alamb commented on code in PR #11322:
URL: https://github.com/apache/arrow-rs/pull/11322#discussion_r4271497618
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arrow-buffer/src/util/bit_util.rs:
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@@ -64,6 +73,29 @@ pub fn compress(value: u64, mask: u64) -> u64 {
#[cfg(not(all(target_arch = "x86_64", target_feature = "bmi2")))]
{
+ compress_portable(value, mask)
+ }
+}
+
+/// [`compress`] without `pext`, by the number of kept bits `k`:
+///
+/// * `k <= 2`: the two lowest kept bits, tested directly
+/// * `k <= 16`: one step per kept bit
+/// * `k >= 62`: `value` with its at most two dropped bits removed
+/// * otherwise [`compress_bytes`]
+///
+/// The two ends need no loop, so no loop exit to mispredict when `k`
+/// varies from word to word.
+// Always: called once a word, so a call would cost more than a sparse word
+#[cfg(any(test, not(all(target_arch = "x86_64", target_feature = "bmi2"))))]
+#[inline(always)]
+fn compress_portable(value: u64, mask: u64) -> u64 {
+ let kept = mask.count_ones();
Review Comment:
Oh boy, we are getting fancy now
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