Merge #441v0.7.5

441: Prepare for v0.7.5 r=newAM a=adamgreig

Currently with cortex-m 0.7.4 it's not possible for stable Rust users to enable the inline-asm feature, even though their compiler might support it, because of the `![cfg_attr(feature = "inline-asm", feature(asm))]` line. I propose a new 0.7.5 release that removes this line, which means users on stable Rust >=1.59 could enable the previously nightly-only feature to get stable inline asm.

I wouldn't enable the feature by default, because that would be a significant MSRV bump, but at least this way more users could enable it before we release 0.8 with the revamped and inline-only asm.

I've also backported the bugfix from #380 which I don't believe is a breaking change.

I haven't had a chance to test this yet so it would be great if someone could try it out and just make sure the inline-asm feature does work before merging.

Any thoughts on anything else worth backporting from 0.8?

Co-authored-by: Adam Greig <adam@adamgreig.com>
Co-authored-by: bors[bot] <26634292+bors[bot]@users.noreply.github.com>

5 files changed, 40 insertions, 18 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ddc03a7..95ed203 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,10 +7,21 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+## [v0.7.5] - 2022-05-15
+
 ### Deprecated
 - the `ptr()` function on all peripherals register blocks in favor of
   the associated constant `PTR` (#386).
 
+### Changed
+
+- The `inline-asm` feature no longer requires a nightly Rust compiler, but
+  does require Rust 1.59 or above.
+
+### Fixed
+- Fixed `singleton!()` statics sometimes ending up in `.data` instead of `.bss` (#364, #380).
+  (Backported from upcoming 0.8 release).
+
 ## [v0.7.4] - 2021-12-31
 
 ### Added
@@ -735,7 +746,8 @@ fn main() {
 - Functions to get the vector table
 - Wrappers over miscellaneous instructions like `bkpt`
 
-[Unreleased]: https://github.com/rust-embedded/cortex-m/compare/v0.7.4...HEAD
+[Unreleased]: https://github.com/rust-embedded/cortex-m/compare/v0.7.5...HEAD
+[v0.7.5]: https://github.com/rust-embedded/cortex-m/compare/v0.7.4...v0.7.5
 [v0.7.4]: https://github.com/rust-embedded/cortex-m/compare/v0.7.3...v0.7.4
 [v0.7.3]: https://github.com/rust-embedded/cortex-m/compare/v0.7.2...v0.7.3
 [v0.7.2]: https://github.com/rust-embedded/cortex-m/compare/v0.7.1...v0.7.2
diff --git a/Cargo.toml b/Cargo.toml
index dcbd3bd..dc0678e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -11,7 +11,7 @@ license = "MIT OR Apache-2.0"
 name = "cortex-m"
 readme = "README.md"
 repository = "https://github.com/rust-embedded/cortex-m"
-version = "0.7.4"
+version = "0.7.5"
 edition = "2018"
 links = "cortex-m"  # prevent multiple versions of this crate to be linked together
 
diff --git a/cortex-m-semihosting/src/lib.rs b/cortex-m-semihosting/src/lib.rs
index a67b84d..721ac45 100644
--- a/cortex-m-semihosting/src/lib.rs
+++ b/cortex-m-semihosting/src/lib.rs
@@ -153,8 +153,7 @@
 //!
 //! ## `inline-asm`
 //!
-//! When this feature is enabled semihosting is implemented using inline assembly (`llvm_asm!`) and
-//! compiling this crate requires nightly.
+//! When this feature is enabled semihosting is implemented using inline assembly (`asm!`).
 //!
 //! When this feature is disabled semihosting is implemented using FFI calls into an external
 //! assembly file and compiling this crate works on stable and beta.
@@ -179,7 +178,6 @@
 //!
 //! [pdf]: http://infocenter.arm.com/help/topic/com.arm.doc.dui0471e/DUI0471E_developing_for_arm_processors.pdf
 
-#![cfg_attr(feature = "inline-asm", feature(llvm_asm))]
 #![deny(missing_docs)]
 #![no_std]
 
