Allow AtomicConsume to be used on targets that do not support atomic

Author: taiki-e

.github/workflows/ci.yml

@@ -86,7 +86,7 @@ jobs:
       - name: dependency tree check
         run: ./ci/dependencies.sh
 
-  # When this job failed, run ci/no_atomic_cas.sh and commit result changes.
+  # When this job failed, run ci/no_atomic.sh and commit result changes.
   # TODO(taiki-e): Ideally, this should be automated using a bot that creates
   #                PR when failed, but there is no bandwidth to implement it
   #                right now...
@@ -96,7 +96,7 @@ jobs:
       - uses: actions/checkout@v2
       - name: Install Rust
         run: rustup update nightly && rustup default nightly
-      - run: ci/no_atomic_cas.sh
+      - run: ci/no_atomic.sh
       - run: git diff --exit-code
 
   # Check formatting.

build.rs

@@ -13,14 +13,19 @@ if [[ "$RUST_VERSION" != "nightly"* ]]; then
     # * `--no-dev-deps` - build without dev-dependencies to avoid https://github.com/rust-lang/cargo/issues/4866
     # * `--exclude benchmarks` - benchmarks doesn't published.
     # * `--skip nightly` - skip `nightly` feature as requires nightly compilers.
-    cargo hack check --all --feature-powerset --no-dev-deps --exclude benchmarks --skip nightly
+    cargo hack build --all --feature-powerset --no-dev-deps --exclude benchmarks --skip nightly
 else
     # On nightly, all feature　combinations should work.
-    cargo hack check --all --feature-powerset --no-dev-deps --exclude benchmarks
+    cargo hack build --all --feature-powerset --no-dev-deps --exclude benchmarks
 
-    # Check for no_std environment.
+    # Build for no_std environment.
+    # thumbv7m-none-eabi supports atomic CAS.
+    # thumbv6m-none-eabi supports atomic, but not atomic CAS.
+    # riscv32i-unknown-none-elf does not support atomic at all.
     rustup target add thumbv7m-none-eabi
     rustup target add thumbv6m-none-eabi
-    cargo hack check --all --feature-powerset --no-dev-deps --exclude benchmarks --target thumbv7m-none-eabi --skip std,default
-    cargo hack check --all --feature-powerset --no-dev-deps --exclude benchmarks --target thumbv6m-none-eabi --skip std,default
+    rustup target add riscv32i-unknown-none-elf
+    cargo hack build --all --feature-powerset --no-dev-deps --exclude benchmarks --target thumbv7m-none-eabi --skip std,default
+    cargo hack build --all --feature-powerset --no-dev-deps --exclude benchmarks --target thumbv6m-none-eabi --skip std,default
+    cargo hack build --all --feature-powerset --no-dev-deps --exclude benchmarks --target riscv32i-unknown-none-elf --skip std,default
 fi

ci/check-features.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+
+# Update the list of targets that do not support atomic/CAS operations.
+#
+# Usage:
+#    ./ci/no_atomic.sh
+
+set -euo pipefail
+IFS=$'\n\t'
+
+cd "$(cd "$(dirname "$0")" && pwd)"/..
+
+file="no_atomic.rs"
+
+{
+    echo "// This file is @generated by $(basename "$0")."
+    echo "// It is not intended for manual editing."
+    echo ""
+} >"$file"
+
+echo "const NO_ATOMIC_CAS: &[&str] = &[" >>"$file"
+for target in $(rustc --print target-list); do
+    res=$(rustc --print target-spec-json -Z unstable-options --target "$target" \
+        | jq -r "select(.\"atomic-cas\" == false)")
+    [[ -z "$res" ]] || echo "    \"$target\"," >>"$file"
+done
+echo "];" >>"$file"
+
+# `"max-atomic-width" == 0` means that atomic is not supported at all.
+{
+    # Only crossbeam-utils actually uses this const.
+    echo "#[allow(dead_code)]"
+    echo "const NO_ATOMIC: &[&str] = &["
+} >>"$file"
+for target in $(rustc --print target-list); do
+    res=$(rustc --print target-spec-json -Z unstable-options --target "$target" \
+        | jq -r "select(.\"max-atomic-width\" == 0)")
+    [[ -z "$res" ]] || echo "    \"$target\"," >>"$file"
+done
+echo "];" >>"$file"

ci/no_atomic.sh

@@ -2,7 +2,7 @@
 
 use std::env;
 
-include!("no_atomic_cas.rs");
+include!("no_atomic.rs");
 
 // The rustc-cfg strings below are *not* public API. Please let us know by
 // opening a GitHub issue if your build environment requires some way to enable
@@ -28,5 +28,5 @@ fn main() {
         println!("cargo:rustc-cfg=crossbeam_no_atomic_cas");
     }
 
-    println!("cargo:rerun-if-changed=no_atomic_cas.rs");
+    println!("cargo:rerun-if-changed=no_atomic.rs");
 }

ci/no_atomic_cas.sh

@@ -0,0 +1 @@
+../no_atomic.rs

crossbeam-epoch/build.rs

@@ -104,10 +104,11 @@ mod primitive {
     pub(crate) use loom::lazy_static;
     pub(crate) use loom::thread_local;
 }
+#[cfg(not(crossbeam_no_atomic_cas))]
 #[cfg(not(crossbeam_loom))]
 #[allow(unused_imports, dead_code)]
 mod primitive {
-    #[cfg(any(feature = "alloc", feature = "std"))]
+    #[cfg(feature = "alloc")]
     pub(crate) mod cell {
         #[derive(Debug)]
         #[repr(transparent)]
@@ -135,14 +136,13 @@ mod primitive {
             }
         }
     }
-    #[cfg(any(feature = "alloc", feature = "std"))]
+    #[cfg(feature = "alloc")]
     pub(crate) mod sync {
         pub(crate) mod atomic {
             pub(crate) use core::sync::atomic::compiler_fence;
             pub(crate) use core::sync::atomic::fence;
             pub(crate) use core::sync::atomic::AtomicUsize;
         }
-        #[cfg(not(crossbeam_no_atomic_cas))]
         pub(crate) use alloc::sync::Arc;
     }
 

crossbeam-epoch/no_atomic.rs

@@ -2,7 +2,7 @@
 
 use std::env;
 
-include!("no_atomic_cas.rs");
+include!("no_atomic.rs");
 
 // The rustc-cfg strings below are *not* public API. Please let us know by
 // opening a GitHub issue if your build environment requires some way to enable
@@ -28,5 +28,5 @@ fn main() {
         println!("cargo:rustc-cfg=crossbeam_no_atomic_cas");
     }
 
-    println!("cargo:rerun-if-changed=no_atomic_cas.rs");
+    println!("cargo:rerun-if-changed=no_atomic.rs");
 }