[debug] Introduce EphemeronHashTable root for locals block lists

This CL introduces a new root that is conceptually a

WeakMap<ScopeInfo, Tuple2<ScopeInfo, StringSet>>

Instead of storing the "locals block list" for debug-evaluate on the
`ScopeInfo` object directly, we will store it instead in a global
WeakMap. This enables us to re-use the "locals block lists" across
multiple debug-evaluate invocations without having to modify the
`ScopeInfo` of `SharedFunctionInfo`s.

R=bmeurer@chromium.org

Doc: https://bit.ly/chrome-devtools-debug-evaluate-design
Bug: chromium:1363561
Change-Id: Ib52f9abd97cf1c8fa3053ff3c61a6062c4b814be
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3902041
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83318}

src/execution/isolate.cc

@@ -94,6 +94,7 @@
 #include "src/objects/slots.h"
 #include "src/objects/smi.h"
 #include "src/objects/source-text-module-inl.h"
+#include "src/objects/string-set-inl.h"
 #include "src/objects/visitors.h"
 #include "src/profiler/heap-profiler.h"
 #include "src/profiler/tracing-cpu-profiler.h"
@@ -6004,6 +6005,45 @@ ExternalPointerHandle Isolate::GetOrCreateWaiterQueueNodeExternalPointer() {
 }
 #endif  // V8_COMPRESS_POINTERS
 
+void Isolate::LocalsBlockListCacheSet(Handle<ScopeInfo> scope_info,
+                                      Handle<ScopeInfo> outer_scope_info,
+                                      Handle<StringSet> locals_blocklist) {
+  Handle<EphemeronHashTable> cache;
+  if (heap()->locals_block_list_cache().IsEphemeronHashTable()) {
+    cache = handle(EphemeronHashTable::cast(heap()->locals_block_list_cache()),
+                   this);
+  } else {
+    CHECK(heap()->locals_block_list_cache().IsUndefined());
+    constexpr int kInitialCapacity = 8;
+    cache = EphemeronHashTable::New(this, kInitialCapacity);
+  }
+  DCHECK(cache->IsEphemeronHashTable());
+
+  Handle<Tuple2> outer_scope_info_and_locals = factory()->NewTuple2(
+      outer_scope_info, locals_blocklist, AllocationType::kYoung);
+
+  cache =
+      EphemeronHashTable::Put(cache, scope_info, outer_scope_info_and_locals);
+  heap()->set_locals_block_list_cache(*cache);
+}
+
+Object Isolate::LocalsBlockListCacheGet(Handle<ScopeInfo> scope_info) {
+  DisallowGarbageCollection no_gc;
+
+  if (!heap()->locals_block_list_cache().IsEphemeronHashTable()) {
+    return ReadOnlyRoots(this).the_hole_value();
+  }
+
+  Object maybe_outer_scope_info_and_locals =
+      EphemeronHashTable::cast(heap()->locals_block_list_cache())
+          .Lookup(scope_info);
+  if (maybe_outer_scope_info_and_locals.IsTheHole()) {
+    return maybe_outer_scope_info_and_locals;
+  }
+
+  return Tuple2::cast(maybe_outer_scope_info_and_locals).value2();
+}
+
 namespace {
 class DefaultWasmAsyncResolvePromiseTask : public v8::Task {
  public:

src/execution/isolate.h

@@ -2036,6 +2036,16 @@ class V8_EXPORT_PRIVATE Isolate final : private HiddenFactory {
   wasm::StackMemory*& wasm_stacks() { return wasm_stacks_; }
 #endif
 
+  // Access to the global "locals block list cache". Caches outer-stack
+  // allocated variables per ScopeInfo for debug-evaluate.
+  // We also store a strong reference to the outer ScopeInfo to keep all
+  // blocklists along a scope chain alive.
+  void LocalsBlockListCacheSet(Handle<ScopeInfo> scope_info,
+                               Handle<ScopeInfo> outer_scope_info,
+                               Handle<StringSet> locals_blocklist);
+  // Returns either `TheHole` or `StringSet`.
+  Object LocalsBlockListCacheGet(Handle<ScopeInfo> scope_info);
+
  private:
   explicit Isolate(std::unique_ptr<IsolateAllocator> isolate_allocator,
                    bool is_shared);

src/heap/setup-heap-internal.cc

@@ -878,6 +878,7 @@ void Heap::CreateInitialObjects() {
   set_feedback_vectors_for_profiling_tools(roots.undefined_value());
   set_pending_optimize_for_test_bytecode(roots.undefined_value());
   set_shared_wasm_memories(roots.empty_weak_array_list());
+  set_locals_block_list_cache(roots.undefined_value());
 #ifdef V8_ENABLE_WEBASSEMBLY
   set_active_continuation(roots.undefined_value());
   set_active_suspender(roots.undefined_value());

src/roots/roots.h

@@ -334,6 +334,8 @@ class Symbol;
     PendingOptimizeForTestBytecode)                                         \
   V(ArrayList, basic_block_profiling_data, BasicBlockProfilingData)         \
   V(WeakArrayList, shared_wasm_memories, SharedWasmMemories)                \
+  /* EphemeronHashTable for debug scopes (local debug evaluate) */          \
+  V(HeapObject, locals_block_list_cache, DebugLocalsBlockListCache)         \
   IF_WASM(V, HeapObject, active_continuation, ActiveContinuation)           \
   IF_WASM(V, HeapObject, active_suspender, ActiveSuspender)                 \
   IF_WASM(V, WeakArrayList, js_to_wasm_wrappers, JSToWasmWrappers)          \