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diff --git a/‎src/strconv/quote.go‎
Lines changed: 7 additions & 0 deletions b/‎src/strconv/quote.go‎
Lines changed: 7 additions & 0 deletions
diff --git a/‎src/strconv/strconv_test.go‎
Lines changed: 10 additions & 0 deletions b/‎src/strconv/strconv_test.go‎
Lines changed: 10 additions & 0 deletions
@@ -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])
 
@@ -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() {
@@ -41,13 +44,20 @@ 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")
 	}
 	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 {