From f84f01c0148cfb2451775fcbf299a2e7b10b1e81 Mon Sep 17 00:00:00 2001
From: Mara Bos <m-ou.se@m-ou.se>
Date: Tue, 6 Oct 2020 20:01:42 +0200
Subject: [PATCH] Use futex-based thread-parker for Wasm32.

---
 library/std/src/sys/wasm/futex_atomics.rs       | 17 +++++++++++++++++
 library/std/src/sys/wasm/mod.rs                 |  2 ++
 library/std/src/sys_common/thread_parker/mod.rs |  6 +++++-
 3 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 library/std/src/sys/wasm/futex_atomics.rs

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 2934ea59ab5..ca50c393c0b 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 {