Support/avr platform

.github/workflows/rust.yml

@@ -16,4 +16,4 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Build
-        run: ./ci.sh
+        run: ./ci.sh

avr-specs/avr-atmega328p.json

@@ -0,0 +1,27 @@
+{
+  "arch": "avr",
+  "atomic-cas": false,
+  "cpu": "atmega328p",
+  "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "eh-frame-header": false,
+  "exe-suffix": ".elf",
+  "executables": true,
+  "late-link-args": {
+    "gcc": [
+      "-lgcc"
+    ]
+  },
+  "linker": "avr-gcc",
+  "linker-is-gnu": true,
+  "llvm-target": "avr-unknown-unknown",
+  "max-atomic-width": 8,
+  "no-default-libraries": false,
+  "pre-link-args": {
+    "gcc": [
+      "-mmcu=atmega328p",
+      "-Wl,--as-needed"
+    ]
+  },
+  "target-c-int-width": "16",
+  "target-pointer-width": "16"
+}

ci.sh

@@ -2,6 +2,9 @@
 
 set -euxo pipefail
 
+rustup toolchain install nightly-2021-01-07 --component rust-src
+cargo +nightly-2021-01-07 build -Zbuild-std=core --target avr-specs/avr-atmega328p.json
+
 cargo build
 cargo build --target thumbv6m-none-eabi
 cargo build --target thumbv7em-none-eabi

rust-toolchain.toml

@@ -8,4 +8,5 @@ targets = [
     "thumbv7em-none-eabi",
     "riscv32imc-unknown-none-elf",
     "riscv32imac-unknown-none-elf",
+    "avr-specs/avr-atmpeg328p.json",
 ]

src/lib.rs

@@ -1,5 +1,7 @@
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![no_std]
+#![cfg_attr(target_arch = "avr", feature(llvm_asm))]
+#![cfg_attr(target_arch = "avr", feature(extended_key_value_attributes))]
 #![doc = include_str!("../README.md")]
 
 pub use bare_metal::CriticalSection;
@@ -118,6 +120,25 @@ cfg_if::cfg_if! {
                 cortex_m::interrupt::enable()
             }
         }
+    } else if #[cfg(target_arch = "avr")] {
+        #[no_mangle]
+        unsafe fn _critical_section_acquire() -> u8 {
+            let mut sreg: u8;
+            llvm_asm!(
+                "in $0, 0x3F
+                 cli"
+                : "=r"(sreg)
+                ::: "volatile"
+            );
+            sreg
+        }
+
+        #[no_mangle]
+        unsafe fn _critical_section_release(token: u8) {
+            if token & 0x80 == 0x80 {
+                llvm_asm!("sei" :::: "volatile");
+            }
+        }
     } else if #[cfg(target_arch = "riscv32")] {
         #[no_mangle]
         unsafe fn _critical_section_acquire() -> u8 {