[patch] Provide a can_compare_and_swap_p target hook.

[Andrew MacLeod]

On 11/04/2014 02:53 PM, Richard Biener wrote:
> On November 4, 2014 7:30:18 PM CET, Andrew MacLeod <amacleod@redhat.com> wrote:
>> On 11/04/2014 12:57 PM, Richard Henderson wrote:
>>> On 11/04/2014 06:56 PM, Andrew MacLeod wrote:
>>>> On 11/04/2014 12:25 PM, Richard Henderson wrote:
>>>>> On 11/04/2014 05:28 PM, Andrew MacLeod wrote:
>>>>>> + bool
>>>>>> + default_can_compare_and_swap_p (machine_mode mode, bool
>> allow_libcall)
>>>>>> + {
>>>>>> +   return can_compare_and_swap_p (mode, allow_libcall);
>>>>>> + }
>>>>> This is silly.  I think the problem you point out can be better
>> fixed by moving
>>>>> the can_compare_and_swap_p prototype elsewhere.
>>>>>
>>>> yeah, except it uses some of the optab table stuff that is static to
>>>> optabs.c...   so the basic functionality remains there.
>>> I said move the prototype.  Of course the implementation remains
>> where it is.
>> prototype is in optabs.h where it belongs since its defined in
>> optabs.c.  :-)
>>
>> I'm not sure why this is much different than something like the
>> targhook
>> for builtin_support_vector_misalignment(), other than we are calling
>> the
>> routine in optabs.c rather than putting the actual code in targhooks.c.
>>
> >from targhooks.c:
>> bool
>> default_builtin_support_vector_misalignment (machine_mode mode,
>> const_tree type, <...>)
>>   {
>>    if (optab_handler (movmisalign_optab, mode) != CODE_FOR_nothing)
>>      return true;
>>    return false;
>> }
>>
>> the idea is to move all the functionality that front ends need into
>> well
>> defined and controlled places so we can increase the separation.  "can
>> perform a  compare_and_swap operation" is clearly a target specific
>> question isn't it?
> I would rather question what is so special about java that it needs to ask that and other frontends not.  Don't we have generic atomics support now?
>
> Richard.
>
True... I don't know if this is a thing that simply predates our current 
level of support or if it is something else that is java specific for 
its builtins.
Don't know enough about java to comment.

aph? Looks like you wrote the originals in 2006...   Can the java CAS 
builtins simply use our current atomic calls rather than doing their own 
thing and querying whether the target has a sync compare and swap operation?

Andrew