[llvm-dev] RFC: Building GlobalISel by default

Thu Jan 26 16:33:42 PST 2017

> On Jan 26, 2017, at 4:12 PM, Quentin Colombet <qcolombet at apple.com> wrote:
> 
> Thanks all the feedbacks.
> 
> I moved forward, the switch happened in r293232.

Great!

> 
> For incremental buildbots, I believe we are going to need to kick a clean build for them to pick it up.
> @Galina, is it something you can do?
> 
> Cheers,
> -Quentin
> 
>> On Jan 25, 2017, at 10:50 AM, Gerolf Hoflehner <ghoflehner at apple.com <mailto:ghoflehner at apple.com>> wrote:
>> 
>> Hey Quentin, reads like you got the community nod and can start the engine …
>> 
>> Cheers
>> Gerolf
>> 
>>> On Jan 24, 2017, at 5:10 PM, Eric Christopher via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> 
>>> 
>>> 
>>> On Thu, Jan 19, 2017 at 1:10 PM Jim Grosbach <grosbach at apple.com <mailto:grosbach at apple.com>> wrote:
>>>> On Jan 19, 2017, at 11:59 AM, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>>>> 
>>>> Hi all,
>>>> 
>>>> Trying to summarize the concerns along side my answers but I believe we are good to do the switch to build GlobalISel by default. In particular, Renato and I exchanged offline and he tried doing the switch on some of the bots he maintained and did not see any overhead/problem for that.
>>>> Let me know if you disagree.
>>>> 
>>>> 
>>>> * What is the impact on compile time?
>>>> 
>>>> Negligible: ~5s for a 19min build. (See http://lists.llvm.org/pipermail/llvm-dev/2017-January/109185.html <http://lists.llvm.org/pipermail/llvm-dev/2017-January/109185.html> for the details.)
>>>> 
>>>> 
>>>> * What is the impact on binary size?
>>>> 
>>>> Negligible: 0 to 1M on a 37M to 104M. (See http://lists.llvm.org/pipermail/llvm-dev/2017-January/109185.html <http://lists.llvm.org/pipermail/llvm-dev/2017-January/109185.html> for the details.)
>>>> 
>>>> 
>>>> * How likely GlobalISel will break on non-related change?
>>>> 
>>>> Unlikely: GlobalISel's APIs are fairly isolated or work on pretty well established low-level APIs. (See http://lists.llvm.org/pipermail/llvm-dev/2017-January/109154.html <http://lists.llvm.org/pipermail/llvm-dev/2017-January/109154.html> for details.)
>>>> 
>>>> 
>>>> * Why GlobalISel wasn’t built from the start?
>>>> 
>>>> One of the requirement was that we don’t impact negatively the footprint of the compiler. Given we were not sure where we were going and in particular we temporarily increased the size of the MachineInstrs and some other core classes during the prototype, we decided to turn it off by default. Those problems have been fixed now.
>>>> 
>>>> 
>>>> * How is the existing GlobalISel implement viewed?
>>>> 
>>>> Last year we finished the proof-of-concept and are moving toward production quality now. We are doing reviews like any other part of the code and rewrite what we believe needs to be rewritten. I’ll go into more details regarding the status in another thread, but one of the thing we are pushing right now is tablegen support in GlobalISel so that we can just reuse the existing td form SDISel.
>>> 
>>> To reinforce Quentin’s point here, if folks have concerns about the suitability of areas of the code for long term maintainability, elegance of design or implementation, or anything else regarding the code quality, please say so. Deep and thorough review is essential to long term success.
>>> 
>>> 
>>> Sadly I didn't have time to review a prototype and am a little lacking in review time right now. I'll have to hope for the best :)
>>> 
>>> -eric
>>>  
>>>> 
>>>> 
>>>> Let me know if I didn’t address some of the concerns.
>>>> 
>>>> Cheers,
>>>> -Quentin
>>>> 
>>>>> On Jan 18, 2017, at 10:56 AM, Eric Christopher <echristo at gmail.com <mailto:echristo at gmail.com>> wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>> On Wed, Jan 18, 2017 at 9:13 AM David Blaikie via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>>> These concerns sound applicable to the situation when GlobalISel is turned on by default and has an effect on code generation.
>>>>> 
>>>>> While it's a library with tests like any other LLVM component I don't see these concerns as being significantly greater risk than any other library in LLVM.
>>>>> 
>>>>> I'm honestly surprised GlobalISel was developed in such a way that it hasn't been built and tested by default for all developers from Day 1. The new pass manager and the ORC JIT are examples of this - there was never any question about when to enable building and testing them. They were built and tested from the moment they went in tree.
>>>>> 
>>>>> 
>>>>> A lot of this is because GlobalISel was a "prototype" that would just be easier to work on in top of tree rather than something that was made with the idea of going live originally. Arguably code review requirements were a bit more lax, etc. This appears to have changed recently, but I'm not sure what changed in how the existing GlobalISel implementation is viewed.
>>>>> 
>>>>> So, what's up? :)
>>>>> 
>>>>> -eric
>>>>> 
>>>>>  
