[wasm] Add a fast path to {FindJumpTablesForRegion}

This method is called in the critical section in {PublishCode}, hence
performance is important here. Since most modules will only have a
single code space anyway, we can use the main jump table in the vast
majority of cases, and avoid taking a lock and iterating another data
structure.

R=ahaas@chromium.org

Bug: v8:10330
Change-Id: I18cbd3b127172963ccc9ec576a0985e874da7865
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2104891
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66721}

src/wasm/wasm-code-manager.cc

@@ -1275,7 +1275,13 @@ void NativeModule::AddCodeSpace(
         WasmCode::kRuntimeStubCount, num_function_slots);
   }
 
-  if (is_first_code_space) main_jump_table_ = jump_table;
+  if (is_first_code_space) {
+    // This can be updated and accessed without locks, since the addition of the
+    // first code space happens during initialization of the {NativeModule},
+    // where no concurrent accesses are possible.
+    main_jump_table_ = jump_table;
+    main_far_jump_table_ = far_jump_table;
+  }
 
   base::MutexGuard guard(&allocation_mutex_);
   code_space_data_.push_back(CodeSpaceData{region, jump_table, far_jump_table});
@@ -1364,6 +1370,18 @@ NativeModule::JumpTablesRef NativeModule::FindJumpTablesForRegion(
         table_end > code_region.begin() ? table_end - code_region.begin() : 0);
     return max_distance < kMaxWasmCodeSpaceSize;
   };
+
+  // Fast path: Try to use {main_jump_table_} and {main_far_jump_table_}.
+  // Access to these fields is possible without locking, since these fields are
+  // initialized on construction of the {NativeModule}.
+  if (main_far_jump_table_ && jump_table_usable(main_far_jump_table_) &&
+      (main_jump_table_ == nullptr || jump_table_usable(main_jump_table_))) {
+    return {
+        main_jump_table_ ? main_jump_table_->instruction_start() : kNullAddress,
+        main_far_jump_table_->instruction_start()};
+  }
+
+  // Otherwise, take the mutex and look for another suitable jump table.
   base::MutexGuard guard(&allocation_mutex_);
   for (auto& code_space_data : code_space_data_) {
     DCHECK_IMPLIES(code_space_data.jump_table, code_space_data.far_jump_table);

src/wasm/wasm-code-manager.h

@@ -679,10 +679,14 @@ class V8_EXPORT_PRIVATE NativeModule final {
   // {WireBytesStorage}, held by background compile tasks.
   std::shared_ptr<OwnedVector<const uint8_t>> wire_bytes_;
 
-  // Jump table used by external calls (from JS). Wasm calls use one of the jump
-  // tables stored in {code_space_data_}.
+  // The first allocated jump table. Always used by external calls (from JS).
+  // Wasm calls might use one of the other jump tables stored in
+  // {code_space_data_}.
   WasmCode* main_jump_table_ = nullptr;
 
+  // The first allocated far jump table.
+  WasmCode* main_far_jump_table_ = nullptr;
+
   // Lazy compile stub table, containing entries to jump to the
   // {WasmCompileLazy} builtin, passing the function index.
   WasmCode* lazy_compile_table_ = nullptr;