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};
 
 macro_rules! read_volatile {
     ($name:ident, $express:expr) => {
@@ -100,6 +101,33 @@ macro_rules! write_volatile {
 }
 
 cfg_match! {
+    all(target_arch = "arm", target_feature = "thumb-mode", target_pointer_width = "32") => {
+        write_volatile!(dst, src, {
+            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)
+            }
+        });
+    }
     _ => {
         write_volatile!(dst, src, intrinsics::volatile_store(dst, src));
     }