libstdc++: Use hardware_destructive_interference_size in atomic.cc
Commit Message
This code was moved from a header into the library, so is no longer
affected by compiler options used when including the headers. We can
just use std::hardware_destructive_interference_size now, and remove the
comment.
libstdc++-v3/ChangeLog:
* src/c++20/atomic.cc (__waitable_state::_S_align): Remove.
(__waitable_state::_M_waiters, __waitable_state::_M_ver): Use
std::hardware_destructive_interference_size for alignment.
---
Tested x86_64-linux.
libstdc++-v3/src/c++20/atomic.cc | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)
Comments
On Tue, May 5, 2026 at 5:00 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> This code was moved from a header into the library, so is no longer
> affected by compiler options used when including the headers. We can
> just use std::hardware_destructive_interference_size now, and remove the
> comment.
>
> libstdc++-v3/ChangeLog:
>
> * src/c++20/atomic.cc (__waitable_state::_S_align): Remove.
> (__waitable_state::_M_waiters, __waitable_state::_M_ver): Use
> std::hardware_destructive_interference_size for alignment.
> ---
>
> Tested x86_64-linux.
>
LGTM.
>
> libstdc++-v3/src/c++20/atomic.cc | 10 ++++------
> 1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++20/atomic.cc
> b/libstdc++-v3/src/c++20/atomic.cc
> index 3e02f437db3c..3fbc05c2e682 100644
> --- a/libstdc++-v3/src/c++20/atomic.cc
> +++ b/libstdc++-v3/src/c++20/atomic.cc
> @@ -289,12 +289,9 @@ namespace
> // The state used by atomic waiting and notifying functions.
> struct __waitable_state
> {
> - // Don't use std::hardware_destructive_interference_size here because
> we
> - // don't want the layout of library types to depend on compiler
> options.
> - static constexpr auto _S_align = 64;
> -
> // Count of threads blocked waiting on this state.
> - alignas(_S_align) __platform_wait_t _M_waiters = 0;
> + alignas(std::hardware_destructive_interference_size)
> + __platform_wait_t _M_waiters = 0;
>
> #ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
> mutex _M_mtx;
> @@ -310,7 +307,8 @@ namespace
> // If we can't do a platform wait on the atomic variable itself,
> // we use this member as a proxy for the atomic variable and we
> // use this for waiting and notifying functions instead.
> - alignas(_S_align) __platform_wait_t _M_ver = 0;
> + alignas(std::hardware_destructive_interference_size)
> + __platform_wait_t _M_ver = 0;
>
> #ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
> __condvar _M_cv;
> --
> 2.54.0
>
>
@@ -289,12 +289,9 @@ namespace
// The state used by atomic waiting and notifying functions.
struct __waitable_state
{
- // Don't use std::hardware_destructive_interference_size here because we
- // don't want the layout of library types to depend on compiler options.
- static constexpr auto _S_align = 64;
-
// Count of threads blocked waiting on this state.
- alignas(_S_align) __platform_wait_t _M_waiters = 0;
+ alignas(std::hardware_destructive_interference_size)
+ __platform_wait_t _M_waiters = 0;
#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
mutex _M_mtx;
@@ -310,7 +307,8 @@ namespace
// If we can't do a platform wait on the atomic variable itself,
// we use this member as a proxy for the atomic variable and we
// use this for waiting and notifying functions instead.
- alignas(_S_align) __platform_wait_t _M_ver = 0;
+ alignas(std::hardware_destructive_interference_size)
+ __platform_wait_t _M_ver = 0;
#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
__condvar _M_cv;