Allow compiler to optimize atomic_modify_at when arguments are statically known

Author: Piroro-hs

src/reg.rs

@@ -1,5 +1,7 @@
 //! Register modifying support
 
+use core::sync::atomic::{compiler_fence, Ordering};
+
 /// Modify specific index of array-like register
 macro_rules! modify_at {
     ($reg:expr, $bitwidth:expr, $index:expr, $value:expr) => {
@@ -21,17 +23,23 @@ pub(crate) unsafe fn atomic_modify_at(reg: *mut u32, bitwidth: u8, index: u8, va
     // Perform volatile atomic `*reg = *reg & !mask_comp | value;`
     let mut res: u32;
     while {
+        compiler_fence(Ordering::SeqCst);
         core::arch::asm!(
             "ldrex {0}, [{1}]",
-            "bics {0}, {2}",
-            "orrs {0}, {3}",
-            "strex {0}, {0}, [{1}]",
             out(reg) res,
             in(reg) reg,
-            in(reg) mask_comp,
-            in(reg) value,
-            options(nostack),
+            options(readonly, preserves_flags, nostack),
+        );
+        compiler_fence(Ordering::SeqCst);
+        res = res & !mask_comp | value;
+        compiler_fence(Ordering::SeqCst);
+        core::arch::asm!(
+            "strex {0}, {0}, [{1}]",
+            inout(reg) res,
+            in(reg) reg,
+            options(preserves_flags, nostack),
         );
+        compiler_fence(Ordering::SeqCst);
         res != 0
     } {}
 }