bytes,strings: in generic Index, use mix of IndexByte and Rabin-Karp

Use IndexByte first, as it allows us to skip lots of bytes quickly.
If IndexByte is generating a lot of false positives, switch over to Rabin-Karp.

Experiments for ppc64le
bytes:
name                             old time/op  new time/op  delta
IndexPeriodic/IndexPeriodic2-2   1.12ms ± 0%  0.18ms ± 0%  -83.54%  (p=0.000 n=10+9)
IndexPeriodic/IndexPeriodic4-2    635µs ± 0%   184µs ± 0%  -71.06%  (p=0.000 n=9+9)
IndexPeriodic/IndexPeriodic8-2    289µs ± 0%   184µs ± 0%  -36.51%  (p=0.000 n=10+9)
IndexPeriodic/IndexPeriodic16-2   133µs ± 0%   183µs ± 0%  +37.68%  (p=0.000 n=10+9)
IndexPeriodic/IndexPeriodic32-2  68.3µs ± 0%  70.2µs ± 0%   +2.76%  (p=0.000 n=10+10)
IndexPeriodic/IndexPeriodic64-2  35.8µs ± 0%  36.6µs ± 0%   +2.17%  (p=0.000 n=8+10)

strings:
name                             old time/op  new time/op  delta
IndexPeriodic/IndexPeriodic2-2    184µs ± 0%   184µs ± 0%   +0.11%  (p=0.029 n=4+4)
IndexPeriodic/IndexPeriodic4-2    184µs ± 0%   184µs ± 0%     ~     (p=0.886 n=4+4)
IndexPeriodic/IndexPeriodic8-2    184µs ± 0%   184µs ± 0%     ~     (p=0.486 n=4+4)
IndexPeriodic/IndexPeriodic16-2   185µs ± 1%   184µs ± 0%     ~     (p=0.343 n=4+4)
IndexPeriodic/IndexPeriodic32-2   184µs ± 0%    69µs ± 0%  -62.37%  (p=0.029 n=4+4)
IndexPeriodic/IndexPeriodic64-2   184µs ± 0%    37µs ± 0%  -80.17%  (p=0.029 n=4+4)

Fixes #22578

Change-Id: If2a4d8554cb96bfd699b58149d13ac294615f8b8
Reviewed-on: https://go-review.googlesource.com/76070
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/src/bytes/bytes_test.go b/src/bytes/bytes_test.go
index 78eca2064a..1e56571c73 100644
--- a/src/bytes/bytes_test.go
+++ b/src/bytes/bytes_test.go
@@ -139,6 +139,9 @@ var indexTests = []BinOpTest{
 	{"barfoobarfooyyyzzzyyyzzzyyyzzzyyyxxxzzzyyy", "x", 33},
 	{"foofyfoobarfoobar", "y", 4},
 	{"oooooooooooooooooooooo", "r", -1},
+	// test fallback to Rabin-Karp.
+	{"oxoxoxoxoxoxoxoxoxoxoxoy", "oy", 22},
+	{"oxoxoxoxoxoxoxoxoxoxoxox", "oy", -1},
 }
 
 var lastIndexTests = []BinOpTest{
@@ -1730,3 +1733,18 @@ func BenchmarkTrimASCII(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkIndexPeriodic(b *testing.B) {
+	key := []byte{1, 1}
+	for _, skip := range [...]int{2, 4, 8, 16, 32, 64} {
+		b.Run(fmt.Sprintf("IndexPeriodic%d", skip), func(b *testing.B) {
+			buf := make([]byte, 1<<16)
+			for i := 0; i < len(buf); i += skip {
+				buf[i] = 1
+			}
+			for i := 0; i < b.N; i++ {
+				Index(buf, key)
+			}
+		})
+	}
+}