// SPDX-License-Identifier: Apache-2.0 OR MIT

/*
Adapted from https://github.com/rust-embedded/msp430.

See also src/imp/msp430.rs.

Refs:
- MSP430x5xx and MSP430x6xx Family User's Guide, Rev. Q
  https://www.ti.com/lit/ug/slau208q/slau208q.pdf

See tests/asm-test/asm/portable-atomic for generated assembly.
*/

#[cfg(not(portable_atomic_no_asm))]
use core::arch::asm;

#[cfg_attr(
    portable_atomic_no_cfg_target_has_atomic,
    cfg(any(test, portable_atomic_no_atomic_cas))
)]
#[cfg_attr(
    not(portable_atomic_no_cfg_target_has_atomic),
    cfg(any(test, not(target_has_atomic = "ptr")))
)]
pub(super) use super::super::msp430 as atomic;

pub(crate) type State = u16;

/// Disables interrupts and returns the previous interrupt state.
#[inline(always)]
pub(crate) fn disable() -> State {
    let sr: State;
    // SAFETY: reading the status register and disabling interrupts are safe.
    // (see module-level comments of interrupt/mod.rs on the safety of using privileged instructions)
    //
    // See "NOTE: Enable and Disable Interrupt" of User's Guide for NOP: https://www.ti.com/lit/ug/slau208q/slau208q.pdf#page=60
    unsafe {
        #[cfg(not(portable_atomic_no_asm))]
        asm!(
            "mov r2, {sr}", // sr = SR
            "dint {{ nop",  // SR.GIE = 0
            sr = out(reg) sr,
            // Do not use `nomem` and `readonly` because prevent subsequent memory accesses from being reordered before interrupts are disabled.
            // Do not use `preserves_flags` because DINT modifies the GIE (global interrupt enable) bit of the status register.
            options(nostack),
        );
        #[cfg(portable_atomic_no_asm)]
        {
            llvm_asm!("mov r2, $0" : "=r"(sr) ::: "volatile");
            llvm_asm!("dint { nop" ::: "memory" : "volatile");
        }
    }
    sr
}

/// Restores the previous interrupt state.
///
/// # Safety
///
/// The state must be the one retrieved by the previous `disable`.
#[inline(always)]
pub(crate) unsafe fn restore(prev_sr: State) {
    // SAFETY: the caller must guarantee that the state was retrieved by the previous `disable`,
    //
    // This clobbers the entire status register, but we never explicitly modify
    // flags within a critical session, and the only flags that may be changed
    // within a critical session are the arithmetic flags that are changed as
    // a side effect of arithmetic operations, etc., which LLVM recognizes,
    // so it is safe to clobber them here.
    // See also the discussion at https://github.com/taiki-e/portable-atomic/pull/40.
    //
    // See "NOTE: Enable and Disable Interrupt" of User's Guide for NOP: https://www.ti.com/lit/ug/slau208q/slau208q.pdf#page=60
    unsafe {
        #[cfg(not(portable_atomic_no_asm))]
        asm!(
            "nop {{ mov {prev_sr}, r2 {{ nop", // SR = prev_sr
            prev_sr = in(reg) prev_sr,
            // Do not use `nomem` and `readonly` because prevent preceding memory accesses from being reordered after interrupts are enabled.
            // Do not use `preserves_flags` because MOV modifies the status register.
            options(nostack),
        );
        #[cfg(portable_atomic_no_asm)]
        llvm_asm!("nop { mov $0, r2 { nop" :: "r"(prev_sr) : "memory" : "volatile");
    }
}
