[Bug c++/105251] New: static assert sizeof(decltype(_together)) <= hardware_constructive_interference_size fails with gcc12

[raj.khem at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105251

            Bug ID: 105251
           Summary: static assert sizeof(decltype(_together)) <=
                    hardware_constructive_interference_size fails with
                    gcc12
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: raj.khem at gmail dot com
  Target Milestone: ---

This testcase below fails to compile with gcc12 on aarch64 but works ok with
gcc11

gcc12
=====
$ aarch64-yoe-linux-musl-g++
--sysroot=/mnt/b/yoe/master/build/tmp/work/cortexa72-yoe-linux-musl/mongodb/4.4.13-r0/recipe-sysroot
a.cpp -std=c++17
a.cpp:35:47: error: static assertion failed: cache line spill
   35 |     static_assert(sizeof(decltype(_together)) <=
hardware_constructive_interference_size,
      |                  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cpp:35:47: note: the comparison reduces to '(256 <= 64)'

=====
a.cpp
=====


#include <cstdint>
#include <cstddef>
#include <new>
#include <algorithm>

#ifdef __cpp_lib_hardware_interference_size
    using std::hardware_constructive_interference_size;
    using std::hardware_destructive_interference_size;
#else
    // 64 bytes on x86-64 │ L1_CACHE_BYTES │ L1_CACHE_SHIFT │
__cacheline_aligned │ ...
    constexpr std::size_t hardware_constructive_interference_size = 64;
    constexpr std::size_t hardware_destructive_interference_size = 64;
#endif

class NetworkCounter {

private:
        template <typename T, size_t alignment>
        struct alignas(alignment) WithAlignment : T {
                using T::T;
        };

    template <typename T, size_t alignment>
    using WithAlignmentAtLeast = WithAlignment<T, std::max(alignof(T),
alignment)>;

    // These two counters are always incremented at the same time, so
    // we place them on the same cache line.
    template <typename T>
        using CacheAligned = WithAlignmentAtLeast<T,
hardware_destructive_interference_size>;
    struct Together {
        long long logicalBytesIn{0};
        long long requests{0};
    };
    CacheAligned<Together> _together{};
    static_assert(sizeof(decltype(_together)) <=
hardware_constructive_interference_size,
                  "cache line spill");
};