[llvm-dev] Definitive list of optimisations at each optimisation level

Tue Jul 4 12:47:48 PDT 2017

On Tue, Jul 4, 2017 at 3:49 AM, Martin J. O'Riordan <
martin.oriordan at movidius.com> wrote:

> Thanks Sean,
>
>
>
> My apologies for previously writing “Thanks Sean and Silva”, I intended
> “Thanks Sean and Ryan” - duh! - sleep deprived having just flown to the USA.
>
>
>
> The dump is very enlightening for me, and very large - I hadn’t realised
> that there were quite so many passes.  At ‘-O0’ there are ~70 passes, but
> at ‘-O1’ it jumps to ~310 passes, and then ~340 passes at ‘-O2’ and ‘-O3’.
> I will have to carefully study these passes to see what optimisations they
> perform (if any), though there are the obvious ones too.
>
>
>
> One thing I observed, is that there are some passes that report:
>
>
>
> Unnamed pass: implement Pass::getPassName()
>
>
>
> This always appears after the final Target ‘AsmPrinter’ pass (“SHAVE
> Assembly Printer” in our case) and appears to be from the ‘FPPassManager’,
> while the other happens after ‘Global Variable Optimizer’ for optimisation
> levels of ‘-O1’ and higher and this seems to be the from the ‘
> MPPassManager’.  I am using the release LLVM v4.0 sources, not the head
> revision.  I guess these are noise and can be ignored.
>

I remember running into these before. I forget exactly what we ended up
discovering they were but I remember that it wasn't remarkable. Vaguely
remembering, I think the GlobalOpt one was something related to it
accessing function AA results or something like that. The reason it didn't
have a name is that it goes through some sort of adapter thing which
doesn't implement getPassName.

-- Sean Silva

>
>
> All the best,
>
>
>
>             MartinO
>
>
>
> *From:* Sean Silva [mailto:chisophugis at gmail.com]
> *Sent:* 29 June 2017 04:15
> *To:* ORiordan, Martin <martin.oriordan at intel.com>
> *Cc:* Ryan Taylor <ryta1203 at gmail.com>; Martin J. O'Riordan <
> martin.oriordan at movidius.com>; llvm-dev <llvm-dev at lists.llvm.org>
>
> *Subject:* Re: [llvm-dev] Definitive list of optimisations at each
> optimisation level
>
>
>
>
>
>
>
> On Mon, Jun 26, 2017 at 5:04 AM, ORiordan, Martin <
> martin.oriordan at intel.com> wrote:
>
> Thanks Sean and Silva.
>
>
>
> I guess what I was seeking was a URL that I could point (non-compiler)
> people at, but I guess no such reference exists.  What I can do if
> reference bot the source manager and use ‘-mllvm -debug-pass=Structure’
> for each optimisation level, and document that.  The downside is that this
> will continuously go out of data with each release.
>
>
>
>
>
> Yeah, the nasty thing is that as each user of LLVM is running potentially
> different pass pipelines, we don't really have a good way to write a
> user-facing (or at least non-compiler-dev-facing) page describing the
> optimizations that they should expect the compiler to do, as the compiler
> developers using LLVM may have changed it. In theory, we could provide such
> a page for the open-source clang. Such a page of course couldn't go into
> massive detail about each pass and as such will likely be mostly describing
> the basics.
>
>
>
> Most of the information that users would care about is likely to not get
> out of date very easily (e.g. we might switch GVN implementations, but the
> way in which they differ isn't going to be relevant to such a document; we
> might switch EarlyCSE to MemorySSA, but that isn't relevant to such a
> document).
>
>
>
> -- Sean Silva
>
>
>
>
>
> All the best,
>
>
>
>             MartinO
>
>
>
> PS: Movidius is now part of Intel, so I will be gradually switching to my
> Intel email address.
>
>
>
> *From:* Ryan Taylor [mailto:ryta1203 at gmail.com]
> *Sent:* Sunday, June 25, 2017 2:59 AM
> *To:* Sean Silva <chisophugis at gmail.com>
> *Cc:* Martin J. O'Riordan <martin.oriordan at movidius.com>; llvm-dev <
> llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] Definitive list of optimisations at each
> optimisation level
>
>
>
> I agree, it's much clearer, it just takes runs at multiple opt levels and
> therefore I don't find it to be a "one stop shop".
>
>
>
> On Jun 24, 2017 8:44 PM, "Sean Silva" <chisophugis at gmail.com> wrote:
>
> Looking at PassManagerBuilder can be useful because there are sometimes
> comments giving some idea of the intent of the particular choice of passes,
> but it can be difficult to see the big picture of all the passes that are
> run because it is very parameterized and split across multiple subroutines.
>
>
>
> Running clang at O0 through O3 with -mllvm -debug-pass=Structure is
> generally more enlightening because then you see everything in one place
> for each optimization level.
>
>
>
> -- Sean Silva
>
>
>
> On Jun 24, 2017 12:54 PM, "Ryan Taylor" <ryta1203 at gmail.com> wrote:
>
> Right but then you'll have to call each opt level. Have u looked in the
> pass manager?
>
>
>
> On Jun 24, 2017 3:52 PM, "Sean Silva via llvm-dev" <
> llvm-dev at lists.llvm.org> wrote:
>
> The most definitive list you can probably hope to get will be obtained by
> passing -mllvm -debug-pass=Structure to a clang invocation.
>
>
>
> -- Sean Silva
>
>
>
> On Jun 24, 2017 7:10 AM, "Martin J. O'Riordan via llvm-dev" <
> llvm-dev at lists.llvm.org> wrote:
>
> I am often asked what optimisations “our” compiler performs at each
> level.  But “our” compiler is actually CLang/LLVM which we have retargeted
> to our proprietary target.
>
>
>
> Most of the work we do is in maintaining our target specific backend.
> Certainly there are optimisations that we do to take best advantage of our
> instruction set during lowering and instruction selection, and we have also
> added a couple of additional passes which manipulate the IR in advance of
> lowering to better shape it for our target.
>
>
>
> But in practice the vast majority of optimisations are contributed by the
> continuously evolving and excellent LLVM target independent passes, and
> have little or nothing to do with the work we do in our backend - though it
> is obviously directed/tuned by the various target call-backs and the target
> cost models.
>
>
>
> Is there a “one stop shop” list of the optimisation passes that LLVM
> performs, and identification of which are enabled by default for each of
> the 4 standard optimisation levels ‘-O0’ thru ‘-O3’?
>
>
>
> The reason that I ask, is that I really don’t have an honest or informed
> answer that I can provide to people when they ask, and I haven’t found a
> definitive statement of this in the LLVM documentation that I could refer
> them to.
>
>
>
> Thanks,
>
>
>
>             MartinO
>
>
>
>
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