[wasm] Optimize EvaluateInitExpression

This should mitigate some of the regressions introduced in
https://chromium-review.googlesource.com/c/v8/v8/+/2972910.
Changes:
- Use a single Zone for all init. expressions in InstanceBuilder.
- Specialize DecodeFunctionBody() for init. expressions.

Bug: v8:11895, chromium:1226551
Change-Id: Ie39f981efeaa89e57f8ccb68903c6e7cc1cb7f09
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3009465
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75597}

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

@@ -2271,12 +2271,20 @@ class WasmFullDecoder : public WasmDecoder<validate, decoding_mode> {
       DCHECK(control_.empty());
       control_.emplace_back(kControlBlock, 0, 0, this->pc_, kReachable);
       Control* c = &control_.back();
-      InitMerge(&c->start_merge, 0, [](uint32_t) -> Value { UNREACHABLE(); });
-      InitMerge(&c->end_merge,
-                static_cast<uint32_t>(this->sig_->return_count()),
-                [&](uint32_t i) {
-                  return Value{this->pc_, this->sig_->GetReturn(i)};
-                });
+      if (decoding_mode == kFunctionBody) {
+        InitMerge(&c->start_merge, 0, [](uint32_t) -> Value { UNREACHABLE(); });
+        InitMerge(&c->end_merge,
+                  static_cast<uint32_t>(this->sig_->return_count()),
+                  [&](uint32_t i) {
+                    return Value{this->pc_, this->sig_->GetReturn(i)};
+                  });
+      } else {
+        DCHECK_EQ(this->sig_->parameter_count(), 0);
+        DCHECK_EQ(this->sig_->return_count(), 1);
+        c->start_merge.arity = 0;
+        c->end_merge.arity = 1;
+        c->end_merge.vals.first = Value{this->pc_, this->sig_->GetReturn(0)};
+      }
       CALL_INTERFACE_IF_OK_AND_REACHABLE(StartFunctionBody, c);
     }
 

src/wasm/module-instantiate.cc

@@ -291,6 +291,7 @@ class InstanceBuilder {
   std::vector<Handle<WasmExceptionObject>> exception_wrappers_;
   Handle<WasmExportedFunction> start_function_;
   std::vector<SanitizedImport> sanitized_imports_;
+  Zone init_expr_zone_;
 
 // Helper routines to print out errors with imports.
 #define ERROR_THROWER_WITH_MESSAGE(TYPE)                                      \
@@ -434,7 +435,8 @@ InstanceBuilder::InstanceBuilder(Isolate* isolate,
       thrower_(thrower),
       module_object_(module_object),
       ffi_(ffi),
-      memory_buffer_(memory_buffer) {
+      memory_buffer_(memory_buffer),
+      init_expr_zone_(isolate_->allocator(), "init. expression zone") {
   sanitized_imports_.reserve(module_->import_table.size());
 }
 
@@ -1538,19 +1540,17 @@ T* InstanceBuilder::GetRawUntaggedGlobalPtr(const WasmGlobal& global) {
 WasmValue InstanceBuilder::EvaluateInitExpression(
     WireBytesRef init, ValueType expected,
     Handle<WasmInstanceObject> instance) {
-  AccountingAllocator allocator;
-  Zone zone(&allocator, "consume_init_expr");
   base::Vector<const byte> module_bytes =
       instance->module_object().native_module()->wire_bytes();
-  FunctionBody body(FunctionSig::Build(&zone, {expected}, {}), init.offset(),
-                    module_bytes.begin() + init.offset(),
+  FunctionBody body(FunctionSig::Build(&init_expr_zone_, {expected}, {}),
+                    init.offset(), module_bytes.begin() + init.offset(),
                     module_bytes.begin() + init.end_offset());
   WasmFeatures detected;
   // We use kFullValidation so we do not have to create another instance of
   // WasmFullDecoder, which would cost us >50Kb binary code size.
   WasmFullDecoder<Decoder::kFullValidation, InitExprInterface, kInitExpression>
-      decoder(&zone, module_, WasmFeatures::All(), &detected, body, module_,
-              isolate_, instance, tagged_globals_, untagged_globals_);
+      decoder(&init_expr_zone_, module_, WasmFeatures::All(), &detected, body,
+              module_, isolate_, instance, tagged_globals_, untagged_globals_);
 
   decoder.DecodeFunctionBody();