The std::latch::max() function assumes that the returned value can be
represented by ptrdiff_t, which is true when __platform_wait_t is int
(e.g. on Linux) but not when it's unsigned long, which is the case for
most other 64-bit targets. We should use the smaller of PTRDIFF_MAX and
std::numeric_limits<__platform_wait_t>::max(). Use std::cmp_less to do a
safe comparison that works for all types. We can also use std::cmp_less
and std::cmp_equal in std::latch::count_down so that we don't need to
deal with comparisons between signed and unsigned.
Also add a missing precondition check to constructor and fix the
existing check in count_down which was duplicated by mistake.
libstdc++-v3/ChangeLog:
PR libstdc++/98749
* include/std/latch (latch::max()): Ensure the return value is
representable as the return type.
(latch::latch(ptrdiff_t)): Add assertion.
(latch::count_down): Fix copy & pasted duplicate assertion. Use
std::cmp_equal to compare __platform_wait_t and ptrdiff_t
values.
(latch::_M_a): Use defined constant for alignment.
* testsuite/30_threads/latch/1.cc: Check max(). Check constant
initialization works for values in the valid range. Check
alignment.
---
Tested x86_64-linux and sparcv9-solaris (where max() was returning -1 so
the new checks i 30_threads/latch/1.cc failed).
Pushed to trunk. This seems safe to backport to gcc-14 too.
It would be nice if the safe integer comparison functions (cmp_less etc)
were in their own <bits/xxx.h> header so that <latch> didn't need to
include all of <utility>, which pulls in <bits/stl_pair.h>.
libstdc++-v3/include/std/latch | 32 +++++++++++++-------
libstdc++-v3/testsuite/30_threads/latch/1.cc | 12 +++++++-
2 files changed, 32 insertions(+), 12 deletions(-)
diff --git a/libstdc++-v3/include/std/latch b/libstdc++-v3/include/std/latch
index 9220580613d2..cf648545629d 100644
--- a/libstdc++-v3/include/std/latch
+++ b/libstdc++-v3/include/std/latch
@@ -41,6 +41,7 @@
#ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
#include <bits/atomic_base.h>
#include <ext/numeric_traits.h>
+#include <utility> // cmp_equal, cmp_less_equal, etc.
namespace std _GLIBCXX_VISIBILITY(default)
{
@@ -51,24 +52,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
public:
static constexpr ptrdiff_t
max() noexcept
- { return __gnu_cxx::__int_traits<__detail::__platform_wait_t>::__max; }
+ {
+ using __gnu_cxx::__int_traits;
+ constexpr auto __max = __int_traits<__detail::__platform_wait_t>::__max;
+ if constexpr (std::cmp_less(__max, __PTRDIFF_MAX__))
+ return __max;
+ return __PTRDIFF_MAX__;
+ }
- constexpr explicit latch(ptrdiff_t __expected) noexcept
- : _M_a(__expected) { }
+ constexpr explicit
+ latch(ptrdiff_t __expected) noexcept
+ : _M_a(__expected)
+ { __glibcxx_assert(__expected >= 0 && __expected <= max()); }
~latch() = default;
+
latch(const latch&) = delete;
latch& operator=(const latch&) = delete;
_GLIBCXX_ALWAYS_INLINE void
count_down(ptrdiff_t __update = 1)
{
- __glibcxx_assert(__update >= 0);
- auto const __old = __atomic_impl::fetch_sub(&_M_a,
- __update, memory_order::release);
- __glibcxx_assert(__update >= 0);
- if (__old == static_cast<__detail::__platform_wait_t>(__update))
+ __glibcxx_assert(__update >= 0 && __update <= max());
+ auto const __old = __atomic_impl::fetch_sub(&_M_a, __update,
+ memory_order::release);
+ if (std::cmp_equal(__old, __update))
__atomic_impl::notify_all(&_M_a);
+ else
+ __glibcxx_assert(std::cmp_less(__update, __old));
}
_GLIBCXX_ALWAYS_INLINE bool
@@ -90,9 +101,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
private:
- // This alignas is not redundant, it increases the alignment for
- // long long on x86.
- alignas(__alignof__(__detail::__platform_wait_t))
__detail::__platform_wait_t _M_a;
+ alignas(__detail::__platform_wait_alignment)
+ __detail::__platform_wait_t _M_a;
};
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
diff --git a/libstdc++-v3/testsuite/30_threads/latch/1.cc
b/libstdc++-v3/testsuite/30_threads/latch/1.cc
index 20c40254c0e6..29984cbf354e 100644
--- a/libstdc++-v3/testsuite/30_threads/latch/1.cc
+++ b/libstdc++-v3/testsuite/30_threads/latch/1.cc
@@ -22,6 +22,16 @@
#ifndef __cpp_lib_latch
# error "Feature-test macro for latch missing in <latch>"
-#elif __cpp_lib_latch!= 201907L
+#elif __cpp_lib_latch != 201907L
# error "Feature-test macro for latch has wrong value in <latch>"
#endif
+
+static_assert(std::latch::max() > 0);
+
+constinit std::latch l0(0);
+constinit std::latch l1(1);
+constinit std::latch l2(std::latch::max());
+
+#ifdef _GLIBCXX_RELEASE
+static_assert(alignof(std::latch) == std::__detail::__platform_wait_alignment);
+#endif
--
2.48.1
