diff --git a/src/build/core/ExecuteRegisterNumber.go b/src/build/core/ExecuteRegisterNumber.go
index e2e4171..c78fe66 100644
--- a/src/build/core/ExecuteRegisterNumber.go
+++ b/src/build/core/ExecuteRegisterNumber.go
@@ -9,6 +9,10 @@ import (
 
 // ExecuteRegisterNumber performs an operation on a register and a number.
 func (f *Function) ExecuteRegisterNumber(operation token.Token, register cpu.Register, number int) error {
+	if !operation.IsAssignment() && !operation.IsComparison() {
+		f.SaveRegister(register)
+	}
+
 	switch operation.Kind {
 	case token.Add, token.AddAssign:
 		f.RegisterNumber(asm.ADD, register, number)
@@ -40,12 +44,12 @@ func (f *Function) ExecuteRegisterNumber(operation token.Token, register cpu.Reg
 	case token.Shr, token.ShrAssign:
 		f.RegisterNumber(asm.SHIFTRS, register, number)
 
-	case token.Equal, token.NotEqual, token.Less, token.LessEqual, token.Greater, token.GreaterEqual:
-		f.RegisterNumber(asm.COMPARE, register, number)
-
 	case token.Assign:
 		f.RegisterNumber(asm.MOVE, register, number)
 
+	case token.Equal, token.NotEqual, token.Less, token.LessEqual, token.Greater, token.GreaterEqual:
+		f.RegisterNumber(asm.COMPARE, register, number)
+
 	default:
 		return errors.New(&errors.InvalidOperator{Operator: operation.Text(f.File.Bytes)}, f.File, operation.Position)
 	}
diff --git a/src/build/core/ExecuteRegisterRegister.go b/src/build/core/ExecuteRegisterRegister.go
index aa8006f..129d452 100644
--- a/src/build/core/ExecuteRegisterRegister.go
+++ b/src/build/core/ExecuteRegisterRegister.go
@@ -9,6 +9,10 @@ import (
 
 // ExecuteRegisterRegister performs an operation on two registers.
 func (f *Function) ExecuteRegisterRegister(operation token.Token, register cpu.Register, operand cpu.Register) error {
+	if !operation.IsAssignment() && !operation.IsComparison() {
+		f.SaveRegister(register)
+	}
+
 	switch operation.Kind {
 	case token.Add, token.AddAssign:
 		f.RegisterRegister(asm.ADD, register, operand)
@@ -34,12 +38,12 @@ func (f *Function) ExecuteRegisterRegister(operation token.Token, register cpu.R
 	case token.Xor, token.XorAssign:
 		f.RegisterRegister(asm.XOR, register, operand)
 
-	case token.Equal, token.NotEqual, token.Less, token.LessEqual, token.Greater, token.GreaterEqual:
-		f.RegisterRegister(asm.COMPARE, register, operand)
-
 	case token.Assign:
 		f.RegisterRegister(asm.MOVE, register, operand)
 
+	case token.Equal, token.NotEqual, token.Less, token.LessEqual, token.Greater, token.GreaterEqual:
+		f.RegisterRegister(asm.COMPARE, register, operand)
+
 	default:
 		return errors.New(&errors.InvalidOperator{Operator: operation.Text(f.File.Bytes)}, f.File, operation.Position)
 	}
diff --git a/src/build/core/ExpressionToRegister.go b/src/build/core/ExpressionToRegister.go
index 1956932..e115758 100644
--- a/src/build/core/ExpressionToRegister.go
+++ b/src/build/core/ExpressionToRegister.go
@@ -12,6 +12,7 @@ import (
 // ExpressionToRegister puts the result of an expression into the specified register.
 func (f *Function) ExpressionToRegister(node *expression.Expression, register cpu.Register) error {
 	if node.IsFolded {
+		f.SaveRegister(register)
 		f.RegisterNumber(asm.MOVE, register, node.Value)
 		return nil
 	}
@@ -24,6 +25,7 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 		err := f.CompileCall(node)
 
 		if register != f.CPU.Output[0] {
+			f.SaveRegister(register)
 			f.RegisterRegister(asm.MOVE, register, f.CPU.Output[0])
 		}
 
@@ -68,6 +70,7 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 	err = f.Execute(node.Token, register, right)
 
 	if register != final {
+		f.SaveRegister(final)
 		f.RegisterRegister(asm.MOVE, final, register)
 		f.FreeRegister(register)
 	}
diff --git a/src/build/core/TokenToRegister.go b/src/build/core/TokenToRegister.go
index a7e32f3..3335daf 100644
--- a/src/build/core/TokenToRegister.go
+++ b/src/build/core/TokenToRegister.go
@@ -20,6 +20,7 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
 		}
 
 		f.UseVariable(variable)
+		f.SaveRegister(register)
 		f.RegisterRegister(asm.MOVE, register, variable.Register)
 		return nil
 
@@ -30,6 +31,7 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
 			return err
 		}
 
+		f.SaveRegister(register)
 		f.RegisterNumber(asm.MOVE, register, number)
 		return nil
 
