[LLVMdev] Artificial deps and stores

[Hal Finkel]

----- Original Message -----
> From: "Jakob Stoklund Olesen" <stoklund at 2pi.dk>
> To: "Andy Trick" <atrick at apple.com>
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Friday, January 17, 2014 6:16:25 PM
> Subject: Re: [LLVMdev] Artificial deps and stores
> 
> 
> 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?

I think that TII->shouldScheduleAdjacent, which is used by MacroFusion in MachineScheduler.cpp is supposed to do this.

 -Hal

> 
> Thanks,
> /jakob
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory