debug/gosym: use sort.Search in findFunc

Use sort.Search instead of open-coding the binary search.
This makes the code a lot easier to work on.

As a bonus, it speeds it up.

name            old time/op    new time/op    delta
115/LineToPC-8    57.4µs ± 5%    59.2µs ± 8%   +3.19%  (p=0.003 n=15+13)
115/PCToLine-8     255ns ± 1%     192ns ± 3%  -24.63%  (p=0.000 n=15+15)

Change-Id: I41da18bfb0e745c40d24e5b96e50dfdd0c3b79f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/353879
Trust: Josh Bleecher Snyder <[email protected]>
Run-TryBot: Josh Bleecher Snyder <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

src/debug/gosym/pclntab.go

@@ -11,6 +11,7 @@ package gosym
 import (
  "bytes"
  "encoding/binary"
+ "sort"
  "sync"
 )
 
@@ -319,25 +320,13 @@ func (t *LineTable) findFunc(pc uint64) funcData {
  if pc < t.uintptr(t.functab) || pc >= t.uintptr(t.functab[len(t.functab)-int(t.ptrsize):]) {
  return funcData{}
  }
-
  // The function table is a list of 2*nfunctab+1 uintptrs,
  // alternating program counters and offsets to func structures.
- f := t.functab
- nf := t.nfunctab
- for nf > 0 {
- m := nf / 2
- fm := f[2*t.ptrsize*m:]
- if t.uintptr(fm) <= pc && pc < t.uintptr(fm[2*t.ptrsize:]) {
- data := t.funcdata[t.uintptr(fm[t.ptrsize:]):]
- return funcData{t: t, data: data}
- } else if pc < t.uintptr(fm) {
- nf = m
- } else {
- f = f[(m+1)*2*t.ptrsize:]
- nf -= m + 1
- }
- }
- return funcData{}
+ idx := sort.Search(int(t.nfunctab), func(i int) bool {
+ return t.uintptr(t.functab[2*i*int(t.ptrsize):]) > pc
+ })
+ idx--
+ return t.funcData(uint32(idx))
 }
 
 // readvarint reads, removes, and returns a varint from *pp.