strconv: pre-allocate in appendQuotedWith

The byte-at-a-time allocation done quoting strings in appendQuotedWith
grows the output incrementally, which is poor behavior for very large
strings. An easy fix is to make sure the buffer has enough room at
least for an unquoted string.

Add a benchmark with a megabyte of non-ASCII data.
	Before: 39 allocations.
	After: 7 allocations.

We could do better by doing a lot more work but this seems like a big
result for little effort.

Fixes #31472.

Change-Id: I852139e0a2bd13722c4dd329ded8ae1759abad5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/172677Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/strconv/quote.go

@@ -25,6 +25,13 @@ func quoteRuneWith(r rune, quote byte, ASCIIonly, graphicOnly bool) string {
 }
 
 func appendQuotedWith(buf []byte, s string, quote byte, ASCIIonly, graphicOnly bool) []byte {
+	// Often called with big strings, so preallocate. If there's quoting,
+	// this is conservative but still helps a lot.
+	if cap(buf)-len(buf) < len(s) {
+		nBuf := make([]byte, len(buf), len(buf)+1+len(s)+1)
+		copy(nBuf, buf)
+		buf = nBuf
+	}
 	buf = append(buf, quote)
 	for width := 0; len(s) > 0; s = s[width:] {
 		r := rune(s[0])

src/strconv/strconv_test.go

@@ -30,6 +30,9 @@ var (
 			AppendFloat(localBuf[:0], 1.23, 'g', 5, 64)
 		}},
 		{0, `AppendFloat(globalBuf[:0], 1.23, 'g', 5, 64)`, func() { AppendFloat(globalBuf[:0], 1.23, 'g', 5, 64) }},
+		// In practice we see 7 for the next one, but allow some slop.
+		// Before pre-allocation in appendQuotedWith, we saw 39.
+		{10, `AppendQuoteToASCII(nil, oneMB)`, func() { AppendQuoteToASCII(nil, string(oneMB)) }},
 		{0, `ParseFloat("123.45", 64)`, func() { ParseFloat("123.45", 64) }},
 		{0, `ParseFloat("123.456789123456789", 64)`, func() { ParseFloat("123.456789123456789", 64) }},
 		{0, `ParseFloat("1.000000000000000111022302462515654042363166809082031251", 64)`, func() {
@@ -41,6 +44,8 @@ var (
 	}
 )
 
+var oneMB []byte // Will be allocated to 1MB of random data by TestCountMallocs.
+
 func TestCountMallocs(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skipping malloc count in short mode")
@@ -48,6 +53,11 @@ func TestCountMallocs(t *testing.T) {
 	if runtime.GOMAXPROCS(0) > 1 {
 		t.Skip("skipping; GOMAXPROCS>1")
 	}
+	// Allocate a big messy buffer for AppendQuoteToASCII's test.
+	oneMB = make([]byte, 1e6)
+	for i := range oneMB {
+		oneMB[i] = byte(i)
+	}
 	for _, mt := range mallocTest {
 		allocs := testing.AllocsPerRun(100, mt.fn)
 		if max := float64(mt.count); allocs > max {