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

Eric Christopher via llvm-dev
Mon, 30 Sep 2019 19:25:42 -0000

On Sat, Sep 28, 2019 at 8:25 AM Sriraman Tallam <tmsriram at google.com>
wrote:
>
>
>
> 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.
>

They're both in the same arena.

>
>>
>> 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.

Of course, but context is helpful for discussion and for others in the
future understanding the discussion.

>
>>
>> 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.
>

Again a question of "why at the object level rather than a low level
IR" seems like something we should really explore the tradeoffs for at
more length.

-eric

> 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
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