cgen: support larger closure parameters (#13040)

vlib/v/gen/c/cheaders.v

@@ -330,6 +330,8 @@ const c_common_macros = '
 
 // returns the number of CPU registers that TYPE takes up
 #define _REG_WIDTH(T) (((sizeof(T) + sizeof(void*) - 1) & ~(sizeof(void*) - 1)) / sizeof(void*))
+// parameters of size <= 2 registers are spilled across those two registers; larger types are passed as one pointer to some stack location
+#define _REG_WIDTH_BOUNDED(T) (_REG_WIDTH(T) <= 2 ? _REG_WIDTH(T) : 1)
 
 #define OPTION_CAST(x) (x)
 

vlib/v/gen/c/fn.v

@@ -467,7 +467,7 @@ fn (mut g Gen) gen_anon_fn(mut node ast.AnonFn) {
 
 	mut size_sb := strings.new_builder(node.decl.params.len * 50)
 	for param in node.decl.params {
-		size_sb.write_string('_REG_WIDTH(${g.typ(param.typ)}) + ')
+		size_sb.write_string('_REG_WIDTH_BOUNDED(${g.typ(param.typ)}) + ')
 	}
 	if g.pref.arch == .amd64 && node.decl.return_type != ast.void_type {
 		size_sb.write_string('(_REG_WIDTH(${g.typ(node.decl.return_type)}) > 2) + ')

vlib/v/tests/closure_test.v

@@ -168,3 +168,33 @@ fn test_closures_with_ifstmt() {
 	assert f(0) == 1
 	assert g() == 1
 }
+
+struct Command {
+	a int = 1234
+	b int = 2345
+	c int = 3456
+	d int = 4567
+	e int = 5678
+}
+
+struct App {}
+
+fn main() {
+	mut app := &App{}
+	eprintln('app ptr: ${u64(app)}')
+	f := fn [mut app] (cmd Command) u64 {
+		p := u64(app)
+		println('>>   p: $p')
+		println('>> cmd: $cmd')
+		assert cmd.a == 1234
+		assert cmd.b == 2345
+		assert cmd.c == 3456
+		assert cmd.d == 4567
+		assert cmd.e == 5678
+		return p
+	}
+	cmd := Command{}
+	res := f(cmd)
+	println('> res: $res | sizeof Command: ${sizeof(Command)}')
+	assert res == u64(app)
+}