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66 lines
2.3 KiB
C++
66 lines
2.3 KiB
C++
// Copyright 2024 The Chromium Authors
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/atomicops.h"
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#include <atomic>
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#include "base/memory/aligned_memory.h"
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namespace base::subtle {
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void RelaxedAtomicWriteMemcpy(base::span<uint8_t> dst,
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base::span<const uint8_t> src) {
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CHECK_EQ(dst.size(), src.size());
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size_t bytes = dst.size();
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uint8_t* dst_byte_ptr = dst.data();
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const uint8_t* src_byte_ptr = src.data();
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// Make sure that we can at least copy byte by byte with atomics.
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static_assert(std::atomic_ref<uint8_t>::required_alignment == 1);
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// Alignment for uintmax_t atomics that we use in the happy case.
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constexpr size_t kDesiredAlignment =
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std::atomic_ref<uintmax_t>::required_alignment;
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// Copy byte-by-byte until `dst_byte_ptr` is not properly aligned for
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// the happy case.
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while (bytes > 0 && !IsAligned(dst_byte_ptr, kDesiredAlignment)) {
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std::atomic_ref<uint8_t>(*dst_byte_ptr)
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.store(*src_byte_ptr, std::memory_order_relaxed);
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// SAFETY: We check above that `dst_byte_ptr` and `src_byte_ptr` point
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// to spans of sufficient size.
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UNSAFE_BUFFERS(++dst_byte_ptr);
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UNSAFE_BUFFERS(++src_byte_ptr);
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--bytes;
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}
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// Happy case where both `src_byte_ptr` and `dst_byte_ptr` are both properly
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// aligned and the largest possible atomic is used for copying.
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if (IsAligned(src_byte_ptr, kDesiredAlignment)) {
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while (bytes >= sizeof(uintmax_t)) {
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std::atomic_ref<uintmax_t>(*reinterpret_cast<uintmax_t*>(dst_byte_ptr))
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.store(*reinterpret_cast<const uintmax_t*>(src_byte_ptr),
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std::memory_order_relaxed);
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// SAFETY: We check above that `dst_byte_ptr` and `src_byte_ptr` point
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// to spans of sufficient size.
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UNSAFE_BUFFERS(dst_byte_ptr += sizeof(uintmax_t));
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UNSAFE_BUFFERS(src_byte_ptr += sizeof(uintmax_t));
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bytes -= sizeof(uintmax_t);
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}
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}
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// Copy what's left after the happy-case byte-by-byte.
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while (bytes > 0) {
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std::atomic_ref<uint8_t>(*dst_byte_ptr)
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.store(*src_byte_ptr, std::memory_order_relaxed);
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// SAFETY: We check above that `dst_byte_ptr` and `src_byte_ptr` point
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// to spans of sufficient size.
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UNSAFE_BUFFERS(++dst_byte_ptr);
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UNSAFE_BUFFERS(++src_byte_ptr);
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--bytes;
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}
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}
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} // namespace base::subtle
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