[LLVMdev] Upstreaming PNaCl's IR simplification passes

[Chandler Carruth]

On Tue, Mar 4, 2014 at 4:12 PM, Sean Silva <silvas at purdue.edu> wrote:

>
>
>
> On Tue, Mar 4, 2014 at 6:17 PM, Chandler Carruth <chandlerc at google.com>wrote:
>
>> On Tue, Mar 4, 2014 at 1:04 PM, Mark Seaborn <mseaborn at chromium.org>wrote:
>>
>>> The PNaCl project has implemented various IR simplification passes that
>>> simplify LLVM IR by lowering complex features to simpler features.  We'd
>>> like to upstream some of these IR passes to LLVM.  We'd like to explore if
>>> this acceptable, and if so, how we should go about doing this.
>>>
>>
>> My question is somewhat different. I'm not questioning whether these are
>> acceptable, I'm questioning why these are interesting and important for the
>> LLVM project.
>>
>> Neither PNaCl nor Emscripten open source projects have extensive
>> developer overlap with the LLVM community, and the developers have not (so
>> far) become super active maintainers of LLVM, although your recent patches
>> to fix some bugs uncovered by PNaCl have been much appreciated. These
>> lowering passes are likely to have few (most likely, zero) in-tree users
>> for the foreseeable future. I'm not enthusiastic about the community taking
>> on the maintenance, update, and code review burden of these.
>>
>> I would point you at the several emails I have written to folks adding
>> new significant features to LLVM about how to offset this by contributing
>> maintenance and improvements to the core infrastructure, fixing bugs and
>> generally making things better sufficient to offset the ongoing complexity
>> cost of the new features. Fortunately, the PNaCl passes seem somewhat less
>> complex than (for instance) the x32 backend, but they seem likely to still
>> add a reasonable amount of complexity. They will certainly be challenging
>> to review and get the design into an acceptable state across the community.
>> At this point, I'm not really optimistic about there being a large enough
>> body of community members excited about getting these passes in to offset
>> these costs. I'm happy to be proven wrong of course, and would also be
>> happy to see you, other PNaCl developers, or Emscripten developers become
>> more active in the community in order to build this trust and establish a
>> good basis for these to go into LLVM.
>>
>>
>>>
>>> The immediate reason is that Emscripten is reusing PNaCl's IR passes for
>>> its new "fastcomp" backend [1].  It would be really useful if PNaCl and
>>> Emscripten could collaborate via upstream LLVM rather than a branch.
>>>
>>
>> While this does seem like a useful thing for your two projects, it isn't
>> clear why this benefits the LLVM community. Perhaps it does, but I'd like
>> to see that clarified.
>>
>
> I think Alon's point about easing the task for students/people learning
> (or playing with) LLVM is pretty strong. People playing around with LLVM
> today are tomorrow's contributors. If we can get them to that feeling of
> "win" faster, they are more likely to stick with the project.
>

Sure, but I don't think this direction is a necessary step there, or even a
very significant one. I don't think any part of this is going to make it
easier to get up and rolling with LLVM for newcomers.



>
>
>>
>>
>>> Some background:  There are two related use cases for these IR
>>> simplification passes:
>>>
>>>  1) Simplifying the task of writing a new LLVM backend.  This is
>>> Emscripten's use case.  The IR simplification passes reduce the number of
>>> cases a backend has to handle, so they would be useful for anyone else
>>> creating a new backend.
>>>
>>
>> If these simplify writing a backend, why wouldn't the patches include
>> commensurate simplifications to LLVM's backends? That would both give them
>> an in-tree customer, and more immediate value to the community and project
>> as a whole.
>>
>
> I'd also like to add:
> If these simplify writing a backend, should there be commensurate changes
> to any relevant documentation for getting started writing backends? (we
> don't have much such documentation though...)
>

Very much so, yes.


>
> (such documentation could also be construed as an in-tree customer if
> indeed this would simplify it).
>

I won't go that far. It won't keep it well tested or correct.


