[wasm] Move JumpBuffer inside StackMemory

The stack memory will be accessed through a global list later, so the
stack pointer should be accessible without the containing
WasmContinuationObject. This also saves some unnecessary allocations and
indirections already.

R=ahaas@chromium.org

Bug: v8:12191
Change-Id: Ic3d71ecadbb13b18f0440049527ba71d657589b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3312486
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78215}

src/diagnostics/objects-printer.cc

@@ -1853,8 +1853,7 @@ void WasmContinuationObject::WasmContinuationObjectPrint(std::ostream& os) {
   PrintHeader(os, "WasmContinuationObject");
   os << "\n - parent: " << parent();
   os << "\n - jmpbuf: " << jmpbuf();
-  os << "\n - managed_stack: " << managed_stack();
-  os << "\n - managed_jmpbuf: " << managed_jmpbuf();
+  os << "\n - stack: " << stack();
   os << "\n";
 }
 

src/runtime/runtime-wasm.cc

@@ -706,9 +706,8 @@ void SyncStackLimit(Isolate* isolate) {
   DisallowGarbageCollection no_gc;
   auto continuation = WasmContinuationObject::cast(
       *isolate->roots_table().slot(RootIndex::kActiveContinuation));
-  auto jmpbuf =
-      Managed<wasm::JumpBuffer>::cast(continuation.managed_jmpbuf()).get();
-  uintptr_t limit = reinterpret_cast<uintptr_t>(jmpbuf->stack_limit);
+  auto stack = Managed<wasm::StackMemory>::cast(continuation.stack()).get();
+  uintptr_t limit = reinterpret_cast<uintptr_t>(stack->jmpbuf()->stack_limit);
   isolate->stack_guard()->SetStackLimit(limit);
 }
 }  // namespace

src/wasm/stacks.h

@@ -45,6 +45,7 @@ class StackMemory {
 
   void* jslimit() { return limit_ + kJSLimitOffsetKB; }
   void* base() { return limit_ + size_; }
+  JumpBuffer* jmpbuf() { return &jmpbuf_; }
 
   // Track external memory usage for Managed<StackMemory> objects.
   size_t owned_size() { return sizeof(StackMemory) + (owned_ ? size_ : 0); }
@@ -68,6 +69,7 @@ class StackMemory {
   byte* limit_;
   size_t size_;
   bool owned_;
+  JumpBuffer jmpbuf_;
 };
 
 }  // namespace wasm

src/wasm/wasm-objects.cc

@@ -1747,18 +1747,14 @@ Handle<WasmContinuationObject> WasmContinuationObject::New(
     HeapObject parent) {
   Handle<WasmContinuationObject> result = Handle<WasmContinuationObject>::cast(
       isolate->factory()->NewStruct(WASM_CONTINUATION_OBJECT_TYPE));
-  auto jmpbuf = std::make_unique<wasm::JumpBuffer>();
-  jmpbuf->stack_limit = stack->jslimit();
-  jmpbuf->sp = stack->base();
-  result->set_jmpbuf(
-      *isolate->factory()->NewForeign(reinterpret_cast<Address>(jmpbuf.get())));
+  stack->jmpbuf()->stack_limit = stack->jslimit();
+  stack->jmpbuf()->sp = stack->base();
+  result->set_jmpbuf(*isolate->factory()->NewForeign(
+      reinterpret_cast<Address>(stack->jmpbuf())));
   size_t external_size = stack->owned_size();
   Handle<Foreign> managed_stack = Managed<wasm::StackMemory>::FromUniquePtr(
       isolate, external_size, std::move(stack));
-  Handle<Foreign> managed_jmpbuf = Managed<wasm::JumpBuffer>::FromUniquePtr(
-      isolate, sizeof(wasm::JumpBuffer), std::move(jmpbuf));
-  result->set_managed_stack(*managed_stack);
-  result->set_managed_jmpbuf(*managed_jmpbuf);
+  result->set_stack(*managed_stack);
   result->set_parent(parent);
   return result;
 }

src/wasm/wasm-objects.tq

@@ -90,9 +90,8 @@ extern class WasmIndirectFunctionTable extends Struct {
 }
 
 extern class WasmContinuationObject extends Struct {
-  managed_stack: Foreign;
-  managed_jmpbuf: Foreign;
-  jmpbuf: Foreign;  // Direct access to managed_jmpbuf's underlying pointer.
+  stack: Foreign;
+  jmpbuf: Foreign;  // Direct access to the stack's jump buffer.
   parent: WasmContinuationObject|Undefined;
 }