Ctor-eval: Don't stop evaluating when an imported global isn't used (#8168)

Resolves the TODO is global-get-init.wast. Previously if a global was
imported it would stop us from evaluating almost anything. After this
change, an imported global doesn't prevent us from optimizing as long as
the global isn't referenced in the constructor that we're targeting.

Part of #8180.

Author: stevenfontanella

src/tools/wasm-ctor-eval.cpp

@@ -71,11 +71,17 @@ class EvallingModuleRunner;
 
 class EvallingImportResolver : public ImportResolver {
 public:
-  EvallingImportResolver() = default;
+  EvallingImportResolver() : stubLiteral({Literal(0)}){};
 
+  // Return an unused stub value. We throw FailToEvalException on reading any
+  // imported globals. We ignore the type and return an i32 literal since some
+  // types can't be created anyway (e.g. ref none).
   Literals* getGlobalOrNull(ImportNames name, Type type) const override {
-    throw FailToEvalException("Accessed imported global");
+    return &stubLiteral;
   }
+
+private:
+  mutable Literals stubLiteral;
 };
 
 class EvallingModuleRunner : public ModuleRunnerBase<EvallingModuleRunner> {
@@ -557,6 +563,9 @@ struct CtorEvalExternalInterface : EvallingModuleRunner::ExternalInterface {
     wasm->updateMaps();
 
     for (auto& oldGlobal : oldGlobals) {
+      if (oldGlobal->imported()) {
+        continue;
+      }
       // Serialize the global's value. While doing so, pass in the name of this
       // global, as we may be able to reuse the global as the defining global
       // for the value. See getSerialization() for more details.

test/ctor-eval/global-get-init.wast

@@ -1,8 +1,16 @@
 (module
-  (import "import" "global" (global $imported i32))
-  (func $test1 (export "test1")
-    ;; This should be safe to eval in theory, but the imported global stops us,
-    ;; so this function will not be optimized out.
-    ;; TODO: perhaps if we never use that global that is ok?
+  ;; an imported global that isn't accessed doesn't stop us from optimizing
+  (import "import" "global" (global $imported (ref i31)))
+  (global $g (mut i32) (i32.const 0))
+  (func $setg (export "setg")
+    (drop (i32.const 1))
+    (global.set $g
+      (i32.add (i32.const 1) (i32.const 2))
+    )
+  )
+
+  (func $keepalive (export "keepalive") (result i32)
+    ;; Keep the global alive so we can see its value.
+    (global.get $g)
   )
 )

test/ctor-eval/global-get-init.wast.ctors

@@ -1 +1 @@
-test1
+setg

test/ctor-eval/global-get-init.wast.out

@@ -1,7 +1,8 @@
 (module
- (type $0 (func))
- (export "test1" (func $test1))
- (func $test1 (type $0)
-  (nop)
+ (type $0 (func (result i32)))
+ (global $g (mut i32) (i32.const 3))
+ (export "keepalive" (func $keepalive))
+ (func $keepalive (type $0) (result i32)
+  (global.get $g)
  )
 )