cmd/link: symbol generation optimizations

After making dwarf generation backed by LSyms there was a performance regression
of about 10%. These changes make on the fly symbol generation faster and
are meant to help mitigate that.

name       old secs    new secs    delta
LinkCmdGo   0.55 ± 9%   0.53 ± 8%  -4.42%   (p=0.000 n=100+99)

name       old MaxRSS  new MaxRSS  delta
LinkCmdGo   152k ± 6%   149k ± 3%  -1.99%    (p=0.000 n=99+97)

Change-Id: Iacca3ec924ce401aa83126bc0b10fe89bedf0ba6
Reviewed-on: https://go-review.googlesource.com/21733
Run-TryBot: Shahar Kohanim <skohanim@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

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

@@ -50,8 +50,9 @@ func Symgrow(ctxt *Link, s *LSym, siz int64) {
 	if int64(len(s.P)) >= siz {
 		return
 	}
-	for cap(s.P) < int(siz) {
-		s.P = append(s.P[:len(s.P)], 0)
+	if cap(s.P) < int(siz) {
+		p := make([]byte, 2*(siz+1))
+		s.P = append(p[:0], s.P...)
 	}
 	s.P = s.P[:siz]
 }
@@ -90,11 +91,8 @@ func Addbytes(ctxt *Link, s *LSym, bytes []byte) int64 {
 		s.Type = obj.SDATA
 	}
 	s.Attr |= AttrReachable
-	s.Size += int64(len(bytes))
-	if int64(int(s.Size)) != s.Size {
-		log.Fatalf("Addbytes size %d too long", s.Size)
-	}
 	s.P = append(s.P, bytes...)
+	s.Size = int64(len(s.P))
 
 	return s.Size
 }
@@ -106,7 +104,15 @@ func adduintxx(ctxt *Link, s *LSym, v uint64, wid int) int64 {
 }
 
 func Adduint8(ctxt *Link, s *LSym, v uint8) int64 {
-	return adduintxx(ctxt, s, uint64(v), 1)
+	off := s.Size
+	if s.Type == 0 {
+		s.Type = obj.SDATA
+	}
+	s.Attr |= AttrReachable
+	s.Size++
+	s.P = append(s.P, v)
+
+	return off
 }
 
 func Adduint16(ctxt *Link, s *LSym, v uint16) int64 {
@@ -1006,16 +1012,14 @@ func Addstring(s *LSym, str string) int64 {
 		s.Type = obj.SNOPTRDATA
 	}
 	s.Attr |= AttrReachable
-	r := int32(s.Size)
-	n := len(str) + 1
+	r := s.Size
 	if s.Name == ".shstrtab" {
 		elfsetstring(str, int(r))
 	}
-	Symgrow(Ctxt, s, int64(r)+int64(n))
-	copy(s.P[r:], str)
-	s.P[int(r)+len(str)] = 0
-	s.Size += int64(n)
-	return int64(r)
+	s.P = append(s.P, str...)
+	s.P = append(s.P, 0)
+	s.Size = int64(len(s.P))
+	return r
 }
 
 // addgostring adds str, as a Go string value, to s. symname is the name of the

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

@@ -88,9 +88,7 @@ func uleb128put(s *LSym, v int64) {
 
 func sleb128put(s *LSym, v int64) {
 	b := appendSleb128(encbuf[:0], v)
-	for _, x := range b {
-		Adduint8(Ctxt, s, x)
-	}
+	Addbytes(Ctxt, s, b)
 }
 
 /*
@@ -552,8 +550,15 @@ func findchild(die *DWDie, name string) *DWDie {
 	return nil
 }
 
+// Used to avoid string allocation when looking up dwarf symbols
+var prefixBuf = []byte(infoprefix)
+
 func find(name string) *LSym {
-	return Linkrlookup(Ctxt, infoprefix+name, 0)
+	n := append(prefixBuf, name...)
+	// The string allocation below is optimized away because it is only used in a map lookup.
+	s := Linkrlookup(Ctxt, string(n), 0)
+	prefixBuf = n[:len(infoprefix)]
+	return s
 }
 
 func mustFind(name string) *LSym {

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

@@ -127,8 +127,7 @@ func addpctab(ftab *LSym, off int32, d *Pcdata) int32 {
 	var start int32
 	if len(d.P) > 0 {
 		start = int32(len(ftab.P))
-		Symgrow(Ctxt, ftab, int64(start)+int64(len(d.P)))
-		copy(ftab.P[start:], d.P)
+		Addbytes(Ctxt, ftab, d.P)
 	}
 	return int32(setuint32(Ctxt, ftab, int64(off), uint32(start)))
 }

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

@@ -54,15 +54,9 @@ func putelfstr(s string) int {
 		s = strings.Replace(s, "·", ".", -1)
 	}
 
-	n := len(s) + 1
-	for len(Elfstrdat)+n > cap(Elfstrdat) {
-		Elfstrdat = append(Elfstrdat[:cap(Elfstrdat)], 0)[:len(Elfstrdat)]
-	}
-
 	off := len(Elfstrdat)
-	Elfstrdat = Elfstrdat[:off+n]
-	copy(Elfstrdat[off:], s)
-
+	Elfstrdat = append(Elfstrdat, s...)
+	Elfstrdat = append(Elfstrdat, 0)
 	return off
 }