[PATCH] [SimplifyCFG] Swap to using TargetTransformInfo for cost analysis.

[Hal Finkel]

----- Original Message -----
> From: "Andrea Di Biagio" <andrea.dibiagio at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "James Molloy" <james at jamesmolloy.co.uk>, reviews+D7506+public+d608f0951474be02 at reviews.llvm.org,
> "llvm-commits at cs.uiuc.edu for LLVM" <llvm-commits at cs.uiuc.edu>
> Sent: Tuesday, February 10, 2015 11:15:39 AM
> Subject: Re: [PATCH] [SimplifyCFG] Swap to using TargetTransformInfo for cost analysis.
> 
> On Tue, Feb 10, 2015 at 5:13 PM, Andrea Di Biagio
> <andrea.dibiagio at gmail.com> wrote:
> > On Tue, Feb 10, 2015 at 5:12 PM, Hal Finkel <hfinkel at anl.gov>
> > wrote:
> >> ----- Original Message -----
> >>> From: "Andrea Di Biagio" <andrea.dibiagio at gmail.com>
> >>> To: "Hal Finkel" <hfinkel at anl.gov>
> >>> Cc: "James Molloy" <james at jamesmolloy.co.uk>,
> >>> reviews+D7506+public+d608f0951474be02 at reviews.llvm.org,
> >>> "llvm-commits at cs.uiuc.edu for LLVM" <llvm-commits at cs.uiuc.edu>
> >>> Sent: Tuesday, February 10, 2015 10:59:15 AM
> >>> Subject: Re: [PATCH] [SimplifyCFG] Swap to using
> >>> TargetTransformInfo for cost analysis.
> >>>
> >>> Hi James, Hal
> >>>
> >>> thanks for the feedback.
> >>>
> >>> In that case, I'll see if I can come up with a fix to improve the
> >>> cost
> >>> of intrinsic calls for x86.
> >>> On x86, the cost of cttz/ctlz would be either TCC_Basic or
> >>> TCC_Expensive based on whether we have certain features.
> >>>
> >>> @Hal: Does it mean that this patch would make the the code I
> >>> added in
> >>> CodeGenPrepare to speculate calls to cttz/ctlz redundant? I see
> >>> that
> >>> with this patch, some of the test cases in
> >>> CodeGen/X86/lzcnt-tzcnt.ll
> >>> are now optimized by SimplifyCFG. The only unoptimized cases are
> >>> those
> >>> where the 'then' block contains extra instructions. I guess D7507
> >>> will
> >>> take care of it.
> >>>
> >>> Please correct me if I am wrong.
> >>
> >> I think you're right; it should get the simple speculation case.
> >> Will it also get the cases where we clear the 'undef on zero'
> >> flag?
> >>
> >>  -Hal
> >
> > Yes, it would work because of this revision:
> 
> Forget my last email...
> 
> It will get the 'undef on zero' flag because of revision 227197.
> http://llvm.org/viewvc/llvm-project?view=revision&revision=227197

Ah, okay, Now I understand. Thanks!

So, yes, it might make some of that logic redundant. However, we can also reuse the TLI interfaces by making TTI's default implementation use those interfaces for providing user costs for those intrinsics.

 -Hal

> 
> >
> >>
> >>>
> >>> Thanks!
> >>> Andrea
> >>>
> >>> On Tue, Feb 10, 2015 at 4:27 PM, Hal Finkel <hfinkel at anl.gov>
> >>> wrote:
> >>> > ----- Original Message -----
> >>> >> From: "James Molloy" <james at jamesmolloy.co.uk>
> >>> >> To: reviews+D7506+public+d608f0951474be02 at reviews.llvm.org,
> >>> >> "james
> >>> >> molloy" <james.molloy at arm.com>, hfinkel at anl.gov,
> >>> >> "t p northover" <t.p.northover at gmail.com>, "Andrea DiBiagio"
> >>> >> <Andrea_DiBiagio at sn.scee.net>
> >>> >> Cc: llvm-commits at cs.uiuc.edu
> >>> >> Sent: Tuesday, February 10, 2015 10:22:07 AM
> >>> >> Subject: Re: [PATCH] [SimplifyCFG] Swap to using
> >>> >> TargetTransformInfo for cost analysis.
> >>> >>
> >>> >>
> >>> >> Hi Andrea,
> >>> >>
> >>> >> Performance on AArch64 showed no major swings. However, it
> >>> >> sounds
> >>> >> to
> >>> >> me like your TargetTransformInfo isn't behaving correctly.
> >>> >> That's
> >>> >> TTI's entire point, and the x86 one should surely return
> >>> >> "TCC_Expensive" for such intrinsic calls.
> >>> >>
> >>> >> I see that the default TTI does return TCC_Basic for almost
> >>> >> all
> >>> >> intrinsics, and that is arguable behaviour. But there's no
> >>> >> reason
> >>> >> you shouldn't override this. In fact, it seems the right thing
> >>> >> to
> >>> >> do.
> >>> >>
> >>> >
> >>> > I agree, x86 should override these. I'll add that we're going
> >>> > to
> >>> > run into this more because we're starting to use the user-cost
> >>> > interface for more things. The target's user-cost
> >>> > implementations
> >>> > are not as well tuned as the cost interfaces used by the
> >>> > vectorizer.
> >>> >
> >>> >  -Hal
> >>> >
> >>> >>
> >>> >> Cheers,
> >>> >>
> >>> >>
> >>> >> James
> >>> >>
> >>> >> On Tue Feb 10 2015 at 2:53:52 PM Andrea Di Biagio <
> >>> >> Andrea_DiBiagio at sn.scee.net > wrote:
> >>> >>
> >>> >>
> >>> >> REPOSITORY
> >>> >> rL LLVM
> >>> >>
> >>> >> http://reviews.llvm.org/D7506
> >>> >>
> >>> >> EMAIL PREFERENCES
> >>> >> http://reviews.llvm.org/ settings/panel/ emailpreferences/
> >>> >>
> >>> >>
> >>> >>
> >>> >> ______________________________ _________________
> >>> >> llvm-commits mailing list
> >>> >> llvm-commits at cs.uiuc.edu
> >>> >> http://lists.cs.uiuc.edu/ mailman/listinfo/llvm-commits
> >>> >>
> >>> >
> >>> > --
> >>> > Hal Finkel
> >>> > Assistant Computational Scientist
> >>> > Leadership Computing Facility
> >>> > Argonne National Laboratory
> >>> > _______________________________________________
> >>> > llvm-commits mailing list
> >>> > llvm-commits at cs.uiuc.edu
> >>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >>>
> >>
> >> --
> >> Hal Finkel
> >> Assistant Computational Scientist
> >> Leadership Computing Facility
> >> Argonne National Laboratory
> 

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