cmd/compile: write type symbols referenced in ptabs

The exported symbol for a plugin can be the only reference to a
type in a program. In particular, "var F func()" will have
the type *func(), which is uncommon.

Fixes #17140

Change-Id: Ide2104edbf087565f5377374057ae54e0c00c57e
Reviewed-on: https://go-review.googlesource.com/29692
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

misc/cgo/testplugin/src/host/host.go

@@ -61,6 +61,15 @@ func main() {
 		log.Fatalf("plugin.Open(%q) failed: %v", subpPath, err)
 	}
 
+	funcVar, err := subp.Lookup("FuncVar")
+	if err != nil {
+		log.Fatalf(`sub/plugin1.Lookup("FuncVar") failed: %v`, err)
+	}
+	called := false
+	*funcVar.(*func()) = func() {
+		called = true
+	}
+
 	readFunc, err = subp.Lookup("ReadCommonX")
 	if err != nil {
 		log.Fatalf(`sub/plugin1.Lookup("ReadCommonX") failed: %v`, err)
@@ -68,6 +77,9 @@ func main() {
 	if got := readFunc.(func() int)(); got != wantX {
 		log.Fatalf("sub/plugin1.ReadCommonX()=%d, want %d", got, wantX)
 	}
+	if !called {
+		log.Fatal("calling ReadCommonX did not call FuncVar")
+	}
 
 	subf, err := subp.Lookup("F")
 	if err != nil {

misc/cgo/testplugin/src/sub/plugin1/plugin1.go

@@ -11,7 +11,10 @@ import "common"
 
 func F() int { return 17 }
 
+var FuncVar = func() {}
+
 func ReadCommonX() int {
+	FuncVar()
 	return common.X
 }
 

src/cmd/compile/internal/gc/reflect.go

@@ -1416,7 +1416,7 @@ func dumptypestructs() {
 			// }
 			nsym := dname(p.s.Name, "", nil, true)
 			ot = dsymptrOffLSym(s, ot, nsym, 0)
-			ot = dsymptrOffLSym(s, ot, Linksym(typesym(p.t)), 0)
+			ot = dsymptrOffLSym(s, ot, Linksym(dtypesym(p.t)), 0)
 		}
 		ggloblLSym(s, int32(ot), int16(obj.RODATA))
 

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

@@ -290,6 +290,11 @@ func (d *deadcodepass) flood() {
 		}
 
 		if strings.HasPrefix(s.Name, "type.") && s.Name[5] != '.' {
+			if len(s.P) == 0 {
+				// Probably a bug. The undefined symbol check
+				// later will give a better error than deadcode.
+				continue
+			}
 			if decodetypeKind(s)&kindMask == kindInterface {
 				for _, sig := range decodeIfaceMethods(d.ctxt.Arch, s) {
 					if d.ctxt.Debugvlog > 1 {