fmt: recycle printers with large buffers

Previously when a printer had a large buffer we dropped both
the buffer and the printer. There is no need to drop the printer
in this case, as a printer with a nil buffer is valid. So we
just drop the buffer and recycle the printer anyway.
This saves one allocation in case the buffer is over the limit.

Also tighten some of the tests for other unrelated cases.

Change-Id: Iba1b6a71ca4691464b8c68ab0b6ab0d4d5d6168c
Reviewed-on: https://go-review.googlesource.com/c/go/+/427395
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Rob Pike <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: CAFxX

src/fmt/fmt_test.go

@@ -1429,11 +1429,16 @@ var mallocTest = []struct {
 }{
 	{0, `Sprintf("")`, func() { Sprintf("") }},
 	{1, `Sprintf("xxx")`, func() { Sprintf("xxx") }},
-	{2, `Sprintf("%x")`, func() { Sprintf("%x", 7) }},
-	{2, `Sprintf("%s")`, func() { Sprintf("%s", "hello") }},
-	{3, `Sprintf("%x %x")`, func() { Sprintf("%x %x", 7, 112) }},
-	{2, `Sprintf("%g")`, func() { Sprintf("%g", float32(3.14159)) }}, // TODO: Can this be 1?
-	{1, `Fprintf(buf, "%s")`, func() { mallocBuf.Reset(); Fprintf(&mallocBuf, "%s", "hello") }},
+	{0, `Sprintf("%x")`, func() { Sprintf("%x", 7) }},
+	{1, `Sprintf("%x")`, func() { Sprintf("%x", 1<<16) }},
+	{3, `Sprintf("%80000s")`, func() { Sprintf("%80000s", "hello") }}, // large buffer (>64KB)
+	{1, `Sprintf("%s")`, func() { Sprintf("%s", "hello") }},
+	{1, `Sprintf("%x %x")`, func() { Sprintf("%x %x", 7, 112) }},
+	{1, `Sprintf("%g")`, func() { Sprintf("%g", float32(3.14159)) }},
+	{0, `Fprintf(buf, "%s")`, func() { mallocBuf.Reset(); Fprintf(&mallocBuf, "%s", "hello") }},
+	{0, `Fprintf(buf, "%x")`, func() { mallocBuf.Reset(); Fprintf(&mallocBuf, "%x", 7) }},
+	{0, `Fprintf(buf, "%x")`, func() { mallocBuf.Reset(); Fprintf(&mallocBuf, "%x", 1<<16) }},
+	{2, `Fprintf(buf, "%80000s")`, func() { mallocBuf.Reset(); Fprintf(&mallocBuf, "%80000s", "hello") }}, // large buffer (>64KB)
 	// If the interface value doesn't need to allocate, amortized allocation overhead should be zero.
 	{0, `Fprintf(buf, "%x %x %x")`, func() {
 		mallocBuf.Reset()

src/fmt/print.go

@@ -172,15 +172,17 @@ func newPrinter() *pp {
 func (p *pp) free() {
 	// Proper usage of a sync.Pool requires each entry to have approximately
 	// the same memory cost. To obtain this property when the stored type
-	// contains a variably-sized buffer, we add a hard limit on the maximum buffer
-	// to place back in the pool.
+	// contains a variably-sized buffer, we add a hard limit on the maximum
+	// buffer to place back in the pool. If the buffer is larger than the
+	// limit, we drop the buffer and recycle just the printer.
 	//
-	// See https://golang.org/issue/23199
-	if cap(p.buf) > 64<<10 {
-		return
+	// See https://golang.org/issue/23199.
+	if cap(p.buf) > 64*1024 {
+		p.buf = nil
+	} else {
+		p.buf = p.buf[:0]
 	}
 
-	p.buf = p.buf[:0]
 	p.arg = nil
 	p.value = reflect.Value{}
 	p.wrappedErr = nil