Fixing performance issue by removing uneeded GroupElement conversions… #1056
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… and hashing by std::hash()
levonpetrosyan93
requested review from
a-bezrukov and
psolstice
and removed request for
a-bezrukov
a-bezrukov
previously approved these changes
Jul 19, 2021
psolstice
requested changes
Jul 20, 2021
| std::size_t GroupElement::lowest_limb() const { | ||
| secp256k1_fe x = reinterpret_cast<secp256k1_gej *>(g_)->x; | ||
| secp256k1_fe_normalize(&x); | ||
| return x.n[0]; |
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64-bit implementation of secp256k1 has 58-bit limb. This will not be uniform distribution in [0, SIZE_MAX] range (because it's less than 2^58). As a result (depending on implementation) unordered map/set may have just 1/64 buckets used decreasing performance. Examining C++ STL source is not for faint hearted so it's easier to just use for hash something like x.n[0] ^ (x.n[1] << 8) which will give you all bits in 64-bit integer and will be uniformly distributed. Then you need to rename it to something like get_hash()
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er, it's 52 bits and hence x.n[0] ^ (x.n[1] << 16) should be used. This will work both in 64-bit and 32-bit cases
psolstice
approved these changes
Jul 21, 2021
a-bezrukov
approved these changes
Jul 21, 2021
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