[PATCH][CilkPlus] Merge libcilkrts from upstream

[Jeff Law]

On 05/11/2016 09:59 AM, Ilya Verbin wrote:
> On Wed, May 11, 2016 at 10:47:49 +0100, Ramana Radhakrishnan wrote:
>>
>>> I've looked at the generated code in more details, and for armv6 this generates
>>> mcr     p15, 0, r0, c7, c10, 5
>>> which is not what __cilkrts_fence uses currently (CP15DSB vs CP15DMB)
>>
>> Wow I hadn't noticed that it was a DSB -  DSB is way too heavy weight. Userland shouldn't need to use this by default IMNSHO. It's needed if you are working on non-cacheable memory or performing cache maintenance operations but I can't imagine cilkplus wanting to do that !
>>
>> http://infocenter.arm.com/help/topic/com.arm.doc.genc007826/Barrier_Litmus_Tests_and_Cookbook_A08.pdf
>>
>> It's almost like the default definitions need to be in terms of the atomic extensions rather than having these written in this form. Folks usually get this wrong !
>>
>>> Looking at arm/sync.md it seems that there is no way to generate CP15DSB.
>>
>> No - there is no way of generating DSB,  DMB's should be sufficient for this purpose. Would anyone know what the semantics of __cilkrts_fence are that require this to be a DSB ?
>
> __cilkrts_fence semantics is identical to __sync_synchronize, so DMB look OK.
>
> Maybe we should just define:
>   #define __cilkrts_fence() __sync_synchronize()
Certainly seems like indirecting through a compiler builtin rather than 
using an ASM would be advisable.

jeff