[Wasm GC] Add experimental support for non-nullable locals (#3932)

This adds a new feature flag, GCNNLocals that enables support for
non-nullable locals. No validation is applied to check that they are
actually assigned before their use yet - this just allows experimentation
to begin.

This feature is not enabled by default even with -all. If we enabled it,
then it would take effect in most of our tests and likely confuse current
users as well. Instead, the flag must be opted in explicitly using
--enable-gc-nn-locals. That is, this is an experimental feature flag,
and as such must be explicitly enabled. (Once the spec stabilizes,
we will remove the feature anyhow when we implement the
final status of non-nullability. )

1 files changed, 27 insertions, 0 deletions

diff --git a/test/lit/passes/ssa-gc-nn-locals.wast b/test/lit/passes/ssa-gc-nn-locals.wast
new file mode 100644
index 000000000..149a0c5c7
--- /dev/null
+++ b/test/lit/passes/ssa-gc-nn-locals.wast
@@ -0,0 +1,27 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt %s -all --ssa --enable-gc-nn-locals -S -o - | filecheck %s
+
+(module
+ ;; CHECK:      (func $nn-locals
+ ;; CHECK-NEXT:  (local $x (ref func))
+ ;; CHECK-NEXT:  (local.set $x
+ ;; CHECK-NEXT:   (ref.func $nn-locals)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (local.get $x)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (local.get $x)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $nn-locals
+  ;; A non-nullable local
+  (local $x (ref func))
+  ;; Set the local, and get it later. The SSA pass will normally handle non-
+  ;; nullability using ref.as_non_null, but with --gc-nn-locals nothing should
+  ;; be done.
+  (local.set $x (ref.func $nn-locals))
+  (drop (local.get $x))
+  (drop (local.get $x))
+ )
+)