cmd/compile: use a boolean as a avoid clobbering flags mov marker

The Value type implements Aux interface because it is being used as a
"avoid clobbering flags" marker by amd64, x86 and s390x SSA parts.

Create a boolean that implements the Aux interface. Use it as the marker
instead. We no longer need Value to implement Aux.

Resolves a TODO.

See CL 275756 for more info.

Change-Id: I8a1eddf7e738b8aa31e82f3c4c590bafd2cdc56b
Reviewed-on: https://go-review.googlesource.com/c/go/+/461156
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Jakub Ciolek <jakub@ciolek.dev>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/compile/internal/amd64/ssa.go

@@ -30,7 +30,7 @@ func ssaMarkMoves(s *ssagen.State, b *ssa.Block) {
 		v := b.Values[i]
 		if flive && (v.Op == ssa.OpAMD64MOVLconst || v.Op == ssa.OpAMD64MOVQconst) {
 			// The "mark" is any non-nil Aux value.
-			v.Aux = v
+			v.Aux = ssa.AuxMark
 		}
 		if v.Type.IsFlags() {
 			flive = false

src/cmd/compile/internal/s390x/ssa.go

@@ -26,7 +26,7 @@ func ssaMarkMoves(s *ssagen.State, b *ssa.Block) {
 		v := b.Values[i]
 		if flive && v.Op == ssa.OpS390XMOVDconst {
 			// The "mark" is any non-nil Aux value.
-			v.Aux = v
+			v.Aux = ssa.AuxMark
 		}
 		if v.Type.IsFlags() {
 			flive = false

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

@@ -729,6 +729,13 @@ type Aux interface {
 	CanBeAnSSAAux()
 }
 
+// for now only used to mark moves that need to avoid clobbering flags
+type auxMark bool
+
+func (auxMark) CanBeAnSSAAux() {}
+
+var AuxMark auxMark
+
 // stringAux wraps string values for use in Aux.
 type stringAux string
 

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

@@ -554,9 +554,6 @@ func (v *Value) removeable() bool {
 	return true
 }
 
-// TODO(mdempsky): Shouldn't be necessary; see discussion at golang.org/cl/275756
-func (*Value) CanBeAnSSAAux() {}
-
 // AutoVar returns a *Name and int64 representing the auto variable and offset within it
 // where v should be spilled.
 func AutoVar(v *Value) (*ir.Name, int64) {

src/cmd/compile/internal/x86/ssa.go

@@ -28,7 +28,7 @@ func ssaMarkMoves(s *ssagen.State, b *ssa.Block) {
 		v := b.Values[i]
 		if flive && v.Op == ssa.Op386MOVLconst {
 			// The "mark" is any non-nil Aux value.
-			v.Aux = v
+			v.Aux = ssa.AuxMark
 		}
 		if v.Type.IsFlags() {
 			flive = false