[llvm-dev] Proposal for O1/Og Optimization and Code Generation Pipeline

Eric Christopher via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 1 19:02:17 PDT 2019

Hi Greg,

Thanks for the detailed response!

On Fri, Mar 29, 2019 at 5:25 AM Greg Bedwell <gregbedwell at gmail.com> wrote:
> Thanks for posting this.  I'm absolutely of the opinion that current -O1 is almost a "worst of all worlds" optimization level, where the performance of the generated code isn't good enough to be particularly useful (for our users at least) but the debug experience is already getting close to being as bad as -O2/3, so I'm personally very happy with your direction of redefining -O1 (especially as that could then open up the way to future enhancements like using PGO data to let us compile everything at -O1 for the build time performance win, except for the critical hot functions that get the full -O2/3 pipeline for the run time performance win).

Excellent. Thanks.

> How will this optimization level interact with LTO (specifically ThinLTO)?  Would -O1 -flto=thin to run through a different, faster LTO pipeline or are we expecting that any everyday development build configuration won't include LTO?

I'll be honest... I hadn't thought about it at all. I'd think that the
new O1 would supersede any O1 and so thin/normal lto invoked with O1
would give you the new pipeline.

> I'm a little bit more on the fence with what this would mean for -Og, as I'd really like to try and come to some sort of community consensus on exactly what -Og should mean and what its aims should be.  If you happen to be at EuroLLVM this year then that would be absolutely perfect timing as I'd already submitted a round table topic to try and start just that process [ http://llvm.org/devmtg/2019-04/#rounds ].  My team's main focus right now is in trying to fix as many -O2 debug experience issues as possible, with the hope that we could consider using an -Og mode to mop up what's left, but we've been surveying our users for a few years now about what they'd find useful in such an optimization level.

Sorry, I won't be able to make it. Conveniently I wrote up a nice
email with my general pipeline discussion. I can do more of a write-up
for debugging metrics if that will help? I've got the start of it
anyhow and since a number of people wanted to talk about it I can do
that this week if you think it will be helpful. I'd prefer to keep the
pass discussions on the mailing list as much as possible so that we
can a) keep our discussions focused, and b) include people who can't
necessarily make particular developer meetings.

> The general consensus is that performance must not be significantly worse than -O2.  We've heard a few numbers thrown around like 5-10% runtime slowdown compared to -O2 being the absolute maximum acceptable level of intrusion for them to consider using such a mode.  I'm not really sure how realistic that is and I'm inclined to think that we could probably stretch that limit a little bit here and there if the debugging experience really was that much better, but I think it gives a good indication of at least what our users are looking for.  Essentially -O2 but with as few changes as we can get away with making to make the debugging experience better.  I know that this is somewhat woolly, so it might be that your proposed pipeline is the closest we can get that matches such an aim, but once we've decided what -Og should mean, I'd like to try and justify any changes with some real data.  I'm willing for my team to contribute as much data as we can.  We've also been using dexter [ http://llvm.org/devmtg/2018-04/slides/Bedwell-Measuring_the_User_Debugging_Experience.pdf ] to target our -O2 debugging improvement work, but hopefully it will be useful to provide another datapoint for the effects on the debugging experience of disabling specific passes.

That's a lot of "general consensus" :)

Of course, I'm approaching it from the perspective of "faster than O0"
rather than "no slower than O2". I'm also not sure whether or not the
game community is unusual here. I also expect that we can adapt or
change existing passes in order to make them more debugging friendly
over time. For example, the differences I suggested with respect to
cfg cleanup in my proposal. I have a lot of real data I've started
collecting here and I'll work on summarizing some of that in a future

Sounds like a general approach of "let's start carving blocks of
marble and then break out the sanding tools" is welcome though :)

> In my mind, -Og probably would incorporate a few things:
> * Tweak certain pass behaviors in order to be more favorable towards debugging [ https://reviews.llvm.org/D59431#1437716 ]
> * Enable features favorable to debugging [ http://llvm.org/devmtg/2017-10/#lightning8 ]
> * Disable whole passes that are known to fundamentally harm the debugging experience if there is no other alternative approach (this proposal?)
> * Still give a decent debug experience when used in conjunction with LTO.

I don't think we're substantially apart here. I'm very much personally
not a fan of extending the lifetime of local variables, but I think
it's absolutely a worthwhile discussion to have :)

> Thanks again for writing up your proposal.  I'm really happy to see movement in this area!

Happy to help. This is something I've wanted to work on for years at
this point :)


> -Greg
> On Fri, 29 Mar 2019 at 02:09, Eric Christopher via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> Hi All,
>> I’ve been thinking about both O1 and Og optimization levels and have a
>> proposal for an improved O1 that I think overlaps in functionality
>> with our desires for Og. The design goal is to rewrite the O1
>> optimization and code generation pipeline to include the set of
>> optimizations that minimizes build and test time while retaining our
>> ability to debug.
>> This isn’t to minimize efforts around optimized debugging or negate O0
>> builds, but rather to provide a compromise mode that encompasses some
>> of the benefits of both. In effect to create a “build mode for
>> everyday development”.
>> This proposal is a first approximation guess on direction. I’ll be
>> exploring different options and combinations, but I think this is a
>> good place to start for discussion. Unless there are serious
>> objections to the general direction I’d like to get started so we can
>> explore and look at the code as it comes through review.
>> Optimization and Code Generation Pipeline
>> The optimization passes chosen fall into a few main categories,
>> redundancy elimination and basic optimization/abstraction elimination.
>> The idea is that these are going to be the optimizations that a
>> programmer would expect to happen without affecting debugging. This
>> means not eliminating redundant calls or non-redundant loads as those
>> could fail in different ways and locations while executing.  These
>> optimizations will also reduce the overall amount of code going to the
>> code generator helping both linker input size and code generation
>> speed.
>> Dead code elimination
>>  - Dead code elimination (ADCE, BDCE)
>>  - Dead store elimination
>>  - Parts of CFG Simplification
>>  - Removing branches and dead code paths and not including commoning
>> and speculation
>> Basic Scalar Optimizations
>>  - Constant propagation including SCCP and IPCP
>>  - Constant merging
>>  - Instruction Combining
>>  - Inlining: always_inline and normal inlining passes
>>  - Memory to register promotion
>>  - CSE of “unobservable” operations
>>  - Reassociation of expressions
>>  - Global optimizations - try to fold globals to constants
>> Loop Optimizations
>> Loop optimizations have some problems around debuggability and
>> observability, but a suggested set of passes would include
>> optimizations that remove abstractions and not ones that necessarily
>> optimize for performance.
>>  - Induction Variable Simplification
>>  - LICM but not promotion
>>  - Trivial Unswitching
>>  - Loop rotation
>>  - Full loop unrolling
>>  - Loop deletion
>> Pass Structure
>> Overall pass ordering will look similar to the existing pass layout in
>> llvm with passes added or subtracted for O1 rather than a new pass
>> ordering. The motivation here is to make the overall proposal easier
>> to understand initially upstream while also maintaining existing pass
>> pipeline synergies between passes.
>> Instruction selection
>> We will use the fast instruction selector (where it exists) for three reasons:
>>  - Significantly faster code generation than llvm’s dag based
>> instruction selection
>>  - Better debugability than selection dag - fewer instructions moved around
>>  - Fast instruction selection has been optimized somewhat and
>> shouldn’t be an outrageous penalty on most architectures
>> Register allocation
>> The fast register allocator should be used for compilation speed.
>> Thoughts?
>> Thanks!
>> -eric
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