diff --git a/src/build/core/ExpressionToRegister.go b/src/build/core/ExpressionToRegister.go
index ef69a6f..fade8b8 100644
--- a/src/build/core/ExpressionToRegister.go
+++ b/src/build/core/ExpressionToRegister.go
@@ -27,7 +27,7 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 	right := node.Children[1]
 	final := register
 
-	if OverwritesRegister(right, register) {
+	if f.UsesRegister(right, register) {
 		register = f.cpu.MustFindFree(f.cpu.General)
 	}
 
diff --git a/src/build/core/OverwritesRegister.go b/src/build/core/OverwritesRegister.go
deleted file mode 100644
index ce950f8..0000000
--- a/src/build/core/OverwritesRegister.go
+++ /dev/null
@@ -1,11 +0,0 @@
-package core
-
-import (
-	"git.akyoto.dev/cli/q/src/build/cpu"
-	"git.akyoto.dev/cli/q/src/build/expression"
-)
-
-// OverwritesRegister returns true if evaluating the expression would overwrite the given register.
-func OverwritesRegister(expr *expression.Expression, register cpu.Register) bool {
-	return !expr.IsLeaf()
-}
diff --git a/src/build/core/UsesRegister.go b/src/build/core/UsesRegister.go
new file mode 100644
index 0000000..5090f63
--- /dev/null
+++ b/src/build/core/UsesRegister.go
@@ -0,0 +1,47 @@
+package core
+
+import (
+	"git.akyoto.dev/cli/q/src/build/ast"
+	"git.akyoto.dev/cli/q/src/build/cpu"
+	"git.akyoto.dev/cli/q/src/build/expression"
+	"git.akyoto.dev/cli/q/src/build/token"
+)
+
+// UsesRegister returns true if evaluating the expression would write or read the given register.
+func (f *Function) UsesRegister(expr *expression.Expression, register cpu.Register) bool {
+	if expr.IsLeaf() {
+		if expr.Token.Kind == token.Number {
+			return false
+		}
+
+		name := expr.Token.Text()
+		variable := f.variables[name]
+		return register == variable.Register
+	}
+
+	if ast.IsFunctionCall(expr) {
+		if register == f.cpu.Output[0] {
+			return true
+		}
+
+		for i, parameter := range expr.Children[1:] {
+			if register == f.cpu.Input[i] {
+				return true
+			}
+
+			if f.UsesRegister(parameter, register) {
+				return true
+			}
+		}
+
+		return false
+	}
+
+	for _, child := range expr.Children {
+		if f.UsesRegister(child, register) {
+			return true
+		}
+	}
+
+	return false
+}