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// Copyright 2025 The Abseil Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "absl/debugging/internal/borrowed_fixup_buffer.h"
#include <assert.h>
#include <limits.h>
#include <stddef.h>
#include <stdint.h>
#include <atomic>
#include <iterator>
#include <utility>
#include "absl/base/attributes.h"
#include "absl/base/config.h"
#include "absl/base/internal/low_level_alloc.h"
#include "absl/hash/hash.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace internal_stacktrace {
// A buffer for holding fix-up information for stack traces of common sizes_.
struct BorrowedFixupBuffer::FixupStackBuffer {
static constexpr size_t kMaxStackElements = 128; // Can be reduced if needed
std::atomic_flag in_use{};
uintptr_t frames[kMaxStackElements];
int sizes[kMaxStackElements];
ABSL_CONST_INIT static FixupStackBuffer g_instances[kNumStaticBuffers];
};
ABSL_CONST_INIT BorrowedFixupBuffer::FixupStackBuffer
BorrowedFixupBuffer::FixupStackBuffer::g_instances[kNumStaticBuffers] = {};
BorrowedFixupBuffer::~BorrowedFixupBuffer() {
if (borrowed_) {
std::move(*this).Unlock();
} else {
base_internal::LowLevelAlloc::Free(frames_);
}
}
BorrowedFixupBuffer::BorrowedFixupBuffer(size_t length)
: borrowed_(0 < length && length <= FixupStackBuffer::kMaxStackElements
? TryLock()
: nullptr) {
if (borrowed_) {
InitViaBorrow();
} else {
InitViaAllocation(length);
}
}
void BorrowedFixupBuffer::InitViaBorrow() {
assert(borrowed_);
frames_ = borrowed_->frames;
sizes_ = borrowed_->sizes;
}
void BorrowedFixupBuffer::InitViaAllocation(size_t length) {
static_assert(alignof(decltype(*frames_)) >= alignof(decltype(*sizes_)),
"contiguous layout assumes decreasing alignment, otherwise "
"padding may be needed in the middle");
assert(!borrowed_);
base_internal::InitSigSafeArena();
void* buf = base_internal::LowLevelAlloc::AllocWithArena(
length * (sizeof(*frames_) + sizeof(*sizes_)),
base_internal::SigSafeArena());
if (buf == nullptr) {
frames_ = nullptr;
sizes_ = nullptr;
return;
}
frames_ = new (buf) uintptr_t[length];
sizes_ = new (static_cast<void*>(static_cast<unsigned char*>(buf) +
length * sizeof(*frames_))) int[length];
}
[[nodiscard]] BorrowedFixupBuffer::FixupStackBuffer*
BorrowedFixupBuffer::TryLock() {
constexpr size_t kNumSlots = std::size(FixupStackBuffer::g_instances);
const size_t i = absl::Hash<const void*>()(this) % kNumSlots;
for (size_t k = 0; k < kNumSlots; ++k) {
auto* instance = &FixupStackBuffer::g_instances[(i + k) % kNumSlots];
// Use memory_order_acquire to ensure that no reads and writes on the
// borrowed buffer are reordered before the borrowing.
if (!instance->in_use.test_and_set(std::memory_order_acquire)) {
return instance;
}
}
return nullptr;
}
void BorrowedFixupBuffer::Unlock() && {
// Use memory_order_release to ensure that no reads and writes on the borrowed
// buffer are reordered after the borrowing.
borrowed_->in_use.clear(std::memory_order_release);
}
} // namespace internal_stacktrace
ABSL_NAMESPACE_END
} // namespace absl
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