llvm dev - Apr 2019 - Interprocedural DSE for -ftrivial-auto-var-init

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.

* 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.

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.
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> On Apr 15, 2019, at 1:51 PM, Vitaly Buka <vitalybuka at google.com>
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.
> 
> * 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.
> 
> 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.

This is great! I’ll try out the patches when you post them, and see if it
resolves the issues I’d been seeing. I don’t think we need benchmark gains fro
this to be worthwhile since variable auto-init adds slightly unusual code. I
think it’s aggravating cases where current DSE was failing in innocuous ways.

> 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.
> 
> 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

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


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