Use AtomicCell<u64> on targets with target_has_atomic less than 64

.github/workflows/ci.yml

@@ -71,6 +71,39 @@ jobs:
         command: test
         args: --doc --features "unstable attributes"
 
+  cross:
+    name: Cross compile
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        target:
+          - i686-unknown-linux-gnu
+          - powerpc-unknown-linux-gnu
+          - powerpc64-unknown-linux-gnu
+          - mips-unknown-linux-gnu
+          - arm-linux-androideabi
+
+    steps:
+    - uses: actions/checkout@master
+
+    - name: Install nightly
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: nightly
+        override: true
+
+    - name: Install cross
+      run: cargo install cross
+
+    - name: check
+      run: cross check --all --target ${{ matrix.target }}
+
+    - name: check unstable
+      run: cross check --all --features unstable --target ${{ matrix.target }}
+
+    - name: test
+      run: cross test --all --features unstable --target ${{ matrix.target }}
+
   check_fmt_and_docs:
     name: Checking fmt and docs
     runs-on: ubuntu-latest

src/sync/atomic.rs

@@ -0,0 +1,37 @@
+pub(crate) use self::imp::AtomicU64;
+
+// `AtomicU64` can only be used on targets with `target_has_atomic` is 64 or greater.
+// Once `cfg_target_has_atomic` feature is stable, we can replace it with
+// `#[cfg(target_has_atomic = "64")]`.
+// Refs: https://github.com/rust-lang/rust/tree/master/src/librustc_target
+#[cfg(not(any(target_arch = "arm", target_arch = "mips", target_arch = "powerpc")))]
+mod imp {
+    pub(crate) use std::sync::atomic::AtomicU64;
+}
+
+#[cfg(any(target_arch = "arm", target_arch = "mips", target_arch = "powerpc"))]
+mod imp {
+    use std::sync::atomic::Ordering;
+
+    use crossbeam_utils::atomic::AtomicCell;
+
+    pub(crate) struct AtomicU64(AtomicCell<u64>);
+
+    impl AtomicU64 {
+        pub(crate) const fn new(val: u64) -> Self {
+            Self(AtomicCell::new(val))
+        }
+
+        pub(crate) fn load(&self, _: Ordering) -> u64 {
+            self.0.load()
+        }
+
+        pub(crate) fn fetch_add(&self, val: u64, _: Ordering) -> u64 {
+            self.0.fetch_add(val)
+        }
+
+        pub(crate) fn fetch_sub(&self, val: u64, _: Ordering) -> u64 {
+            self.0.fetch_sub(val)
+        }
+    }
+}

src/sync/mod.rs

@@ -182,6 +182,10 @@ pub use rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 mod mutex;
 mod rwlock;
 
+cfg_default! {
+    pub(crate) mod atomic;
+}
+
 cfg_unstable! {
     pub use barrier::{Barrier, BarrierWaitResult};
     pub use channel::{channel, Sender, Receiver};

src/task/task_id.rs

@@ -1,5 +1,7 @@
 use std::fmt;
-use std::sync::atomic::{AtomicU64, Ordering};
+use std::sync::atomic::Ordering;
+
+use crate::sync::atomic::AtomicU64;
 
 /// A unique identifier for a task.
 ///