[wasm] Fix reference type global initialization by index.

This fixes initialization of reference type global variables (i.e.
anyref and except_ref) based on an index of another global. It extends
the existing support to exception types, fixes the logic, and also fixes
a missing write barrier.

R=ahaas@chromium.org
TEST=mjsunit/wasm/exceptions-global
BUG=v8:8091

Change-Id: Ia91f1ea03be24fadf3023a5acdd073badb8dcd93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1539581
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60525}

src/wasm/module-instantiate.cc

@@ -1199,25 +1199,21 @@ void InstanceBuilder::InitGlobals() {
                              SKIP_WRITE_BARRIER);
         break;
       case WasmInitExpr::kGlobalIndex: {
-        if (global.type == ValueType::kWasmAnyRef) {
-          DCHECK(enabled_.anyref);
-          int other_offset =
-              module_->globals[global.init.val.global_index].offset;
-
-          tagged_globals_->set(global.offset,
-                               tagged_globals_->get(other_offset),
-                               SKIP_WRITE_BARRIER);
-        }
         // Initialize with another global.
         uint32_t new_offset = global.offset;
         uint32_t old_offset =
             module_->globals[global.init.val.global_index].offset;
         TRACE("init [globals+%u] = [globals+%d]\n", global.offset, old_offset);
-        size_t size = (global.type == kWasmI64 || global.type == kWasmF64)
-                          ? sizeof(double)
-                          : sizeof(int32_t);
-        memcpy(raw_buffer_ptr(untagged_globals_, new_offset),
-               raw_buffer_ptr(untagged_globals_, old_offset), size);
+        if (ValueTypes::IsReferenceType(global.type)) {
+          DCHECK(enabled_.anyref || enabled_.eh);
+          tagged_globals_->set(new_offset, tagged_globals_->get(old_offset));
+        } else {
+          size_t size = (global.type == kWasmI64 || global.type == kWasmF64)
+                            ? sizeof(double)
+                            : sizeof(int32_t);
+          memcpy(raw_buffer_ptr(untagged_globals_, new_offset),
+                 raw_buffer_ptr(untagged_globals_, old_offset), size);
+        }
         break;
       }
       case WasmInitExpr::kNone:

test/mjsunit/wasm/exceptions-global.js

@@ -136,3 +136,19 @@ load("test/mjsunit/wasm/wasm-module-builder.js");
   let exception2 = mutable_global.value = "an even fancier exception";
   assertThrowsEquals(() => instance.exports.rethrow_except_ref(), exception2);
 })();*/
+
+// Test custom initialization index for a global "except_ref" variable.
+(function TestGlobalExceptRefInitIndex() {
+  print(arguments.callee.name);
+  let builder = new WasmModuleBuilder();
+  let g1_index = builder.addImportedGlobal("m", "exn", kWasmExceptRef);
+  let g2 = builder.addGlobal(kWasmExceptRef);
+  g2.init_index = g1_index;  // Initialize {g2} to equal {g1}.
+  builder.addFunction('push_and_return_except_ref', kSig_e_v)
+      .addBody([kExprGetGlobal, g2.index])
+      .exportFunc();
+  let exception = { x: "my fancy exception" };
+  let instance = builder.instantiate({ "m": { "exn": exception }});
+
+  assertSame(exception, instance.exports.push_and_return_except_ref());
+})();