unicode/utf8: make DecodeRune{,InString} inlineable

This change makes the fast path for ASCII characters inlineable in
DecodeRune and DecodeRuneInString and removes most instances of manual
inlining at call sites.

Here are some benchmark results (no change to allocations):

goos: darwin
goarch: amd64
pkg: unicode/utf8
cpu: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
                             │     old      │                 new                  │
                             │    sec/op    │    sec/op     vs base                │
DecodeASCIIRune-8              2.4545n ± 2%   0.6253n ± 2%  -74.52% (p=0.000 n=20)
DecodeJapaneseRune-8            3.988n ± 1%    4.023n ± 1%   +0.86% (p=0.050 n=20)
DecodeASCIIRuneInString-8      2.4675n ± 1%   0.6264n ± 2%  -74.61% (p=0.000 n=20)
DecodeJapaneseRuneInString-8    3.992n ± 1%    4.001n ± 1%        ~ (p=0.625 n=20)
geomean                         3.134n         1.585n       -49.43%

Note: when #61502 gets resolved, DecodeRune and DecodeRuneInString should
be reverted to their idiomatic implementations.

Fixes #31666
Updates #48195

Change-Id: I4be25c4f52417dc28b3a7bd72f1b04018470f39d
GitHub-Last-Rev: 2e352a0045027e059be79cdb60241b5cf35fec71
GitHub-Pull-Request: golang/go#75181
Reviewed-on: https://go-review.googlesource.com/c/go/+/699675
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

2 files changed, 9 insertions, 37 deletions

diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index ce2e004910..9a7f4ee3c9 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -528,11 +528,7 @@ func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
 	// more efficient, possibly due to cache effects.
 	start := -1 // valid span start if >= 0
 	for i := 0; i < len(s); {
-		size := 1
-		r := rune(s[i])
-		if r >= utf8.RuneSelf {
-			r, size = utf8.DecodeRune(s[i:])
-		}
+		r, size := utf8.DecodeRune(s[i:])
 		if f(r) {
 			if start >= 0 {
 				spans = append(spans, span{start, i})
@@ -614,11 +610,7 @@ func Map(mapping func(r rune) rune, s []byte) []byte {
 	// fine. It could also shrink but that falls out naturally.
 	b := make([]byte, 0, len(s))
 	for i := 0; i < len(s); {
-		wid := 1
-		r := rune(s[i])
-		if r >= utf8.RuneSelf {
-			r, wid = utf8.DecodeRune(s[i:])
-		}
+		r, wid := utf8.DecodeRune(s[i:])
 		r = mapping(r)
 		if r >= 0 {
 			b = utf8.AppendRune(b, r)
@@ -917,11 +909,7 @@ func LastIndexFunc(s []byte, f func(r rune) bool) int {
 func indexFunc(s []byte, f func(r rune) bool, truth bool) int {
 	start := 0
 	for start < len(s) {
-		wid := 1
-		r := rune(s[start])
-		if r >= utf8.RuneSelf {
-			r, wid = utf8.DecodeRune(s[start:])
-		}
+		r, wid := utf8.DecodeRune(s[start:])
 		if f(r) == truth {
 			return start
 		}
@@ -1052,10 +1040,7 @@ func trimLeftASCII(s []byte, as *asciiSet) []byte {
 
 func trimLeftUnicode(s []byte, cutset string) []byte {
 	for len(s) > 0 {
-		r, n := rune(s[0]), 1
-		if r >= utf8.RuneSelf {
-			r, n = utf8.DecodeRune(s)
-		}
+		r, n := utf8.DecodeRune(s)
 		if !containsRune(cutset, r) {
 			break
 		}
@@ -1251,19 +1236,10 @@ hasUnicode:
 	t = t[i:]
 	for len(s) != 0 && len(t) != 0 {
 		// Extract first rune from each.
-		var sr, tr rune
-		if s[0] < utf8.RuneSelf {
-			sr, s = rune(s[0]), s[1:]
-		} else {
-			r, size := utf8.DecodeRune(s)
-			sr, s = r, s[size:]
-		}
-		if t[0] < utf8.RuneSelf {
-			tr, t = rune(t[0]), t[1:]
-		} else {
-			r, size := utf8.DecodeRune(t)
-			tr, t = r, t[size:]
-		}
+		sr, size := utf8.DecodeRune(s)
+		s = s[size:]
+		tr, size := utf8.DecodeRune(t)
+		t = t[size:]
 
 		// If they match, keep going; if not, return false.
 
diff --git a/src/bytes/iter.go b/src/bytes/iter.go
index b2abb2c9ba..a4ece881d2 100644
--- a/src/bytes/iter.go
+++ b/src/bytes/iter.go
@@ -117,11 +117,7 @@ func FieldsFuncSeq(s []byte, f func(rune) bool) iter.Seq[[]byte] {
 	return func(yield func([]byte) bool) {
 		start := -1
 		for i := 0; i < len(s); {
-			size := 1
-			r := rune(s[i])
-			if r >= utf8.RuneSelf {
-				r, size = utf8.DecodeRune(s[i:])
-			}
+			r, size := utf8.DecodeRune(s[i:])
 			if f(r) {
 				if start >= 0 {
 					if !yield(s[start:i:i]) {