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

[Jonathan Wakely]

On 03/03/21 14:34 +0000, Jonathan Wakely wrote:
>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)

Oh, except that is_scalar is surprisingly expensive to instantiate
(its defined in a really expensive way) and since we control all uses
of this constant, I don't think we need it. It's only ever used from
atomic waiting functions which are only defined for scalar types.

>>+	  && 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
>>