2 files changed, 49 insertions, 4 deletions
diff --git a/src/unicode/utf8/utf8.go b/src/unicode/utf8/utf8.go
index 01cad1cc81..68283341d9 100644
--- a/src/unicode/utf8/utf8.go
+++ b/src/unicode/utf8/utf8.go
@@ -155,6 +155,20 @@ func FullRuneInString(s string) bool {
 // out of range, or is not the shortest possible UTF-8 encoding for the
 // value. No other validation is performed.
 func DecodeRune(p []byte) (r rune, size int) {
+	// Inlineable fast path for ASCII characters; see #48195.
+	// This implementation is weird but effective at rendering the
+	// function inlineable.
+	for _, b := range p {
+		if b < RuneSelf {
+			return rune(b), 1
+		}
+		break
+	}
+	r, size = decodeRuneSlow(p)
+	return
+}
+
+func decodeRuneSlow(p []byte) (r rune, size int) {
 	n := len(p)
 	if n < 1 {
 		return RuneError, 0
@@ -203,6 +217,18 @@ func DecodeRune(p []byte) (r rune, size int) {
 // out of range, or is not the shortest possible UTF-8 encoding for the
 // value. No other validation is performed.
 func DecodeRuneInString(s string) (r rune, size int) {
+	// Inlineable fast path for ASCII characters; see #48195.
+	// This implementation is a bit weird but effective at rendering the
+	// function inlineable.
+	if s != "" && s[0] < RuneSelf {
+		return rune(s[0]), 1
+	} else {
+		r, size = decodeRuneInStringSlow(s)
+	}
+	return
+}
+
+func decodeRuneInStringSlow(s string) (rune, int) {
 	n := len(s)
 	if n < 1 {
 		return RuneError, 0
diff --git a/src/unicode/utf8/utf8_test.go b/src/unicode/utf8/utf8_test.go
index aece0fab73..bf4f074ffd 100644
--- a/src/unicode/utf8/utf8_test.go
+++ b/src/unicode/utf8/utf8_test.go
@@ -747,18 +747,37 @@ func BenchmarkAppendInvalidRuneNegative(b *testing.B) {
 
 func BenchmarkDecodeASCIIRune(b *testing.B) {
 	a := []byte{'a'}
-	for i := 0; i < b.N; i++ {
-		DecodeRune(a)
+	for range b.N {
+		runeSink, sizeSink = DecodeRune(a)
 	}
 }
 
 func BenchmarkDecodeJapaneseRune(b *testing.B) {
 	nihon := []byte("本")
-	for i := 0; i < b.N; i++ {
-		DecodeRune(nihon)
+	for range b.N {
+		runeSink, sizeSink = DecodeRune(nihon)
+	}
+}
+
+func BenchmarkDecodeASCIIRuneInString(b *testing.B) {
+	a := "a"
+	for range b.N {
+		runeSink, sizeSink = DecodeRuneInString(a)
 	}
 }
 
+func BenchmarkDecodeJapaneseRuneInString(b *testing.B) {
+	nihon := "本"
+	for range b.N {
+		runeSink, sizeSink = DecodeRuneInString(nihon)
+	}
+}
+
+var (
+	runeSink rune
+	sizeSink int
+)
+
 // boolSink is used to reference the return value of benchmarked
 // functions to avoid dead code elimination.
 var boolSink bool