cmd/link/internal/ld: avoid Reloc copies in range loops

Copying sym.Reloc in loops hurts performance as
it has 48 byte size (on 64-bit platforms).

There are quite many symbols and each of them has more than 1
relocation (so, it's possible to have more than 1kk relocs).
The're also traversed more than once in some code paths.

By using pointers to them, copies are avoided.

For linking "hello world" example from net/http:

	name      old time/op  new time/op  delta
	Linker-4   530ms ± 2%   521ms ± 3%  -1.80%  (p=0.000 n=17+20)

Change-Id: I6518aec69d6adcd137f84b5c089ceab4cb4ea2dd
Reviewed-on: https://go-review.googlesource.com/113636
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -105,7 +105,8 @@ func hostArchive(ctxt *Link, name string) {
 	for any {
 		var load []uint64
 		for _, s := range ctxt.Syms.Allsym {
-			for _, r := range s.R {
+			for i := range s.R {
+				r := &s.R[i] // Copying sym.Reloc has measurable impact on peformance
 				if r.Sym != nil && r.Sym.Type == sym.SXREF {
 					if off := armap[r.Sym.Name]; off != 0 && !loaded[off] {
 						load = append(load, off)

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

@@ -772,7 +772,8 @@ func Datblk(ctxt *Link, addr int64, size int64) {
 		if ctxt.LinkMode != LinkExternal {
 			continue
 		}
-		for _, r := range sym.R {
+		for i := range sym.R {
+			r := &sym.R[i] // Copying sym.Reloc has measurable impact on peformance
 			rsname := ""
 			if r.Sym != nil {
 				rsname = r.Sym.Name

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

@@ -245,8 +245,8 @@ func (d *deadcodepass) init() {
 				// but we do keep the symbols it refers to.
 				exports := d.ctxt.Syms.ROLookup("go.plugin.exports", 0)
 				if exports != nil {
-					for _, r := range exports.R {
-						d.mark(r.Sym, nil)
+					for i := range exports.R {
+						d.mark(exports.R[i].Sym, nil)
 					}
 				}
 			}

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

@@ -1061,7 +1061,8 @@ func getCompilationDir() string {
 func importInfoSymbol(ctxt *Link, dsym *sym.Symbol) {
 	dsym.Attr |= sym.AttrNotInSymbolTable | sym.AttrReachable
 	dsym.Type = sym.SDWARFINFO
-	for _, r := range dsym.R {
+	for i := range dsym.R {
+		r := &dsym.R[i] // Copying sym.Reloc has measurable impact on peformance
 		if r.Type == objabi.R_DWARFSECREF && r.Sym.Size == 0 {
 			if ctxt.BuildMode == BuildModeShared {
 				// These type symbols may not be present in BuildModeShared. Skip.
@@ -1090,7 +1091,8 @@ func collectAbstractFunctions(ctxt *Link, fn *sym.Symbol, dsym *sym.Symbol, absf
 
 	// Walk the relocations on the primary subprogram DIE and look for
 	// references to abstract funcs.
-	for _, reloc := range dsym.R {
+	for i := range dsym.R {
+		reloc := &dsym.R[i] // Copying sym.Reloc has measurable impact on peformance
 		candsym := reloc.Sym
 		if reloc.Type != objabi.R_DWARFSECREF {
 			continue
@@ -1801,7 +1803,8 @@ func collectlocs(ctxt *Link, syms []*sym.Symbol, units []*compilationUnit) []*sy
 	empty := true
 	for _, u := range units {
 		for _, fn := range u.funcDIEs {
-			for _, reloc := range fn.R {
+			for i := range fn.R {
+				reloc := &fn.R[i] // Copying sym.Reloc has measurable impact on peformance
 				if reloc.Type == objabi.R_DWARFSECREF && strings.HasPrefix(reloc.Sym.Name, dwarf.LocPrefix) {
 					reloc.Sym.Attr |= sym.AttrReachable | sym.AttrNotInSymbolTable
 					syms = append(syms, reloc.Sym)

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

@@ -503,7 +503,8 @@ func (ctxt *Link) loadlib() {
 		// objects, try to read them from the libgcc file.
 		any := false
 		for _, s := range ctxt.Syms.Allsym {
-			for _, r := range s.R {
+			for i := range s.R {
+				r := &s.R[i] // Copying sym.Reloc has measurable impact on peformance
 				if r.Sym != nil && r.Sym.Type == sym.SXREF && r.Sym.Name != ".got" {
 					any = true
 					break