[llvm-dev] [RFC] Add IR level interprocedural outliner for code size.

Evgeny Astigeevich via llvm-dev llvm-dev at lists.llvm.org
Mon Jul 31 08:46:57 PDT 2017

Hi Chris,

> One particular disagreement that I think very much needs to be revisited in this thread was Jessica's proposal of a pipeline of:
> 1. IR outline
> 2. Inline
> 3. MIR outline

IMHO, there is no need to restrict a place of the Outliner in the pipeline at the moment. I hope people representing different architectures will try different configurations and the best will be chosen. I’d like to try the pipeline configuration:

1.       Inline

2.       IR optimizations

3.       IR outline

4.       MIR optimizations

5.       MIR outline

I think this configuration allows to apply as many IR optimizations, especially those which reduce code size, as possible and then extract commonly used code into functions. I am also interested in some kind of Oz LTO with the IR Outliner enabled.

Evgeny Astigeevich

Senior Compiler Engineer

Compilation Tools


From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of River Riddle via llvm-dev
Sent: Saturday, July 29, 2017 6:33 AM
To: Chris Bieneman
Cc: llvm-dev
Subject: Re: [llvm-dev] [RFC] Add IR level interprocedural outliner for code size.

Hi Chris,

It's okay to put this on the spot because posting the patches was meant to help further the discussion that kind of stalled previously.

On Fri, Jul 28, 2017 at 9:58 PM, Chris Bieneman <beanz at apple.com<mailto:beanz at apple.com>> wrote:
Apologies for delayed joining of this discussion, but I had a few notes from this thread that I really wanted to chime in about.


I don't mean to put you on the spot, but I do want to start on a semantic issue. In several places in the thread you used the words "we" and "our" to imply that you're not alone in writing this (which is totally fine), but your initial thread presented this as entirely your own work. So, when you said things like "we feel there's an advantage to being at the IR level", can you please clarify who is "we"?

 In regards to the words "we" and "our", I am referring to myself. My writing style tends to shift between the usage of those words. I'll avoid any kind of confusion in the future.

Given that there are a number of disagreements and opinions floating around I think it benefits us all to speak clearly about who is taking what stances.

One particular disagreement that I think very much needs to be revisited in this thread was Jessica's proposal of a pipeline of:

  1.  IR outline
  2.  Inline
  3.  MIR outline
In your response to that proposal you dismissed it out of hand with "feelings" but not data. Given that the proposal came from Jessica (a community member with significant relevant experience in outlining), and it was also recognized as interesting by Eric Christopher (a long-time member of the community with wide reaching expertise), I think dismissing it may have been a little premature.

I dismissed the idea of an outliner at the machine level being able to catch bad inlining decisions. Given the loss of information between the two I felt it was a little optimistic to rely on a very late pass being able to reverse those decisions, especially coupled with the fact that the current machine outliner requires exact equivalence. I don't disagree with the proposal of an example : outline, inline, outline: pipeline, but the idea of being able to catch inlining decisions given the circumstances seemed optimistic to me. From there I went ahead and implemented a generic interface for outlining that can be shared between IR/Machine level so that such a pipeline could be more feasible.

I also want to visit a few procedural notes.

Mehdi commented on the thread that it wouldn't be fair to ask for a comparative study because the MIR outliner didn't have one. While I don't think anyone is asking for a comparative study, I want to point out that I think it is completely fair. If a new contributor approached the community with a new SROA pass and wanted to land it in-tree it would be appropriate to ask for a comparative analysis against the existing pass. How is this different?

The real question comes from what exactly you want to define as a "comparative analysis". When posting the patch I included additional performance data( found here goo.gl/5k6wsP<http://goo.gl/5k6wsP>) that includes benchmarking and comparisons between the outliner that I am proposing and the machine outliner on a wide variety of benchmarks. The proposed outliner performs quite favorable in comparison. As for feature comparison, the proposed outliner has many features currently missing from the machine outliner:
 - parameterization
 - outputs
 - relaxed equivalence(machine outliner requires exact)
 - usage of profile data
 - support for opt remarks

 The machine outliner currently only supports X86 and AArch64, the IR outliner can/should support all targets immediately without the requirement of ABI restrictions(mno-red-zone is required for the machine outliner).
 At the IR level we have much more opportunity to find congruent instructions than at the machine level given the possible variation at that level: RA, instruction selection, instruction scheduling, etc.

I am more than willing to do a comparative analysis but I'm not quite sure what the expectation for one is.

Adding a new IR outliner is a different situation from when the MIR one was added. When the MIR outliner was introduced there was no in-tree analog. When someone comes to the community with something that has no existing in-tree analog it isn't fair to necessarily ask them to implement it multiple different ways to prove their solution is the best. However, as a community, we do still exercise the right to reject contributions we disagree with, and we frequently request changes to the implementation (as is shown every time someone tries to add SPIR-V support).

I perfectly agree :)

In the LLVM community we have a long history of approaching large contributions (especially ones from new contributors) with scrutiny and discussion. It would be a disservice to the project to forget that.

River, as a last note. I see that you've started uploading patches to Phabricator, and I know you're relatively new to the community. When uploading patches it helps to include appropriate reviewers so that the right people see the patches as they come in. To that end can you please include Jessica as a reviewer? Given her relevant domain experience I think her feedback on the patches will be very valuable.