>
>>
>>
>>>
>>>  2) Using a subset of LLVM IR as a stable distribution format for
>>> portable executables.  This is PNaCl's use case.  PNaCl's IR subset omits
>>> various complex IR features, which we lower using the IR simplification
>>> passes [2].  Renderscript is an example of another project that uses IR as
>>> a stable distribution format, though I think currently Renderscript is not
>>> subsetting IR much.
>>>
>>
>> Given that the bitcode is stable, I don't understand why this is
>> important. What technical problems are you solving other than making the IR
>> match some predetermined form chosen by PNaCl?
>>
>>
>>>
>>> Some examples of PNaCl's IR simplification passes are:
>>>
>>
>> I have a bunch of questions about the specific passes you mention.
>> Perhaps these questions are better answered in the review thread for the
>> patches, but they are at least things that I would think about and try to
>> address if and when you send out the code review.
>>
>>
>>>
>>>  * Calling conventions lowering:  ExpandVarArgs and ExpandByVal lower
>>> varargs and by-value argument passing respectively.  They would be useful
>>> for any backend that doesn't want to implement varargs or by-value calling
>>> conventions.
>>>
>>
>> Why wouldn't these be applicable to existing backends? What is hard about
>> the existing representations?
>>
>>
>>>
>>>  * Instruction-level lowering:
>>>     * ExpandStructRegs splits up struct values into scalars, removing
>>> the "insertvalue" and "extractvalue" instructions.
>>>
>>
>> There are already passes that do this outside of function arguments and
>> return values. Why is a new one needed? How do you handle the
>> overflow-detecting operations?
>>
>>
>>
>>>     * PromoteIntegers legalizes integer types (e.g. i30 is converted to
>>> i32).
>>>
>>
>> Does it split up too-wide integers? Do we really want another integer
>> legalization framework in LLVM? I am actually interested in doing (partial)
>> legalization in the IR during lowering (codegenprep time) in order to
>> simplify the backend, but I don't think we should develop such a framework
>> independently of the legalization currently used in the backends.
>>
>>
>>>
>>>  * Module-level lowering:  This implements, at the IR level,
>>> functionality that is traditionally provided by "ld".  e.g. ExpandCtors
>>> lowers llvm.global_ctors to the __init_array_start and __init_array_end
>>> symbols that are used by C libraries at startup.
>>>
>>
>> This doesn't make any sense to me. The IR representation is strictly
>> simpler. It is trivially lowered in a backend. I don't understand what this
>> would benefit.
>>
>
> It might be simpler to do in the backend, but I think that the point is
> that it is a recurring cost in every backend; in particular for backends
> written by people starting out/playing around with LLVM (i.e. potential
> future contributors), where any potential performance loss is acceptable
> for the sake of simplifying things.
>

I don't understand this at all.

We have a *target independent* backend. There is only one, so there should
be no recurring cost.

If people are writing a totally independent backend, then the cost of
handling this very trivial construct is ... completely unimportant compared
to the challenge of a new backend.


Also, I don't think this is about performance at all. Today, we have a
clear declarative construct that marks a special "on startup" thing with a
clear spec in the langref. With this patch we'll have an ad-hoc implicit
contract with an implementation detail of some systems libc ABIs. I don't
see how the latter is easier on any level.


>
>
>>
>>
>>> There seems to be plenty of precedent for IR-to-IR lowering passes --
>>> LLVM already contains passes such as LowerInvoke, LowerSwitch and
>>> LowerAtomic.
>>>
>>
>> Note that these are quite different -- they lower from a front-end
>> convenient form toward the canonical IR form. You are talking about
>> something totally different that deals with target-oriented lowering. The
>> correct place to look for analogies is CodeGenPrep.
>>
>>
>>> The PNaCl team (which I'm a member of) is happy to take on the work of
>>> maintaining this code, such as updating it as LLVM IR evolves and doing
>>> code reviews.  We would upstream this gradually, pass by pass, so the
>>> changes would be manageable.
>>>
>>
>> While this is appreciated, the PNaCl team should work to much more
>> actively contribute to the core of LLVM if it wants to be trusted to
>> maintain this code.
>>
>
> Is eliben still on the PNaCl team? (e.g. <
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-June/063010.html>)
>

Nope.


>
> I'd also like to point out that IR-level passes are pretty much LLVM's
> strongest point of decoupling and modularization, so of all code changes to
> have no in-tree users (if indeed there are none), this is probably a
> best-case scenario from a maintainability perspective (especially if it
> becomes the point of collaboration for Emscripten and PNaCl).
>

Yep, its definitely a best case scenario. Note that I started off saying
that this was less complex than the proposed x32 changes. I think IR passes
are reasonably well factored for this.

However, it does still have a cost. Having fixed bugs in RegionInfo (prior
to the current excellent Polly bots) and deleted a large number of stale IR
passes that were not used, they cause confusion and ongoing maintenance
headaches. These aren't extreme, they are imminently surmountable even! But
we do need to have something to overcome them, and currently I'm not seeing
it.
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