[PATCH] Fix a performance problem in gep(gep ...) merging

[Xinliang David Li]

Mark, sounds great. Can you send the patch to Wei for testing ?

David

On Wed, Apr 15, 2015 at 11:34 AM, Mark Heffernan <meheff at google.com> wrote:

> There was a discussion on the mailing list about the exact issue this
> patch is addressing.  We (Google team working on NVPTX backend) ran into
> this same issue as did someone (Francois Pichet) working with the ppc
> backend.  Here's the link to the discussion:
>
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-March/083398.html
>
> I initially proposed what your patch does.  That is, don't merge geps
> unless the indices are constant (in which case the add is folded so no
> additional instructions are potentially created in loops).  However, after
> some discussion we came to the conclusion that the gep merging is a good
> canonicalization which we want to do.  Instead a better approach might be
> to split GEPs (undoing the early GEP merge) very late, right before
> codegen, if it is determined to be profitable.  This way, all the
> intermediate passes get the advantage of the canonicalization, while still
> being able to do LICM/CSE on GEP subexpressions which the gep merge had
> prevented.
>
> I have a patch I am in the last stages of testing which will perform this
> optimization in the pass SeparateConstOffsetFromGEP (to be renamed
> ReassociateGEPs... or maybe SplitGEPs now that I think about it).  I ran my
> patch with your example from bug 23163 and it looks to perform better than
> the prevent gep merging approach does.  With head compiler I get 29
> instructions in the inner loop (this matches what you report in the bug).
> You mention disabling merging reduces the loop instructions down to 25
> instructions.  With the patch I am working on, it gets reduced down to 16
> instructions.
>
> The pass is currently enabled only for select backends: nvptx, arm (-O3),
> and ppc (-O3).   For testing with your bug I added it in x86 addIRPasses()
> followed by CSE and LICM (these extra passes do nothing on your example
> without the GEP splitting pass).
>
> I should have the patch out for review later in the week.
>
> Sorry for the duplicate effort here.  I should have filed a bug on this
> issue before diving in :-(
>
> Mark
>
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