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[ruby/json] Extract json_fast_memcpy16 for readability

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https://github.com/ruby/json/commit/1b276c8623
This commit is contained in:
Jean Boussier 2026-01-16 18:43:08 +01:00 committed by git
parent 456ef9140a
commit 3164d4e8a2
3 changed files with 37 additions and 28 deletions
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34
ext/json/generator/generator.c
View File

@ -288,6 +288,8 @@ static inline void escape_UTF8_char(search_state *search, unsigned char ch_len)
ALWAYS_INLINE(static) char *copy_remaining_bytes(search_state *search, unsigned long vec_len, unsigned long len)
{
RBIMPL_ASSERT_OR_ASSUME(len < vec_len);
// Flush the buffer so everything up until the last 'len' characters are unflushed.
search_flush(search);
@ -303,37 +305,13 @@ ALWAYS_INLINE(static) char *copy_remaining_bytes(search_state *search, unsigned
// Optimistically copy the remaining 'len' characters to the output FBuffer. If there are no characters
// to escape, then everything ends up in the correct spot. Otherwise it was convenient temporary storage.
#if defined(__has_builtin) && __has_builtin(__builtin_memcpy)
#ifdef RBIMPL_ASSERT_OR_ASSUME
RBIMPL_ASSERT_OR_ASSUME(len < 16);
#endif
if (vec_len == 16 && len >= 4) {
// If __builtin_memcpy is available, use it to copy between SIMD_MINIMUM_THRESHOLD and vec_len-1 bytes.
// These copies overlap. The first copy will copy the first 8 (or 4) bytes. The second copy will copy
// the last 8 (or 4) bytes but overlap with the first copy. The overlapping bytes will be in the correct
// position in both copies.
// Please do not attempt to replace __builtin_memcpy with memcpy without profiling and/or looking at the
// generated assembly. On clang-specifically (tested on Apple clang version 17.0.0 (clang-1700.0.13.3)),
// when using memcpy, the compiler will notice the only difference is a 4 or 8 and generate a conditional
// select instruction instead of direct loads and stores with a branch. This ends up slower than the branch
// plus two loads and stores generated when using __builtin_memcpy.
if (len >= 8) {
__builtin_memcpy(s, search->ptr, 8);
__builtin_memcpy(s + len - 8, search->ptr + len - 8, 8);
} else {
__builtin_memcpy(s, search->ptr, 4);
__builtin_memcpy(s + len - 4, search->ptr + len - 4, 4);
}
if (vec_len == 16) {
RBIMPL_ASSERT_OR_ASSUME(len >= SIMD_MINIMUM_THRESHOLD);
json_fast_memcpy16(s, search->ptr, len);
} else {
MEMCPY(s, search->ptr, char, len);
}
#else
MEMCPY(s, search->ptr, char, len);
#endif
return s;
}

4
ext/json/json.h
View File

@ -5,6 +5,10 @@
#include "ruby/encoding.h"
#include <stdint.h>
#ifndef RBIMPL_ASSERT_OR_ASSUME
# define RBIMPL_ASSERT_OR_ASSUME(x)
#endif
#if defined(RUBY_DEBUG) && RUBY_DEBUG
# define JSON_ASSERT RUBY_ASSERT
#else

27
ext/json/simd/simd.h
View File

@ -60,6 +60,33 @@ static inline int trailing_zeros(int input)
#define SIMD_MINIMUM_THRESHOLD 4
ALWAYS_INLINE(static) void json_fast_memcpy16(char *dst, const char *src, size_t len)
{
RBIMPL_ASSERT_OR_ASSUME(len < 16);
RBIMPL_ASSERT_OR_ASSUME(len >= SIMD_MINIMUM_THRESHOLD); // 4
#if defined(__has_builtin) && __has_builtin(__builtin_memcpy)
// If __builtin_memcpy is available, use it to copy between SIMD_MINIMUM_THRESHOLD (4) and vec_len-1 (15) bytes.
// These copies overlap. The first copy will copy the first 8 (or 4) bytes. The second copy will copy
// the last 8 (or 4) bytes but overlap with the first copy. The overlapping bytes will be in the correct
// position in both copies.
// Please do not attempt to replace __builtin_memcpy with memcpy without profiling and/or looking at the
// generated assembly. On clang-specifically (tested on Apple clang version 17.0.0 (clang-1700.0.13.3)),
// when using memcpy, the compiler will notice the only difference is a 4 or 8 and generate a conditional
// select instruction instead of direct loads and stores with a branch. This ends up slower than the branch
// plus two loads and stores generated when using __builtin_memcpy.
if (len >= 8) {
__builtin_memcpy(dst, src, 8);
__builtin_memcpy(dst + len - 8, src + len - 8, 8);
} else {
__builtin_memcpy(dst, src, 4);
__builtin_memcpy(dst + len - 4, src + len - 4, 4);
}
#else
MEMCPY(dst, src, char, len);
#endif
}
#if defined(__ARM_NEON) || defined(__ARM_NEON__) || defined(__aarch64__) || defined(_M_ARM64)
#include <arm_neon.h>