alamb commented on code in PR #10968:
URL: https://github.com/apache/arrow-rs/pull/10968#discussion_r4115925246
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arrow-buffer/src/util/bit_util.rs:
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@@ -113,29 +116,40 @@ pub fn compress(value: u64, mask: u64) -> u64 {
/// assert_eq!(expand(compress(value, mask), mask), value & mask);
/// ```
#[inline]
-pub fn expand(mut value: u64, mask: u64) -> u64 {
- if value == 0 {
- return 0;
- }
-
- let mut zeros = !mask;
- if zeros.count_ones() <= 32 {
- // Insert zeros from low to high; excess input bits shift out.
- while zeros != 0 {
- let lower = (1_u64 << zeros.trailing_zeros()) - 1;
- value = (value & lower) | ((value & !lower) << 1);
- zeros &= zeros - 1;
+pub fn expand(value: u64, mask: u64) -> u64 {
+ #[cfg(all(target_arch = "x86_64", target_feature = "bmi2"))]
+ {
+ // SAFETY: the `bmi2` target feature is statically enabled for this
+ // build, so the `pdep` instruction is guaranteed to be available.
+ unsafe { std::arch::x86_64::_pdep_u64(value, mask) }
Review Comment:
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