Reshuffling of custom argument registers ignored

[saagarjha]

Version and Platform (required):

• Binary Ninja Version: 3.6.4548-dev Personal (72b6028d)
• OS: macOS
• OS Version: 14.0 (23A339)
• CPU Architecture: Apple silicon

Bug Description: If I reorder the arguments of a function such that it gets its arguments from other "parameter" registers (seemingly, the ones that are typically used for arguments), then it does not actually show up as a change in signature for the function and things that call it don't show the parameters passed correctly.

Steps To Reproduce: Please provide all steps required to reproduce the behavior:

1. Compile the following code:

_foo:
	bl _bar
	add x0, x0, x1
	ret

_bar:
	add x0, x2, x3
	ret

2. Load into Binary Ninja. Set the type for _bar to be int64_t _bar(int64_t @ x2, int64_t @ x3) (e.g. two non-standard parameters coming from specific registers).
3. Set the type for _foo to be int64_t _foo(int64_t, int64_t, int64_t, int64_t) (standard AAPCS calling convention with four parameters, first ends up unused).

Expected Behavior: I expect the signatures to be displayed as I specified. _foo does but _bar shows int64_t _bar(int64_t arg1, int64_t arg2) without the register assignments. I expect the following decompilation for _foo:

int64_t _foo(int64_t arg1, int64_t arg2, int64_t arg3, int64_t arg4)
{
      return (_bar(arg3, arg4) + arg3);
}

Instead I get:

int64_t _foo(int64_t arg1, int64_t arg2, int64_t arg3, int64_t arg4)
{
      return (_bar(arg1, arg2) + arg2);
}

This suggests that the register assignments for _bar have not been done correctly, since it thinks that _bar takes parameters using AAPCS and not the convention I specified.

Screenshots:

Additional Information: The actual reason I want this is for Objective-C retain/release stubs which take parameters in-nonstandard registers. This is my script:

for function in filter(lambda f: f.name.startswith("_objc_retain_"), bv.functions):
	register = function.name.removeprefix("_objc_retain_")
	function.type = f"id {function.name}(id @ {register})"

for function in filter(lambda f: f.name.startswith("_objc_release_"), bv.functions):
	register = function.name.removeprefix("_objc_release_")
	function.type = f"void {function.name}(id @ {register})"

This works for registers like x20 but not x1.