[wasm-gc] Experiment: ref.cast_nop_static

This adds a non-standard, unsafe instruction for performance
experiments: ref.cast_nop_static behaves like ref.cast_static
as far as static types are concerned, but emits no code.

Bug: v8:7748
Change-Id: Ic5797a941146a06d7c6ff249d8e29919145d8ea1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3639206Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80471}

src/wasm/function-body-decoder-impl.h

@@ -1881,6 +1881,7 @@ class WasmDecoder : public Decoder {
           case kExprRttCanon:
           case kExprRefTestStatic:
           case kExprRefCastStatic:
+          case kExprRefCastNopStatic:
           case kExprBrOnCastStatic:
           case kExprBrOnCastStaticFail: {
             IndexImmediate<validate> imm(decoder, pc + length, "type index");
@@ -2053,6 +2054,7 @@ class WasmDecoder : public Decoder {
           case kExprArrayLen:
           case kExprRefTestStatic:
           case kExprRefCastStatic:
+          case kExprRefCastNopStatic:
           case kExprBrOnCastStatic:
           case kExprBrOnCastStaticFail:
             return {1, 1};
@@ -4500,6 +4502,35 @@ class WasmFullDecoder : public WasmDecoder<validate, decoding_mode> {
         Push(value);
         return opcode_length;
       }
+      case kExprRefCastNopStatic: {
+        // Temporary non-standard instruction, for performance experiments.
+        if (!VALIDATE(this->enabled_.has_ref_cast_nop())) {
+          this->DecodeError(
+              "Invalid opcode 0xfb48 (enable with "
+              "--experimental-wasm-ref-cast-nop)");
+          return 0;
+        }
+        IndexImmediate<validate> imm(this, this->pc_ + opcode_length,
+                                     "type index");
+        if (!this->ValidateType(this->pc_ + opcode_length, imm)) return 0;
+        opcode_length += imm.length;
+        Value obj = Peek(0);
+        if (!VALIDATE(IsSubtypeOf(obj.type, kWasmFuncRef, this->module_) ||
+                      IsSubtypeOf(obj.type,
+                                  ValueType::Ref(HeapType::kData, kNullable),
+                                  this->module_) ||
+                      obj.type.is_bottom())) {
+          PopTypeError(0, obj, "subtype of (ref null func) or (ref null data)");
+          return 0;
+        }
+        Value value = CreateValue(ValueType::Ref(
+            imm.index,
+            obj.type.is_bottom() ? kNonNullable : obj.type.nullability()));
+        CALL_INTERFACE_IF_OK_AND_REACHABLE(Forward, obj, &value);
+        Drop(obj);
+        Push(value);
+        return opcode_length;
+      }
       case kExprRefCast:
       case kExprRefCastStatic: {
         NON_CONST_ONLY

src/wasm/wasm-feature-flags.h

@@ -36,6 +36,7 @@
     "assume ref.cast always succeeds and skip the related type check "         \
     "(unsafe)",                                                                \
     false)                                                                     \
+  V(ref_cast_nop, "enable unsafe ref.cast_nop instruction", false)             \
   V(skip_null_checks,                                                          \
     "skip null checks for call.ref and array and struct operations (unsafe)",  \
     false)                                                                     \

src/wasm/wasm-opcodes-inl.h

@@ -422,6 +422,7 @@ constexpr const char* WasmOpcodes::OpcodeName(WasmOpcode opcode) {
     CASE_OP(RefTestStatic, "ref.test_static")
     CASE_OP(RefCast, "ref.cast")
     CASE_OP(RefCastStatic, "ref.cast_static")
+    CASE_OP(RefCastNopStatic, "ref.cast_nop_static")
     CASE_OP(BrOnCast, "br_on_cast")
     CASE_OP(BrOnCastStatic, "br_on_cast_static")
     CASE_OP(BrOnCastFail, "br_on_cast_fail")

src/wasm/wasm-opcodes.h

@@ -707,6 +707,7 @@ bool V8_EXPORT_PRIVATE IsJSCompatibleSignature(const FunctionSig* sig,
   V(RefCastStatic, 0xfb45, _)                                                  \
   V(BrOnCastStatic, 0xfb46, _)                                                 \
   V(BrOnCastStaticFail, 0xfb47, _)                                             \
+  V(RefCastNopStatic, 0xfb48, _)                                               \
   V(RefIsFunc, 0xfb50, _)                                                      \
   V(RefIsData, 0xfb51, _)                                                      \
   V(RefIsI31, 0xfb52, _)                                                       \

test/mjsunit/wasm/gc-experiments.js

+// Copyright 2022 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --experimental-wasm-gc --experimental-wasm-ref-cast-nop
+
+d8.file.execute("test/mjsunit/wasm/wasm-module-builder.js");
+
+(function TestRefCastNop() {
+  var builder = new WasmModuleBuilder();
+  let struct = builder.addStruct([makeField(kWasmI32, true)]);
+
+  builder.addFunction("main", kSig_i_i)
+    .addLocals(wasmOptRefType(kWasmDataRef), 1)
+    .addBody([
+      kExprLocalGet, 0,
+      kGCPrefix, kExprStructNew, struct,
+      kExprLocalSet, 1,
+      kExprLocalGet, 1,
+      kGCPrefix, kExprRefCastNopStatic, struct,
+      kGCPrefix, kExprStructGet, struct, 0,
+  ]).exportFunc();
+
+  var instance = builder.instantiate();
+  assertEquals(42, instance.exports.main(42));
+})();

test/mjsunit/wasm/wasm-module-builder.js

@@ -505,6 +505,7 @@ let kExprRefTestStatic = 0x44;
 let kExprRefCastStatic = 0x45;
 let kExprBrOnCastStatic = 0x46;
 let kExprBrOnCastStaticFail = 0x47;
+let kExprRefCastNopStatic = 0x48;
 let kExprRefIsFunc = 0x50;
 let kExprRefIsData = 0x51;
 let kExprRefIsI31 = 0x52;