto type = func->getLocalType(i);
           if (func->isParam(i)) {
 #ifdef RSE_DEBUG
             std::cout << "new param value for " << i << '\n';
 #endif
             start[i] = getUniqueValue();
+          } else if (type.isRef() && !type.isNullable()) {
+#ifdef RSE_DEBUG
+            std::cout << "new unique value for non-nullable " << i << '\n';
+#endif
+            start[i] = getUniqueValue();
           } else {
-            start[i] =
-              getLiteralValue(Literal::makeZeros(func->getLocalType(i)));
+            start[i] = getLiteralValue(Literal::makeZeros(type));
           }
         }
       } else {
diff --git a/test/lit/passes/rse-gc.wast b/test/lit/passes/rse-gc.wast
new file mode 100644
index 000000000..7bea92260
--- /dev/null
+++ b/test/lit/passes/rse-gc.wast
@@ -0,0 +1,15 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt %s --rse --enable-gc-nn-locals -all -S -o - | filecheck %s
+
+(module
+ ;; CHECK:      (func $10
+ ;; CHECK-NEXT:  (local $1 (ref func))
+ ;; CHECK-NEXT:  (nop)
+ ;; CHECK-NEXT: )
+ (func $10
+  ;; A non-nullable local. The pass should ignore it (as we cannot optimize
+  ;; anything here anyhow: the code must assign to the local before reading from
+  ;; it, so no sets can be redundant in that sense).
+  (local $1 (ref func))
+ )
+)