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- // Copyright 2019 The Chromium Authors. All rights reserved.
- // Use of this source code is governed by a BSD-style license that can be
- // found in the LICENSE file.
- #include "base/android/reached_addresses_bitset.h"
- #include "base/android/library_loader/anchor_functions.h"
- #include "base/android/library_loader/anchor_functions_buildflags.h"
- #include "base/check_op.h"
- #include "base/numerics/safe_conversions.h"
- namespace base {
- namespace android {
- namespace {
- constexpr size_t kBitsPerElement = sizeof(uint32_t) * 8;
- // Below an array of uint32_t in BSS is introduced and then casted to an array
- // of std::atomic<uint32_t>. In C++20 constructing an std::atomic is not
- // 'trivial'. See https://github.com/microsoft/STL/issues/661 for reasons of
- // this change in the standard.
- //
- // Assert that both types have the same size. The sizes do not have to match
- // according to a note in [atomics.types.generic] in C++17. With this assertion
- // in place it is unlikely that the constructor produces the value other than
- // (uint32_t)0.
- static_assert(sizeof(uint32_t) == sizeof(std::atomic<uint32_t>), "");
- // Keep the array in BSS only for non-official builds to avoid potential harm to
- // data locality and unspecified behavior from the reinterpret_cast below.
- // In order to start new experiments with base::Feature(ReachedCodeProfiler) on
- // Canary/Dev this array will need to be reintroduced to official builds. When
- // doing so, don't forget to update `kConfigurationSupported` in
- // `reached_code_profiler.cc`
- #if BUILDFLAG(SUPPORTS_CODE_ORDERING) && !defined(OFFICIAL_BUILD)
- // Enough for 1 << 29 bytes of code, 512MB.
- constexpr size_t kTextBitfieldSize = 1 << 20;
- uint32_t g_text_bitfield[kTextBitfieldSize];
- #endif
- } // namespace
- // static
- ReachedAddressesBitset* ReachedAddressesBitset::GetTextBitset() {
- #if BUILDFLAG(SUPPORTS_CODE_ORDERING) && !defined(OFFICIAL_BUILD)
- static ReachedAddressesBitset text_bitset(
- kStartOfText, kEndOfText,
- reinterpret_cast<std::atomic<uint32_t>*>(g_text_bitfield),
- kTextBitfieldSize);
- return &text_bitset;
- #else
- return nullptr;
- #endif
- }
- void ReachedAddressesBitset::RecordAddress(uintptr_t address) {
- // |address| is outside of the range.
- if (address < start_address_ || address >= end_address_)
- return;
- size_t offset = static_cast<size_t>(address - start_address_);
- uint32_t offset_index = checked_cast<uint32_t>(offset / kBytesGranularity);
- // Atomically set the corresponding bit in the array.
- std::atomic<uint32_t>* element = reached_ + (offset_index / kBitsPerElement);
- // First, a racy check. This saves a CAS if the bit is already set, and
- // allows the cache line to remain shared across CPUs in this case.
- uint32_t value = element->load(std::memory_order_relaxed);
- uint32_t mask = 1 << (offset_index % kBitsPerElement);
- if (value & mask)
- return;
- element->fetch_or(mask, std::memory_order_relaxed);
- }
- std::vector<uint32_t> ReachedAddressesBitset::GetReachedOffsets() const {
- std::vector<uint32_t> offsets;
- const size_t elements = NumberOfReachableElements();
- for (size_t i = 0; i < elements; ++i) {
- uint32_t element = reached_[i].load(std::memory_order_relaxed);
- // No reached addresses at this element.
- if (element == 0)
- continue;
- for (size_t j = 0; j < 32; ++j) {
- if (!((element >> j) & 1))
- continue;
- size_t offset_index = i * 32 + j;
- size_t offset = offset_index * kBytesGranularity;
- offsets.push_back(checked_cast<uint32_t>(offset));
- }
- }
- return offsets;
- }
- ReachedAddressesBitset::ReachedAddressesBitset(
- uintptr_t start_address,
- uintptr_t end_address,
- std::atomic<uint32_t>* storage_ptr,
- size_t storage_size)
- : start_address_(start_address),
- end_address_(end_address),
- reached_(storage_ptr) {
- DCHECK_LE(start_address_, end_address_);
- DCHECK_LE(NumberOfReachableElements(), storage_size * kBitsPerElement);
- }
- size_t ReachedAddressesBitset::NumberOfReachableElements() const {
- size_t reachable_bits =
- (end_address_ + kBytesGranularity - 1) / kBytesGranularity -
- start_address_ / kBytesGranularity;
- return (reachable_bits + kBitsPerElement - 1) / kBitsPerElement;
- }
- } // namespace android
- } // namespace base
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