bytes, strings: optimize Cut for single-byte separators

src/bytes/bytes.go

@@ -1330,6 +1330,12 @@ func Index(s, sep []byte) int {
 //
 // Cut returns slices of the original slice s, not copies.
 func Cut(s, sep []byte) (before, after []byte, found bool) {
+	if len(sep) == 1 {
+		if i := IndexByte(s, sep[0]); i >= 0 {
+			return s[:i], s[i+1:], true
+		}
+		return s, nil, false
+	}
 	if i := Index(s, sep); i >= 0 {
 		return s[:i], s[i+len(sep):], true
 	}

src/bytes/bytes_test.go

@@ -2263,3 +2263,31 @@ func TestClone(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkCut(b *testing.B) {
+	b.ReportAllocs()
+
+	for _, skip := range [...]int{2, 4, 8, 16, 32, 64} {
+		s := Repeat(append(append(Repeat([]byte(" "), skip), 'a', 'a'), Repeat([]byte(" "), skip)...), 1<<16/skip)
+		b.Run(fmt.Sprintf("Cut-One/%d", skip), func(b *testing.B) {
+			for range b.N {
+				_, _, _ = Cut(s, []byte{'a'})
+			}
+		})
+		b.Run(fmt.Sprintf("Cut-Two/%d", skip), func(b *testing.B) {
+			for range b.N {
+				_, _, _ = Cut(s, []byte{'a', 'a'})
+			}
+		})
+		b.Run(fmt.Sprintf("Cut-One-Nil/%d", skip), func(b *testing.B) {
+			for range b.N {
+				_, _, _ = Cut(s, []byte{'c'})
+			}
+		})
+		b.Run(fmt.Sprintf("Cut-Two-Nil/%d", skip), func(b *testing.B) {
+			for range b.N {
+				_, _, _ = Cut(s, []byte{'c', 'c'})
+			}
+		})
+	}
+}

src/internal/stringslite/strings.go

@@ -103,6 +103,12 @@ func Index(s, substr string) int {
 }
 
 func Cut(s, sep string) (before, after string, found bool) {
+	if len(sep) == 1 {
+		if i := IndexByte(s, sep[0]); i >= 0 {
+			return s[:i], s[i+1:], true
+		}
+		return s, "", false
+	}
 	if i := Index(s, sep); i >= 0 {
 		return s[:i], s[i+len(sep):], true
 	}

src/strings/strings_test.go

@@ -2071,3 +2071,31 @@ func BenchmarkReplaceAll(b *testing.B) {
 		stringSink = ReplaceAll("banana", "a", "<>")
 	}
 }
+
+func BenchmarkCut(b *testing.B) {
+	b.ReportAllocs()
+
+	for _, skip := range [...]int{2, 4, 8, 16, 32, 64} {
+		s := Repeat(Repeat(" ", skip)+"aa"+Repeat(" ", skip), 1<<16/skip)
+		b.Run(fmt.Sprintf("Cut-One/%d", skip), func(b *testing.B) {
+			for range b.N {
+				_, _, _ = Cut(s, "a")
+			}
+		})
+		b.Run(fmt.Sprintf("Cut-Two/%d", skip), func(b *testing.B) {
+			for range b.N {
+				_, _, _ = Cut(s, "aa")
+			}
+		})
+		b.Run(fmt.Sprintf("Cut-One-Nil/%d", skip), func(b *testing.B) {
+			for range b.N {
+				_, _, _ = Cut(s, "c")
+			}
+		})
+		b.Run(fmt.Sprintf("Cut-Two-Nil/%d", skip), func(b *testing.B) {
+			for range b.N {
+				_, _, _ = Cut(s, "cc")
+			}
+		})
+	}
+}