[wasm] Remove --wasm-shared-engine runtime flag.

With the recent removal of the --wasm-shared-code flag, it became
effectively impossible to turn off this flag. Hence its functionality
became mandatory and the ability to turn off sharing of {WasmEngine}
process-wide has to be removed as well.

R=clemensb@chromium.org

Change-Id: I7c25e909e49134a226d6a9fe9c42f0ecd9d02a69
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1864935
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64322}

src/flags/flag-definitions.h

@@ -730,9 +730,6 @@ DEFINE_BOOL(wasm_no_stack_checks, false,
 DEFINE_BOOL(wasm_math_intrinsics, true,
             "intrinsify some Math imports into wasm")
 
-DEFINE_BOOL(wasm_shared_engine, true,
-            "shares one wasm engine between all isolates within a process")
-DEFINE_IMPLICATION(future, wasm_shared_engine)
 DEFINE_BOOL(wasm_trap_handler, true,
             "use signal handlers to catch out of bounds memory access in wasm"
             " (currently Linux x86_64 only)")

src/runtime/runtime-test.cc

@@ -1231,7 +1231,7 @@ RUNTIME_FUNCTION(Runtime_CloneWasmModule) {
   CONVERT_ARG_HANDLE_CHECKED(WasmModuleObject, module_object, 0);
 
   Handle<WasmModuleObject> new_module_object =
-      wasm::WasmEngine::GetWasmEngine()->ImportNativeModule(
+      isolate->wasm_engine()->ImportNativeModule(
           isolate, module_object->shared_native_module());
 
   return *new_module_object;

src/wasm/wasm-engine.cc

@@ -959,20 +959,17 @@ DEFINE_LAZY_LEAKY_OBJECT_GETTER(std::shared_ptr<WasmEngine>,
 
 // static
 void WasmEngine::InitializeOncePerProcess() {
-  if (!FLAG_wasm_shared_engine) return;
   *GetSharedWasmEngine() = std::make_shared<WasmEngine>();
 }
 
 // static
 void WasmEngine::GlobalTearDown() {
-  if (!FLAG_wasm_shared_engine) return;
   GetSharedWasmEngine()->reset();
 }
 
 // static
 std::shared_ptr<WasmEngine> WasmEngine::GetWasmEngine() {
-  if (FLAG_wasm_shared_engine) return *GetSharedWasmEngine();
+  return *GetSharedWasmEngine();
-  return std::make_shared<WasmEngine>();
 }
 
 // {max_mem_pages} is declared in wasm-limits.h.

src/wasm/wasm-engine.h

@@ -215,8 +215,9 @@ class V8_EXPORT_PRIVATE WasmEngine {
   static void InitializeOncePerProcess();
   static void GlobalTearDown();
 
-  // Constructs a WasmEngine instance. Depending on whether we are sharing
+  // Returns a reference to the WasmEngine shared by the entire process. Try to
-  // engines this might be a pointer to a new instance or to a shared one.
+  // use {Isolate::wasm_engine} instead if it is available, which encapsulates
+  // engine lifetime decisions during Isolate bootstrapping.
   static std::shared_ptr<WasmEngine> GetWasmEngine();
 
  private:

test/cctest/wasm/test-wasm-serialization.cc

@@ -272,7 +272,6 @@ TEST(BlockWasmCodeGenAtDeserialization) {
 }
 
 UNINITIALIZED_TEST(CompiledWasmModulesTransfer) {
-  FlagScope<bool> flag_scope_engine(&FLAG_wasm_shared_engine, true);
   i::wasm::WasmEngine::InitializeOncePerProcess();
   v8::internal::AccountingAllocator allocator;
   Zone zone(&allocator, ZONE_NAME);

test/mjsunit/regress/wasm/regress-8533.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --wasm-shared-engine --no-wasm-disable-structured-cloning --allow-natives-syntax --experimental-wasm-threads
+// Flags: --no-wasm-disable-structured-cloning --allow-natives-syntax --experimental-wasm-threads
 
 load('test/mjsunit/wasm/wasm-module-builder.js');
 

test/mjsunit/wasm/worker-module.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --wasm-shared-engine --no-wasm-disable-structured-cloning
+// Flags: --no-wasm-disable-structured-cloning
 
 load("test/mjsunit/wasm/wasm-module-builder.js");