[wasm] Use a single source of truth for feedback vector size

The number of feedback vector slots is currently stored in the
{WasmFunction}, returned in the {WasmCompilationResult}, and implicitly
stored as the size of the {call_targets} vector in
{FunctionTypeFeedback}.

This CL uses the latter as the source of truth, encapsulated in a new
{NumFeedbackSlots} function. This can be updated when adding new kinds
of feedback that need additional slots.
For now, the implementation of {NumFeedbackSlots} requires taking a
mutex, which we can hopefully avoid when productionizing speculative
inlining. We also take the mutex on every Liftoff compilation, which
adds synchronization between concurrent compilation which we previously
tried very hard to avoid (because it introduced significant overhead for
eager compilation).

As a nice side-effect, this CL reduces the per-function overhead by 8
bytes, independent of enabled features.

R=jkummerow@chromium.org

Bug: v8:13209
Change-Id: I2fe5f7fe73154328032a3f0961e88d068c5d07ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899299Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83253}

src/wasm/baseline/liftoff-compiler.cc

@@ -556,12 +556,6 @@ class LiftoffCompiler {
     return __ GetTotalFrameSlotCountForGC();
   }
 
-  int GetFeedbackVectorSlots() const {
-    // The number of call instructions is capped by max function size.
-    static_assert(kV8MaxWasmFunctionSize < std::numeric_limits<int>::max() / 2);
-    return static_cast<int>(encountered_call_instructions_.size()) * 2;
-  }
-
   void unsupported(FullDecoder* decoder, LiftoffBailoutReason reason,
                    const char* detail) {
     DCHECK_NE(kSuccess, reason);
@@ -7709,7 +7703,6 @@ WasmCompilationResult ExecuteLiftoffCompilation(
   if (auto* debug_sidetable = compiler_options.debug_sidetable) {
     *debug_sidetable = debug_sidetable_builder->GenerateDebugSideTable();
   }
-  result.feedback_vector_slots = compiler->GetFeedbackVectorSlots();
 
   if (V8_UNLIKELY(v8_flags.trace_wasm_compilation_times)) {
     base::TimeDelta time = base::TimeTicks::Now() - start_time;

src/wasm/function-compiler.h

@@ -57,7 +57,6 @@ struct WasmCompilationResult {
   ExecutionTier result_tier;
   Kind kind = kFunction;
   ForDebugging for_debugging = kNoDebugging;
-  int feedback_vector_slots = 0;
 };
 
 class V8_EXPORT_PRIVATE WasmCompilationUnit final {

src/wasm/module-compiler.cc

@@ -1247,13 +1247,14 @@ bool CompileLazy(Isolate* isolate, Handle<WasmInstanceObject> instance,
   }
 
   // Allocate feedback vector if needed.
-  if (result.feedback_vector_slots > 0) {
+  int feedback_vector_slots = NumFeedbackSlots(module, func_index);
+  if (feedback_vector_slots > 0) {
     DCHECK(v8_flags.wasm_speculative_inlining);
     // We have to save the native_module on the stack, in case the allocation
     // triggers a GC and we need the module to scan WasmCompileLazy stack frame.
     *out_native_module = native_module;
-    Handle<FixedArray> vector = isolate->factory()->NewFixedArrayWithZeroes(
-        result.feedback_vector_slots);
+    Handle<FixedArray> vector =
+        isolate->factory()->NewFixedArrayWithZeroes(feedback_vector_slots);
     instance->feedback_vectors().set(
         declared_function_index(module, func_index), *vector);
   }

src/wasm/module-decoder-impl.h

@@ -787,7 +787,6 @@ class ModuleDecoderTemplate : public Decoder {
                                         import->index,  // func_index
                                         0,              // sig_index
                                         {0, 0},         // code
-                                        0,              // feedback slots
                                         true,           // imported
                                         false,          // exported
                                         false});        // declared
@@ -884,7 +883,6 @@ class ModuleDecoderTemplate : public Decoder {
                                     func_index,  // func_index
                                     0,           // sig_index
                                     {0, 0},      // code
-                                    0,           // feedback slots
                                     false,       // imported
                                     false,       // exported
                                     false});     // declared

