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editorial

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joaquintides
2022-10-15 10:18:47 +02:00
parent 9da61e9050
commit e663812f24
1 changed files with 13 additions and 12 deletions
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25
include/boost/unordered/detail/foa.hpp
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@@ -85,7 +85,7 @@ namespace foa{
* quadratic probing. * quadratic probing.
* - Each group has an associated 16B metadata word holding reduced hash * - Each group has an associated 16B metadata word holding reduced hash
* values and overflow information. Reduced hash values are used to * values and overflow information. Reduced hash values are used to
* accelerate lookup within the group by using SIMD or 64-bit word * accelerate lookup within the group by using 128-bit SIMD or 64-bit word
* operations. * operations.
*/ */
@@ -113,10 +113,10 @@ namespace foa{
* the table), probing stops at the first non-overflowed group. Having 8 * the table), probing stops at the first non-overflowed group. Having 8
* bits for signalling overflow makes it very likely that we stop at the * bits for signalling overflow makes it very likely that we stop at the
* current group (this happens when no element with the same (h%8) value * current group (this happens when no element with the same (h%8) value
* has overflowed in the same group), saving us an additional group check * has overflowed in the group), saving us an additional group check even
* even under high-load/high-erase conditions. * under high-load/high-erase conditions.
* *
* When looking for an element with hash value h, match(n) returns a bitmask * When looking for an element with hash value h, match(h) returns a bitmask
* signalling which slots have the same reduced hash value. If available, * signalling which slots have the same reduced hash value. If available,
* match uses SSE2 or (little endian) Neon 128-bit SIMD operations. On non-SIMD * match uses SSE2 or (little endian) Neon 128-bit SIMD operations. On non-SIMD
* scenarios, the logical layout described above is physically mapped to two * scenarios, the logical layout described above is physically mapped to two
@@ -601,8 +601,9 @@ inline void prefetch(const void* p)
#endif #endif
} }
/* foa::table uses a size policy to control the permissible sizes of the group /* foa::table uses a size policy to obtain the permissible sizes of the group
* array (and, by implication, the element array) and the hash->group mapping. * array (and, by implication, the element array) and to do the hash->group
* mapping.
* *
* - size_index(n) returns an unspecified "index" number used in other policy * - size_index(n) returns an unspecified "index" number used in other policy
* operations. * operations.
@@ -621,8 +622,8 @@ inline void prefetch(const void* p)
* of the hash value as the position in the group array. Using a size index * of the hash value as the position in the group array. Using a size index
* defined as i = (bits in std::size_t) - n, we have an unbeatable * defined as i = (bits in std::size_t) - n, we have an unbeatable
* implementation of position(hash) as hash>>i. We've chosen to select the * implementation of position(hash) as hash>>i. We've chosen to select the
* most significant bits of hash for positioning as multiplication-based mixing * most significant bits of hash for positioning because multiplication-based
* tends to yield better entropy in the high part of its result. * mixing tends to yield better entropy in the high part of its result.
*/ */
struct pow2_size_policy struct pow2_size_policy
@@ -650,8 +651,8 @@ struct pow2_size_policy
}; };
/* Quadratic prober over a power-of-two range using triangular numbers. /* Quadratic prober over a power-of-two range using triangular numbers.
* mask in next(mask) is expected to be the range size minus one (and since * mask in next(mask) must be the range size minus one (and since size is 2^n,
* size is 2^n, mask has exactly its n first bits set to 1). * mask has exactly its n first bits set to 1).
*/ */
struct pow2_quadratic_prober struct pow2_quadratic_prober
@@ -703,7 +704,7 @@ struct xmx_mix
template<typename,typename,typename,typename> template<typename,typename,typename,typename>
class table; class table;
/* table_iterators keeps two pointers: /* table_iterator keeps two pointers:
* *
* - A pointer p to the element slot. * - A pointer p to the element slot.
* - A pointer pc to the n-th byte of the associated group metadata, where n * - A pointer pc to the n-th byte of the associated group metadata, where n
@@ -713,7 +714,7 @@ class table;
* pointer pg to the group, and the position n, but that would increase * pointer pg to the group, and the position n, but that would increase
* sizeof(table_iterator) by 4/8 bytes. In order to make this compact * sizeof(table_iterator) by 4/8 bytes. In order to make this compact
* representation possible, it is required that group objects are aligned * representation possible, it is required that group objects are aligned
* to its size, so that we can recover pg and n as * to their size, so that we can recover pg and n as
* *
* - n = pc%sizeof(group) * - n = pc%sizeof(group)
* - pg = pc-n * - pg = pc-n