[PATCH][LoopVectorizer] Restrict the unroll factor of reductions in loops

[Hal Finkel]

----- Original Message -----
> From: "James Molloy" <james at jamesmolloy.co.uk>
> To: "Gerolf Hoflehner" <ghoflehner at apple.com>, "Arnold Schwaighofer" <aschwaighofer at apple.com>
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "James Molloy" <james.molloy at arm.com>, "llvm-commits" <llvm-commits at cs.uiuc.edu>
> Sent: Monday, August 11, 2014 11:17:49 AM
> Subject: Re: [PATCH][LoopVectorizer] Restrict the unroll factor of reductions in loops
> 
> 
> Hi Arnold, Gerolf, Hal,
> 
> 
> Good idea about a tuning option. Attached is a patch that implements
> that tuning option. Is it OK to commit?

Looks good from my end.

Thanks again,
Hal

> 
> 
> Gerolf, did you want me to add "|| isCyclone()" to
> AArch64TargetTransformInfo?
> 
> 
> Cheers,
> 
> 
> James
> 
> 
> 
> On 11 August 2014 04:31, Gerolf Hoflehner < ghoflehner at apple.com >
> wrote:
> 
> 
> On Typhoon the gain for libquantum is almost 2%, and about one
> percent on hmmer. No regression on CINT2006. This is O3 LTO, ref
> input.
> 
> -Gerolf
> 
> 
> 
> On Aug 8, 2014, at 4:57 PM, Gerolf Hoflehner < ghoflehner at apple.com >
> wrote:
> 
> > I second a tuning option at least in the short term. It is usually
> > hard to get it right, though. So longer term this is a case for
> > dynamic versioning that invokes different versions of the code at
> > run-time depending on the trip count.
> > 
> > -Gerolf
> > 
> > On Aug 8, 2014, at 12:30 PM, Hal Finkel < hfinkel at anl.gov > wrote:
> > 
> >> ----- Original Message -----
> >>> From: "James Molloy" < james.molloy at arm.com >
> >>> To: "Arnold Schwaighofer" < aschwaighofer at apple.com >
> >>> Cc: "llvm-commits" < llvm-commits at cs.uiuc.edu >
> >>> Sent: Friday, August 8, 2014 9:37:38 AM
> >>> Subject: [PATCH][LoopVectorizer] Restrict the unroll factor of
> >>> reductions in loops
> >>> 
> >>> 
> >>> 
> >>> 
> >>> 
> >>> Hi Arnold,
> >>> 
> >>> 
> >>> 
> >>> Attached are two patches. The first ups the maximum unroll factor
> >>> on
> >>> AArch64 from 2 to 4, for C-A57 only at the moment as that’s all
> >>> I’ve
> >>> got data for. This gives us significant wins – ~14% on
> >>> 462.libquantum at least.
> >>> 
> >>> 
> >>> 
> >>> However it also causes some regressions. The second patch makes
> >>> the
> >>> loop vectorizer a bit more conservative with its unroll factor.
> >>> The
> >>> problem is purely for reductions within loops. The regressions
> >>> I’ve
> >>> seen are small (but runtime-known) trip count loops within a loop
> >>> nest. A loop unroll factor of 2 is fine, but above 2 the
> >>> reduction
> >>> variable fixup logic after the loop increases the critical path
> >>> length and resource usage. For most loops this isn’t a problem,
> >>> but
> >>> small loops in a larger loop nest will execute this fixup code
> >>> many
> >>> times.
> >> 
> >> Can you please add a flag for this? I anticipate needing to tune
> >> it.
> >> 
> >> Also, it seems to me that this is exactly the kind of thing that
> >> would benefit from profiling information (so we can determine if
> >> the inner loop is likely to have a large trip count). Can the
> >> current infrastructure do this? Also, maybe in cases where the
> >> inner loop count is not a function of the outer loop, we might
> >> 'unswitch' it so that we get the unrolled inner loop only when
> >> actually profitable.
> >> 
> >> Thanks again,
> >> Hal
> >> 
> >>> 
> >>> 
> >>> 
> >>> The heuristic is: if this is a (scalar) reduction, and the loop
> >>> is
> >>> nested, clamp the UF to a maximum of 2. With 2, we still get wins
> >>> but we only add one fadd/fmul to the critical path.
> >>> 
> >>> 
> >>> 
> >>> Please take a look.
> >>> 
> >>> 
> >>> 
> >>> Cheers,
> >>> 
> >>> 
> >>> 
> >>> James
> >>> _______________________________________________
> >>> 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
> > 
> 
> 
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> 

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