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[hash] Enable better hash for integers

Browse source 
https://github.com/harfbuzz/harfbuzz/pull/4228#issuecomment-1565079537
This commit is contained in:
Behdad Esfahbod 2023-05-27 10:13:53 -06:00
parent 20c564bc76
commit 00900f761f
1 changed files with 3 additions and 10 deletions
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13
src/hb-algs.hh
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@ -314,24 +314,17 @@ struct
template <typename T> constexpr auto
impl (const T& v, hb_priority<2>) const HB_RETURN (uint32_t, hb_deref (v).hash ())
#if 0
// The following, unfortunately, while keeps the probing chains short, slows
// down the overall hash table performance. Not because of the extra operation
// itself in my opinion, but something else going on that we have not been able
// to track down. So for now, this is disabled. Discuss:
// https://github.com/harfbuzz/harfbuzz/pull/4228
// Horrible: std:hash() of integers seems to be identity in gcc / clang?!
// https://github.com/harfbuzz/harfbuzz/pull/4228
//
// For performance characteristics see:
// https://github.com/harfbuzz/harfbuzz/pull/4228#issuecomment-1565079537
template <typename T,
hb_enable_if (std::is_integral<T>::value && sizeof (T) <= sizeof (uint32_t))> constexpr auto
impl (const T& v, hb_priority<1>) const HB_RETURN (uint32_t, v * 2654435761 /* Knuh's multiplicative hash */)
template <typename T,
hb_enable_if (std::is_integral<T>::value && sizeof (T) > sizeof (uint32_t))> constexpr auto
impl (const T& v, hb_priority<1>) const HB_RETURN (uint32_t, (v ^ (v >> 32)) * 2654435761 /* Knuth's multiplicative hash */)
#endif
template <typename T> constexpr auto
impl (const T& v, hb_priority<0>) const HB_RETURN (uint32_t, std::hash<hb_decay<decltype (hb_deref (v))>>{} (hb_deref (v)))