[dev.link] cmd/link: fix macho host objects

Small updates to macho loader, fixing some misunderstandings I had about
using the new object file format.

Change-Id: I9224b01ca327e3a087ebfa36800bd6eef6abcc80
Reviewed-on: https://go-review.googlesource.com/c/go/+/204097Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -168,7 +168,7 @@ func canLinkHostObj(ctxt *Link) bool {
 	if !*flagNewobj {
 		return true
 	}
-	return ctxt.IsELF
+	return ctxt.IsELF || objabi.GOOS == "darwin"
 }
 
 // mustLinkExternal reports whether the program being linked requires

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

@@ -1653,11 +1653,12 @@ func ldobj(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string,
 	if magic&^1 == 0xfeedface || magic&^0x01000000 == 0xcefaedfe {
 		if *flagNewobj {
 			ldmacho := func(ctxt *Link, f *bio.Reader, pkg string, length int64, pn string) {
-				err := loadmacho.Load(ctxt.loader, ctxt.Arch, ctxt.Syms, f, pkg, length, pn)
+				textp, err := loadmacho.Load(ctxt.loader, ctxt.Arch, ctxt.Syms, f, pkg, length, pn)
 				if err != nil {
 					Errorf(nil, "%v", err)
 					return
 				}
+				ctxt.Textp = append(ctxt.Textp, textp...)
 			}
 			return ldhostobj(ldmacho, ctxt.HeadType, f, pkg, length, pn, file)
 		} else {

src/cmd/link/internal/loader/loader.go

@@ -511,7 +511,7 @@ func (l *Loader) OuterSym(i Sym) Sym {
 	return 0
 }
 
-// SubSym gets sub symbols for a previously loaded host object symbol.
+// SubSym gets the subsymbol for host object loaded symbols.
 func (l *Loader) SubSym(i Sym) Sym {
 	sym := l.Syms[i]
 	if sym != nil && sym.Sub != nil {

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

@@ -424,31 +424,24 @@ func macholoadsym(m *ldMachoObj, symtab *ldMachoSymtab) int {
 	return 0
 }
 
-func Load(l *loader.Loader, arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length int64, pn string) error {
-	lookup := func(name string, version int) *sym.Symbol {
-		// Check to see if we've already defined the symbol.
-		if i := l.Lookup(name, version); i != 0 {
-			return l.Syms[i]
-		}
-		// Not defined, let's make one.
-		if s := l.AddExtSym(name, version); s == 0 {
+func Load(l *loader.Loader, arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length int64, pn string) ([]*sym.Symbol, error) {
+	newSym := func(name string, version int) *sym.Symbol {
+		i := l.Lookup(name, version)
+		if i != 0 {
+			return l.LoadSymbol(name, version, syms)
+		}
+		if i = l.AddExtSym(name, version); i == 0 {
 			panic("AddExtSym returned bad index")
-		} else if int(s) != len(l.Syms) {
-			panic("unexpected length of loaded symbols")
 		}
 		newSym := syms.Newsym(name, version)
-		l.Syms = append(l.Syms, newSym)
+		l.Syms[i] = newSym
 		return newSym
 	}
-	_, err := load(arch, syms.IncVersion(), lookup, f, pkg, length, pn)
-	return err
+	return load(arch, syms.IncVersion(), newSym, f, pkg, length, pn)
 }
 
 func LoadOld(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, pkg string, length int64, pn string) (textp []*sym.Symbol, err error) {
-	lookup := func(name string, version int) *sym.Symbol {
-		return syms.Lookup(name, version)
-	}
-	return load(arch, syms.IncVersion(), lookup, f, pkg, length, pn)
+	return load(arch, syms.IncVersion(), syms.Lookup, f, pkg, length, pn)
 }
 
 // load the Mach-O file pn from f.