cmd/link: don't mark a symbol's GoType reachable when -linkshared

In CL 231397, we stopped marking symbols' GoType reachable in
general, but not when -linkshared. It was left as a TODO. This CL
addresses it.

The problem was that the type names are mangled in the shared
library, so we need to mangle the name consistently in the
executable as well (regardless of whether the symbol is reachable
or not), so that the GCProg generation code can find the
corresponding symbol from the shared library.

Change-Id: I1040747402929a983ec581109f1681a77893682e
Reviewed-on: https://go-review.googlesource.com/c/go/+/255964
Trust: Cherry Zhang <[email protected]>
Run-TryBot: Cherry Zhang <[email protected]>
Reviewed-by: Jeremy Faller <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: cherrymui

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

@@ -174,13 +174,9 @@ func (d *deadcodePass) flood() {
 		naux := d.ldr.NAux(symIdx)
 		for i := 0; i < naux; i++ {
 			a := d.ldr.Aux(symIdx, i)
-			if a.Type() == goobj.AuxGotype && !d.ctxt.linkShared {
+			if a.Type() == goobj.AuxGotype {
 				// A symbol being reachable doesn't imply we need its
 				// type descriptor. Don't mark it.
-				// TODO: when -linkshared, the GCProg generation code
-				// seems to need it. I'm not sure why. I think it could
-				// just reach to the type descriptor's data without
-				// requiring to mark it reachable.
 				continue
 			}
 			d.mark(a.Sym(), symIdx)

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

@@ -831,7 +831,12 @@ func (ctxt *Link) mangleTypeSym() {
 
 	ldr := ctxt.loader
 	for s := loader.Sym(1); s < loader.Sym(ldr.NSym()); s++ {
-		if !ldr.AttrReachable(s) {
+		if !ldr.AttrReachable(s) && !ctxt.linkShared {
+			// If -linkshared, the GCProg generation code may need to reach
+			// out to the shared library for the type descriptor's data, even
+			// the type descriptor itself is not actually needed at run time
+			// (therefore not reachable). We still need to mangle its name,
+			// so it is consistent with the one stored in the shared library.
 			continue
 		}
 		name := ldr.SymName(s)