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 <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/debug/gosym/pclntab.go b/src/debug/gosym/pclntab.go
index b2bd914ad7600d84b065df243ab094a519732dea..4d312d22f12e302ee37efd0e4e4065c6d5e2fd94 100644 (file)
--- a/src/debug/gosym/pclntab.go
+++ b/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.