Optimize bit count polyfills #2914
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src/support/bits.cpp
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| template<> int PopCount<uint64_t>(uint64_t v) { | ||
| return PopCount((uint32_t)v) + PopCount((uint32_t)(v >> 32)); | ||
| return PopCount((uint32_t)v) + (v >> 32 ? PopCount((uint32_t)(v >> 32)) : 0); |
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What is the motivation for this change? This seems slightly harder to read than before.
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Main motivation is consistency with CountLeadingZeros for 64-bit and potentially speedup calculation when high part of 64-bit is zero
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I think the consistency with CountLeadingZeroes is a non-goal here because they are fundamentally different optimizations. If we could rewrite CountLeadingZeroes to match PopCount, I think we would want to do that for improved readability.
If we're going to sacrifice some readability for performance, it would be good to see that the performance difference is measurable rather than hypothetical.
This PR LGTM other than this point, so it might be nice to split this out and land the rest.
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Alright. I switched to prev implementation for PopCount<uint64_t>
| template<typename T> bool IsPowerOf2(T v) { | ||
| return v != 0 && (v & (v - 1)) == 0; | ||
| } |
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Nice 👍 I like that this lets us delete code.
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usually popcnt(x) == 1 (btw it include x != 0 case also) faster but only if we have native popcnt support. Alos LLVM & GCC smart enought to replace pattern above to popcnt(x) == 1 if it possible
| template<> int PopCount<uint64_t>(uint64_t v) { | ||
| #if __has_builtin(__builtin_popcountll) || defined(__GNUC__) | ||
| return __builtin_popcountll(v); | ||
| #if __has_builtin(__builtin_popcount) || defined(__GNUC__) || defined(_MSC_VER) |
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use __has_builtin(__builtin_popcount) instead __has_builtin(__builtin_popcountll) due to clang-format forcing line terminator and carry defined(_MSC_VER) to new line and this looks weird.
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| template<typename T, typename U> inline static T RotateLeft(T val, U count) { | ||
| T mask = sizeof(T) * CHAR_BIT - 1; | ||
| auto value = typename std::make_unsigned<T>::type(val); |
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pretty important cast to unsigned. Otherwise not LLVM nor GCC can fold this to single rol / rot op
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