strings: make use of sizeclasses in (*Builder).Grow

Fixes #64833

Change-Id: Ice3f5dfab65f5525bc7a6f57ddeaabda8d64dfa3
GitHub-Last-Rev: 38f1d6c19d
GitHub-Pull-Request: golang/go#64835
Reviewed-on: https://go-review.googlesource.com/c/go/+/552135
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/bytes/bytes.go

@@ -525,7 +525,7 @@ func Join(s [][]byte, sep []byte) []byte {
 		n += len(v)
 	}
 
-	b := bytealg.MakeNoZero(n)
+	b := bytealg.MakeNoZero(n)[:n:n]
 	bp := copy(b, s[0])
 	for _, v := range s[1:] {
 		bp += copy(b[bp:], sep)
@@ -610,7 +610,7 @@ func Repeat(b []byte, count int) []byte {
 			chunkMax = len(b)
 		}
 	}
-	nb := bytealg.MakeNoZero(n)
+	nb := bytealg.MakeNoZero(n)[:n:n]
 	bp := copy(nb, b)
 	for bp < n {
 		chunk := bp
@@ -640,7 +640,7 @@ func ToUpper(s []byte) []byte {
 			// Just return a copy.
 			return append([]byte(""), s...)
 		}
-		b := bytealg.MakeNoZero(len(s))
+		b := bytealg.MakeNoZero(len(s))[:len(s):len(s)]
 		for i := 0; i < len(s); i++ {
 			c := s[i]
 			if 'a' <= c && c <= 'z' {
@@ -670,7 +670,7 @@ func ToLower(s []byte) []byte {
 		if !hasUpper {
 			return append([]byte(""), s...)
 		}
-		b := bytealg.MakeNoZero(len(s))
+		b := bytealg.MakeNoZero(len(s))[:len(s):len(s)]
 		for i := 0; i < len(s); i++ {
 			c := s[i]
 			if 'A' <= c && c <= 'Z' {

src/internal/bytealg/bytealg.go

@@ -111,7 +111,8 @@ func LastIndexRabinKarp[T string | []byte](s, sep T) int {
 	return -1
 }
 
-// MakeNoZero makes a slice of length and capacity n without zeroing the bytes.
+// MakeNoZero makes a slice of length n and capacity of at least n Bytes
+// without zeroing the bytes (including the bytes between len and cap).
 // It is the caller's responsibility to ensure uninitialized bytes
 // do not leak to the end user.
 func MakeNoZero(n int) []byte

src/runtime/slice.go

@@ -366,5 +366,6 @@ func bytealg_MakeNoZero(len int) []byte {
 	if uintptr(len) > maxAlloc {
 		panicmakeslicelen()
 	}
-	return unsafe.Slice((*byte)(mallocgc(uintptr(len), nil, false)), len)
+	cap := roundupsize(uintptr(len), true)
+	return unsafe.Slice((*byte)(mallocgc(uintptr(cap), nil, false)), cap)[:len]
 }

src/strings/builder.go

@@ -15,6 +15,10 @@ import (
 // Do not copy a non-zero Builder.
 type Builder struct {
 	addr *Builder // of receiver, to detect copies by value
+
+	// External users should never get direct access to this buffer, since
+	// the slice at some point will be converted to a string using unsafe, also
+	// data between len(buf) and cap(buf) might be uninitialized.
 	buf []byte
 }
 

src/strings/builder_test.go

@@ -385,3 +385,16 @@ func BenchmarkBuildString_ByteBuffer(b *testing.B) {
 		}
 	})
 }
+
+func TestBuilderGrowSizeclasses(t *testing.T) {
+	s := Repeat("a", 19)
+	allocs := testing.AllocsPerRun(100, func() {
+		var b Builder
+		b.Grow(18)
+		b.WriteString(s)
+		_ = b.String()
+	})
+	if allocs > 1 {
+		t.Fatalf("unexpected amount of allocations: %v, want: 1", allocs)
+	}
+}