[dev.ssa] cmd/compile/internal/ssa: handle returns correctly

Make sure that return blocks take a store as their control.  Without
this, code was getting inserted between the return and exit blocks.

Use AEND to mark the end of code.  The live variable analysis gets
confused when routines end like:
    JMP earlier
    RET
because the RET is unreachable.  The RET was incorrectly added to the
last basic block, rendering the JMP invisible to the CFG builder.

Change-Id: I91b32c8b37075347243ff039b4e4385856fba7cd
Reviewed-on: https://go-review.googlesource.com/14398Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -425,7 +425,7 @@ func newcfg(firstp *obj.Prog) []*BasicBlock {
 
 	bb := newblock(firstp)
 	cfg = append(cfg, bb)
-	for p := firstp; p != nil; p = p.Link {
+	for p := firstp; p != nil && p.As != obj.AEND; p = p.Link {
 		Thearch.Proginfo(p)
 		if p.To.Type == obj.TYPE_BRANCH {
 			if p.To.Val == nil {
@@ -453,7 +453,7 @@ func newcfg(firstp *obj.Prog) []*BasicBlock {
 	// contained instructions until a label is reached.  Add edges
 	// for branches and fall-through instructions.
 	for _, bb := range cfg {
-		for p := bb.last; p != nil; p = p.Link {
+		for p := bb.last; p != nil && p.As != obj.AEND; p = p.Link {
 			if p.Opt != nil && p != bb.last {
 				break
 			}
@@ -462,6 +462,8 @@ func newcfg(firstp *obj.Prog) []*BasicBlock {
 			// Stop before an unreachable RET, to avoid creating
 			// unreachable control flow nodes.
 			if p.Link != nil && p.Link.As == obj.ARET && p.Link.Mode == 1 {
+				// TODO: remove after SSA is done.  SSA does not
+				// generate any unreachable RET instructions.
 				break
 			}
 

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

@@ -116,8 +116,11 @@ func buildssa(fn *Node) (ssafn *ssa.Func, usessa bool) {
 	s.stmtList(fn.Nbody)
 
 	// fallthrough to exit
-	if b := s.endBlock(); b != nil {
+	if s.curBlock != nil {
+		m := s.mem()
+		b := s.endBlock()
 		b.Kind = ssa.BlockRet
+		b.Control = m
 		b.AddEdgeTo(s.exit)
 	}
 
@@ -575,8 +578,10 @@ func (s *state) stmt(n *Node) {
 
 	case ORETURN:
 		s.stmtList(n.List)
+		m := s.mem()
 		b := s.endBlock()
 		b.Kind = ssa.BlockRet
+		b.Control = m
 		b.AddEdgeTo(s.exit)
 
 	case OCONTINUE, OBREAK:
@@ -2631,8 +2636,6 @@ func genssa(f *ssa.Func, ptxt *obj.Prog, gcargs, gclocals *Sym) {
 		p.To.Val = s.deferTarget
 	}
 
-	Pc.As = obj.ARET // overwrite AEND
-
 	if logProgs {
 		for p := ptxt; p != nil; p = p.Link {
 			var s string

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

@@ -63,8 +63,11 @@ func checkFunc(f *Func) {
 			if len(b.Succs) != 1 {
 				f.Fatalf("ret block %s len(Succs)==%d, want 1", b, len(b.Succs))
 			}
-			if b.Control != nil {
-				f.Fatalf("ret block %s has non-nil control %s", b, b.Control.LongString())
+			if b.Control == nil {
+				f.Fatalf("ret block %s has nil control %s", b)
+			}
+			if !b.Control.Type.IsMemory() {
+				f.Fatalf("ret block %s has non-memory control value %s", b, b.Control.LongString())
 			}
 			if b.Succs[0].Kind != BlockExit {
 				f.Fatalf("ret block %s has successor %s, not Exit", b, b.Succs[0].Kind)

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -369,9 +369,11 @@ var genericOps = []opData{
 //     kind           control    successors
 //   ------------------------------------------
 //     Exit        return mem                []
+//      Ret        return mem            [exit]
 //    Plain               nil            [next]
 //       If   a boolean Value      [then, else]
-//     Call               mem  [nopanic, panic]  (control opcode should be OpCall or OpStaticCall)
+//     Call               mem   [nopanic, exit]  (control opcode should be OpCall or OpStaticCall)
+//    First               nil    [always,never]
 
 var genericBlocks = []blockData{
 	{name: "Exit"},  // no successors.  There should only be 1 of these.