Re: [PATCH] libstdc++: Remove some more unconditional uses of atomics

[Christophe Lyon <christophe.lyon@linaro.org>]

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Hi,


On Wed, 13 Sept 2023 at 14:32, Jonathan Wakely <jwakely@redhat.com> wrote:

> Tested x86_64-linux and aarch64-linux. I intend to push this to trunk.
>
> -- >8 --
>
> These atomics cause linker errors on arm4t where __sync_synchronize is
> not defined. For single-threaded targets we don't need the atomics.
>
>
I ran the tests on arm-eabi default config (so, armv4t) with this patch,
and here is the list of remaining UNRESOLVED tests:
 29_atomics/atomic/compare_exchange_padding.cc
29_atomics/atomic/cons/value_init.cc
29_atomics/atomic_float/value_init.cc
29_atomics/atomic_integral/cons/value_init.cc
29_atomics/atomic_ref/compare_exchange_padding.cc
29_atomics/atomic_ref/generic.cc
29_atomics/atomic_ref/integral.cc
29_atomics/atomic_ref/pointer.cc
experimental/polymorphic_allocator/construct_pair.cc

all of them are due to undefined reference to __sync_synchronize
(some also reference __atomic_compare_exchange_4, etc...)


IIUC, this should not be the case for
experimental/polymorphic_allocator/construct_pair.cc ?
The reference for __sync_synchronize is near the beginning of test0[123]
from a call to __atomic_load_n line 835 of atomic_base.h
not sure where it comes from, the .loc directive indicates line 28 of the
testcase which is the opening brace

HTH

Christophe


libstdc++-v3/ChangeLog:
>
>         * include/experimental/io_context (io_context)
> [!_GLIBCXX_HAS_GTHREADS]:
>         Use a plain integer for _M_work_count for single-threaded
>         targets.
>         * src/c++17/memory_resource.cc [!_GLIBCXX_HAS_GTHREADS]
>         (atomic_mem_res): Use unsynchronized type for single-threaded
>         targets.
> ---
>  libstdc++-v3/include/experimental/io_context |  4 ++
>  libstdc++-v3/src/c++17/memory_resource.cc    | 49 ++++++++++----------
>  2 files changed, 29 insertions(+), 24 deletions(-)
>
> diff --git a/libstdc++-v3/include/experimental/io_context
> b/libstdc++-v3/include/experimental/io_context
> index c59f8c8e73b..c878d5a7025 100644
> --- a/libstdc++-v3/include/experimental/io_context
> +++ b/libstdc++-v3/include/experimental/io_context
> @@ -562,7 +562,11 @@ inline namespace v1
>         }
>        };
>
> +#ifdef _GLIBCXX_HAS_GTHREADS
>      atomic<count_type>         _M_work_count;
> +#else
> +    count_type                 _M_work_count;
> +#endif
>      mutable execution_context::mutex_type              _M_mtx;
>      queue<function<void()>>    _M_op;
>      bool                       _M_stopped = false;
> diff --git a/libstdc++-v3/src/c++17/memory_resource.cc
> b/libstdc++-v3/src/c++17/memory_resource.cc
> index c0c7cf0cf83..63856eadaf5 100644
> --- a/libstdc++-v3/src/c++17/memory_resource.cc
> +++ b/libstdc++-v3/src/c++17/memory_resource.cc
> @@ -27,9 +27,9 @@
>  #include <atomic>
>  #include <bit>                         // has_single_bit, bit_ceil,
> bit_width
>  #include <new>
> +#include <bits/move.h>                 // std::__exchange
>  #if ATOMIC_POINTER_LOCK_FREE != 2
>  # include <bits/std_mutex.h>   // std::mutex, std::lock_guard
> -# include <bits/move.h>                // std::__exchange
>  #endif
>
>  #if __has_cpp_attribute(clang::require_constant_initialization)
> @@ -94,10 +94,31 @@ namespace pmr
>
>      __constinit constant_init<newdel_res_t> newdel_res{};
>      __constinit constant_init<null_res_t> null_res{};
> -#if ATOMIC_POINTER_LOCK_FREE == 2
> +
> +#ifndef _GLIBCXX_HAS_GTHREADS
> +# define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
> +    // Single-threaded, no need for synchronization
> +    struct atomic_mem_res
> +    {
> +      constexpr
> +      atomic_mem_res(memory_resource* r) : val(r) { }
> +
> +      memory_resource* val;
> +
> +      memory_resource* load(std::memory_order) const
> +      {
> +       return val;
> +      }
> +
> +      memory_resource* exchange(memory_resource* r, std::memory_order)
> +      {
> +       return std::__exchange(val, r);
> +      }
> +    };
> +#elif ATOMIC_POINTER_LOCK_FREE == 2
>      using atomic_mem_res = atomic<memory_resource*>;
>  # define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
> -#elif defined(_GLIBCXX_HAS_GTHREADS)
> +#else
>      // Can't use pointer-width atomics, define a type using a mutex
> instead:
>      struct atomic_mem_res
>      {
> @@ -123,27 +144,7 @@ namespace pmr
>         return std::__exchange(val, r);
>        }
>      };
> -#else
> -# define _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
> -    // Single-threaded, no need for synchronization
> -    struct atomic_mem_res
> -    {
> -      constexpr
> -      atomic_mem_res(memory_resource* r) : val(r) { }
> -
> -      memory_resource* val;
> -
> -      memory_resource* load(std::memory_order) const
> -      {
> -       return val;
> -      }
> -
> -      memory_resource* exchange(memory_resource* r, std::memory_order)
> -      {
> -       return std::__exchange(val, r);
> -      }
> -    };
> -#endif // ATOMIC_POINTER_LOCK_FREE == 2
> +#endif
>
>  #ifdef _GLIBCXX_ATOMIC_MEM_RES_CAN_BE_CONSTANT_INITIALIZED
>      __constinit constant_init<atomic_mem_res>
> default_res{&newdel_res.obj};
> --
> 2.41.0
>
>