Re: [PATCH] libatomic: Provide gthr.h default implementation

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

On 30.05.23 11:53, Richard Biener wrote:
> On Tue, May 23, 2023 at 11:28 AM Sebastian Huber
> <sebastian.huber@embedded-brains.de>  wrote:
>> On 10.01.23 16:38, Sebastian Huber wrote:
>>> On 19/12/2022 17:02, Sebastian Huber wrote:
>>>> Build libatomic for all targets.  Use gthr.h to provide a default
>>>> implementation.  If the thread model is "single", then this
>>>> implementation will
>>>> not work if for example atomic operations are used for thread/interrupt
>>>> synchronization.
>>> Is this and the related -fprofile-update=atomic patch something for GCC 14?
>> Now that the GCC 14 development is in progress, what about this patch?
> Sorry, there doesn't seem to be a main maintainer for libatomic and your patch
> touches targets which didn't have it before.
> 
> Can you explain how this affects the ABI of targets not having (needing?!)
> libatomic?  It might help if you can say this is still opt-in and targets not
> building libatomic right now would not with your patch and targets already
> building libatomic have no changes with your patch.
> 
> That said - what kind of ABI implications has providing libatomic support
> for a target that didn't do so before?

Sorry for the missing context. The root problem I want to solve is 
getting gcov support for multi-threaded applications. For this we need 
atomic 64-bit operations, see also:

https://gcc.gnu.org/pipermail/gcc-patches/2022-December/608620.html

The libatomic patch lets it build for every target. Targets with no 
explicit support will use the gthr.h API to provide a default 
implementation.

An alternative would be to use the RTEMS approach which uses the 
following API (provided by Newlib <machine/_libatomic.h> for RTEMS):

#include <sys/cdefs.h>
#include <sys/_types.h>

__BEGIN_DECLS

__uint32_t _Libatomic_Protect_start(void *);

void _Libatomic_Protect_end(void *, __uint32_t);

void _Libatomic_Lock_n(void *, __size_t);

void _Libatomic_Unlock_n(void *, __size_t);

__END_DECLS

We could also leave libatomic as is, but then you may get unresolved 
references if you use -fprofile-update=atomic with the patch mentioned 
above.

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