cmd/compile: reuseable cache of Prog structs

Reuseable cache of Prog entries.

Improves compiler speed by ~10%.

Update #13646

Change-Id: I01bd8606540d989ea8b8ba5131d1275ba380d976
Reviewed-on: https://go-review.googlesource.com/19868
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/link.go

@@ -618,6 +618,10 @@ type Link struct {
 	Data  *LSym
 	Etext *LSym
 	Edata *LSym
+
+	// Cache of Progs
+	allocIdx int
+	progs    [10000]Prog
 }
 
 func (ctxt *Link) Diag(format string, args ...interface{}) {

src/cmd/internal/obj/objfile.go

@@ -318,6 +318,7 @@ func Flushplist(ctxt *Link) {
 	ctxt.Plist = nil
 	ctxt.Plast = nil
 	ctxt.Curp = nil
+	ctxt.freeProgs()
 }
 
 func Writeobjfile(ctxt *Link, b *Biobuf) {

src/cmd/internal/obj/util.go

@@ -325,10 +325,23 @@ func (p *Prog) String() string {
 }
 
 func (ctxt *Link) NewProg() *Prog {
-	p := new(Prog) // should be the only call to this; all others should use ctxt.NewProg
+	var p *Prog
+	if i := ctxt.allocIdx; i < len(ctxt.progs) {
+		p = &ctxt.progs[i]
+		ctxt.allocIdx = i + 1
+	} else {
+		p = new(Prog) // should be the only call to this; all others should use ctxt.NewProg
+	}
 	p.Ctxt = ctxt
 	return p
 }
+func (ctxt *Link) freeProgs() {
+	s := ctxt.progs[:ctxt.allocIdx]
+	for i := range s {
+		s[i] = Prog{}
+	}
+	ctxt.allocIdx = 0
+}
 
 func (ctxt *Link) Line(n int) string {
 	return ctxt.LineHist.LineString(n)