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Re: [Patch] [Aarch64] PR 86538 - Define __ARM_FEATURE_LSE if LSE is available


On 24/07/18 22:55, Steve Ellcey wrote:
> On Tue, 2018-07-24 at 22:04 +0100, James Greenhalgh wrote:
>>  
>>
>> I'd say this patch isn't desirable for trunk. I'd be interested in use cases
>> that need a static decision on presence of LSE that are not better expressed
>> using higher level language features.
>>
>> Thanks,
>> James
> 
> How about when building the higher level features?  Right now,
> in sysdeps/aarch64/atomic-machine.h, we
> hardcode ATOMIC_EXCHANGE_USES_CAS to 0.  If we had __ARM_FEATURE_LSE we
> could use that to determine if we wanted to set
> ATOMIC_EXCHANGE_USES_CAS to 0 or 1 which would affect the call
> generated in nptl/pthread_spin_lock.c.  That would be useful if we
> built a lipthread specifically for a platform that had LSE.
> 
> Steve Ellcey
> sellcey@cavium.com
> 

If there is a case for such a define, it needs to be made with the ACLE
specification maintainers.  I don't think GCC should be ploughing a
separate furrow here.

So make your case to the ACLE maintainers.  If that adopts a pre-define,
then implementing it in GCC would go through on the nod.

R.


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