Implemented function calls

src/build/Finalize.go

@@ -1,12 +1,18 @@
 package build
 
 import (
+	"git.akyoto.dev/cli/q/src/build/arch/x64"
 	"git.akyoto.dev/cli/q/src/build/asm"
+	"git.akyoto.dev/cli/q/src/build/os/linux"
 )
 
 // Finalize generates the final machine code.
 func Finalize(functions map[string]*Function) ([]byte, []byte) {
-	a := asm.New()
+	a := entry()
+
+	main := functions["main"]
+	delete(functions, "main")
+	a.Merge(&main.Assembler)
 
 	for _, f := range functions {
 		a.Merge(&f.Assembler)
@@ -15,3 +21,16 @@ func Finalize(functions map[string]*Function) ([]byte, []byte) {
 	code, data := a.Finalize()
 	return code, data
 }
+
+// entry returns the entry point of the executable.
+// The only job of the entry function is to call `main` and exit cleanly.
+// The reason we call `main` instead of using `main` itself is to place
+// a return address on the stack, which allows return statements in `main`.
+func entry() *asm.Assembler {
+	entry := asm.New()
+	entry.Call("main")
+	entry.MoveRegisterNumber(x64.SyscallArgs[0], linux.Exit)
+	entry.MoveRegisterNumber(x64.SyscallArgs[1], 0)
+	entry.Syscall()
+	return entry
+}