Add options to asm

- Use `nomem` when neither memory access nor reordering prevention is needed.
- Use `nostack` when not pushing data to the stack.
- Use `preserves_flags` when not modifying the status register.
  refs: https://doc.rust-lang.org/nightly/unstable-book/language-features/asm-experimental-arch.html#flags-covered-by-preserves_flags
- Add comments explaining why specific options are not used.

src/asm.rs

@@ -6,14 +6,15 @@ use crate::asm;
 #[inline(always)]
 pub fn nop() {
     unsafe {
-        asm!("nop");
+        asm!("nop", options(nostack, preserves_flags));
     }
 }
 
 /// A compiler fence, prevents instruction reordering.
 #[inline(always)]
 pub fn barrier() {
     unsafe {
-        asm!("");
+        // Do not use `nomem` and `readonly` because prevent preceding and subsequent memory accesses from being reordered.
+        asm!("", options(nostack, preserves_flags));
     }
 }

src/interrupt.rs

@@ -10,7 +10,9 @@ pub fn disable() {
     match () {
         #[cfg(target_arch = "msp430")]
         () => unsafe {
-            asm!("dint {{ nop");
+            // 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.
+            asm!("dint {{ nop", options(nostack));
         },
         #[cfg(not(target_arch = "msp430"))]
         () => {}
@@ -29,7 +31,9 @@ pub unsafe fn enable() {
     match () {
         #[cfg(target_arch = "msp430")]
         () => {
-            asm!("nop {{ eint {{ nop");
+            // Do not use `nomem` and `readonly` because prevent preceding memory accesses from being reordered after interrupts are enabled.
+            // Do not use `preserves_flags` because eint modifies the GIE (global interrupt enable) bit of the status register.
+            asm!("nop {{ eint {{ nop", options(nostack));
         }
         #[cfg(not(target_arch = "msp430"))]
         () => {}

src/register/pc.rs

@@ -7,7 +7,7 @@ use crate::asm;
 pub fn read() -> u16 {
     let r;
     unsafe {
-        asm!("mov R0, {0}", out(reg) r);
+        asm!("mov R0, {0}", out(reg) r, options(nomem, nostack, preserves_flags));
     }
     r
 }

src/register/sp.rs

@@ -7,7 +7,7 @@ use crate::asm;
 pub fn read() -> u16 {
     let r;
     unsafe {
-        asm!("mov R1, {0}", out(reg) r);
+        asm!("mov R1, {0}", out(reg) r, options(nomem, nostack, preserves_flags));
     }
     r
 }

src/register/sr.rs

@@ -79,7 +79,7 @@ impl Sr {
 pub fn read() -> Sr {
     let r: u16;
     unsafe {
-        asm!("mov R2, {0}", out(reg) r);
+        asm!("mov R2, {0}", out(reg) r, options(nomem, nostack, preserves_flags));
     }
     Sr { bits: r }
 }