Try allowing globals to be imported in ctor eval

Author: stevenfontanella

src/tools/wasm-ctor-eval.cpp

@@ -73,9 +73,15 @@ class EvallingImportResolver : public ImportResolver {
 public:
   EvallingImportResolver() = default;
 
+  // We throw FailToEvalException on reading these. Provide a stub value that
+  // passes import validation.
   Literals* getGlobalOrNull(ImportNames name, Type type) const override {
-    throw FailToEvalException("Accessed imported global");
+    auto [it, _] = stubLiterals.try_emplace(type, Literals({Literal(type)}));
+    return &it->second;
   }
+
+private:
+  mutable std::unordered_map<Type, Literals> stubLiterals;
 };
 
 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
+  ;; an imported global that isn't accessed doesn't stop us from optimizing
   (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?
+  (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)
  )
 )