[dev.ssa] cmd/compile/internal/ssa: fix defer in functions with no return

The after-defer test jumps to a deferreturn site.  Some functions
(those with infinite loops) have no deferreturn site.  Add one
so we have one to jump to.

Change-Id: I505e7f3f888f5e7d03ca49a3477b41cf1f78eb8a
Reviewed-on: https://go-review.googlesource.com/14349
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -2621,6 +2621,12 @@ func genssa(f *ssa.Func, ptxt *obj.Prog, gcargs, gclocals *Sym) {
 	for _, br := range s.branches {
 		br.p.To.Val = s.bstart[br.b.ID]
 	}
+	if s.deferBranches != nil && s.deferTarget == nil {
+		// This can happen when the function has a defer but
+		// no return (because it has an infinite loop).
+		s.deferReturn()
+		Prog(obj.ARET)
+	}
 	for _, p := range s.deferBranches {
 		p.To.Val = s.deferTarget
 	}
@@ -3463,20 +3469,7 @@ func (s *genState) genBlock(b, next *ssa.Block) {
 	case ssa.BlockExit:
 	case ssa.BlockRet:
 		if Hasdefer != 0 {
-			// Deferred calls will appear to be returning to
-			// the CALL deferreturn(SB) that we are about to emit.
-			// However, the stack trace code will show the line
-			// of the instruction byte before the return PC.
-			// To avoid that being an unrelated instruction,
-			// insert an actual hardware NOP that will have the right line number.
-			// This is different from obj.ANOP, which is a virtual no-op
-			// that doesn't make it into the instruction stream.
-			s.deferTarget = Pc
-			Thearch.Ginsnop()
-			p := Prog(obj.ACALL)
-			p.To.Type = obj.TYPE_MEM
-			p.To.Name = obj.NAME_EXTERN
-			p.To.Sym = Linksym(Deferreturn.Sym)
+			s.deferReturn()
 		}
 		Prog(obj.ARET)
 	case ssa.BlockCall:
@@ -3537,6 +3530,23 @@ func (s *genState) genBlock(b, next *ssa.Block) {
 	}
 }
 
+func (s *genState) deferReturn() {
+	// Deferred calls will appear to be returning to
+	// the CALL deferreturn(SB) that we are about to emit.
+	// However, the stack trace code will show the line
+	// of the instruction byte before the return PC.
+	// To avoid that being an unrelated instruction,
+	// insert an actual hardware NOP that will have the right line number.
+	// This is different from obj.ANOP, which is a virtual no-op
+	// that doesn't make it into the instruction stream.
+	s.deferTarget = Pc
+	Thearch.Ginsnop()
+	p := Prog(obj.ACALL)
+	p.To.Type = obj.TYPE_MEM
+	p.To.Name = obj.NAME_EXTERN
+	p.To.Sym = Linksym(Deferreturn.Sym)
+}
+
 // addAux adds the offset in the aux fields (AuxInt and Aux) of v to a.
 func addAux(a *obj.Addr, v *ssa.Value) {
 	if a.Type != obj.TYPE_MEM {

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

@@ -17,12 +17,18 @@ import (
 // TODO: move all these tests elsewhere?
 // Perhaps teach test/run.go how to run them with a new action verb.
 func runTest(t *testing.T, filename string) {
+	doTest(t, filename, "run")
+}
+func buildTest(t *testing.T, filename string) {
+	doTest(t, filename, "build")
+}
+func doTest(t *testing.T, filename string, kind string) {
 	if runtime.GOARCH != "amd64" {
 		t.Skipf("skipping SSA tests on %s for now", runtime.GOARCH)
 	}
 	testenv.MustHaveGoBuild(t)
 	var stdout, stderr bytes.Buffer
-	cmd := exec.Command("go", "run", filepath.Join("testdata", filename))
+	cmd := exec.Command("go", kind, filepath.Join("testdata", filename))
 	cmd.Stdout = &stdout
 	cmd.Stderr = &stderr
 	// TODO: set GOGC=off until we have stackmaps
@@ -70,3 +76,5 @@ func TestMap(t *testing.T) { runTest(t, "map_ssa.go") }
 func TestRegalloc(t *testing.T) { runTest(t, "regalloc_ssa.go") }
 
 func TestString(t *testing.T) { runTest(t, "string_ssa.go") }
+
+func TestDeferNoReturn(t *testing.T) { buildTest(t, "deferNoReturn_ssa.go") }

src/cmd/compile/internal/gc/testdata/deferNoReturn_ssa.go

@@ -0,0 +1,21 @@
+// compile
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that a defer in a function with no return
+// statement will compile correctly.
+
+package main
+
+func deferNoReturn_ssa() {
+	defer func() { println("returned") }()
+	for {
+		println("loop")
+	}
+}
+
+func main() {
+	deferNoReturn_ssa()
+}