>>>>> The question of when to turn these things (GlobalISel, New PM, ORC, etc) is a very different question and one I'd expect to be staged in the way you're describing, Renato. Provide a flag, off by default. Ask/encourage people to test it out. Eventually flip the default - encouraging people to unflip manually and report if they have trouble. Maintain this state until reported issues are addressed/wait for a grace period, then remove the flag/old code entirely.
>>>>> 
>>>>> 
>>>>> On Sun, Jan 15, 2017 at 5:15 AM Renato Golin via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>>> On 15 January 2017 at 04:24, Michael Kuperstein
>>>>> <michael.kuperstein at gmail.com <mailto:michael.kuperstein at gmail.com>> wrote:
>>>>> > Regarding "we shouldn't enable it because it will make the bots slower" -
>>>>> > well, yes, but that's just postponing the inevitable. We will enable
>>>>> > GlobalISel eventually, and there will probably be a very long time-frame
>>>>> > during which both are enabled concurrently.
>>>>> 
>>>>> No magic solutions expected. I just don't want to crank all bots up to
>>>>> 11 on day one. We start with the fast bots, when they're happy, we
>>>>> move on until we're confident that the whole thing is stable, then we
>>>>> make it build by default.
>>>>> 
>>>>> There are two problems here:
>>>>> 
>>>>> 1. Environmental issues.
>>>>> 
>>>>> We do find more of those than we'd hope. Different compilers, linkers,
>>>>> libraries that produce not only crashes, but unstable builds and
>>>>> unpredictable or irreproducible test failures. The environment on our
>>>>> bots are wildly different (on purpose) and dealing with different
>>>>> unpredictable issues all at once is something that I'm not willing to
>>>>> take on lightly. Due to the nature of buildbots and some of our
>>>>> hardware and the fact that virtually all ARM/AArch64 bots are ours,
>>>>> this will be something that Linaro will pay the price in full.
>>>>> 
>>>>> 2. Experimental nature.
>>>>> 
>>>>> If the past serves as a guide, experimental features at the core of
>>>>> LLVM will be largely independent until they start to merge, when they
>>>>> can become a huge issue and be in merging state for months, if not
>>>>> years, or worse, never be merged. Right now, we're not at that state,
>>>>> we're still largely independent, so your worries regarding the
>>>>> development tests (which also hit buildbots at a different pace
>>>>> because of the environment issues above), will be lower for now, but
>>>>> will increase massively soon enough.
>>>>> 
>>>>> There is the argument that having the bots building means we'll have
>>>>> *less* teething issues and the people building GlobalISel (which also
>>>>> includes Linaro) will have a lot less work. But there's a cost to be
>>>>> paid, and that cost is a lot higher on the ARM/AArch64 side than x86,
>>>>> just because most developers use an x86 as their main machine.
>>>>> 
>>>>> 
>>>>> > Still, as a strawman proposal - would it make sense to add
>>>>> > LLVM_BUILD_GLOBAL_ISEL_EXPERIMENTAL, or something of that sort? The more
>>>>> > mature ports will build under LLVM_BUILD_GLOBAL_ISEL (which will be enabled
>>>>> > by default), the less mature ones only under
>>>>> > LLVM_BUILD_GLOBAL_ISEL_EXPERIMENTAL (which won't be), and moving a port from
>>>>> > GLOBAL_ISEL_EXPERIMENTAL to GLOBAL_ISEL would be equivalent to marking a
>>>>> > regular backend non-experimental. We can start with just AArch64 in the
>>>>> > non-experimental category, and move the others as they become more stable.
>>>>> > Or is this too much complexity for what we gain?
>>>>> 
>>>>> It is.
>>>>> 
>>>>> We already separate that by not building any other back-end than ARM
>>>>> and AArch64. So I think we can do that without the added complexity.
>>>>> 
>>>>> I'm not trying to block the move, I'm just trying to make it smother
>>>>> and raise awareness that the cost we pay for any large move is not
>>>>> well distributed.
>>>>> 
>>>>> For the past few months I have reverted patches that only broke the
>>>>> ARM bots and it was clear on most of those instances which patch was
>>>>> to blame, but people didn't bother and after 8, 12 sometimes 24 hours,
>>>>> I reverted. This is not a nice place to be in, and given that
>>>>> resurgence lately, I am honestly afraid adding more things will make
>>>>> it worse.
>>>>> 
>>>>> A red bot can't catch new errors. A slow red bot can go on red for
>>>>> weeks (and we have seen it numerous times) because once you fix bug
>>>>> #1, #2 has made #3 appear and not be warned.
>>>>> 
>>>>> This is not everyone, and I do appreciate all the people that helped
>>>>> us and worried about the breakages, but it's a trend that is
>>>>> increasing, not decreasing. Our team is too small to cope with further
>>>>> increases, so if the community is willing to do its part, I think we
>>>>> can make it work.
>>>>> 
>>>>> But it has to be slow and steady.
>>>>> 
>>>>> cheers,
>>>>> --renato
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>>>> 
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