Merge #652

652: Remove use of basepri register on thumbv8m.base r=AfoHT a=neonquill

The basepri register appears to be aviable on thumbv8m.main but not thumbv8m.base. At the very least, attempting to compile against a Cortex-M23 based Microchip ATSAML10E16A generates an error:

```
error[E0432]: unresolved import `cortex_m::register::basepri`
  --> /Users/dwatson/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-rtic-1.1.3/src/export.rs:25:5
   |
25 | use cortex_m::register::basepri;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `basepri` in `register`
```

I wasn't sure if it made more sense to replace the `armv7m` config flag with something related to basepri availability or to get closer to matching the cortex-m use of several architecture specific flags. In the end i chose to make the minimal change possible and just narrowed the existing `thumbv8m` check.

Context:
[cortex-m:src/register/mod.rs](4e90862520/src/register/mod.rs (L33)):
```
#[cfg(all(not(armv6m), not(armv8m_base)))]
pub mod basepri;
```

[cortex-m:build.rs](4e90862520/build.rs (L21)):
```
    } else if target.starts_with("thumbv8m.base") {
        println!("cargo:rustc-cfg=cortex_m");
        println!("cargo:rustc-cfg=armv8m");
        println!("cargo:rustc-cfg=armv8m_base");
```

Co-authored-by: David Watson <david@neonquill.com>

macros/src/codegen/assertions.rs

@@ -22,15 +22,16 @@ pub fn codegen(app: &App, analysis: &Analysis, extra: &Extra) -> Vec<TokenStream
     }
 
     let device = &extra.device;
-    let arm_v6_checks: Vec<_> = app
+    let chunks_name = util::priority_mask_chunks_ident();
+    let no_basepri_checks: Vec<_> = app
         .hardware_tasks
         .iter()
         .filter_map(|(_, task)| {
             if !util::is_exception(&task.args.binds) {
                 let interrupt_name = &task.args.binds;
                 Some(quote!(
-                    if (#device::Interrupt::#interrupt_name as u32) > 31 {
-                        ::core::panic!("An interrupt above value 31 is used while in armv6");
+                    if (#device::Interrupt::#interrupt_name as usize) >= (#chunks_name * 32) {
+                        ::core::panic!("An interrupt out of range is used while in armv6 or armv8m.base");
                     }
                 ))
             } else {
@@ -41,8 +42,8 @@ pub fn codegen(app: &App, analysis: &Analysis, extra: &Extra) -> Vec<TokenStream
 
     let const_check = quote! {
         const _CONST_CHECK: () = {
-            if rtic::export::is_armv6() {
-                #(#arm_v6_checks)*
+            if !rtic::export::have_basepri() {
+                #(#no_basepri_checks)*
             } else {
                 // TODO: Add armv7 checks here
             }

macros/src/codegen/shared_resources.rs

@@ -118,6 +118,8 @@ pub fn codegen(
     let device = &extra.device;
     let mut uses_exceptions_with_resources = false;
 
+    let mut mask_ids = Vec::new();
+
     for (&priority, name) in interrupt_ids.chain(app.hardware_tasks.values().flat_map(|task| {
         if !util::is_exception(&task.args.binds) {
             Some((&task.args.priority, &task.args.binds))
@@ -147,12 +149,13 @@ pub fn codegen(
     })) {
         let v = prio_to_masks.entry(priority - 1).or_insert(Vec::new());
         v.push(quote!(#device::Interrupt::#name as u32));
+        mask_ids.push(quote!(#device::Interrupt::#name as u32));
     }
 
-    // Call rtic::export::create_mask([u32; N]), where the array is the list of shifts
+    // Call rtic::export::create_mask([Mask; N]), where the array is the list of shifts
 
     let mut mask_arr = Vec::new();
-    // NOTE: 0..3 assumes max 4 priority levels according to M0 spec
+    // NOTE: 0..3 assumes max 4 priority levels according to M0, M23 spec
     for i in 0..3 {
         let v = if let Some(v) = prio_to_masks.get(&i) {
             v.clone()
@@ -165,18 +168,26 @@ pub fn codegen(
         ));
     }
 
+    // Generate a constant for the number of chunks needed by Mask.
+    let chunks_name = util::priority_mask_chunks_ident();
+    mod_app.push(quote!(
+        #[doc(hidden)]
+        #[allow(non_upper_case_globals)]
+        const #chunks_name: usize = rtic::export::compute_mask_chunks([#(#mask_ids),*]);
+    ));
+
     let masks_name = util::priority_masks_ident();
     mod_app.push(quote!(
         #[doc(hidden)]
         #[allow(non_upper_case_globals)]
-        const #masks_name: [u32; 3] = [#(#mask_arr),*];
+        const #masks_name: [rtic::export::Mask<#chunks_name>; 3] = [#(#mask_arr),*];
     ));
 
     if uses_exceptions_with_resources {
         mod_app.push(quote!(
             #[doc(hidden)]
             #[allow(non_upper_case_globals)]
-            const __rtic_internal_V6_ERROR: () = rtic::export::v6_panic();
+            const __rtic_internal_V6_ERROR: () = rtic::export::no_basepri_panic();
         ));
     }
 

macros/src/codegen/util.rs

@@ -253,6 +253,11 @@ pub fn static_shared_resource_ident(name: &Ident) -> Ident {
     mark_internal_name(&format!("shared_resource_{}", name))
 }
 
+/// Generates an Ident for the number of 32 bit chunks used for Mask storage.
+pub fn priority_mask_chunks_ident() -> Ident {
+    mark_internal_name("MASK_CHUNKS")
+}
+
 pub fn priority_masks_ident() -> Ident {
     mark_internal_name("MASKS")
 }