[LLVMdev] some superoptimizer results

[Philip Reames]

On 07/22/2015 01:28 PM, Sean Silva wrote:
>
>
> On Wed, Jul 22, 2015 at 12:54 PM, Hal Finkel <hfinkel at anl.gov 
> <mailto:hfinkel at anl.gov>> wrote:
>
>     One thing that is important to consider is where in the pipeline
>     these kinds of optimizations fit. We normally try to put the IR
>     into a canonical simplified form in the mid-level optimizer. This
>     form is supposed to be whatever is most useful for exposing other
>     optimizations, and for lowering, but only in a generic sense. We
>     do have some optimizations near the end of our pipeline
>     (vectorization, partial unrolling, etc.) that consider
>     target-specific properties, but only because the alternative is
>     doing those loop optimizations after instruction selection.
>
>     Considering ILP and other pipeline-level costs are something we
>     generally consider only in the SelectionDAG and after. If these
>     are IR optimizations, then I'm not sure that considering ILP, etc.
>     is the right metric -- so long as the transformations are
>     sufficiently reversible to allow of efficient lowering afterward.
>
>
> Agreed. It might just be that these initial results are from the 
> "burn-in" specifically targeting short simple sequences, but most of 
> the transformations in the link seem to be things that, if applicable, 
> we would want to do in the backend instead of in the middle-end.
Looking through the items, I see a number which are suitable for mid 
level canonicalization.  For example, the two for converting and/cmps 
into truncs seem like good candidates.  We need to make sure to apply 
judgement here, but not *all* of these are backend specific.



>
> -- Sean Silva
>
>
>      -Hal
>
>     ----- Original Message -----
>     > From: "Sean Silva" <chisophugis at gmail.com
>     <mailto:chisophugis at gmail.com>>
>     > To: "John Regehr" <regehr at cs.utah.edu <mailto:regehr at cs.utah.edu>>
>     > Cc: llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>
>     > Sent: Wednesday, July 22, 2015 2:35:51 PM
>     > Subject: Re: [LLVMdev] some superoptimizer results
>     >
>     >
>     >
>     > Are you taking into account critical path length? Because e.g. for:
>     >
>     >
>     >
>     > %0:i64 = var
>     > %1:i1 = slt 18446744073709551615:i64, %0
>     > %2:i64 = subnsw 0:i64, %0
>     > %3:i64 = select %1, %0, %2
>     > infer %3
>     > %4:i64 = ashr %0, 63:i64
>     > %5:i64 = add %0, %4
>     > %6:i64 = xor %5, %4
>     > result %6
>     >
>     >
>     > In the former case, the cmp and sub are independent, so can be
>     > executed in parallel, while in the latter case all 3 instructions
>     > are dependent. So the former case can execute in 2 cycles while the
>     > latter takes 3. Modern OoO chips do in fact exploit this kind of
>     > thing.
>     >
>     >
>     > -- Sean Silva
>     >
>     >
>     >
>     >
>     > On Wed, Jul 22, 2015 at 10:15 AM, John Regehr <
>     regehr at cs.utah.edu <mailto:regehr at cs.utah.edu> >
>     > wrote:
>     >
>     >
>     > We (the folks working on Souper) would appreciate any feedback on
>     > these IR-level superoptimizer results:
>     >
>     > http://blog.regehr.org/extra_files/souper-jul-15.html
>     <https://urldefense.proofpoint.com/v2/url?u=http-3A__blog.regehr.org_extra-5Ffiles_souper-2Djul-2D15.html&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=Zws35DV5cT6VNt0_Wc8MbZqd8-UXh4q602LshCf-frE&s=2cBkApfDIxWcm7AqVD7-qdjeHG3okw8lDLBn_URJO2s&e=>
>     >
>     > My impression is that while there's clearly plenty of material in
>     > here that doesn't want to get implemented in an opt pass, there are
>     > a number of gems hiding in there that are worth implementing.
>     >
>     > Blog post containing additional explanation and caveats is here:
>     >
>     > http://blog.regehr.org/archives/1252
>     <https://urldefense.proofpoint.com/v2/url?u=http-3A__blog.regehr.org_archives_1252&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=Zws35DV5cT6VNt0_Wc8MbZqd8-UXh4q602LshCf-frE&s=Ek-DIAbJ7gqXWn_mfOpgfQE3i2dh1D_5peAOqtO1oYc&e=>
>     >
>     > Thanks!
>     >
>     > John
>     >
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>
>     --
>     Hal Finkel
>     Assistant Computational Scientist
>     Leadership Computing Facility
>     Argonne National Laboratory
>
>
>
>
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