cmd/compile: fix type of static closure pointer

  var x *X = ...
  defer x.foo()

As part of the defer, we need to calculate &(*X).foo·f.  This expression
is the address of the static closure that will call (*X).foo when a
pointer to that closure is used in a call/defer/go.  This pointer is not
currently properly typed in SSA.  It is a pointer type, but the base
type is nil, not a proper type.

This turns out not to be a problem currently because we never use the
type of these SSA values.  But I'm trying to change that (to be able to
spill them) in CL 28391.  To fix, use uint8 as the fake type of the
closure.

Change-Id: Ieee388089c9af398ed772ee8c815122c347cb633
Reviewed-on: https://go-review.googlesource.com/29444
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -2788,9 +2788,15 @@ func (s *state) call(n *Node, k callKind) *ssa.Value {
 			sym = fn.Sym
 			break
 		}
+		// Make a name n2 for the function.
+		// fn.Sym might be sync.(*Mutex).Unlock.
+		// Make a PFUNC node out of that, then evaluate it.
+		// We get back an SSA value representing &sync.(*Mutex).Unlock·f.
+		// We can then pass that to defer or go.
 		n2 := newname(fn.Sym)
 		n2.Class = PFUNC
 		n2.Lineno = fn.Lineno
+		n2.Type = Types[TUINT8] // dummy type for a static closure. Could use runtime.funcval if we had it.
 		closure = s.expr(n2)
 		// Note: receiver is already assigned in n.List, so we don't
 		// want to set it here.

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

@@ -1141,6 +1141,9 @@ func ptrto(t *Type) *Type {
 	if Tptr == 0 {
 		Fatalf("ptrto: no tptr")
 	}
+	if t == nil {
+		Fatalf("ptrto: nil ptr")
+	}
 	// Reduce allocations by pre-creating common cases.
 	if !initPtrtoDone {
 		initPtrto()