// The aarch64 calling conventions save the return PC one i64 above the FP and
// the previous FP is pointed to by the current FP:
//
// > Each frame shall link to the frame of its caller by means of a frame record
// > of two 64-bit values on the stack [...] The frame record for the innermost
// > frame [...] shall be pointed to by the frame pointer register (FP). The
// > lowest addressed double-word shall point to the previous frame record and the
// > highest addressed double-word shall contain the value passed in LR on entry
// > to the current function.
//
// - AAPCS64 section 6.2.3 The Frame Pointer[0]
pub unsafe fn get_next_older_pc_from_fp(fp: usize) -> usize {
    let mut pc = *(fp as *mut usize).offset(1);

    // The return address might be signed, so we need to strip the highest bits
    // (where the authentication code might be located) in order to obtain a
    // valid address. We use the `XPACLRI` instruction, which is executed as a
    // no-op by processors that do not support pointer authentication, so that
    // the implementation is backward-compatible and there is no duplication.
    // However, this instruction requires the LR register for both its input and
    // output.
    std::arch::asm!(
        "mov lr, {pc}",
        "xpaclri",
        "mov {pc}, lr",
        pc = inout(reg) pc,
        out("lr") _,
        options(nomem, nostack, preserves_flags, pure),
    );

    pc
}

// And the current frame pointer points to the next older frame pointer.
pub const NEXT_OLDER_FP_FROM_FP_OFFSET: usize = 0;

pub fn reached_entry_sp(fp: usize, entry_sp: usize) -> bool {
    fp >= entry_sp
}

pub fn assert_entry_sp_is_aligned(sp: usize) {
    assert_eq!(sp % 16, 0, "stack should always be aligned to 16");
}

pub fn assert_fp_is_aligned(_fp: usize) {
    // From AAPCS64, section 6.2.3 The Frame Pointer[0]:
    //
    // > The location of the frame record within a stack frame is not specified.
    //
    // So this presumably means that the FP can have any alignment, as its
    // location is not specified and nothing further is said about constraining
    // alignment.
    //
    // [0]: https://github.com/ARM-software/abi-aa/blob/2022Q1/aapcs64/aapcs64.rst#the-frame-pointer
}
