Repository: go
Issue: cmd/pprof: weblist gives wrong flat count
Labels: NeedsInvestigation,compiler/runtime

### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.17.6 windows/amd64
</pre>

### Does this issue reproduce with the latest release?
Yes.

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\a3367\AppData\Local\go-build
set GOENV=C:\Users\a3367\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\a3367\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\a3367\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.17.6
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=NUL
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\a3367\AppData\Local\Temp\go-build2334633044=/tmp/go-build -gno-record-gcc-switches
gdb --version: GNU gdb (GDB) 8.1
</pre></details>

### What did you do?
I profiled the standard library's `png.Encode` function and looked at this attached profile:

[png_encode.zip](https://github.com/golang/go/files/8368856/png_encode.zip)

I then ran both `weblist` and `list` on the `png.filter` function.

### What did you expect to see?
I expect both `weblist` and `list` to show the same result.

### What did you see instead?
I saw `list` showing the correct result, with the sum of the `flat` time of `png.abs8` in `png.filter` tallying with the total time spent on `png.abs8`.
In particular, the below log shows that `png.abs8` took 1.56 seconds in total.
```
PS C:\Users\a3367\work\misc\seg\output> go tool pprof .\png_encode.pprof
Type: cpu
Time: Mar 29, 2022 at 1:28pm (CST)
Duration: 31.43s, Total samples = 17.53s (55.77%)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 16.92s, 96.52% of 17.53s total
Dropped 91 nodes (cum <= 0.09s)
Showing top 10 nodes out of 29
      flat  flat%   sum%        cum   cum%
     7.96s 45.41% 45.41%     13.07s 74.56%  image/png.filter
        2s 11.41% 56.82%      2.01s 11.47%  image/png.abs
     1.74s  9.93% 66.74%      1.74s  9.93%  runtime.memmove
     1.56s  8.90% 75.64%      1.56s  8.90%  image/png.abs8
     1.44s  8.21% 83.86%      3.46s 19.74%  image/png.paeth
     0.90s  5.13% 88.99%      0.90s  5.13%  hash/adler32.update
     0.80s  4.56% 93.55%      0.81s  4.62%  compress/flate.(*deflateFast).matchLen
     0.29s  1.65% 95.21%      0.31s  1.77%  compress/flate.(*huffmanEncoder).bitCounts
     0.12s  0.68% 95.89%      0.12s  0.68%  runtime.asyncPreempt
     0.11s  0.63% 96.52%      1.04s  5.93%  compress/flate.(*deflateFast).encode
(pprof)
```

Adding the time spent on `png.abs8` in `png.filter` also shows the same 1.56 seconds.
```
(pprof) list png.filter
Total: 17.53s
ROUTINE ======================== image/png.filter in C:\Program Files\Go\src\image\png\writer.go
     7.96s     13.07s (flat, cum) 74.56% of Total
         .          .    202:   return len(b), nil
         .          .    203:}
         .          .    204:
         .          .    205:// Chooses the filter to use for encoding the current row, and applies it.
         .          .    206:// The return value is the index of the filter and also of the row in cr that has had it applied.
      10ms       10ms    207:func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {
         .          .    208:   // We try all five filter types, and pick the one that minimizes the sum of absolute differences.
         .          .    209:   // This is the same heuristic that libpng uses, although the filters are attempted in order of
         .          .    210:   // estimated most likely to be minimal (ftUp, ftPaeth, ftNone, ftSub, ftAverage), rather than
         .          .    211:   // in their enumeration order (ftNone, ftSub, ftUp, ftAverage, ftPaeth).
         .          .    212:   cdat0 := cr[0][1:]
     110ms      110ms    213:   cdat1 := cr[1][1:]
         .          .    214:   cdat2 := cr[2][1:]
         .          .    215:   cdat3 := cr[3][1:]
         .          .    216:   cdat4 := cr[4][1:]
         .          .    217:   pdat := pr[1:]
         .          .    218:   n := len(cdat0)
         .          .    219:
         .          .    220:   // The up filter.
         .          .    221:   sum := 0
     310ms      310ms    222:   for i := 0; i < n; i++ {
     1.02s      1.03s    223:           cdat2[i] = cdat0[i] - pdat[i]
     230ms      940ms    224:           sum += abs8(cdat2[i])
         .          .    225:   }
         .          .    226:   best := sum
         .          .    227:   filter := ftUp
         .          .    228:
         .          .    229:   // The Paeth filter.
         .          .    230:   sum = 0
         .          .    231:   for i := 0; i < bpp; i++ {
         .          .    232:           cdat4[i] = cdat0[i] - pdat[i]
         .          .    233:           sum += abs8(cdat4[i])
         .          .    234:   }
     790ms      790ms    235:   for i := bpp; i < n; i++ {
     3.49s      6.99s    236:           cdat4[i] = cdat0[i] - paeth(cdat0[i-bpp], pdat[i], pdat[i-bpp])
     240ms      760ms    237:           sum += abs8(cdat4[i])
     180ms      180ms    238:           if sum >= best {
         .          .    239:                   break
         .          .    240:           }
         .          .    241:   }
      40ms       40ms    242:   if sum < best {
         .          .    243:           best = sum
         .          .    244:           filter = ftPaeth
         .          .    245:   }
         .          .    246:
         .          .    247:   // The none filter.
         .          .    248:   sum = 0
         .          .    249:   for i := 0; i < n; i++ {
         .          .    250:           sum += abs8(cdat0[i])
      10ms       10ms    251:           if sum >= best {
         .          .    252:                   break
         .          .    253:           }
         .          .    254:   }
         .          .    255:   if sum < best {
         .          .    256:           best = sum
         .          .    257:           filter = ftNone
         .          .    258:   }
         .          .    259:
         .          .    260:   // The sub filter.
         .          .    261:   sum = 0
         .          .    262:   for i := 0; i < bpp; i++ {
         .          .    263:           cdat1[i] = cdat0[i]
         .          .    264:           sum += abs8(cdat1[i])
         .          .    265:   }
     190ms      210ms    266:   for i := bpp; i < n; i++ {
     990ms         1s    267:           cdat1[i] = cdat0[i] - cdat0[i-bpp]
      80ms      390ms    268:           sum += abs8(cdat1[i])
     150ms      150ms    269:           if sum >= best {
         .          .    270:                   break
         .          .    271:           }
         .          .    272:   }
         .          .    273:   if sum < best {
         .          .    274:           best = sum
         .          .    275:           filter = ftSub
         .          .    276:   }
         .          .    277:
         .          .    278:   // The average filter.
         .          .    279:   sum = 0
         .          .    280:   for i := 0; i < bpp; i++ {
      70ms       80ms    281:           cdat3[i] = cdat0[i] - pdat[i]/2
         .          .    282:           sum += abs8(cdat3[i])
         .          .    283:   }
         .          .    284:   for i := bpp; i < n; i++ {
      30ms       30ms    285:           cdat3[i] = cdat0[i] - uint8((int(cdat0[i-bpp])+int(pdat[i]))/2)
      10ms       30ms    286:           sum += abs8(cdat3[i])
      10ms       10ms    287:           if sum >= best {
         .          .    288:                   break
         .          .    289:           }
         .          .    290:   }
         .          .    291:   if sum < best {
         .          .    292:           filter = ftAverage
(pprof)
```

However, `weblist` shows the wrong result, it seems to exagerate the result.
[Pprof listing.zip](https://github.com/golang/go/files/8368908/Pprof.listing.zip)
For example, on line 268, `list` shows only 80ms spent, whereas `weblist` shows 360ms spent.