On Thu, 6 Jun 2024 at 03:37, Deev Patel <dkp10...@gmail.com> wrote:
>
> Things like std::atomic<double> are currently unable to be created in a
> constexpr context with clang 18 and c++20. Example compilation error
> ```
> external/com_google_tcmalloc/tcmalloc/parameters.cc:223:17: error: variable
> does not have a constant initializer
> 223 | Parameters::peak_sampling_heap_growth_fraction_(1.1);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> external/com_google_tcmalloc/tcmalloc/parameters.cc:222:1: note: required by
> 'constinit' specifier here
> 222 | ABSL_CONST_INIT std::atomic<double>
> | ^~~~~~~~~~~~~~~
> external/com_google_absl/absl/base/attributes.h:743:25: note: expanded from
> macro 'ABSL_CONST_INIT'
> 743 | #define ABSL_CONST_INIT constinit
> | ^~~~~~~~~
> external/local_config_clang/crosstool/extra_tools/lib/gcc/x86_64-linux-gnu/14.1.0/../../../../include/c++/14.1.0/bits/atomic_base.h:1286:9:
> note: non-constexpr function '__clear_padding<double>' cannot be used in a
> constant expression
> 1286 | { __atomic_impl::__clear_padding(_M_fp); }
> | ^
> external/local_config_clang/crosstool/extra_tools/lib/gcc/x86_64-linux-gnu/14.1.0/../../../../include/c++/14.1.0/atomic:1644:38:
> note: in call to '__atomic_float(1.100000e+00)'
> 1644 | atomic(double __fp) noexcept : __atomic_float<double>(__fp)
> ```
>
> This patch adds the necessary constexpr specifier.
>
> - Deev Patel
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index 062f1549740..5e89f66f620 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -968,7 +968,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
>
> template<typename _Tp>
> - _GLIBCXX_ALWAYS_INLINE _Tp*
> + constexpr _GLIBCXX_ALWAYS_INLINE _Tp*
Thanks. I'm pretty sure the constexpr should go after the
always_inline attribute, but I'll take care of it.
> __clear_padding(_Tp& __val) noexcept
> {
> auto* __ptr = std::__addressof(__val);
