cyb70289 commented on code in PR #39403:
URL: https://github.com/apache/arrow/pull/39403#discussion_r1439173153
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cpp/src/arrow/util/bit_util.h:
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@@ -167,10 +167,8 @@ constexpr int64_t CoveringBytes(int64_t offset, int64_t
length) {
// Returns the 'num_bits' least-significant bits of 'v'.
static inline uint64_t TrailingBits(uint64_t v, int num_bits) {
- if (ARROW_PREDICT_FALSE(num_bits == 0)) return 0;
if (ARROW_PREDICT_FALSE(num_bits >= 64)) return v;
- int n = 64 - num_bits;
- return (v << n) >> n;
+ return ((v >> num_bits) << num_bits) ^ v;
Review Comment:
What about `return v & ~(-1ULL << num_bits);` ?
It enables gcc to optimize the code with bmi2 `bzhi`.
https://godbolt.org/z/oq9zx4nhf
And looks it's slightly faster even without bmi2.
https://quick-bench.com/q/rgBQzUFls9IP48xm3JiRPtJoI_M
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