[patch] Provide a can_compare_and_swap_p target hook.

Thu Nov 6 17:57:00 GMT 2014

On 11/04/2014 03:13 PM, Andrew MacLeod wrote:
> >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
It looks like java is deciding whether or not GCC can inline atomic 
operations or not, and if it can't, doesn't want the atomic 
operations... which presumably means there is no dependency on 
libatomic at runtime.
A call to can_compare_and_swap_p(mode) is analogous to a compile time 
version of folding atomic_always_lock_free(mode) to a constant... 
Frankly that seems like a reasonable question for some front end to 
ask... and elect not to emit atomic calls if so desired. (which is what 
java is doing I think)
whether it still needs to do that is a question for some java person.
Andrew