bytes, strings: add ASCII fast path to EqualFold

This commit adds an ASCII fast path to bytes/strings EqualFold that
roughly doubles performance when all characters are ASCII.

It also changes strings.EqualFold to use `for range` for the first
string since this is ~10% faster than using utf8.DecodeRuneInString for
both (see #31666).

Performance (similar results on arm64 and amd64):

name                        old time/op  new time/op  delta
EqualFold/Tests-10           238ns ± 0%   172ns ± 1%  -27.91%  (p=0.000 n=10+10)
EqualFold/ASCII-10          20.5ns ± 0%   9.7ns ± 0%  -52.73%  (p=0.000 n=10+10)
EqualFold/UnicodePrefix-10  86.5ns ± 0%  77.6ns ± 0%  -10.37%  (p=0.000 n=10+10)
EqualFold/UnicodeSuffix-10  86.8ns ± 2%  71.3ns ± 0%  -17.88%  (p=0.000 n=10+8)

Change-Id: I058f3f97a08dc04d65af895674d85420f920abe1
Reviewed-on: https://go-review.googlesource.com/c/go/+/425459
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

1 files changed, 28 insertions, 5 deletions

diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index 9323ff988d..210bd9e44b 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -1556,13 +1556,36 @@ func TestEqualFold(t *testing.T) {
 }
 
 func BenchmarkEqualFold(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		for _, tt := range EqualFoldTests {
-			if out := EqualFold(tt.s, tt.t); out != tt.out {
-				b.Fatal("wrong result")
+	b.Run("Tests", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			for _, tt := range EqualFoldTests {
+				if out := EqualFold(tt.s, tt.t); out != tt.out {
+					b.Fatal("wrong result")
+				}
 			}
 		}
-	}
+	})
+
+	const s1 = "abcdefghijKz"
+	const s2 = "abcDefGhijKz"
+
+	b.Run("ASCII", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			EqualFold(s1, s2)
+		}
+	})
+
+	b.Run("UnicodePrefix", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			EqualFold("αβδ"+s1, "ΑΒΔ"+s2)
+		}
+	})
+
+	b.Run("UnicodeSuffix", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			EqualFold(s1+"αβδ", s2+"ΑΒΔ")
+		}
+	})
 }
 
 var CountTests = []struct {