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//===-- Memcpy implementation for arm ---------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_INLINE_MEMCPY_H
#define LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_INLINE_MEMCPY_H
#include "src/__support/macros/attributes.h" // LIBC_INLINE
#include "src/__support/macros/optimization.h" // LIBC_LOOP_NOUNROLL
#include "src/string/memory_utils/utils.h" // memcpy_inline, distance_to_align
#include <stddef.h> // size_t
// https://libc.llvm.org/compiler_support.html
// Support for [[likely]] / [[unlikely]]
// [X] GCC 12.2
// [X] Clang 12
// [ ] Clang 11
#define LIBC_ATTR_LIKELY [[likely]]
#define LIBC_ATTR_UNLIKELY [[unlikely]]
#if defined(LIBC_COMPILER_IS_CLANG)
#if LIBC_COMPILER_CLANG_VER < 1200
#undef LIBC_ATTR_LIKELY
#undef LIBC_ATTR_UNLIKELY
#define LIBC_ATTR_LIKELY
#define LIBC_ATTR_UNLIKELY
#endif
#endif
namespace LIBC_NAMESPACE_DECL {
namespace {
LIBC_INLINE_VAR constexpr size_t kWordSize = sizeof(uint32_t);
enum Strategy {
ForceWordLdStChain,
AssumeWordAligned,
AssumeUnaligned,
};
template <size_t bytes, Strategy strategy = AssumeUnaligned>
LIBC_INLINE void copy_and_bump_pointers(Ptr &dst, CPtr &src) {
if constexpr (strategy == AssumeUnaligned) {
memcpy_inline<bytes>(assume_aligned<1>(dst), assume_aligned<1>(src));
} else if constexpr (strategy == AssumeWordAligned) {
static_assert(bytes >= kWordSize);
memcpy_inline<bytes>(assume_aligned<kWordSize>(dst),
assume_aligned<kWordSize>(src));
} else if constexpr (strategy == ForceWordLdStChain) {
// We restrict loads/stores to 4 byte to prevent the use of load/store
// multiple (LDM, STM) and load/store double (LDRD, STRD). First, they may
// fault (see notes below) and second, they use more registers which in turn
// adds push/pop instructions in the hot path.
static_assert((bytes % kWordSize == 0) && (bytes >= kWordSize));
LIBC_LOOP_UNROLL
for (size_t i = 0; i < bytes / kWordSize; ++i) {
const size_t offset = i * kWordSize;
memcpy_inline<kWordSize>(dst + offset, src + offset);
}
}
// In the 1, 2, 4 byte copy case, the compiler can fold pointer offsetting
// into the load/store instructions.
// e.g.,
// ldrb r3, [r1], #1
// strb r3, [r0], #1
dst += bytes;
src += bytes;
}
LIBC_INLINE void copy_bytes_and_bump_pointers(Ptr &dst, CPtr &src,
const size_t size) {
LIBC_LOOP_NOUNROLL
for (size_t i = 0; i < size; ++i)
*dst++ = *src++;
}
template <size_t block_size, Strategy strategy>
LIBC_INLINE void copy_blocks_and_update_args(Ptr &dst, CPtr &src,
size_t &size) {
LIBC_LOOP_NOUNROLL
for (size_t i = 0; i < size / block_size; ++i)
copy_and_bump_pointers<block_size, strategy>(dst, src);
// Update `size` once at the end instead of once per iteration.
size %= block_size;
}
LIBC_INLINE CPtr bitwise_or(CPtr a, CPtr b) {
return cpp::bit_cast<CPtr>(cpp::bit_cast<uintptr_t>(a) |
cpp::bit_cast<uintptr_t>(b));
}
LIBC_INLINE auto misaligned(CPtr a) {
return distance_to_align_down<kWordSize>(a);
}
} // namespace
// Implementation for Cortex-M0, M0+, M1.
// Notes:
// - It compiles down to 196 bytes, but 220 bytes when used through `memcpy`
// that also needs to return the `dst` ptr.
// - These cores do not allow for unaligned loads/stores.
// - When `src` and `dst` are coaligned, we start by aligning them and perform
// bulk copies. We let the compiler know the pointers are aligned so it can
// use load/store multiple (LDM, STM). This significantly increase throughput
// but it also requires more registers and push/pop instructions. This impacts
// latency for small size copies.
