Rollup merge of #77619 - fusion-engineering-forks:wasm-parker, r=dtolnay

Use futex-based thread-parker for Wasm32.

This uses the existing `sys_common/thread_parker/futex.rs` futex-based thread parker (that was already used for Linux) for wasm32 as well (if the wasm32 atomics target feature is enabled, which is not the case by default).

Wasm32 provides the basic futex operations as instructions: https://webassembly.github.io/threads/syntax/instructions.html

These are now exposed from `sys::futex::{futex_wait, futex_wake}`, just like on Linux. So, `thread_parker/futex.rs` stays completely unmodified.

3 files changed, 24 insertions, 1 deletions

diff --git a/library/std/src/sys/wasm/futex_atomics.rs b/library/std/src/sys/wasm/futex_atomics.rs
new file mode 100644
index 00000000000..3d8bf42f725
--- /dev/null
+++ b/library/std/src/sys/wasm/futex_atomics.rs
@@ -0,0 +1,17 @@
+use crate::arch::wasm32;
+use crate::convert::TryInto;
+use crate::sync::atomic::AtomicI32;
+use crate::time::Duration;
+
+pub fn futex_wait(futex: &AtomicI32, expected: i32, timeout: Option<Duration>) {
+    let timeout = timeout.and_then(|t| t.as_nanos().try_into().ok()).unwrap_or(-1);
+    unsafe {
+        wasm32::memory_atomic_wait32(futex as *const AtomicI32 as *mut i32, expected, timeout);
+    }
+}
+
+pub fn futex_wake(futex: &AtomicI32) {
+    unsafe {
+        wasm32::memory_atomic_notify(futex as *const AtomicI32 as *mut i32, 1);
+    }
+}
diff --git a/library/std/src/sys/wasm/mod.rs b/library/std/src/sys/wasm/mod.rs
index 18295e1129a..11c6896f050 100644
--- a/library/std/src/sys/wasm/mod.rs
+++ b/library/std/src/sys/wasm/mod.rs
@@ -55,6 +55,8 @@ cfg_if::cfg_if! {
         pub mod mutex;
         #[path = "rwlock_atomics.rs"]
         pub mod rwlock;
+        #[path = "futex_atomics.rs"]
+        pub mod futex;
     } else {
         #[path = "../unsupported/condvar.rs"]
         pub mod condvar;
diff --git a/library/std/src/sys_common/thread_parker/mod.rs b/library/std/src/sys_common/thread_parker/mod.rs
index 23c17c8e2cf..5e75ac65de4 100644
--- a/library/std/src/sys_common/thread_parker/mod.rs
+++ b/library/std/src/sys_common/thread_parker/mod.rs
@@ -1,5 +1,9 @@
 cfg_if::cfg_if! {
-    if #[cfg(any(target_os = "linux", target_os = "android"))] {
+    if #[cfg(any(
+        target_os = "linux",
+        target_os = "android",
+        all(target_arch = "wasm32", target_feature = "atomics"),
+    ))] {
         mod futex;
         pub use futex::Parker;
     } else {