[Wasm GC] Enable GlobalTypeOptimization (#4305)

Now that all known issues with that pass are fixed, enable it by
default. This adds it in a place that seems to make sense on j2wasm,
but in general multiple cycles of optimization will be needed.

This adds a test showing that we run this pass and that it helps
ConstantFieldPropagation by running before it.

2 files changed, 50 insertions, 1 deletions

diff --git a/src/passes/pass.cpp b/src/passes/pass.cpp
index fd31dada8..3b0faa6c3 100644
--- a/src/passes/pass.cpp
+++ b/src/passes/pass.cpp
@@ -524,7 +524,12 @@ void PassRunner::addDefaultGlobalOptimizationPrePasses() {
   }
   if (wasm->features.hasGC() && getTypeSystem() == TypeSystem::Nominal &&
       options.optimizeLevel >= 2) {
-    // TODO: investigate enabling --gto and --remove-module-elements before cfp
+    // Global type optimization can remove fields that are not needed, which can
+    // remove ref.funcs that were once assigned to vtables but are no longer
+    // needed, which can allow more code to be removed globally. After those,
+    // constant field propagation can be more effective.
+    addIfNoDWARFIssues("gto");
+    addIfNoDWARFIssues("remove-unused-module-elements");
     addIfNoDWARFIssues("cfp");
   }
 }
diff --git a/test/lit/passes/gto_and_cfp_in_O.wast b/test/lit/passes/gto_and_cfp_in_O.wast
new file mode 100644
index 000000000..dcff13a8c
--- /dev/null
+++ b/test/lit/passes/gto_and_cfp_in_O.wast
@@ -0,0 +1,44 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
+
+;; RUN: foreach %s %t wasm-opt -O -all --nominal -S -o - | filecheck %s
+
+;; Test that -O, with nominal typing + GC enabled, will run global type
+;; optimization in conjunction with constant field propagation etc.
+
+(module
+  (type $struct (struct_subtype (field (mut funcref)) (field (mut i32)) data))
+
+  (global $glob (ref $struct) (struct.new $struct
+    (ref.func $by-ref)
+    (i32.const 100)
+  ))
+
+  (func $by-ref
+    ;; This function is kept alive by the reference in $glob. After we remove
+    ;; the field that the funcref is written to, we remove the funcref, which
+    ;; means this function can be removed.
+    ;;
+    ;; Once it is removed, this write no longer exists, and does not hamper
+    ;; constant field propagation from inferring the value of the i32 field.
+    (struct.set $struct 1
+      (global.get $glob)
+      (i32.const 200)
+    )
+  )
+
+  ;; CHECK:      (type $none_=>_i32 (func_subtype (result i32) func))
+
+  ;; CHECK:      (export "main" (func $main))
+
+  ;; CHECK:      (func $main (; has Stack IR ;) (result i32)
+  ;; CHECK-NEXT:  (i32.const 100)
+  ;; CHECK-NEXT: )
+  (func $main (export "main") (result i32)
+    ;; After all the above optimizations, we can infer that $main should simply
+    ;; return 100.
+    (struct.get $struct 1
+      (global.get $glob)
+    )
+  )
+)
+