tokio: improve detection of whether a target supports AtomicU64 (#5284)

tokio/build.rs

@@ -24,10 +24,25 @@ const CONST_MUTEX_NEW_PROBE: &str = r#"
 }
 "#;
 
+const TARGET_HAS_ATOMIC_PROBE: &str = r#"
+{
+    #[cfg(target_has_atomic = "ptr")]
+    let _ = (); 
+}
+"#;
+
+const TARGET_ATOMIC_U64_PROBE: &str = r#"
+{
+    use std::sync::atomic::AtomicU64 as _;
+}
+"#;
+
 fn main() {
     let mut enable_const_thread_local = false;
     let mut enable_addr_of = false;
+    let mut enable_target_has_atomic = false;
     let mut enable_const_mutex_new = false;
+    let mut target_needs_atomic_u64_fallback = false;
 
     match AutoCfg::new() {
         Ok(ac) => {
@@ -66,6 +81,27 @@ fn main() {
                 }
             }
 
+            // The `target_has_atomic` cfg was stabilized in 1.60.
+            if ac.probe_rustc_version(1, 61) {
+                enable_target_has_atomic = true;
+            } else if ac.probe_rustc_version(1, 60) {
+                // This compiler claims to be 1.60, but there are some nightly
+                // compilers that claim to be 1.60 without supporting the
+                // feature. Explicitly probe to check if code using them
+                // compiles.
+                //
+                // The oldest nightly that supports the feature is 2022-02-11.
+                if ac.probe_expression(TARGET_HAS_ATOMIC_PROBE) {
+                    enable_target_has_atomic = true;
+                }
+            }
+
+            // If we can't tell using `target_has_atomic`, tell if the target
+            // has `AtomicU64` by trying to use it.
+            if !enable_target_has_atomic && !ac.probe_expression(TARGET_ATOMIC_U64_PROBE) {
+                target_needs_atomic_u64_fallback = true;
+            }
+
             // The `Mutex::new` method was made const in 1.63.
             if ac.probe_rustc_version(1, 64) {
                 enable_const_mutex_new = true;
@@ -109,6 +145,14 @@ fn main() {
         autocfg::emit("tokio_no_addr_of")
     }
 
+    if !enable_target_has_atomic {
+        // To disable this feature on compilers that support it, you can
+        // explicitly pass this flag with the following environment variable:
+        //
+        // RUSTFLAGS="--cfg tokio_no_target_has_atomic"
+        autocfg::emit("tokio_no_target_has_atomic")
+    }
+
     if !enable_const_mutex_new {
         // To disable this feature on compilers that support it, you can
         // explicitly pass this flag with the following environment variable:
@@ -117,6 +161,14 @@ fn main() {
         autocfg::emit("tokio_no_const_mutex_new")
     }
 
+    if target_needs_atomic_u64_fallback {
+        // To disable this feature on compilers that support it, you can
+        // explicitly pass this flag with the following environment variable:
+        //
+        // RUSTFLAGS="--cfg tokio_no_atomic_u64"
+        autocfg::emit("tokio_no_atomic_u64")
+    }
+
     let target = ::std::env::var("TARGET").unwrap_or_default();
 
     // We emit cfgs instead of using `target_family = "wasm"` that requires Rust 1.54.

tokio/src/macros/cfg.rs

@@ -461,14 +461,14 @@ macro_rules! cfg_not_coop {
 macro_rules! cfg_has_atomic_u64 {
     ($($item:item)*) => {
         $(
-            #[cfg(not(any(
-                target_arch = "arm",
-                target_arch = "mips",
-                target_arch = "powerpc",
-                target_arch = "riscv32",
-                tokio_wasm,
-                tokio_no_atomic_u64,
-            )))]
+            #[cfg_attr(
+                not(tokio_no_target_has_atomic),
+                cfg(all(target_has_atomic = "64", not(tokio_no_atomic_u64))
+            ))]
+            #[cfg_attr(
+                tokio_no_target_has_atomic,
+                cfg(not(tokio_no_atomic_u64))
+            )]
             $item
         )*
     }
@@ -477,14 +477,14 @@ macro_rules! cfg_has_atomic_u64 {
 macro_rules! cfg_not_has_atomic_u64 {
     ($($item:item)*) => {
         $(
-            #[cfg(any(
-                target_arch = "arm",
-                target_arch = "mips",
-                target_arch = "powerpc",
-                target_arch = "riscv32",
-                tokio_wasm,
-                tokio_no_atomic_u64,
+            #[cfg_attr(
+                not(tokio_no_target_has_atomic),
+                cfg(any(not(target_has_atomic = "64"), tokio_no_atomic_u64)
             ))]
+            #[cfg_attr(
+                tokio_no_target_has_atomic,
+                cfg(tokio_no_atomic_u64)
+            )]
             $item
         )*
     }