diff --git a/src/cmd/compile/internal/amd64/ssa.go b/src/cmd/compile/internal/amd64/ssa.go
index 4bc9dae831..fbd0a9e46c 100644
--- a/src/cmd/compile/internal/amd64/ssa.go
+++ b/src/cmd/compile/internal/amd64/ssa.go
@@ -731,7 +731,11 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		p.To.Sym = gc.Duffcopy
 		p.To.Offset = v.AuxInt
 
-	case ssa.OpCopy, ssa.OpAMD64MOVQconvert, ssa.OpAMD64MOVLconvert: // TODO: use MOVQreg for reg->reg copies instead of OpCopy?
+	case ssa.OpAMD64MOVQconvert, ssa.OpAMD64MOVLconvert:
+		if v.Args[0].Reg() != v.Reg() {
+			v.Fatalf("MOVXconvert should be a no-op")
+		}
+	case ssa.OpCopy: // TODO: use MOVQreg for reg->reg copies instead of OpCopy?
 		if v.Type.IsMemory() {
 			return
 		}
diff --git a/src/cmd/compile/internal/x86/ssa.go b/src/cmd/compile/internal/x86/ssa.go
index 54a76bda2f..a491599c58 100644
--- a/src/cmd/compile/internal/x86/ssa.go
+++ b/src/cmd/compile/internal/x86/ssa.go
@@ -604,7 +604,11 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		p.To.Sym = gc.Duffcopy
 		p.To.Offset = v.AuxInt
 
-	case ssa.OpCopy, ssa.Op386MOVLconvert: // TODO: use MOVLreg for reg->reg copies instead of OpCopy?
+	case ssa.Op386MOVLconvert:
+		if v.Args[0].Reg() != v.Reg() {
+			v.Fatalf("MOVLconvert should be a no-op")
+		}
+	case ssa.OpCopy: // TODO: use MOVLreg for reg->reg copies instead of OpCopy?
 		if v.Type.IsMemory() {
 			return
 		}