cmd/link: use string map for symbols with single version

Reduces link time by ~3%

Results with gc on:
name       old s/op   new s/op   delta
LinkCmdGo  0.82 ± 2%  0.78 ± 2%  -3.90%  (p=0.000 n=17+17)
LinkJuju   7.11 ± 7%  6.87 ± 6%  -3.41%  (p=0.012 n=20+19)

Less noisy results with gc turned off:
name       old s/op   new s/op   delta
LinkCmdGo  0.66 ± 2%  0.64 ± 2%  -3.14%  (p=0.000 n=18+20)
LinkJuju   5.91 ± 1%  5.72 ± 2%  -3.17%  (p=0.000 n=20+20)

Change-Id: I4cac7933b0b22d0aee18255e1ab54550ad364593
Reviewed-on: https://go-review.googlesource.com/20478Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>

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

@@ -171,7 +171,13 @@ type Link struct {
 	Bso       *obj.Biobuf
 	Windows   int32
 	Goroot    string
-	Hash       map[symVer]*LSym
+
+	// Map for fast access of symbols based on name.
+	HashName map[string]*LSym
+	// Fallback map based also on version, for symbols
+	// with more than one version (see func _lookup).
+	HashVersion map[symVer]*LSym
+
 	Allsym     []*LSym
 	Nsymbol    int32
 	Tlsg       *LSym

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

@@ -58,7 +58,8 @@ var headers = []struct {
 
 func linknew(arch *LinkArch) *Link {
 	ctxt := &Link{
-		Hash:    make(map[symVer]*LSym, 100000), // preallocate about 2mb for hash
+		HashName:    make(map[string]*LSym, 100000), // preallocate about 2mb for hash
+		HashVersion: make(map[symVer]*LSym),
 		Allsym:      make([]*LSym, 0, 100000),
 		Arch:        arch,
 		Version:     obj.HistVersion,
@@ -182,17 +183,32 @@ type symVer struct {
 }
 
 func _lookup(ctxt *Link, symb string, v int, creat int) *LSym {
-	s := ctxt.Hash[symVer{symb, v}]
+	// Most symbols have only a single version, and a string key
+	// is faster to search for. So we store the first symbol in HashName,
+	// keyed only by symbol name. If there are name collisions, the
+	// alternate versions are stored in the spill over map
+	// HashVersion.
+	s, exist := ctxt.HashName[symb]
 	if s != nil {
+		if int(s.Version) == v {
 			return s
 		}
+		s = ctxt.HashVersion[symVer{symb, v}]
+		if s != nil {
+			return s
+		}
+	}
 	if creat == 0 {
 		return nil
 	}
 
 	s = linknewsym(ctxt, symb, v)
 	s.Extname = s.Name
-	ctxt.Hash[symVer{symb, v}] = s
+	if exist {
+		ctxt.HashVersion[symVer{symb, v}] = s
+	} else {
+		ctxt.HashName[symb] = s
+	}
 	return s
 }