[llvm-dev] Regarding basic block layout/code placement optimizations of profile guided optimization (PGO)

[Xiaowan Dong via llvm-dev]

Dear Davide,

We noticed that C^3 will only be available for the upcoming versions of
LLVM. I was wondering what it is like in LLVM 6.0.1?

Thank you so much!

Best regards,
Xiaowan

On Tue, Aug 7, 2018 at 5:14 PM, Davide Italiano <davide at freebsd.org> wrote:

> On Tue, Aug 7, 2018 at 11:53 AM Xiaowan Dong via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > I would like to learn the details regarding what exactly PGO does for
> basic block layout/code placement optimizations in llvm. Could you please
> point me to some descriptions? Is it close to this paper (Karl Pettis and
> Robert C. Hansen. 1990. Profile guided code positioning.  PLDI'90)
> https://urldefense.proofpoint.com/v2/url?u=http-3A__perso.
> ensta-2Dparistech.fr_-7Ebmonsuez_Cours_B6-2D4_Articles_papers15.pdf-3F&d=
> DwIFaQ&c=kbmfwr1Yojg42sGEpaQh5ofMHBeTl9EI2eaqQZhHbOU&r=
> 9lVsUv0m4rDQCgSupIkpgbizkVWsSr2-oDgbaUgY_C0&m=YPozo_Hjjg-
> 5VCjvXCWDF1LsHc3pKWNWySkv3nXqSFc&s=Hbba3m9Gul1G8YndUAW1N9qQpS3nHs
> 9DA7c0CJVNebg&e= Whether it is purely intra-procedural or include
> inter-procedural optimizations?
> >
> > Thank you so much!
> >
>
> lld implements C^3 which is basically a variation of Pettis-Hansen.
> The CGO 17 paper describes the approach with a great level of detail
> https://urldefense.proofpoint.com/v2/url?u=https-3A__
> research.fb.com_wp-2Dcontent_uploads_2017_01_cgo2017-
> 2Dhfsort-2Dfinal1.pdf-3F&d=DwIFaQ&c=kbmfwr1Yojg42sGEpaQh5ofMHBeTl9
> EI2eaqQZhHbOU&r=9lVsUv0m4rDQCgSupIkpgbizkVWsSr2-oDgbaUgY_C0&m=YPozo_Hjjg-
> 5VCjvXCWDF1LsHc3pKWNWySkv3nXqSFc&s=pIdMmcvpHh75XIH29J98lMchoSRMHF
> 4PjYjsw5fWV2c&e=
> (the algorithm per-se is fairly straightforward).
> Pointers to the code at https://urldefense.proofpoint.
> com/v2/url?u=https-3A__reviews.llvm.org_D36351&d=DwIFaQ&c=
> kbmfwr1Yojg42sGEpaQh5ofMHBeTl9EI2eaqQZhHbOU&r=
> 9lVsUv0m4rDQCgSupIkpgbizkVWsSr2-oDgbaUgY_C0&m=YPozo_Hjjg-
> 5VCjvXCWDF1LsHc3pKWNWySkv3nXqSFc&s=zyCFsKdWzhdEH2AAV0y8itEL3f3MCq
> _97koJeZEBsGY&e=
> You can also find the details in Michael's original RFC
> https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.
> llvm.org_pipermail_llvm-2Ddev_2017-2DJune_114178.html&d=DwIFaQ&c=
> kbmfwr1Yojg42sGEpaQh5ofMHBeTl9EI2eaqQZhHbOU&r=
> 9lVsUv0m4rDQCgSupIkpgbizkVWsSr2-oDgbaUgY_C0&m=YPozo_Hjjg-
> 5VCjvXCWDF1LsHc3pKWNWySkv3nXqSFc&s=DGrA_unYXo8wrHhtd-
> cK2GiyQOfsYqvIz016-wcV4iU&e=
>
> There's also a pass in the backend (codegen) to perform profile-driven
> Machine BB placement (CodeGen/MachineBlockPlacement.cpp).
>
> --
> Davide
>
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