dynamic_bitset<> find_next() improvements #1
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added 3 commits
January 12, 2014 21:17
Instead of masking all bits in the current block that were already visited and then finding the lowest remaining bit, it shifts the current block directly to the first unvisited bit and identifies the lowest set bit. It requires fewer operations that the original implementation and should be faster. See https://svn.boost.org/trac/boost/ticket/5159.
__builtin_ctz[l][l]() is built-in gcc/icc/clang operation to find the lowest bit, _BitScanForward[64]() is MSVC builtin one. It replaces the default arithmetic-based lowest_bit() implementation for the types that support this operation. The patch should improve the efficiency of dynamic_bitset<> traversing. See https://svn.boost.org/trac/boost/ticket/5158. Multiplatform support uses bits&pieces from boost/multiprecision/detail/bitscan.hpp.
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I need to do more work for testing to get these patches submitted. |
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Has dynamic_bitset<> hash actually been implemented? I am using a std::unordered_map<dynamic_bitset<>, double> but the compiler does not find a hash function. |
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Why is this closed? I needed the hash functionality here at least. |
This was referenced Apr 26, 2019
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Looks like the ball got dropped on this one. Instead of re-opening it, will (re)submit one PR per commit. The lowest_bit optimization probably needs more preprocessor conditionals to support other platforms, similar to recent changes made for popcnt acceleration. |
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This pull request should provide some performance improvements to dynamic_bitset<>::find_next() by simplifying bit operations and using built-in instructions for finding the lowest bit. It also provides dynamic_bitset<> hash implementation (so it could be used by unordered_map/set<>).
There were corresponding tickets in boost.org trac for years already:
But there were no response from the maintainers. Also my post to boost.org mailing list had seen no feedback.
So this is my attempt to get some feedback via the new Github-based boost infrastructure.