[PATCH 2/5] Atomic __platform_wait: accept any 32-bit type, not just int

[Jonathan Wakely]

On 26/02/21 07:59 -0800, Thiago Macieira via Libstdc++ wrote:
>The kernel doesn't care what we store in those 32 bits, only that they are
>comparable. This commit adds:
> * pointers and long on 32-bit architectures
> * unsigned
> * untyped enums and typed enums on int & unsigned int
> * float
>
>We're not using FUTEX_OP anywhere today. The kernel reserves 4 bits for
>this field but has only used 6 values so far, so it can be extended to
>unsigned compares in the future, if needed.

And this one for GCC 11 too.

> libstdc++-v3/include/bits/atomic_wait.h | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
>diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
>index 424fccbe4c5..1c6bda2e2b6 100644
>--- a/libstdc++-v3/include/bits/atomic_wait.h
>+++ b/libstdc++-v3/include/bits/atomic_wait.h
>@@ -65,7 +65,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     template<typename _Tp>
>       inline constexpr bool __platform_wait_uses_type
> #ifdef _GLIBCXX_HAVE_LINUX_FUTEX
>-	= is_same_v<remove_cv_t<_Tp>, __platform_wait_t>;
>+	= is_scalar_v<remove_cv_t<_Tp>> && sizeof(_Tp) == sizeof(__platform_wait_t)
>+	  && alignof(_Tp) >= alignof(__platform_wait_t);
> #else
> 	= false;
> #endif
>@@ -91,13 +92,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>     template<typename _Tp>
>       void
>-      __platform_wait(const _Tp* __addr, __platform_wait_t __val) noexcept
>+      __platform_wait(const _Tp* __addr, _Tp __val) noexcept
>       {
> 	for(;;)
> 	  {
> 	    auto __e = syscall (SYS_futex, static_cast<const void*>(__addr),
> 				  static_cast<int>(__futex_wait_flags::__wait_private),
>-				    __val, nullptr);
>+				  static_cast<__platform_wait_t>(__val), nullptr);
> 	    if (!__e || errno == EAGAIN)
> 	      break;
> 	    else if (errno != EINTR)
>-- 
>2.30.1
>