[llvm-dev] [RFC] design doc for straight-line scalar optimizations

Eli Bendersky via llvm-dev llvm-dev at lists.llvm.org
Tue Aug 25 08:52:44 PDT 2015

On Mon, Aug 24, 2015 at 6:43 PM, escha via llvm-dev <llvm-dev at lists.llvm.org
> wrote:

> On Aug 24, 2015, at 11:10 AM, Jingyue Wu via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> Hi,
> As you may have noticed, since last year, we (Google's CUDA compiler team)
> have contributed quite a lot to the effort of optimizing LLVM for CUDA
> programs. I think it's worthwhile to write some docs to wrap them up for
> two reasons.
> 1) Whoever wants to understand or work on these optimizations has some
> detailed docs instead of just source code to refer to.
> 2) RFC on how to improve these optimizations so that other targets can
> benefit from them as well. They are currently mostly restricted to the
> NVPTX backend, but I see many potentials to generalize them.
> So, I started from this overdue design doc
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.google.com_document_d_1momWzKFf4D6h8H3YlfgKQ3qeZy5ayvMRh6yR-2DXn2hUE_edit-3Fusp-3Dsharing&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=szS1_DDBoKCtS8B5df7mJg&m=TggebUNOWYFU5W3tKpC_z1CkNT9MN05aBwWloSru2NI&s=vmPxp-RDJuf_ZN5X7LNlV10JwuHK5Pt1ljn96IenW-o&e=> on the
> straight-line scalar optimizations. I will send out more docs on other
> optimizations later. Please feel free to comment.
> Thanks,
> Jingyue
> Out of curiosity, is there any plan to make the NVPTX-originated passes
> (separateconstantoffsetfromgep, slsr, naryreassociate) more generic? They
> seem very specialized for the nVidia GPU addressing modes despite the
> generic names, and in my tests tend to pessimize our target more often than
> not for that reason.
> It’d be really nice to have something more generic, and I might look into
> helping with that sort of thing in the future if it becomes important for
> us.
> —escha
To add to Jingyue's answer - the reason these passes are not more generic
is very pragmatic - we've just optimized them for the NVIDIA targets we
care about and can run extensive benchmarking on. There's absolutely no
problem generalizing them if someone's interested - in fact, we'd be happy
to see that happen. This is what open-source is for :) IIRC some of the
optimization work was already generalized by the AMD backend folks, and
more can be done for sure. While PTX has its specific characteristics, many
of the general issues with GPU-oriented optimizations are common to other
GPU architectures and can be generalized in IR level passes.

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