// - When `src` and `dst` are misaligned, we align `dst` and recompose words
// using multiple aligned loads. `load_aligned` takes care of endianness
// issues.
[[maybe_unused]] LIBC_INLINE void inline_memcpy_arm_low_end(Ptr dst, CPtr src,
size_t size) {
if (size >= 8) {
if (const size_t offset = distance_to_align_up<kWordSize>(dst))
LIBC_ATTR_UNLIKELY {
copy_bytes_and_bump_pointers(dst, src, offset);
size -= offset;
}
const auto src_alignment = distance_to_align_down<kWordSize>(src);
if (src_alignment == 0)
LIBC_ATTR_LIKELY {
// Both `src` and `dst` are now word-aligned.
copy_blocks_and_update_args<64, AssumeWordAligned>(dst, src, size);
copy_blocks_and_update_args<16, AssumeWordAligned>(dst, src, size);
copy_blocks_and_update_args<4, AssumeWordAligned>(dst, src, size);
}
else {
// `dst` is aligned but `src` is not.
LIBC_LOOP_NOUNROLL
while (size >= kWordSize) {
// Recompose word from multiple loads depending on the alignment.
const uint32_t value =
src_alignment == 2
? load_aligned<uint32_t, uint16_t, uint16_t>(src)
: load_aligned<uint32_t, uint8_t, uint16_t, uint8_t>(src);
memcpy_inline<kWordSize>(assume_aligned<kWordSize>(dst), &value);
dst += kWordSize;
src += kWordSize;
size -= kWordSize;
}
}
// Up to 3 bytes may still need to be copied.
// Handling them with the slow loop below.
}
copy_bytes_and_bump_pointers(dst, src, size);
}
// Implementation for Cortex-M3, M4, M7, M23, M33, M35P, M52 with hardware
// support for unaligned loads and stores.
// Notes:
// - It compiles down to 266 bytes.
// - `dst` and `src` are not `__restrict` to prevent the compiler from
// reordering loads/stores.
// - We keep state variables to a strict minimum to keep everything in the free
// registers and prevent costly push / pop.
// - If unaligned single loads/stores to normal memory are supported, unaligned
// accesses for load/store multiple (LDM, STM) and load/store double (LDRD,
// STRD) instructions are generally not supported and will still fault so we
// make sure to restrict unrolling to word loads/stores.
[[maybe_unused]] LIBC_INLINE void inline_memcpy_arm_mid_end(Ptr dst, CPtr src,
size_t size) {
if (misaligned(bitwise_or(src, dst)))
LIBC_ATTR_UNLIKELY {
if (size < 8)
LIBC_ATTR_UNLIKELY {
if (size & 1)
copy_and_bump_pointers<1>(dst, src);
if (size & 2)
copy_and_bump_pointers<2>(dst, src);
if (size & 4)
copy_and_bump_pointers<4>(dst, src);
return;
}
if (misaligned(src))
LIBC_ATTR_UNLIKELY {
const size_t offset = distance_to_align_up<kWordSize>(dst);
if (offset & 1)
copy_and_bump_pointers<1>(dst, src);
if (offset & 2)
copy_and_bump_pointers<2>(dst, src);
size -= offset;
}
}
copy_blocks_and_update_args<64, ForceWordLdStChain>(dst, src, size);
copy_blocks_and_update_args<16, ForceWordLdStChain>(dst, src, size);
copy_blocks_and_update_args<4, AssumeUnaligned>(dst, src, size);
if (size & 1)
copy_and_bump_pointers<1>(dst, src);
if (size & 2)
LIBC_ATTR_UNLIKELY
copy_and_bump_pointers<2>(dst, src);
}
[[maybe_unused]] LIBC_INLINE void inline_memcpy_arm(void *__restrict dst_,
const void *__restrict src_,
size_t size) {
Ptr dst = cpp::bit_cast<Ptr>(dst_);
CPtr src = cpp::bit_cast<CPtr>(src_);
#ifdef __ARM_FEATURE_UNALIGNED
return inline_memcpy_arm_mid_end(dst, src, size);
#else
return inline_memcpy_arm_low_end(dst, src, size);
#endif
}
} // namespace LIBC_NAMESPACE_DECL
// Cleanup local macros
#undef LIBC_ATTR_LIKELY
#undef LIBC_ATTR_UNLIKELY
#endif // LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_INLINE_MEMCPY_H
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