cmd/link: refactor symbol lookup

Calling the read only Linkrlookup will now not cause the name
string to escape. So a lookup can be performed on a []byte
casted to a string without allocating. This will help a followup
cl and it is also much simpler and cleaner.
Performance not impacted by this.

name       old s/op   new s/op   delta
LinkCmdGo  0.51 ± 6%  0.51 ± 5%   ~     (p=0.192 n=98+98)

Change-Id: I7846ba3160eb845a3a29cbf0be703c47369ece16
Reviewed-on: https://go-review.googlesource.com/21187Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -450,7 +450,7 @@ func ldelf(f *obj.Biobuf, pkg string, length int64, pn string) {
 		fmt.Fprintf(&Bso, "%5.2f ldelf %s\n", obj.Cputime(), pn)
 	}
 
-	Ctxt.Version++
+	Ctxt.IncVersion()
 	base := int32(obj.Boffset(f))
 
 	var add uint64

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

@@ -429,7 +429,7 @@ func ldmacho(f *obj.Biobuf, pkg string, length int64, pn string) {
 	var rp *Reloc
 	var name string
 
-	Ctxt.Version++
+	Ctxt.IncVersion()
 	base := obj.Boffset(f)
 	if obj.Bread(f, hdr[:]) != len(hdr) {
 		goto bad

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

@@ -133,7 +133,7 @@ func ldpe(f *obj.Biobuf, pkg string, length int64, pn string) {
 	}
 
 	var sect *PeSect
-	Ctxt.Version++
+	Ctxt.IncVersion()
 	base := int32(obj.Boffset(f))
 
 	peobj := new(PeObj)

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

@@ -171,11 +171,8 @@ type Link struct {
 	Windows   int32
 	Goroot    string
 
-	// 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
+	// Symbol lookup based on name and indexed by version.
+	Hash []map[string]*LSym
 
 	Allsym     []*LSym
 	Tlsg       *LSym
@@ -212,6 +209,11 @@ func (ctxt *Link) FixedFrameSize() int64 {
 	}
 }
 
+func (l *Link) IncVersion() {
+	l.Version++
+	l.Hash = append(l.Hash, make(map[string]*LSym))
+}
+
 type LinkArch struct {
 	ByteOrder binary.ByteOrder
 	Name      string

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

@@ -116,7 +116,7 @@ const (
 
 func ldobjfile(ctxt *Link, f *obj.Biobuf, pkg string, length int64, pn string) {
 	start := obj.Boffset(f)
-	ctxt.Version++
+	ctxt.IncVersion()
 	var buf [8]uint8
 	obj.Bread(f, buf[:])
 	if string(buf[:]) != startmagic {

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

@@ -58,11 +58,13 @@ var headers = []struct {
 
 func linknew(arch *LinkArch) *Link {
 	ctxt := &Link{
-		HashName:    make(map[string]*LSym, 100000), // preallocate about 2mb for hash
-		HashVersion: make(map[symVer]*LSym),
+		Hash: []map[string]*LSym{
+			// preallocate about 2mb for hash of
+			// non static symbols
+			make(map[string]*LSym, 100000),
+		},
 		Allsym: make([]*LSym, 0, 100000),
 		Arch:   arch,
-		Version:     obj.HistVersion,
 		Goroot: obj.Getgoroot(),
 	}
 
@@ -158,7 +160,7 @@ func linknew(arch *LinkArch) *Link {
 	return ctxt
 }
 
-func linknewsym(ctxt *Link, symb string, v int) *LSym {
+func linknewsym(ctxt *Link, name string, v int) *LSym {
 	batch := ctxt.LSymBatch
 	if len(batch) == 0 {
 		batch = make([]LSym, 1000)
@@ -169,55 +171,28 @@ func linknewsym(ctxt *Link, symb string, v int) *LSym {
 	s.Dynid = -1
 	s.Plt = -1
 	s.Got = -1
-	s.Name = symb
+	s.Name = name
 	s.Version = int16(v)
 	ctxt.Allsym = append(ctxt.Allsym, s)
 
 	return s
 }
 
-type symVer struct {
-	sym string
-	ver int
-}
-
-func _lookup(ctxt *Link, symb string, v int, creat int) *LSym {
-	// 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}]
+func Linklookup(ctxt *Link, name string, v int) *LSym {
+	m := ctxt.Hash[v]
+	s := m[name]
 	if s != nil {
 		return s
 	}
-	}
-	if creat == 0 {
-		return nil
-	}
-
-	s = linknewsym(ctxt, symb, v)
+	s = linknewsym(ctxt, name, v)
 	s.Extname = s.Name
-	if exist {
-		ctxt.HashVersion[symVer{symb, v}] = s
-	} else {
-		ctxt.HashName[symb] = s
-	}
+	m[name] = s
 	return s
 }
 
-func Linklookup(ctxt *Link, name string, v int) *LSym {
-	return _lookup(ctxt, name, v, 1)
-}
-
 // read-only lookup
 func Linkrlookup(ctxt *Link, name string, v int) *LSym {
-	return _lookup(ctxt, name, v, 0)
+	return ctxt.Hash[v][name]
 }
 
 func Headstr(v int) string {