Lift singleton from cortex-m

xtensa-lx/Cargo.toml

@@ -11,6 +11,7 @@ keywords     = ["lx", "peripheral", "register", "xtensa"]
 links        = "xtensa-lx"
 
 [dependencies]
+critical-section = "1.0.0"
 document-features = "0.2.10"
 
 [features]

xtensa-lx/src/lib.rs

@@ -110,3 +110,10 @@ pub fn is_debugger_attached() -> bool {
 pub fn debug_break() {
     unsafe { asm!("break 1, 15", options(nostack)) };
 }
+
+/// Used to reexport items for use in macros. Do not use directly.
+/// Not covered by semver guarantees.
+#[doc(hidden)]
+pub mod _export {
+    pub use critical_section;
+}

xtensa-lx/src/macros.rs

@@ -26,27 +26,31 @@
 /// ```
 #[macro_export]
 macro_rules! singleton {
-    (: $ty:ty = $expr:expr) => {
-        $crate::interrupt::free(|_| {
-            static mut VAR: Option<$ty> = None;
+    ($(#[$meta:meta])* $name:ident: $ty:ty = $expr:expr) => {
+        $crate::_export::critical_section::with(|_| {
+            // 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`...
+            $(#[$meta])*
+            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 {
                 let expr = $expr;
 
                 #[allow(unsafe_code)]
                 unsafe {
-                    VAR = Some(expr)
-                }
-
-                #[allow(unsafe_code)]
-                unsafe {
-                    VAR.as_mut()
+                    $name.1 = true;
+                    Some($name.0.write(expr))
                 }
             }
         })
     };
+    ($(#[$meta:meta])* : $ty:ty = $expr:expr) => {
+        $crate::singleton!($(#[$meta])* VAR: $ty = $expr)
+    };
 }