[LLVMdev] Artificial deps and stores

[Hal Finkel]

----- Original Message -----
> From: "Andrew Trick" <atrick at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Friday, January 17, 2014 6:03:46 PM
> Subject: Re: Artificial deps and stores
> 
> 
> 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. You
> could
> 
> (a) Just nuke it and verify PPC and x86 benchmarks. If all looks
> good, then I’ll confirm by running arm.

r206795, thanks! Maybe one day I won't be so far behind on my TODO list ;)

 -Hal

> 
> (b) Make it target specific and add a FIXME to remove after
> performance analysis
> 
> -Andy
> 
> > Thanks again,
> > Hal
> > 
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> 
> 

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