[llvm-dev] Interprocedural DSE for -ftrivial-auto-var-init

[Vitaly Buka via llvm-dev]

 Sorry for delay, I was busy with other stuff.
 CTMark results.

 dse is the current DSE.
 dsem is my experimental module level DSE.
 dsem runs after dse, so it's additionally deleted stores.

-O3
 dse - Number of stores deleted                                    3033
 dsem - Number of deleted writes                                  3148

-O3 -ftrivial-auto-var-init=pattern
 dse - Number of stores deleted                                    5618
 dsem - Number of deleted writes                                3840

-O3 -flto
 dse - Number of stores deleted                                    3985
 dsem - Number of deleted writes                                  3838

-O3 -flto -ftrivial-auto-var-init=pattern
 dse - Number of stores deleted                                    6461
 dsem - Number of deleted writes                                4215

-Os
  dse - Number of stores deleted                                    1443
  dsem - Number of deleted writes                                  1517

-Os -ftrivial-auto-var-init=pattern
  dse - Number of stores deleted                                    3951
  dsem - Number of deleted writes                                2259

-Oz
  dse - Number of stores deleted                                    1072
  dsem - Number of deleted writes                                   574

-Oz -ftrivial-auto-var-init=pattern
  dse - Number of stores deleted                                    3420
  dsem - Number of deleted writes                                1637


*From: *Amara Emerson <aemerson at apple.com>
*Date: *Tue, Apr 16, 2019 at 12:10 PM
*To: *Vitaly Buka
*Cc: *Alexander Potapenko, llvm-dev, Peter Collingbourne

Can you post numbers for how many stores get eliminated from CTMark?
>
> On Apr 16, 2019, at 11:45 AM, Vitaly Buka <vitalybuka at google.com> wrote:
>
> I tried -Os and effect of new approach significantly increases.
> I run regular DSE and immediately myDSE. With -Os myDSE removes more than
> 50% of DSE number.
> Which is expected as -Os inlines less and regular DSE can't remove over
> function call.
>
> On Tue, Apr 16, 2019 at 7:11 AM Alexander Potapenko <glider at google.com>
> wrote:
>
>> On Mon, Apr 15, 2019 at 11:02 PM Amara Emerson via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> >
>> > > On Apr 15, 2019, at 1:51 PM, Vitaly Buka via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>> > >
>> > > Hi JF,
>> > >
>> > >    I've heard that you are interested DSE improvements and maybe we
>> need to be in sync.
>> > >    So far I experimented with following DSE improvements:
>> > >
>> > > * Cross-block DSE, it eliminates additional 7% stores comparing to
>> existing DSE. But it's not visible on benchmarks.
>> > I take it you couldn’t see any runtime impact? If there’s code size
>> improvements that could also be useful, CTMark in the llvm test suite is a
>> useful subset of benchmarks to check this on (as a baseline use -Os to
>> compare code size).
>> >
>> > Thanks,
>> > Amara
>> > >
>> > > * Cross-block + Interprocedural analysis to annotate each function
>> argument with:
>> > >   - can read before write
>> > >   - will always write
>> > > This annotations gets me 20% stores deleted additional to the current
>> DSE.
>> I believe we can only benefit from removing extra stores.
>> Hot functions in existing benchmarks are probably optimized good
>> enough already, but speeding up the long tail is also important.
>> Also, at least the repro in
>> https://bugs.llvm.org/show_bug.cgi?id=40527 has been extracted from a
>> real kernel benchmark (hackbench), where this extra store costed us
>> 0.45%
>>
>> > > This is on LLVM codebase with -ftrivial-auto-var-init=patter.
>> > >
>> > > As-is it's less than I expected, so I would like to find good
>> benchmark to decide if we should work to make production code from my
>> experiment.
>> > >
>> > > So now I am also planing to try to extend that to whole program
>> analysis.
>> > > I will cleanup my code and upload this during this weak, if anyone
>> wants to try.
>> > >
>> > > Vitaly.
>> > > _______________________________________________
>> > > LLVM Developers mailing list
>> > > llvm-dev at lists.llvm.org
>> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>> --
>> Alexander Potapenko
>> Software Engineer
>>
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>
>
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