[llvm-dev] Llvm-mca library.

Fri May 3 04:42:00 PDT 2019

Hi Sjoerd,

On Fri, May 3, 2019 at 8:19 AM Sjoerd Meijer via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> I read that out-of-order cores are supported. How about in-order cores?
> Would it be easy/difficult to add support for that?
>
>
Cheers,
> Sjoerd.
>
>
I don't think that it would be difficult to support in-order superscalar
cores.
However, it would require a different llvm-mca pipeline of stages. That is
because some stages (and simulated hardware components) work under the
assumption that the processor is out-of-order (example: the dispatch stage
and the retire stage).
That being sadi, it would be a bit more complicated to add support
instruction itineraries. At the moment, the tool doesn't understand
itineraries.

If there is interest in supporting in-order cores, then we should probably
raise a bug for it.

-Andrea

------------------------------
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Matt
> Davis via llvm-dev <llvm-dev at lists.llvm.org>
> *Sent:* 02 May 2019 19:59
> *To:* Shoaib Meenai; lorenzo chelini; llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Llvm-mca library.
>
>
> Hi Lorenzo,
>
>
>
> I’ll answer the first question you have:
>
>
>
> > I would like to use llvm-mca to estimate the IPC of a given code region.
> I am interested in the library version (
> https://reviews.llvm.org/D50929?id=162210
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D50929-3Fid-3D162210&d=DwMFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=Ehy5cbFrTOTrtNpj4jTjwnMXspCxyoUuKpn3m8pxGpg&s=iEwPb0T8FqcGV-IscWqa3SAw64L-buGNLYvMLOgWoi4&e=>)
> but I have troubles understanding how to use it. Could you please point me
> to some documentations or (better) some code examples if any?
>
>
>
> The best example of the llvm-mca library being used is the actual llvm-mca
> tool itself.  If you look in llvm/tools/llvm-mca/llvm-mca.cpp you’ll see
> that it is just a large program that uses the llvm-mca API and library.
> You’ll want to look at main() where the default pipline is created and the
> viewers are associated to that pipeline instance.
>
>    1. You’ll need to construct an mca::Context and use that to create a
>    mca::Pipeline instance.  For starters, I suggest just using the default
>    “out-of-order” pipeline.  “createDefaultPipeline”
>    2. Next, register some handlers take a look at the HWEventListener
>    class in include/llvm/MCA.  Use “Pipeline::addEventListener” to register
>    for the events you might be interested in, such as pipeline stalls.
>
> llvm-mca.cpp does this on behalf of the Viewers.  If you trace what the
> viewers are doing in the source file, you’ll probably
>
> want to do something similar.
>
>    1. Call “Pipeline::run” to start your experiment and collect
>    performance information.
>
>
>
> Your second question:
>
>
>
> > Reading the documentation (
> https://llvm.org/docs/CommandGuide/llvm-mca.html
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_docs_CommandGuide_llvm-2Dmca.html&d=DwMFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=Ehy5cbFrTOTrtNpj4jTjwnMXspCxyoUuKpn3m8pxGpg&s=5845DcPA4uSN_lgPE_SlH6XwRSShIfPIvYR1t8AZHpU&e=>)
> it also seems that llvm-mca does not take into account the cache hierarchy.
> Does this mean that the tool assumes all the loads/stores hit the L1 cache?
>
>
>
> As you pointed out, MCA does not really model the cache hierarchy.  You
> might want to take a look at the Load/Store unit documentation as well:
> http://llvm.org/doxygen/classllvm_1_1mca_1_1LSUnit.html
>
>
>
> You probably already saw this from the CommandGuide for llvm-mca, but I’ll
> repeat it here:
>
> “The LSUnit does not attempt to predict if a load or store hits or misses
> the L1 cache. It only knows if an instruction “MayLoad” and/or “MayStore.”
> For loads, the scheduling model provides an “optimistic” load-to-use
> latency (which usually matches the load-to-use latency for when there is a
> hit in the L1D).”
>
>
>
> Hope that helps,
>
> -Matt
>
>
>
>
>
> *From: *Shoaib Meenai <smeenai at fb.com>
> *Date: *Thursday, May 2, 2019 at 11:34 AM
> *To: *lorenzo chelini <l.chelini at icloud.com>, "llvm-dev at lists.llvm.org" <
> llvm-dev at lists.llvm.org>, Matt Davis <nullptr at fb.com>
> *Subject: *Re: [llvm-dev] Llvm-mca library.
>
>
>
> (Adding Matt Davis, who should be able to help out here.)
>
>
>
> *From: *llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of lorenzo
> chelini via llvm-dev <llvm-dev at lists.llvm.org>
> *Reply-To: *lorenzo chelini <l.chelini at icloud.com>
> *Date: *Thursday, May 2, 2019 at 4:15 AM
> *To: *"llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>
> *Subject: *[llvm-dev] Llvm-mca library.
>
>
>
> Hi All,
>
>
>
> I would like to use llvm-mca to estimate the IPC of a given code region. I
> am interested in the library version (
> https://reviews.llvm.org/D50929?id=162210
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D50929-3Fid-3D162210&d=DwMFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=Ehy5cbFrTOTrtNpj4jTjwnMXspCxyoUuKpn3m8pxGpg&s=iEwPb0T8FqcGV-IscWqa3SAw64L-buGNLYvMLOgWoi4&e=>)
> but I have troubles understanding how to use it. Could you please point me
> to some documentations or (better) some code examples if any?
>
>
>
> Reading the documentation (
> https://llvm.org/docs/CommandGuide/llvm-mca.html
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_docs_CommandGuide_llvm-2Dmca.html&d=DwMFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=Ehy5cbFrTOTrtNpj4jTjwnMXspCxyoUuKpn3m8pxGpg&s=5845DcPA4uSN_lgPE_SlH6XwRSShIfPIvYR1t8AZHpU&e=>)
> it also seems that llvm-mca does not take into account the cache hierarchy.
> Does this mean that the tool assumes all the loads/stores hit the L1 cache?
>
>
>
> Looking forward to hearing from you.
>
> Best regards,
>
>
>
> Lorenzo Chelini.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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