[llvm-dev] [RFC] Propeller: A frame work for Post Link Optimizations

Sriraman Tallam via llvm-dev
Sat, 28 Sep 2019 15:25:02 -0000

On Fri, Sep 27, 2019 at 10:36 PM Eric Christopher <echristo at gmail.com>
wrote:

> On Fri, Sep 27, 2019 at 2:08 PM Sriraman Tallam via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > On Fri, Sep 27, 2019 at 1:16 PM Eli Friedman <efriedma at
quicinc.com>
> wrote:
> > >
> > > > -----Original Message-----
> > > > From: Sriraman Tallam <tmsriram at google.com>
> > > > Sent: Friday, September 27, 2019 9:43 AM
> > > > To: Eli Friedman <efriedma at quicinc.com>
> > > > Cc: Xinliang David Li <xinliangli at gmail.com>;
llvm-dev <
> llvm-dev at lists.llvm.org>
> > > > Subject: [EXT] Re: [llvm-dev] [RFC] Propeller: A frame work
for Post
> Link
> > > > Optimizations
> > > >
> > > > >
> > > > > > > Why are you proposing to add a bunch of
options to clang to
> manipulate
> > > > LLVM
> > > > > > code generation, given none of those options are
actually used
> for Propeller
> > > > > > workflows?
> > > > > >
> > > > > > Where do you suggest labelling and section options
should
> exist?  We
> > > > > > looked at  basic block sections to be similar to
function
> sections in
> > > > > > terms of option handling?
> > > > >
> > > > > Generating bitcode with/without propeller doesn't
actually affect
> the
> > > > generated bitcode, right?  So you could say that Propeller
is
> enabled with "-Wl,--
> > > > enable-propeller", and not change clang at all.
I'm not a fan of
> adding options
> > > > just because we can.  If basic block sections are only used
as a
> sort of secret
> > > > handshake between the propeller compiler and the propeller
linker,
> we can
> > > > change that interface later, if we want; if we expose it as
a clang
> option, we're
> > > > committing to making basic block sections continue to work
the same
> way until
> > > > the end of time.
> > > >
> > > > The generated MIR code is slightly different as the later
passes have
> > > > more CFI instructions, basic block labels and extra direct
jumps
> which
> > > > would have been fall throughs otherwise.  So, some llvm
option is
> > > > needed.
> > >
> > > At link (LTO codegen) time, yes.  But clang's bitcode
generation
> doesn't change; only LTO code generation in lld changes.
> >
> > I see what you mean here.
> >
> > >
> > > > I envisioned that basic block sections could also be useful
on its
> own
> > > > outside of propeller.   Hence, we made it like
function-sections with
> > > > a separate option -fbasicblock-sections.  The propeller
option is an
> > > > umbrella option to make it easy to invoke Propeller.
> > >
> > > I don't think this is really true.  Basic block labels means
"insert
> labels that are useful for propeller profiling", and basic block
sections
> means "split the function into multiple chunks that are convenient for
> propeller optimization".  So it's really only useful for
propeller-like
> workflows.  And I'd be concerned that future changes to propeller could
> affect the behavior of these options in the future, if the behavior
isn't
> specified in some rigorous way.
> >
> > The idea of basic block sections was seeded by Rui Ueyama.  When basic
> > block sections was originally looked at, Propeller was not designed.
> > We looked at basic block sections as finer granularity than function
> > sections.  Section prefix based code layout where you just create
> > smaller code sections and let the linker do what it does today would
> > have much better results with basic block sections.  Symbol ordering
> > file with basic block sections to do random orderings can be done
> > without invoking Propeller.  Function sections has found uses after it
> > was invented like Identical Code Folding.
> >
>
> I'm not sure that function sections are entirely an apt comparison
> here for a few reasons:
>
>  - function sections mostly gave an easy way to do ICF - using MIR or
> LLVM IR are also fairly effective means as is just using the linker to
>

I did not mean to suggest we should do folding with basic block sections
and I am aware of the issues, though I am not ruling out the possibility.
We had briefly brainstormed this.

AFAICT, Function Sections was not invented to do ICF.  Function sections
was used for linker garbage collection and global code layout and when we
first worked on ICF for gold much later, we found function sections to be
useful without which it would have been much  harder.



> compare bytes. Using function sections can still run into language
> specific problems as well, hence why icf=safe vs icf=all. Though pcc's
>

Agreed, and I would like to humbly suggest that I added those options to
gold originally and am aware of that.


> recent work helps out a little bit via significant address
> determination in the front end
>  - order files and other similar mechanisms work based on externally
> visible symbols and are fairly straightforward to use in an ABI safe
> way (no function -> function fallthrough for example)
>  - basic block sections can run into the multiple entry point problems
> depending on how they're laid out as well as other ABI and visibility
> issues


> This can likely be done safely, but there are definitely some concerns
> here with how you want to enable the use of basic block
> labels/sections.
>

I would like to expose basic block sections independently and let me talk
more about one more effort we are trying to do.  Independent of Propeller,
we are looking at a machine learning based approach to do code layout at
link time.  I won't go into the details of the features but having basic
block sections allows us to naturally feed the native object files into a
black box that would try several different reorderings of the sections and
learn a model.  We looked at basic block sections as the building block on
top of which Propeller was built.  I don't think it would be the only
application, just my opinion.

Thanks
Sri





>
> -eric
>
>
> > >
> > > In any case, if we're adding flags to clang other than
"enable
> propeller", I think we should have a separate thread on cfe-dev to
discuss
> them. We don't expose every possible LLVM optimization and code
generation
> option through clang. Users have a strong expectation of stability for
> clang options, and that means we need to evaluate each new option
> carefully, to decide whether it's something we really want to support
> indefinitely.
> >
> > Understood.
> >
> > Thanks
> > Sri
> >
> > >
> > > -Eli
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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