Effects.h: Fix RefAs (#4312)

We marked that as only trapping if the input as nullable. But ref.as_func
will trap if it isn't a func, for example.

We could in theory try to check if a trap is possible, like checking if the
input is already non-nullable or already a function, etc., but we have
optimization passes to get rid of RefAs when they are not needed
anyhow, so there is no point to duplicate that here.

3 files changed, 77 insertions, 3 deletions

diff --git a/src/ir/effects.h b/src/ir/effects.h
index f24f0ac3f..ced8e63cb 100644
--- a/src/ir/effects.h
+++ b/src/ir/effects.h
@@ -725,9 +725,15 @@ private:
     }
     void visitRefAs(RefAs* curr) {
       // traps when the arg is not valid
-      if (curr->value->type.isNullable()) {
-        parent.implicitTrap = true;
-      }
+      parent.implicitTrap = true;
+      // Note: We could be more precise here and report the lack of a possible
+      // trap if the input is non-nullable (and also of the right kind for
+      // RefAsFunc etc.). However, we have optimization passes that will
+      // remove a RefAs in such a case (in OptimizeInstructions, and also
+      // Vacuum in trapsNeverHappen mode), so duplicating that code here would
+      // only help until the next time those optimizations run. As a tradeoff,
+      // we keep the code here simpler, but it does mean another optimization
+      // cycle may be needed in some cases.
     }
   };
 
diff --git a/test/lit/passes/vacuum-gc.wast b/test/lit/passes/vacuum-gc.wast
new file mode 100644
index 000000000..8ca40bee8
--- /dev/null
+++ b/test/lit/passes/vacuum-gc.wast
@@ -0,0 +1,51 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt %s --vacuum -all -S -o - | filecheck %s
+
+(module
+  ;; CHECK:      (func $drop-ref-as (param $x anyref)
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.as_non_null
+  ;; CHECK-NEXT:    (local.get $x)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.as_func
+  ;; CHECK-NEXT:    (local.get $x)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.as_data
+  ;; CHECK-NEXT:    (local.get $x)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.as_i31
+  ;; CHECK-NEXT:    (local.get $x)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $drop-ref-as (param $x anyref)
+    ;; Without -tnh, we must assume all ref_as* can have a trap effect, and so
+    ;; we cannot remove anything here.
+    (drop
+      (ref.as_non_null
+        (local.get $x)
+      )
+    )
+    (drop
+      (ref.as_func
+        (local.get $x)
+      )
+    )
+    (drop
+      (ref.as_data
+        (local.get $x)
+      )
+    )
+    (drop
+      (ref.as_i31
+        (local.get $x)
+      )
+    )
+  )
+)
diff --git a/test/lit/passes/vacuum-tnh.wast b/test/lit/passes/vacuum-tnh.wast
index 4820a55bc..83cb298c2 100644
--- a/test/lit/passes/vacuum-tnh.wast
+++ b/test/lit/passes/vacuum-tnh.wast
@@ -24,6 +24,23 @@
       )
     )
 
+    ;; Other ref.as* as well.
+    (drop
+      (ref.as_func
+        (local.get $y)
+      )
+    )
+    (drop
+      (ref.as_data
+        (local.get $y)
+      )
+    )
+    (drop
+      (ref.as_i31
+        (local.get $y)
+      )
+    )
+
     ;; Ignore unreachable code.
     (drop
       (unreachable)