Reduced number of register moves

src/core/CompileReturn.go

@@ -18,7 +18,13 @@ func (f *Function) CompileReturn(node *ast.Return) error {
 	}
 
 	for i := len(node.Values) - 1; i >= 0; i-- {
-		f.SaveRegister(f.CPU.Output[i])
+		for j := range i {
+			if f.UsesRegister(node.Values[j], f.CPU.Output[i]) {
+				f.SaveRegister(f.CPU.Output[i])
+				break
+			}
+		}
+
 		typ, err := f.ExpressionToRegister(node.Values[i], f.CPU.Output[i])
 
 		if err != nil {

src/core/UsesRegister.go

@@ -4,12 +4,23 @@ import (
 	"git.akyoto.dev/cli/q/src/ast"
 	"git.akyoto.dev/cli/q/src/cpu"
 	"git.akyoto.dev/cli/q/src/expression"
+	"git.akyoto.dev/cli/q/src/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() {
-		return false
+		if expr.Token.Kind != token.Identifier {
+			return false
+		}
+
+		variable := f.VariableByName(expr.Token.Text(f.File.Bytes))
+
+		if variable == nil || variable.Register != register {
+			return false
+		}
+
+		return true
 	}
 
 	if ast.IsFunctionCall(expr) {