internal/pkgbits: fix performance of rawReloc

There was a TODO about quadratic performance, and indeed,
it can get bad.  Added a map, made some integers that are
unlikely to exceed a few million into 32-bit integers.

Change-Id: I6facf2eabc00483e943b326ca8dcae2f778093da
Reviewed-on: https://go-review.googlesource.com/c/go/+/422297
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 11 insertions, 7 deletions

diff --git a/src/internal/pkgbits/encoder.go b/src/internal/pkgbits/encoder.go
index 07695b5751..3859b0f091 100644
--- a/src/internal/pkgbits/encoder.go
+++ b/src/internal/pkgbits/encoder.go
@@ -151,8 +151,9 @@ func (pw *PkgEncoder) NewEncoderRaw(k RelocKind) Encoder {
 type Encoder struct {
 	p *PkgEncoder
 
-	Relocs []RelocEnt
-	Data   bytes.Buffer // accumulated element bitstream data
+	Relocs   []RelocEnt
+	RelocMap map[RelocEnt]uint32
+	Data     bytes.Buffer // accumulated element bitstream data
 
 	encodingRelocHeader bool
 
@@ -214,15 +215,18 @@ func (w *Encoder) rawVarint(x int64) {
 }
 
 func (w *Encoder) rawReloc(r RelocKind, idx Index) int {
-	// TODO(mdempsky): Use map for lookup; this takes quadratic time.
-	for i, rEnt := range w.Relocs {
-		if rEnt.Kind == r && rEnt.Idx == idx {
-			return i
+	e := RelocEnt{r, idx}
+	if w.RelocMap != nil {
+		if i, ok := w.RelocMap[e]; ok {
+			return int(i)
 		}
+	} else {
+		w.RelocMap = make(map[RelocEnt]uint32)
 	}
 
 	i := len(w.Relocs)
-	w.Relocs = append(w.Relocs, RelocEnt{r, idx})
+	w.RelocMap[e] = uint32(i)
+	w.Relocs = append(w.Relocs, e)
 	return i
 }