// Copyright 2020 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef INCLUDE_CPPGC_INTERNAL_GC_INFO_H_
#define INCLUDE_CPPGC_INTERNAL_GC_INFO_H_
#include <atomic>
#include <cstdint>
#include <type_traits>
#include "cppgc/internal/finalizer-trait.h"
#include "cppgc/internal/logging.h"
#include "cppgc/internal/name-trait.h"
#include "cppgc/trace-trait.h"
#include "v8config.h" // NOLINT(build/include_directory)
namespace cppgc::internal {
using GCInfoIndex = uint16_t;
static constexpr GCInfoIndex kMaxGCInfoIndex = 1 << 14;
static constexpr GCInfoIndex kMinGCInfoIndex = 1;
struct GCInfo final {
constexpr GCInfo(FinalizationCallback finalize, TraceCallback trace,
NameCallback name)
: finalize(finalize), trace(trace), name(name) {}
FinalizationCallback finalize;
TraceCallback trace;
NameCallback name;
size_t padding = 0;
};
inline HeapObjectName GetHiddenName(
const void*, HeapObjectNameForUnnamedObject name_retrieval_mode) {
return {
NameProvider::kHiddenName,
name_retrieval_mode == HeapObjectNameForUnnamedObject::kUseHiddenName};
}
#if defined(CPPGC_ENABLE_OBJECT_SECTION_GCINFO)
#if defined(__APPLE__)
// Use __attribute__((visibility("hidden"))) explicitly here, since otherwise
// the compiler will generate GOT-indirected loads. Although the linker emits
// these symbols with `protected` visibility (same as `default`, i.e. exported,
// but without GOT/PLT indirection), the compiler is not aware of that and
// assumes any extern "C" has the `default` visibility.
extern "C" __attribute__((visibility("hidden"))) const GCInfo
__start_gc_info_section[] __asm("section$start$__DATA$gc_info_section");
extern "C" __attribute__((visibility("hidden"))) const GCInfo
__stop_gc_info_section[] __asm("section$end$__DATA$gc_info_section");
#define CPPGC_GC_INFO_SECTION \
__attribute__((section("__DATA,gc_info_section"), used))
#else // defined(__APPLE__)
extern "C" __attribute__((visibility("hidden")))
const GCInfo __start_gc_info_section[];
extern "C" __attribute__((visibility("hidden")))
const GCInfo __stop_gc_info_section[];
#define CPPGC_GC_INFO_SECTION __attribute__((section("gc_info_section"), used))
#endif // defined(__APPLE__)
class V8_EXPORT GCInfoTableSection final {
public:
GCInfoTableSection() = delete;
V8_INLINE static GCInfoIndex Index(const GCInfo& info) {
return &info - __start_gc_info_section + 1;
}
V8_INLINE static const GCInfo& GCInfoFromIndex(GCInfoIndex index) {
CPPGC_DCHECK(index >= 1);
CPPGC_DCHECK(
static_cast<size_t>(index - 1) <
static_cast<size_t>(__stop_gc_info_section - __start_gc_info_section));
return __start_gc_info_section[index - 1];
}
};
#else // defined(CPPGC_ENABLE_OBJECT_SECTION_GCINFO)
struct V8_EXPORT EnsureGCInfoIndexTrait final {
// Acquires a new GC info object and updates `registered_index` with the index
// that identifies that new info accordingly.
template <typename T>
V8_INLINE static GCInfoIndex EnsureIndex(
std::atomic<GCInfoIndex>& registered_index) {
return EnsureGCInfoIndexTraitDispatch<T>{}(registered_index);
}
private:
template <typename T, bool = FinalizerTrait<T>::HasFinalizer(),
bool = NameTrait<T>::HasNonHiddenName()>
struct EnsureGCInfoIndexTraitDispatch;
static GCInfoIndex V8_PRESERVE_MOST
EnsureGCInfoIndex(std::atomic<GCInfoIndex>&, TraceCallback,
FinalizationCallback, NameCallback);
static GCInfoIndex V8_PRESERVE_MOST EnsureGCInfoIndex(
std::atomic<GCInfoIndex>&, TraceCallback, FinalizationCallback);
static GCInfoIndex V8_PRESERVE_MOST
EnsureGCInfoIndex(std::atomic<GCInfoIndex>&, TraceCallback, NameCallback);
static GCInfoIndex V8_PRESERVE_MOST
EnsureGCInfoIndex(std::atomic<GCInfoIndex>&, TraceCallback);
};
#define DISPATCH(has_finalizer, has_non_hidden_name, function) \
template <typename T> \
struct EnsureGCInfoIndexTrait::EnsureGCInfoIndexTraitDispatch< \
T, has_finalizer, has_non_hidden_name> { \
V8_INLINE GCInfoIndex \
operator()(std::atomic<GCInfoIndex>& registered_index) { \
return function; \
} \
};
// ------------------------------------------------------- //
// DISPATCH(has_finalizer, has_non_hidden_name, function) //
// ------------------------------------------------------- //
DISPATCH(true, true, //
EnsureGCInfoIndex(registered_index, //
TraceTrait<T>::Trace, //
FinalizerTrait<T>::kCallback, //
NameTrait<T>::GetName)) //
DISPATCH(true, false, //
EnsureGCInfoIndex(registered_index, //
TraceTrait<T>::Trace, //
FinalizerTrait<T>::kCallback)) //
DISPATCH(false, true, //
EnsureGCInfoIndex(registered_index, //
TraceTrait<T>::Trace, //
NameTrait<T>::GetName)) //
DISPATCH(false, false, //
EnsureGCInfoIndex(registered_index, //
TraceTrait<T>::Trace)) //
#undef DISPATCH
#endif // defined(CPPGC_ENABLE_OBJECT_SECTION_GCINFO)
// Trait determines how the garbage collector treats objects wrt. to traversing,
// finalization, and naming.
