Rollup merge of #39107 - llogiq:branchless_filter_count, r=alexcrichton

branchless .filter(_).count()

I found that the branchless version is only slower if we have little to no branch misses, which usually isn't the case. I notice speedups between -5% (perfect prediction) and 60% (real world data).

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libcoretest/iter.rs b/src/libcoretest/iter.rs
index 6d02f76c33d..c7833dbd156 100644
--- a/src/libcoretest/iter.rs
+++ b/src/libcoretest/iter.rs
@@ -192,6 +192,12 @@ fn test_iterator_enumerate_count() {
 }
 
 #[test]
+fn test_iterator_filter_count() {
+    let xs = [0, 1, 2, 3, 4, 5, 6, 7, 8];
+    assert_eq!(xs.iter().filter(|&&x| x % 2 == 0).count(), 5);
+}
+
+#[test]
 fn test_iterator_peekable() {
     let xs = vec![0, 1, 2, 3, 4, 5];
     let mut it = xs.iter().cloned().peekable();