Add thumb asm volatile writes

Author: RossSmyth

library/core/src/ptr/mod.rs

@@ -425,7 +425,6 @@ pub use non_null::NonNull;
 mod unique;
 #[unstable(feature = "ptr_internals", issue = "none")]
 pub use unique::Unique;
-
 #[stable(feature = "volatile", since = "1.9.0")]
 pub use volatile::{read_volatile, write_volatile};
 

library/core/src/ptr/volatile.rs

@@ -1,4 +1,5 @@
-use crate::{mem::SizedTypeProperties, cfg_match, intrinsics};
+use crate::mem::SizedTypeProperties;
+use crate::{cfg_match, intrinsics};
 
 /// Performs a volatile read of the value from `src` without moving it. This
 /// leaves the memory in `src` unchanged.
@@ -203,6 +204,33 @@ pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
             ) => crate::ub_checks::maybe_is_aligned_and_not_null(addr, align, is_zst)
         );
         cfg_match! {
+            all(target_arch = "arm", target_feature = "thumb-mode", target_pointer_width = "32") => {
+                {
+                    use crate::arch::asm;
+                    use crate::mem::MaybeUninit;
+
+                    match size_of::<T>() {
+                        1 =>
+                            asm!(
+                                "strb {val}, [{dest}]",
+                                val = in(reg) intrinsics::transmute_unchecked::<T, MaybeUninit<u8>>(src),
+                                dest = in(reg) dst
+                            ),
+
+                        2 => asm!(
+                            "strh {val}, [{dest}]",
+                            val = in(reg) intrinsics::transmute_unchecked::<T, MaybeUninit<u16>>(src),
+                            dest = in(reg) dst,
+                        ),
+                        4 => asm!(
+                            "str {val}, [{dest}]",
+                            val = in(reg) intrinsics::transmute_unchecked::<T, MaybeUninit<u32>>(src),
+                            dest = in(reg) dst
+                        ),
+                        _ => intrinsics::volatile_store(dst, src)
+                    }
+                }
+            }
             _ => {
                 intrinsics::volatile_store(dst, src);
             }