Hi Aaron,
On Thu, 2025-09-04 at 14:10 -0400, Aaron Merey wrote:
> On Thu, Sep 4, 2025 at 10:40 AM Mark Wielaard <m...@klomp.org> wrote:
> > On Mon, 2025-09-01 at 20:31 -0400, Aaron Merey wrote:
> > > __libdw_dieabbrev uses the abbrev_lock rwlock to synchronize access to the
> > > Dwarf_Die abbrev field as well as its lazy loading. Calls to
> > > rwlock_wrlock
> > > and unlock incur significant performance overhead even in single-threaded
> > > cases.
> > >
> > > This patch implements thread safety in __libdw_dieabbrev with GCC __atomic
> > > builtins instead of an rwlock, improving performance.
> >
> > configure checks for C11 plus atomics support. So you should be able to
> > just use #include <stdatomic.h> and atomic_compare_exchange* functions.
>
> My understanding is that stdatomic.h atomic_* functions are only
> compatible with values that are declared _Atomic, while the __atomic
> builtins can be used with non-atomic types. Since we can't change the
> declaration of Dwarf_Die.abbrev without breaking compatibility, we
> have to stick with __atomic builtins.
Meh. But yeah. Changing the type in a public struct is not good. "the
atomic version of type-name may have a different size, alignment, and
object representation". We still have the long int padding__ field, but
that might of course also change size when Atomic is added. We could
maybe static assert the Atomic and non-Atomic types are exactly the
same, but that seems also a bit fragile :{
So, sigh, lets go with the __atomic functions for now. Pleas add a
comment to __libdw_dieabbrev explaining this.
Thanks,
Mark
