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Load string scan words with memcpy and use MSVC bit-scan intrinsics

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Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Niels Lohmann committed 2026-10-09 16:01:48 +02:00
1 parent 43c75bef51
commit 0e729be261
2 files changed
+26 -4
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include/nlohmann/detail/bit_ops.hpp
+19 -4
View File
@@ -9,8 +9,9 @@
#pragma once
#include <cstdint> // uint64_t
#if !defined(__SIZEOF_INT128__) && defined(_MSC_VER) && (defined(_M_X64) || defined(_M_ARM64))
#include <intrin0.h> // __umulh, _umul128
#include <cstring> // memcpy
#if defined(_MSC_VER) && (defined(_M_X64) || defined(_M_ARM64)) && (!defined(__SIZEOF_INT128__) || (!defined(__GNUC__) && !defined(__clang__)))
#include <intrin0.h> // __umulh, _umul128, _BitScanForward64, _BitScanReverse64
#endif
#include <nlohmann/detail/abi_macros.hpp>
@@ -28,6 +29,10 @@ inline int count_leading_zeros(std::uint64_t x) noexcept
{
#if defined(__GNUC__) || defined(__clang__)
return __builtin_clzll(x);
#elif defined(_MSC_VER) && (defined(_M_X64) || defined(_M_ARM64))
unsigned long index = 0;
_BitScanReverse64(&index, x);
return 63 - static_cast<int>(index);
#else
int n = 0;
for (int shift = 32; shift != 0; shift >>= 1)
@@ -47,6 +52,10 @@ inline int count_trailing_zeros(std::uint64_t x) noexcept
{
#if defined(__GNUC__) || defined(__clang__)
return __builtin_ctzll(x);
#elif defined(_MSC_VER) && (defined(_M_X64) || defined(_M_ARM64))
unsigned long index = 0;
_BitScanForward64(&index, x);
return static_cast<int>(index);
#else
int n = 0;
for (int shift = 32; shift != 0; shift >>= 1)
@@ -94,14 +103,20 @@ inline uint128_parts full_multiplication(std::uint64_t a, std::uint64_t b) noexc
#endif
}
/// eight bytes as a little-endian word (compilers fold this into one load on
/// little-endian targets)
/// eight bytes as a little-endian word (a single load on little-endian targets)
inline std::uint64_t read_eight_bytes(const unsigned char* b) noexcept
{
#if defined(_MSC_VER) || defined(__x86_64__) || defined(__i386__) || (defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
// the byte order already matches (all MSVC targets are little-endian)
std::uint64_t result = 0;
std::memcpy(&result, b, sizeof(result));
return result;
#else
return static_cast<std::uint64_t>(b[0]) | (static_cast<std::uint64_t>(b[1]) << 8u)
| (static_cast<std::uint64_t>(b[2]) << 16u) | (static_cast<std::uint64_t>(b[3]) << 24u)
| (static_cast<std::uint64_t>(b[4]) << 32u) | (static_cast<std::uint64_t>(b[5]) << 40u)
| (static_cast<std::uint64_t>(b[6]) << 48u) | (static_cast<std::uint64_t>(b[7]) << 56u);
#endif
}
/// eight bytes as a little-endian word