@@ -37,6 +39,7 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
 		data := t.Bytes(f.File.Bytes)
 		data = String(data)
 		label := f.AddBytes(data)
+		f.SaveRegister(register)
 		f.RegisterLabel(asm.MOVE, register, label)
 		return nil
 
diff --git a/src/build/register/RegisterLabel.go b/src/build/register/RegisterLabel.go
index 7778e5a..70690bc 100644
--- a/src/build/register/RegisterLabel.go
+++ b/src/build/register/RegisterLabel.go
@@ -6,10 +6,6 @@ import (
 )
 
 func (f *Machine) RegisterLabel(mnemonic asm.Mnemonic, register cpu.Register, label string) {
-	if f.RegisterIsUsed(register) && isDestructive(mnemonic) {
-		f.SaveRegister(register)
-	}
-
 	f.Assembler.RegisterLabel(mnemonic, register, label)
 
 	if mnemonic == asm.MOVE {
diff --git a/src/build/register/RegisterNumber.go b/src/build/register/RegisterNumber.go
index a6b5aa6..32b552b 100644
--- a/src/build/register/RegisterNumber.go
+++ b/src/build/register/RegisterNumber.go
@@ -7,10 +7,6 @@ import (
 )
 
 func (f *Machine) RegisterNumber(mnemonic asm.Mnemonic, a cpu.Register, b int) {
-	if f.RegisterIsUsed(a) && isDestructive(mnemonic) {
-		f.SaveRegister(a)
-	}
-
 	// The `MOVE` operation is very flexible and works with any type of immediate number.
 	if mnemonic == asm.MOVE {
 		f.Assembler.RegisterNumber(mnemonic, a, b)
diff --git a/src/build/register/RegisterRegister.go b/src/build/register/RegisterRegister.go
index a969ca5..5f87783 100644
--- a/src/build/register/RegisterRegister.go
+++ b/src/build/register/RegisterRegister.go
@@ -10,10 +10,6 @@ func (f *Machine) RegisterRegister(mnemonic asm.Mnemonic, a cpu.Register, b cpu.
 		return
 	}
 
-	if f.RegisterIsUsed(a) && isDestructive(mnemonic) {
-		f.SaveRegister(a)
-	}
-
 	f.Assembler.RegisterRegister(mnemonic, a, b)
 
 	if mnemonic == asm.MOVE {
diff --git a/src/build/register/isDestructive.go b/src/build/register/isDestructive.go
deleted file mode 100644
index 9305367..0000000
--- a/src/build/register/isDestructive.go
+++ /dev/null
@@ -1,12 +0,0 @@
-package register
-
-import "git.akyoto.dev/cli/q/src/build/asm"
-
-func isDestructive(mnemonic asm.Mnemonic) bool {
-	switch mnemonic {
-	case asm.MOVE, asm.ADD, asm.SUB, asm.MUL, asm.DIV, asm.MODULO, asm.AND, asm.OR, asm.XOR, asm.SHIFTL, asm.SHIFTRS, asm.NEGATE:
-		return true
-	default:
-		return false
-	}
-}
diff --git a/src/build/token/Kind.go b/src/build/token/Kind.go
index 6bbaff8..1b45363 100644
--- a/src/build/token/Kind.go
+++ b/src/build/token/Kind.go
@@ -31,12 +31,14 @@ const (
 	Shr                         // >>
 	LogicalAnd                  // &&
 	LogicalOr                   // ||
+	_comparisons                // <comparisons>
 	Equal                       // ==
+	NotEqual                    // !=
 	Less                        // <
 	Greater                     // >
-	NotEqual                    // !=
 	LessEqual                   // <=
 	GreaterEqual                // >=
+	_comparisonsEnd             // </comparisons>
 	Define                      // :=
 	Period                      // .
 	Call                        // x()
diff --git a/src/build/token/Token.go b/src/build/token/Token.go
index 3885218..0476cbd 100644
--- a/src/build/token/Token.go
+++ b/src/build/token/Token.go
@@ -28,6 +28,11 @@ func (t Token) IsAssignment() bool {
 	return t.Kind > _assignments && t.Kind < _assignmentsEnd
 }
 
+// IsComparison returns true if the token is a comparison operator.
+func (t Token) IsComparison() bool {
+	return t.Kind > _comparisons && t.Kind < _comparisonsEnd
+}
+
 // IsExpressionStart returns true if the token starts an expression.
 func (t Token) IsExpressionStart() bool {
 	return t.Kind == GroupStart || t.Kind == ArrayStart || t.Kind == BlockStart
diff --git a/tests/programs/op-assign.q b/tests/programs/op-assign.q
new file mode 100644
index 0000000..496b3a5
--- /dev/null
+++ b/tests/programs/op-assign.q
@@ -0,0 +1,9 @@
+main() {
+	f(10)
+}
+
+f(new) {
+	old := new
+	new -= 1
+	assert new != old
+}
\ No newline at end of file
diff --git a/tests/programs_test.go b/tests/programs_test.go
index 043dbe9..4f2be2d 100644
--- a/tests/programs_test.go
+++ b/tests/programs_test.go
@@ -26,6 +26,7 @@ var programs = []struct {
 	{"return", "", "", 0},
 	{"math", "", "", 0},
 	{"precedence", "", "", 0},
+	{"op-assign", "", "", 0},
 	{"binary", "", "", 0},
 	{"octal", "", "", 0},
 	{"hexadecimal", "", "", 0},