runtime/pprof: compute memory profile block size using sampled values

Fixes #26638.

Change-Id: I3c18d1298d99af8ea8c00916303efd2b5a5effc7
Reviewed-on: https://go-review.googlesource.com/126336Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/pprof/protomem.go

@@ -56,8 +56,8 @@ func writeHeapProto(w io.Writer, p []runtime.MemProfileRecord, rate int64, defau
 		values[0], values[1] = scaleHeapSample(r.AllocObjects, r.AllocBytes, rate)
 		values[2], values[3] = scaleHeapSample(r.InUseObjects(), r.InUseBytes(), rate)
 		var blockSize int64
-		if values[0] > 0 {
-			blockSize = values[1] / values[0]
+		if r.AllocObjects > 0 {
+			blockSize = r.AllocBytes / r.AllocObjects
 		}
 		b.pbSample(values, locs, func() {
 			if blockSize != 0 {

src/runtime/pprof/protomem_test.go

@@ -48,7 +48,7 @@ func TestConvertMemProfile(t *testing.T) {
 				{ID: 3, Mapping: map2, Address: addr2 + 1},
 				{ID: 4, Mapping: map2, Address: addr2 + 2},
 			},
-			NumLabel: map[string][]int64{"bytes": {829411}},
+			NumLabel: map[string][]int64{"bytes": {512 * 1024}},
 		},
 		{
 			Value: []int64{1, 829411, 0, 0},
@@ -57,7 +57,7 @@ func TestConvertMemProfile(t *testing.T) {
 				{ID: 6, Mapping: map1, Address: addr1 + 2},
 				{ID: 7, Mapping: map2, Address: addr2 + 3},
 			},
-			NumLabel: map[string][]int64{"bytes": {829411}},
+			NumLabel: map[string][]int64{"bytes": {512 * 1024}},
 		},
 	}
 	for _, tc := range []struct {