[dev.ssa] cmd/compile: treat control ops as live at end of block

Failure to treat control ops as live can lead
to them being eliminated when they live in
other blocks.

Change-Id: I604a1977a3d3884b1f4516bea4e15885ce38272d
Reviewed-on: https://go-review.googlesource.com/13138Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -137,6 +137,13 @@ func checkFunc(f *Func) {
 		}
 	}
 
+	for _, b := range f.Blocks {
+		if b.Control != nil {
+			if !valueMark[b.Control.ID] {
+				f.Fatalf("control value for %s is missing: %v", b, b.Control)
+			}
+		}
+	}
 	for _, id := range f.bid.free {
 		if blockMark[id] {
 			f.Fatalf("used block b%d in free list", id)

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

@@ -440,6 +440,9 @@ func live(f *Func) [][]ID {
 			// Start with known live values at the end of the block
 			s.clear()
 			s.addAll(live[b.ID])
+			if b.Control != nil {
+				s.add(b.Control.ID)
+			}
 
 			// Propagate backwards to the start of the block
 			// Assumes Values have been scheduled.
@@ -456,7 +459,7 @@ func live(f *Func) [][]ID {
 			}
 
 			// for each predecessor of b, expand its list of live-at-end values
-			// inv: s contains the values live at the start of b (excluding phi inputs)
+			// invariant: s contains the values live at the start of b (excluding phi inputs)
 			for i, p := range b.Preds {
 				t.clear()
 				t.addAll(live[p.ID])

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

+// 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.
+
+package ssa
+
+import "testing"
+
+func TestLiveControlOps(t *testing.T) {
+	c := testConfig(t)
+	f := Fun(c, "entry",
+		Bloc("entry",
+			Valu("mem", OpArg, TypeMem, 0, ".mem"),
+			Valu("x", OpAMD64MOVBconst, TypeInt8, 0, 1),
+			Valu("y", OpAMD64MOVBconst, TypeInt8, 0, 2),
+			Valu("a", OpAMD64TESTB, TypeBool, 0, nil, "x", "y"),
+			Valu("b", OpAMD64TESTB, TypeBool, 0, nil, "y", "x"),
+			If("a", "if", "exit"),
+		),
+		Bloc("if",
+			If("b", "plain", "exit"),
+		),
+		Bloc("plain",
+			Goto("exit"),
+		),
+		Bloc("exit",
+			Exit("mem"),
+		),
+	)
+	regalloc(f.f)
+	checkFunc(f.f)
+}