[LLVMdev] Artificial deps and stores

[Jakob Stoklund Olesen]

On Jan 17, 2014, at 4:03 PM, Andrew Trick <atrick at apple.com> wrote:

> 
> On Jan 17, 2014, at 3:54 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
>> Andy, et al.,
>> 
>> In ScheduleDAGInstrs::buildSchedGraph, the code for handling stores has this:
>> 
>>     if (!ExitSU.isPred(SU))
>>       // Push store's up a bit to avoid them getting in between cmp
>>       // and branches.
>>       ExitSU.addPred(SDep(SU, SDep::Artificial));
>> 
>> This code does not seem to be in any way specific to compares; and in any case, at least on the PPC A2, scheduling stores in between the compare and the branch would not be a bad thing (because the core is in order, and the compare has a 2-cycle latency, so if there is nothing else to do, a store would not be a bad thing to put there).
>> 
>> Can you explain the motivation for this (why or for what it is bad), and what else it might do (aside from the commented cmp/branch pairing)? I'm wondering if we should make this target dependent.
> 
> I don’t agree with the existing comment. It’s possible that somewhere, maybe in target specific code, we make use of the extra store->exit edge, but I can’t remember any reason for it now.

Do you have another mechanism for encouraging macro fusion?

Thanks,
/jakob