[Wasm GC] Full precompute support for GC (#3803)

The precompute pass ignored all reference types, but that was overly
pessimistic: we can precompute some of them, namely a null and a
reference to a function are fully precomputable, etc.

To allow that to work, add missing integration in getFallthrough as well.

With this, we can precompute quite a lot of field accesses in the existing
-Oz testcase, as can be seen from the output. That testcase runs
--fuzz-exec so it prints out all those logged values, proving they have
not changed.

1 files changed, 37 insertions, 0 deletions

diff --git a/test/lit/passes/precompute-gc.wast b/test/lit/passes/precompute-gc.wast
new file mode 100644
index 000000000..9402aa068
--- /dev/null
+++ b/test/lit/passes/precompute-gc.wast
@@ -0,0 +1,37 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt %s --remove-unused-names --precompute-propagate -all -S -o - \
+;; RUN:   | filecheck %s
+
+(module
+ ;; CHECK:      (func $test-fallthrough (result i32)
+ ;; CHECK-NEXT:  (local $x funcref)
+ ;; CHECK-NEXT:  (local.set $x
+ ;; CHECK-NEXT:   (block (result funcref)
+ ;; CHECK-NEXT:    (drop
+ ;; CHECK-NEXT:     (call $test-fallthrough)
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:    (ref.null func)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (i32.const 1)
+ ;; CHECK-NEXT: )
+ (func $test-fallthrough (result i32)
+  (local $x funcref)
+  (local.set $x
+   ;; the fallthrough value should be used. for that to be possible with a block
+   ;; we need for it not to have a name, which is why --remove-unused-names is
+   ;; run
+   (block (result (funcref))
+    ;; make a call so the block is not trivially removable
+    (drop
+     (call $test-fallthrough)
+    )
+    (ref.null func)
+   )
+  )
+  ;; the null in the local should be propagated to here
+  (ref.is_null
+   (local.get $x)
+  )
+ )
+)