@@ -213,7 +211,7 @@ pub unsafe fn syscall1(_nr: usize, _arg: usize) -> usize {
         #[cfg(all(thumb, feature = "inline-asm", not(feature = "no-semihosting")))]
         () => {
             let mut nr = _nr;
-            llvm_asm!("bkpt 0xAB" : "+{r0}"(nr) : "{r1}"(_arg) :: "volatile");
+            core::arch::asm!("bkpt #0xab", inout("r0") nr, in("r1") _arg, options(nostack, preserves_flags));
             nr
         }
 
diff --git a/src/lib.rs b/src/lib.rs
index beff6e8..4790f98 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -22,7 +22,8 @@
 //! - Some of the `register` API only becomes available only when `inline-asm` is enabled. Check the
 //! API docs for details.
 //!
-//! The disadvantage is that `inline-asm` requires a nightly toolchain.
+//! The disadvantage is that `inline-asm` requires a Rust version at least 1.59 to use the `asm!()`
+//! macro. In the future 0.8 and above versions of `cortex-m`, this feature will always be enabled.
 //!
 //! ## `cm7-r0p1`
 //!
@@ -55,7 +56,6 @@
 //! This crate is guaranteed to compile on stable Rust 1.38 and up. It *might*
 //! compile with older versions but that may change in any new patch release.
 
-#![cfg_attr(feature = "inline-asm", feature(asm))]
 #![deny(missing_docs)]
 #![no_std]
 #![allow(clippy::identity_op)]
diff --git a/src/macros.rs b/src/macros.rs
index 66b75b1..512c932 100644
--- a/src/macros.rs
+++ b/src/macros.rs
@@ -30,9 +30,12 @@ macro_rules! iprintln {
 /// `None` variant the caller must ensure that the macro is called from a function that's executed
 /// at most once in the whole lifetime of the program.
 ///
-/// # Note
+/// # Notes
 /// This macro is unsound on multi core systems.
 ///
+/// For debuggability, you can set an explicit name for a singleton.  This name only shows up the
+/// the debugger and is not referencable from other code.  See example below.
+///
 /// # Example
 ///
 /// ``` no_run
@@ -50,15 +53,24 @@ macro_rules! iprintln {
 /// fn alias() -> &'static mut bool {
 ///     singleton!(: bool = false).unwrap()
 /// }
+///
+/// fn singleton_with_name() {
+///     // A name only for debugging purposes
+///     singleton!(FOO_BUFFER: [u8; 1024] = [0u8; 1024]);
+/// }
 /// ```
 #[macro_export]
 macro_rules! singleton {
-    (: $ty:ty = $expr:expr) => {
+    ($name:ident: $ty:ty = $expr:expr) => {
         $crate::interrupt::free(|_| {
-            static mut VAR: Option<$ty> = None;
+            // this is a tuple of a MaybeUninit and a bool because using an Option here is
+            // problematic:  Due to niche-optimization, an Option could end up producing a non-zero
+            // initializer value which would move the entire static from `.bss` into `.data`...
+            static mut $name: (::core::mem::MaybeUninit<$ty>, bool) =
+                (::core::mem::MaybeUninit::uninit(), false);
 
             #[allow(unsafe_code)]
-            let used = unsafe { VAR.is_some() };
+            let used = unsafe { $name.1 };
             if used {
                 None
             } else {
@@ -66,16 +78,16 @@ macro_rules! singleton {
 
                 #[allow(unsafe_code)]
                 unsafe {
-                    VAR = Some(expr)
-                }
-
-                #[allow(unsafe_code)]
-                unsafe {
-                    VAR.as_mut()
+                    $name.1 = true;
+                    $name.0 = ::core::mem::MaybeUninit::new(expr);
+                    Some(&mut *$name.0.as_mut_ptr())
                 }
             }
         })
     };
+    (: $ty:ty = $expr:expr) => {
+        $crate::singleton!(VAR: $ty = $expr)
+    };
 }
 
 /// ``` compile_fail