[dev.ssa] cmd/compile: fix -N build

The OpSB hack didn't quite work.  We need to really
CSE these ops to make regalloc happy.

Change-Id: I9f4d7bfb0929407c84ee60c9e25ff0c0fbea84af
Reviewed-on: https://go-review.googlesource.com/19083
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -92,7 +92,7 @@ var passes = [...]pass{
 	{"decompose", decompose, true},
 	{"opt", opt, true},                // TODO: split required rules and optimizing rules
 	{"opt deadcode", deadcode, false}, // remove any blocks orphaned during opt
-	{"generic cse", cse, false},
+	{"generic cse", cse, true},
 	{"nilcheckelim", nilcheckelim, false},
 	{"generic deadcode", deadcode, false},
 	{"fuse", fuse, false},

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

@@ -10,6 +10,34 @@ import "sort"
 // Values are just relinked, nothing is deleted.  A subsequent deadcode
 // pass is required to actually remove duplicate expressions.
 func cse(f *Func) {
+	if !f.Config.optimize {
+		// Don't do CSE in this case.  But we need to do
+		// just a little bit, to combine multiple OpSB ops.
+		// Regalloc gets very confused otherwise.
+		var sb *Value
+	outer:
+		for _, b := range f.Blocks {
+			for _, v := range b.Values {
+				if v.Op == OpSB {
+					sb = v
+					break outer
+				}
+			}
+		}
+		if sb == nil {
+			return
+		}
+		for _, b := range f.Blocks {
+			for _, v := range b.Values {
+				for i, a := range v.Args {
+					if a.Op == OpSB {
+						v.Args[i] = sb
+					}
+				}
+			}
+		}
+		return
+	}
 	// Two values are equivalent if they satisfy the following definition:
 	// equivalent(v, w):
 	//   v.op == w.op

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

@@ -316,12 +316,6 @@ func (s *regAllocState) assignReg(r register, v *Value, c *Value) {
 		fmt.Printf("assignReg %s %s/%s\n", registers[r].Name(), v, c)
 	}
 	if s.regs[r].v != nil {
-		if v.Op == OpSB && !v.Block.Func.Config.optimize {
-			// Rewrite rules may introduce multiple OpSB, and with
-			// -N they don't get CSEd.  Ignore the extra assignments.
-			s.f.setHome(c, &registers[r])
-			return
-		}
 		s.f.Fatalf("tried to assign register %d to %s/%s but it is already used by %s", r, v, c, s.regs[r].v)
 	}