src/wasm/module-instantiate.cc

@@ -744,8 +744,7 @@ MaybeHandle<WasmInstanceObject> InstanceBuilder::Build() {
     instance->set_feedback_vectors(*vectors);
     for (int i = 0; i < num_functions; i++) {
       int func_index = module_->num_imported_functions + i;
-      int slots =
-          base::Relaxed_Load(&module_->functions[func_index].feedback_slots);
+      int slots = NumFeedbackSlots(module_, func_index);
       if (slots == 0) continue;
       if (v8_flags.trace_wasm_speculative_inlining) {
         PrintF("[Function %d (declared %d): allocating %d feedback slots]\n",

src/wasm/wasm-code-manager.cc

@@ -2336,16 +2336,6 @@ std::vector<std::unique_ptr<WasmCode>> NativeModule::AddCompiledCode(
   for (auto& result : results) {
     DCHECK(result.succeeded());
     total_code_space += RoundUp<kCodeAlignment>(result.code_desc.instr_size);
-    if (result.result_tier == ExecutionTier::kLiftoff) {
-      int index = result.func_index;
-      int* slots = &module()->functions[index].feedback_slots;
-#if DEBUG
-      int current_value = base::Relaxed_Load(slots);
-      DCHECK(current_value == 0 ||
-             current_value == result.feedback_vector_slots);
-#endif
-      base::Relaxed_Store(slots, result.feedback_vector_slots);
-    }
   }
   base::Vector<byte> code_space;
   NativeModule::JumpTablesRef jump_tables;

src/wasm/wasm-module.cc

@@ -678,4 +678,16 @@ size_t GetWireBytesHash(base::Vector<const uint8_t> wire_bytes) {
       kZeroHashSeed);
 }
 
+int NumFeedbackSlots(const WasmModule* module, int func_index) {
+  if (!v8_flags.wasm_speculative_inlining) return 0;
+  // TODO(clemensb): Avoid the mutex once this ships, or at least switch to a
+  // shared mutex.
+  base::MutexGuard type_feedback_guard{&module->type_feedback.mutex};
+  auto it = module->type_feedback.feedback_for_function.find(func_index);
+  if (it == module->type_feedback.feedback_for_function.end()) return 0;
+  // The number of call instructions is capped by max function size.
+  static_assert(kV8MaxWasmFunctionSize < std::numeric_limits<int>::max() / 2);
+  return static_cast<int>(2 * it->second.call_targets.size());
+}
+
 }  // namespace v8::internal::wasm

src/wasm/wasm-module.h

@@ -63,10 +63,6 @@ struct WasmFunction {
   uint32_t func_index;     // index into the function table.
   uint32_t sig_index;      // index into the signature table.
   WireBytesRef code;       // code of this function.
-  // Required number of slots in a feedback vector. Marked {mutable} because
-  // this is computed late (by Liftoff compilation), when the rest of the
-  // {WasmFunction} is typically considered {const}.
-  mutable int feedback_slots;
   bool imported;
   bool exported;
   bool declared;
@@ -783,6 +779,9 @@ size_t PrintSignature(base::Vector<char> buffer, const wasm::FunctionSig*,
 V8_EXPORT_PRIVATE size_t
 GetWireBytesHash(base::Vector<const uint8_t> wire_bytes);
 
+// Get the required number of feedback slots for a function.
+int NumFeedbackSlots(const WasmModule* module, int func_index);
+
 }  // namespace v8::internal::wasm
 
 #endif  // V8_WASM_WASM_MODULE_H_

test/cctest/wasm/wasm-run-utils.cc

@@ -158,7 +158,6 @@ uint32_t TestingModuleBuilder::AddFunction(const FunctionSig* sig,
                                      index,    // func_index
                                      0,        // sig_index
                                      {0, 0},   // code
-                                     0,        // feedback slots
                                      false,    // imported
                                      false,    // exported
                                      false});  // declared

test/common/wasm/wasm-interpreter.cc

@@ -4240,7 +4240,6 @@ ControlTransferMap WasmInterpreter::ComputeControlTransfersForTesting(
                         0,       // func_index
                         0,       // sig_index
                         {0, 0},  // code
-                        0,       // feedback slots
                         false,   // imported
                         false,   // exported
                         false};  // declared

test/unittests/wasm/function-body-decoder-unittest.cc

@@ -183,7 +183,6 @@ class TestModuleBuilder {
          static_cast<uint32_t>(mod.functions.size()),  // func_index
          sig_index,                                    // sig_index
          {0, 0},                                       // code
-         0,                                            // feedback slots
          false,                                        // import
          false,                                        // export
          declared});                                   // declared