[dev.link] cmd/link: resolve ABI aliases for external symbols

ABI alias references in Go symbols are resolved during
loadObjFull. But for external symbols they are not resolved. If
there is a reference from an external symbol to a Go ABIInternal
symbol, this reference will be invalid as it is not resolved.

The old code resolve ABI aliases in the deadcode pass. But the
new deadcode pass doesn't do it, as it works with indices instead
of Symbols. We do this in LoadFull.

This makes all internal cgo linking tests pass on Mach-O.

Change-Id: Iac6c084c03f5ddbcc9455527800ce7ed7313f9a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/204698
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

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

@@ -2620,7 +2620,7 @@ func (ctxt *Link) loadlibfull() {
 
 func (ctxt *Link) dumpsyms() {
 	for _, s := range ctxt.Syms.Allsym {
-		fmt.Printf("%s %s %p\n", s, s.Type, s)
+		fmt.Printf("%s %s %p %v %v\n", s, s.Type, s, s.Attr.Reachable(), s.Attr.OnList())
 		for i := range s.R {
 			fmt.Println("\t", s.R[i].Type, s.R[i].Sym)
 		}

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

@@ -753,6 +753,21 @@ func (l *Loader) LoadFull(arch *sys.Arch, syms *sym.Symbols) {
 	for _, o := range l.objs[1:] {
 		loadObjFull(l, o.r)
 	}
+
+	// Resolve ABI aliases for external symbols. This is only
+	// needed for internal cgo linking.
+	// (The old code does this in deadcode, but deadcode2 doesn't
+	// do this.)
+	for i := l.extStart; i <= l.max; i++ {
+		if s := l.Syms[i]; s != nil && s.Attr.Reachable() {
+			for ri := range s.R {
+				r := &s.R[ri]
+				if r.Sym != nil && r.Sym.Type == sym.SABIALIAS {
+					r.Sym = r.Sym.R[0].Sym
+				}
+			}
+		}
+	}
 }
 
 // ExtractSymbols grabs the symbols out of the loader for work that hasn't been