[dev.ssa] cmd/compile: add and use SSAReg

This will be needed by other architectures as well.
Put a cleaner encapsulation around it.

Change-Id: I0ac25d600378042b2233301678e9d037e20701d8
Reviewed-on: https://go-review.googlesource.com/24154
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -381,8 +381,7 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 	case ssa.OpARMMOVWaddr:
 		if v.Aux == nil {
 			// MOVW $off(SP), R
-			reg := v.Args[0].Block.Func.RegAlloc[v.Args[0].ID].(*ssa.Register)
-			if reg.Name() != "SP" {
+			if reg := gc.SSAReg(v.Args[0]); reg.Name() != "SP" {
 				v.Fatalf("arg/auto symbol with non-SP base register %s", reg.Name())
 			}
 			p := gc.Prog(arm.AMOVW)
@@ -402,14 +401,11 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		default:
 			v.Fatalf("aux is of unknown type %T", v.Aux)
 		case *ssa.ExternSymbol:
-			reg := v.Args[0].Block.Func.RegAlloc[v.Args[0].ID].(*ssa.Register)
-			if reg.Name() != "SB" {
+			if reg := gc.SSAReg(v.Args[0]); reg.Name() != "SB" {
 				v.Fatalf("extern symbol with non-SB base register %s", reg.Name())
 			}
-		case *ssa.ArgSymbol,
-			*ssa.AutoSymbol:
-			reg := v.Args[0].Block.Func.RegAlloc[v.Args[0].ID].(*ssa.Register)
-			if reg.Name() != "SP" {
+		case *ssa.ArgSymbol, *ssa.AutoSymbol:
+			if reg := gc.SSAReg(v.Args[0]); reg.Name() != "SP" {
 				v.Fatalf("arg/auto symbol with non-SP base register %s", reg.Name())
 			}
 		}

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

@@ -4231,17 +4231,18 @@ func (s *state) extendIndex(v *ssa.Value, panicfn *Node) *ssa.Value {
 	return s.newValue1(op, Types[TINT], v)
 }
 
-// SSARegNum returns the register (in cmd/internal/obj numbering) to
-// which v has been allocated. Panics if v is not assigned to a
-// register.
-// TODO: Make this panic again once it stops happening routinely.
-func SSARegNum(v *ssa.Value) int16 {
+// SSAReg returns the register to which v has been allocated.
+func SSAReg(v *ssa.Value) *ssa.Register {
 	reg := v.Block.Func.RegAlloc[v.ID]
 	if reg == nil {
-		v.Unimplementedf("nil regnum for value: %s\n%s\n", v.LongString(), v.Block.Func)
-		return 0
+		v.Fatalf("nil register for value: %s\n%s\n", v.LongString(), v.Block.Func)
 	}
-	return Thearch.SSARegToReg[reg.(*ssa.Register).Num]
+	return reg.(*ssa.Register)
+}
+
+// SSARegNum returns the register number (in cmd/internal/obj numbering) to which v has been allocated.
+func SSARegNum(v *ssa.Value) int16 {
+	return Thearch.SSARegToReg[SSAReg(v).Num]
 }
 
 // AutoVar returns a *Node and int64 representing the auto variable and offset within it