I accidentally posted without any reviewers at first, I've been going back through and adding people I missed.

Thank you,
I appreciate the feedback and welcome all critical discussion about the right way to move forward.
 River Riddle

On Jul 26, 2017, at 1:52 PM, River Riddle via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hey Sanjoy,

On Wed, Jul 26, 2017 at 1:41 PM, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

On Wed, Jul 26, 2017 at 12:54 PM, Sean Silva <chisophugis at gmail.com<mailto:chisophugis at gmail.com>> wrote:
> The way I interpret Quentin's statement is something like:
> - Inlining turns an interprocedural problem into an intraprocedural problem
> - Outlining turns an intraprocedural problem into an interprocedural problem
> Insofar as our intraprocedural analyses and transformations are strictly
> more powerful than interprocedural, then there is a precise sense in which
> inlining exposes optimization opportunities while outlining does not.

While I think our intra-proc optimizations are *generally* more
powerful, I don't think they are *always* more powerful.  For
instance, LICM (today) won't hoist full regions but it can hoist
single function calls.  If we can extract out a region into a
readnone+nounwind function call then LICM will hoist it to the
preheader if the safety checks pass.

> Actually, for his internship last summer River wrote a profile-guided
> outliner / partial inliner (it didn't try to do deduplication; so it was
> more like PartialInliner.cpp). IIRC he found that LLVM's interprocedural
> analyses were so bad that there were pretty adverse effects from many of the
> outlining decisions. E.g. if you outline from the left side of a diamond,
> that side basically becomes a black box to most LLVM analyses and forces
> downstream dataflow meet points to give an overly conservative result, even
> though our standard intraprocedural analyses would have happily dug through
> the left side of the diamond if the code had not been outlined.
> Also, River's patch (the one in this thread) does parameterized outlining.
> For example, two sequences containing stores can be outlined even if the
> corresponding stores have different pointers. The pointer to be loaded from
> is passed as a parameter to the outlined function. In that sense, the
> outlined function's behavior becomes a conservative approximation of both
> which in principle loses precision.

Can we outline only once we've already done all of these optimizations
that outlining would block?

  The outliner is able to run at any point in the interprocedural pipeline. There are currently two locations: Early outlining(pre inliner) and late outlining(practically the last pass to run). It is configured to run either Early+Late, or just Late.

> I like your EarlyCSE example and it is interesting that combined with
> functionattrs it can make a "cheap" pass get a transformation that an
> "expensive" pass would otherwise be needed. Are there any cases where we
> only have the "cheap" pass and thus the outlining would be essential for our
> optimization pipeline to get the optimization right?
> The case that comes to mind for me is cases where we have some cutoff of
> search depth. Reducing a sequence to a single call (+ functionattr
> inference) can essentially summarize the sequence and effectively increase
> search depth, which might give more results. That seems like a bit of a weak
> example though.

I don't know if River's patch outlines entire control flow regions at
a time, but if it does then we could use cheap basic block scanning
analyses for things that would normally require CFG-level analysis.

  The current patch currently just supports outlining from within a single block. Although, I had a working prototype for Region based outlining, I kept it from this patch for simplicity. So its entirely possible to add that kind of functionality because I've already tried.
  River Riddle

-- Sanjoy

> -- Sean Silva
> On Wed, Jul 26, 2017 at 12:07 PM, Sanjoy Das via llvm-dev
> <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
>> Hi,
>> On Wed, Jul 26, 2017 at 10:10 AM, Quentin Colombet via llvm-dev
>> <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
>> > No, I mean in terms of enabling other optimizations in the pipeline like
>> > vectorizer. Outliner does not expose any of that.
>> I have not made a lot of effort to understand the full discussion here (so
>> what
>> I say below may be off-base), but I think there are some cases where
>> outlining
>> (especially working with function-attrs) can make optimization easier.
>> It can help transforms that duplicate code (like loop unrolling and
>> inlining) be
>> more profitable -- I'm thinking of cases where unrolling/inlining would
>> have to
>> duplicate a lot of code, but after outlining would require duplicating
>> only a
>> few call instructions.
>> It can help EarlyCSE do things that require GVN today:
>> void foo() {
>>   ... complex computation that computes func()
>>   ... complex computation that computes func()
>> }
>> outlining=>
>> int func() { ... }
>> void foo() {
>>   int x = func();
>>   int y = func();
>> }
>> functionattrs=>
>> int func() readonly { ... }
>> void foo(int a, int b) {
>>   int x = func();
>>   int y = func();
>> }
>> earlycse=>
>> int func(int t) readnone { ... }
>> void foo(int a, int b) {
>>   int x = func(a);
>>   int y = x;
>> }
>> GVN will catch this, but EarlyCSE is (at least supposed to be!) cheaper.
>> Once we have an analysis that can prove that certain functions can't trap,
>> outlining can allow LICM etc. to speculate entire outlined regions out of
>> loops.
>> Generally, I think outlining exposes information that certain regions of
>> the
>> program are doing identical things.  We should expect to get some mileage
>> out of
>> this information.
>> -- Sanjoy
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