cmd/pprof: make runtime.reflectcall followable by pprof

[thwd]

Context: http://stackoverflow.com/q/31419307/532430

Proposal: allow pprof to follow reflective function invocations via runtime.reflectcall so that code "after the jump" can be profiled.

I think the idea is simple enough but if anything was unclear just ask and I'll try to clarify.

[ianlancetaylor]

To be honest, I'm not sure this has to be a proposal. I think the current behaviour is a bug. I don't know what is involved in fixing it, but I can't imagine that anybody would object to a good fix.

[adg]

Agree, this needn't be a proposal.

[rsc]

This is a bug in pprof alone. The full information is in the profile, but the elision of runtime frames is getting confused by the reflect call and eliding everything below it. You can see the extra information using -runtime.

$ cat x_test.go
package x

import (
    "reflect"
    "testing"
)

func TestReflect(t *testing.T) {
    v := reflect.ValueOf(f)
    for i := 0; i < 10000; i++ {
        v.Call(nil)
    }
}

var x [][]byte

func f() {
    x = append(x, make([]byte, 10000))
}
$ go test -memprofile=x.prof x_test.go
ok      command-line-arguments  0.093s
$ go tool pprof -top x.test x.prof
100.50MB of 100.50MB total (  100%)
      flat  flat%   sum%        cum   cum%
  100.50MB   100%   100%   100.50MB   100%  reflect.Value.call
         0     0%   100%   100.50MB   100%  command-line-arguments.TestReflect
         0     0%   100%   100.50MB   100%  reflect.Value.Call
         0     0%   100%   100.50MB   100%  runtime.goexit
         0     0%   100%   100.50MB   100%  testing.tRunner
$ go tool pprof -top -runtime x.test x.prof
100.50MB of 100.50MB total (  100%)
Dropped 1 node (cum <= 0.50MB)
      flat  flat%   sum%        cum   cum%
  100.50MB   100%   100%   100.50MB   100%  runtime.makeslice
         0     0%   100%   100.50MB   100%  command-line-arguments.TestReflect
         0     0%   100%   100.50MB   100%  command-line-arguments.f
         0     0%   100%   100.50MB   100%  reflect.Value.Call
         0     0%   100%   100.50MB   100%  reflect.Value.call
         0     0%   100%   100.50MB   100%  runtime.call32
         0     0%   100%   100.50MB   100%  runtime.goexit
         0     0%   100%   100.50MB   100%  testing.tRunner
$

[rsc]

We ran out of time for this in Go 1.6.