1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
|
// Copyright 2024 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/atomicops.h"
#include <atomic>
#include "base/memory/aligned_memory.h"
namespace base::subtle {
void RelaxedAtomicWriteMemcpy(base::span<uint8_t> dst,
base::span<const uint8_t> src) {
CHECK_EQ(dst.size(), src.size());
size_t bytes = dst.size();
uint8_t* dst_byte_ptr = dst.data();
const uint8_t* src_byte_ptr = src.data();
// Make sure that we can at least copy byte by byte with atomics.
static_assert(std::atomic_ref<uint8_t>::required_alignment == 1);
// Alignment for uintmax_t atomics that we use in the happy case.
constexpr size_t kDesiredAlignment =
std::atomic_ref<uintmax_t>::required_alignment;
// Copy byte-by-byte until `dst_byte_ptr` is not properly aligned for
// the happy case.
while (bytes > 0 && !IsAligned(dst_byte_ptr, kDesiredAlignment)) {
std::atomic_ref<uint8_t>(*dst_byte_ptr)
.store(*src_byte_ptr, std::memory_order_relaxed);
// SAFETY: We check above that `dst_byte_ptr` and `src_byte_ptr` point
// to spans of sufficient size.
UNSAFE_BUFFERS(++dst_byte_ptr);
UNSAFE_BUFFERS(++src_byte_ptr);
--bytes;
}
// Happy case where both `src_byte_ptr` and `dst_byte_ptr` are both properly
// aligned and the largest possible atomic is used for copying.
if (IsAligned(src_byte_ptr, kDesiredAlignment)) {
while (bytes >= sizeof(uintmax_t)) {
std::atomic_ref<uintmax_t>(*reinterpret_cast<uintmax_t*>(dst_byte_ptr))
.store(*reinterpret_cast<const uintmax_t*>(src_byte_ptr),
std::memory_order_relaxed);
// SAFETY: We check above that `dst_byte_ptr` and `src_byte_ptr` point
// to spans of sufficient size.
UNSAFE_BUFFERS(dst_byte_ptr += sizeof(uintmax_t));
UNSAFE_BUFFERS(src_byte_ptr += sizeof(uintmax_t));
bytes -= sizeof(uintmax_t);
}
}
// Copy what's left after the happy-case byte-by-byte.
while (bytes > 0) {
std::atomic_ref<uint8_t>(*dst_byte_ptr)
.store(*src_byte_ptr, std::memory_order_relaxed);
// SAFETY: We check above that `dst_byte_ptr` and `src_byte_ptr` point
// to spans of sufficient size.
UNSAFE_BUFFERS(++dst_byte_ptr);
UNSAFE_BUFFERS(++src_byte_ptr);
--bytes;
}
}
} // namespace base::subtle
|
