cmd/link: batch allocations of Lsym

Reduces best of 10 linking of cmd/go by ~5%

Change-Id: If673b877ee12595dae517d7eb48430451e5cadba
Reviewed-on: https://go-review.googlesource.com/20060Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/internal/ld/link.go

@@ -158,6 +158,7 @@ type Link struct {
 	Nhistfile  int32
 	Filesyms   *LSym
 	Moduledata *LSym
+	LSymBatch  []LSym
 }
 
 // The smallest possible offset from the hardware stack pointer to a local

src/cmd/link/internal/ld/sym.go

@@ -157,8 +157,12 @@ func linknew(arch *LinkArch) *Link {
 }
 
 func linknewsym(ctxt *Link, symb string, v int) *LSym {
-	s := new(LSym)
-	*s = LSym{}
+	batch := ctxt.LSymBatch
+	if len(batch) == 0 {
+		batch = make([]LSym, 1000)
+	}
+	s := &batch[0]
+	ctxt.LSymBatch = batch[1:]
 
 	s.Dynid = -1
 	s.Plt = -1