boostorg/unordered
1
0
Fork 1
mirror of https://github.com/boostorg/unordered.git synced 2025-07-30 03:17:15 +02:00
Code Issues Packages Projects Releases Wiki Activity

Rename functions used to efficiently calculate modulo, update associated comments

Browse Source
This commit is contained in:
Christian Mazakas
2022-06-03 09:26:29 -07:00
parent 8a74b192b0
commit 0106ed3d91
1 changed files with 19 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

29
include/boost/unordered/detail/fca.hpp
View File

@ -212,25 +212,34 @@ namespace boost {
} }
#if defined(BOOST_UNORDERED_FCA_FASTMOD_SUPPORT) #if defined(BOOST_UNORDERED_FCA_FASTMOD_SUPPORT)
// https://github.com/lemire/fastmod // We emulate the techniques taken from:
// Faster Remainder by Direct Computation: Applications to Compilers and
// Software Libraries
// https://arxiv.org/abs/1902.01961
//
// In essence, use fancy math to directly calculate the remainder (aka
// modulo) exploiting how compilers transform division
//
#if defined(_MSC_VER) #if defined(_MSC_VER)
static inline uint64_t mul128_u32(uint64_t lowbits, uint32_t d) static inline uint64_t get_remainder(uint64_t fractional, uint32_t d)
{ {
return __umulh(lowbits, d); // use fancy msvc instrinsic when available instead of using `>> 64`
//
return __umulh(fractional, d);
} }
#else #else
static inline uint64_t mul128_u32(uint64_t lowbits, uint32_t d) static inline uint64_t get_remainder(uint64_t fractional, uint32_t d)
{ {
__extension__ typedef unsigned __int128 uint128; __extension__ typedef unsigned __int128 uint128;
return static_cast<uint64_t>(((uint128)lowbits * d) >> 64); return static_cast<uint64_t>(((uint128)fractional * d) >> 64);
} }
#endif /* defined(_MSC_VER) */ #endif /* defined(_MSC_VER) */
static inline uint32_t fastmod_u32(uint32_t a, uint64_t M, uint32_t d) static inline uint32_t fast_modulo(uint32_t a, uint64_t M, uint32_t d)
{ {
uint64_t lowbits = M * a; uint64_t fractional = M * a;
return (uint32_t)(mul128_u32(lowbits, d)); return (uint32_t)(get_remainder(fractional, d));
} }
#endif /* defined(BOOST_UNORDERED_FCA_FASTMOD_SUPPORT) */ #endif /* defined(BOOST_UNORDERED_FCA_FASTMOD_SUPPORT) */
@ -241,13 +250,13 @@ namespace boost {
#if defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T) #if defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T)
std::size_t sizes_under_32bit = inv_sizes32_len; std::size_t sizes_under_32bit = inv_sizes32_len;
if (BOOST_LIKELY(size_index < sizes_under_32bit)) { if (BOOST_LIKELY(size_index < sizes_under_32bit)) {
return fastmod_u32(uint32_t(hash) + uint32_t(hash >> 32), return fast_modulo(uint32_t(hash) + uint32_t(hash >> 32),
inv_sizes32[size_index], uint32_t(sizes[size_index])); inv_sizes32[size_index], uint32_t(sizes[size_index]));
} else { } else {
return positions[size_index - sizes_under_32bit](hash); return positions[size_index - sizes_under_32bit](hash);
} }
#else #else
return fastmod_u32( return fast_modulo(
hash, inv_sizes32[size_index], uint32_t(sizes[size_index])); hash, inv_sizes32[size_index], uint32_t(sizes[size_index]));
#endif /* defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T) */ #endif /* defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T) */
#else #else