[libc++] Use Fuchsia futex operations

[frobtech]

The basic futex operations have always been available on Fuchsia.
Wire them up to properly support C++20 atomic notify_*/wait.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Roland McGrath (frobtech)

Changes

The basic futex operations have always been available on Fuchsia.
Wire them up to properly support C++20 atomic notify_*/wait.

---

Full diff: https://github.com/llvm/llvm-project/pull/133571.diff

3 Files Affected:

• (modified) libcxx/include/__atomic/contention_t.h (+1-1)
• (modified) libcxx/include/__cxx03/__atomic/contention_t.h (+1-1)
• (modified) libcxx/src/atomic.cpp (+22)

diff --git a/libcxx/include/__atomic/contention_t.h b/libcxx/include/__atomic/contention_t.h
index 5b42a0125f875..cd3a8e4b29094 100644
--- a/libcxx/include/__atomic/contention_t.h
+++ b/libcxx/include/__atomic/contention_t.h
@@ -19,7 +19,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if defined(__linux__) || (defined(_AIX) && !defined(__64BIT__))
+#if defined(__linux__) || defined(__Fuchsia__) || (defined(_AIX) && !defined(__64BIT__))
 using __cxx_contention_t _LIBCPP_NODEBUG = int32_t;
 #else
 using __cxx_contention_t _LIBCPP_NODEBUG = int64_t;
diff --git a/libcxx/include/__cxx03/__atomic/contention_t.h b/libcxx/include/__cxx03/__atomic/contention_t.h
index a97f0668da2fe..3fc3ccef78df5 100644
--- a/libcxx/include/__cxx03/__atomic/contention_t.h
+++ b/libcxx/include/__cxx03/__atomic/contention_t.h
@@ -19,7 +19,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if defined(__linux__) || (defined(_AIX) && !defined(__64BIT__))
+#if defined(__linux__) || defined(__Fuchsia__) || (defined(_AIX) && !defined(__64BIT__))
 using __cxx_contention_t = int32_t;
 #else
 using __cxx_contention_t = int64_t;
diff --git a/libcxx/src/atomic.cpp b/libcxx/src/atomic.cpp
index c1af8d6f95aae..6de355b534b08 100644
--- a/libcxx/src/atomic.cpp
+++ b/libcxx/src/atomic.cpp
@@ -41,6 +41,10 @@
 // OpenBSD has no indirect syscalls
 #  define _LIBCPP_FUTEX(...) futex(__VA_ARGS__)
 
+#elif defined(__Fuchsia__)
+
+#  include <zircon/syscalls.h>
+
 #else // <- Add other operating systems here
 
 // Baseline needs no new headers
@@ -101,6 +105,24 @@ static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const vo
   _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr), UMTX_OP_WAKE, __notify_one ? 1 : INT_MAX, nullptr, nullptr);
 }
 
+#elif defined(__Fuchsia__)
+
+static inline zx_futex_t const* __libcpp_zx_futex(__cxx_atomic_contention_t const volatile* ptr) {
+  // Implicitly link against the vDSO system call ABI without requiring the
+  // final link to specify -lzircon explicitly when statically linking libc++.
+#  pragma comment(lib, "zircon")
+  return const_cast<zx_futex_t const*>(reinterpret_cast<zx_futex_t const volatile*>(&ptr->__a_value));
+}
+
+static void
+__libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) {
+  _zx_futex_wait(__libcpp_zx_futex(__ptr), __val, ZX_HANDLE_INVALID, ZX_TIME_INFINITE);
+}
+
+static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, bool __notify_one) {
+  _zx_futex_wake(__libcpp_zx_futex(__ptr), __notify_one ? 1 : UINT32_MAX);
+}
+
 #else // <- Add other operating systems here
 
 // Baseline is just a timed backoff

[philnik777]

Please don't modify the C++03 headers for now.

[huixie90]

Might be bit unfortunate, but isn’t this an ABI break for your platform ?