template <typename T>
struct GCInfoTrait final {
#if defined(CPPGC_ENABLE_OBJECT_SECTION_GCINFO)
static const GCInfo gc_info;
V8_INLINE static GCInfoIndex Index() {
static_assert(sizeof(T), "T must be fully defined");
return GCInfoTableSection::Index(gc_info);
}
#else // defined(CPPGC_ENABLE_OBJECT_SECTION_GCINFO)
V8_INLINE static GCInfoIndex Index() {
static_assert(sizeof(T), "T must be fully defined");
static std::atomic<GCInfoIndex>
registered_index; // Uses zero initialization.
GCInfoIndex index = registered_index.load(std::memory_order_acquire);
if (V8_UNLIKELY(!index)) {
index = EnsureGCInfoIndexTrait::EnsureIndex<T>(registered_index);
CPPGC_DCHECK(index != 0);
CPPGC_DCHECK(index == registered_index.load(std::memory_order_acquire));
}
return index;
}
#endif // defined(CPPGC_ENABLE_OBJECT_SECTION_GCINFO)
static constexpr void CheckCallbacksAreDefined() {
// No USE() macro available.
(void)static_cast<TraceCallback>(TraceTrait<T>::Trace);
(void)static_cast<FinalizationCallback>(FinalizerTrait<T>::kCallback);
(void)static_cast<NameCallback>(NameTrait<T>::GetName);
}
};
// Fold types based on finalizer behavior. Note that finalizer characteristics
// align with trace behavior, i.e., destructors are virtual when trace methods
// are and vice versa.
template <typename T, typename ParentMostGarbageCollectedType>
struct GCInfoFolding final {
static constexpr bool kHasVirtualDestructorAtBase =
std::has_virtual_destructor_v<ParentMostGarbageCollectedType>;
static constexpr bool kBothTypesAreTriviallyDestructible =
std::is_trivially_destructible_v<ParentMostGarbageCollectedType> &&
std::is_trivially_destructible_v<T>;
static constexpr bool kHasCustomFinalizerDispatchAtBase =
internal::HasFinalizeGarbageCollectedObject<
ParentMostGarbageCollectedType>::value;
#ifdef CPPGC_SUPPORTS_OBJECT_NAMES
static constexpr bool kWantsDetailedObjectNames = true;
#else // !CPPGC_SUPPORTS_OBJECT_NAMES
static constexpr bool kWantsDetailedObjectNames = false;
#endif // !CPPGC_SUPPORTS_OBJECT_NAMES
// Always true. Forces the compiler to resolve callbacks which ensures that
// both modes don't break without requiring compiling a separate
// configuration. Only a single GCInfo (for `ResultType` below) will actually
// be instantiated but existence (and well-formedness) of all callbacks is
// checked.
static constexpr bool WantToFold() {
if constexpr ((kHasVirtualDestructorAtBase ||
kBothTypesAreTriviallyDestructible ||
kHasCustomFinalizerDispatchAtBase) &&
!kWantsDetailedObjectNames) {
GCInfoTrait<T>::CheckCallbacksAreDefined();
GCInfoTrait<ParentMostGarbageCollectedType>::CheckCallbacksAreDefined();
return true;
}
return false;
}
// Folding would regress name resolution when deriving names from C++
// class names as it would just folds a name to the base class name.
using ResultType =
std::conditional_t<WantToFold(), ParentMostGarbageCollectedType, T>;
};
#if defined(CPPGC_ENABLE_OBJECT_SECTION_GCINFO)
template <typename T>
const GCInfo GCInfoTrait<T>::gc_info CPPGC_GC_INFO_SECTION = GCInfo(
FinalizerTrait<T>::kCallback, TraceTrait<T>::Trace,
NameTrait<T>::HasNonHiddenName() ? NameTrait<T>::GetName : GetHiddenName);
#endif // defined(CPPGC_ENABLE_OBJECT_SECTION_GCINFO)
} // namespace cppgc::internal
#endif // INCLUDE_CPPGC_INTERNAL_GC_INFO_H_