[llvm-dev] [GlobalISel][AArch64] Toward flipping the switch for O0: Please give it a try!

Fri May 19 10:06:18 PDT 2017

Hi Diana,

> On May 19, 2017, at 1:33 AM, Diana Picus <diana.picus at linaro.org> wrote:
> 
> On 18 May 2017 at 19:09, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>> Hi Diana,
>> 
>>> On May 18, 2017, at 1:15 AM, Diana Picus <diana.picus at linaro.org> wrote:
>>> 
>>> On 18 May 2017 at 09:06, Kristof Beyls <Kristof.Beyls at arm.com> wrote:
>>>> I think Diana found that when enabling r299283, the bootstrap failed with
>>>> llvm-tblgen segfaulting.
>>>> So there clearly is some work required there.
>>> 
>>> Indeed.
>>> 
>>> @Quentin, what is the status of that patch?
>> 
>> Initially it was meant as a prototype to investigate how we could address those issues at O0. I didn’t mean to publish it.
>> 
>>> Have you been working on
>>> it since then?
>> 
>> No, I haven’t touched it and I honestly didn’t plan to do that.
>> 
>>> Should I investigate the segfault more and send you a
>>> reproducer?
>> 
>> Depends, do you guys want to pursue in that direction?
> 
> Not necessarily, if you think a better reg alloc scheme will do the
> trick then I see no point in complicating the pass pipeline for now.
> 
>> My though was that it is probably best to rely on a better reg allocator (basic or greedy) scheme for O0. I believe the only concern may be compile time but it may  just fly (+ I have ideas how to make it better pretty easy).
>> 
>>> Is this patch the way forward, or do you have other ideas
>>> for reducing the stack usage?
>> 
>> Better reg alloc scheme for O0 :).
> 
> Ok, let's go with that then :)

Sounds good, let me push a patch that would allow to use the greedy allocator at O0 and see if that’s sufficient. Then, we will look at the compile time that change will imply.

Cheers,
-Quentin

> 
>> 
>>> 
>>> Thanks,
>>> Diana
>>> 
>>>> 
>>>> Thanks,
>>>> 
>>>> Kristof
>>>> 
>>>> 
>>>> On 16 May 2017, at 14:06, Diana Picus via llvm-dev <llvm-dev at lists.llvm.org>
>>>> wrote:
>>>> 
>>>> Turns out it really is a GlobalISel issue - we eat up too much stack
>>>> space because all the constants used in the switches are stored on the
>>>> stack. We need to fix this somehow before changing the default. I'll
>>>> try to give it a run with Quentin's r299283 on top to see if it helps.
>>>> 
>>>> Cheers,
>>>> Diana
>>>> 
>>>> On 15 May 2017 at 09:38, Diana Picus <diana.picus at linaro.org> wrote:
>>>> 
>>>> Got another one: https://bugs.llvm.org/show_bug.cgi?id=33036
>>>> 
>>>> I'm still investigating whether this is an actual GlobalISel issue or
>>>> something else (I'll also start a build on a more recent revision to
>>>> see how that behaves).
>>>> 
>>>> On 12 May 2017 at 20:06, Eric Christopher <echristo at gmail.com> wrote:
>>>> 
>>>> Agreed. That was probably just luck before :)
>>>> 
>>>> -eric
>>>> 
>>>> On Fri, May 12, 2017 at 5:22 AM Diana Picus via llvm-dev
>>>> <llvm-dev at lists.llvm.org> wrote:
>>>> 
>>>> 
>>>> It turns out that can be fixed by adding -lm to the link line, so I
>>>> will probably convert it into a test-suite bug.
>>>> 
>>>> I don't suppose it's crucial to handle the fabs intrinsic nicely at -O0.
>>>> 
>>>> On 12 May 2017 at 13:44, Diana Picus <diana.picus at linaro.org> wrote:
>>>> 
>>>> Hi,
>>>> 
>>>> I ran into a little snag on the test-suite:
>>>> https://bugs.llvm.org/show_bug.cgi?id=33021
>>>> It boils down to GlobalISel generating calls to fabs instead of using
>>>> FABSDr (so we get undefined references).
>>>> 
>>>> Cheers,
>>>> Diana
>>>> 
>>>> On 11 May 2017 at 18:40, Quentin Colombet <qcolombet at apple.com> wrote:
>>>> 
>>>> Hi Diana,
>>>> 
>>>> Thanks for the summary.
>>>> 
>>>> On May 11, 2017, at 4:01 AM, Diana Picus <diana.picus at linaro.org>
>>>> wrote:
>>>> 
>>>> Hi all,
>>>> 
>>>> I'm still running some validation on this, I'll send an email when
>>>> it's done. If that goes well I don't have anything against making the
>>>> switch.
>>>> 
>>>> For the record, here's a summary of issues that were deferred for
>>>> later on (some of which are optimization-ish and we might decide to
>>>> never do at -O0 at all):
>>>> * Crash in RegBankSelect for half fp types:
>>>> https://bugs.llvm.org/show_bug.cgi?id=32560
>>>> 
>>>> 
>>>> I’ll have a look.
>>>> 
>>>> * Improving constant placement:
>>>> http://bugs.llvm.org/show_bug.cgi?id=32561
>>>> 
>>>> 
>>>> I’ve commented in the PR to mention the localizer technic I was playing
>>>> with, if someone wants to give it a try.
>>>> 
>>>> * Fancy switch lowering
>>>> * Transforming division-by-constant-power-of-2 into right shift
>>>> 
>>>> 
>>>> AFAICT all the other issues that were brought up were fixed (yay!).
>>>> 
>>>> Cheers,
>>>> Diana
>>>> 
>>>> 
>>>> Cheers,
>>>> -Quentin
>>>> 
>>>> 
>>>> 
>>>> On 11 May 2017 at 08:44, Kristof Beyls via llvm-dev
>>>> <llvm-dev at lists.llvm.org> wrote:
>>>> 
>>>> 
>>>> On 10 May 2017, at 17:36, Quentin Colombet <qcolombet at apple.com> wrote:
>>>> 
>>>> 
>>>> MultiSource/Benchmarks/mediabench/mpeg2/mpeg2dec/mpeg2decode (46%):
>>>> Function
>>>> Reference_IDCT: Probably due to creating all constants in the entry BB
>>>> +
>>>> spilling floating point data through an X register:
>>>> 
>>>> FastISel:
>>>> fadd d0, d1, d0
>>>> str d0, [sp,#528]
>>>> GlobalISel:
>>>> fadd d0, d1, d0
>>>> fmov x9, d0
>>>> stur x9, [x29,#-48]
>>>> 
>>>> 
>>>> Good finding, I forgot to do stores in my previous fix. I’ll do them
>>>> shortly.
>>>> 
>>>> 
>>>> Should be fixed by r302679
>>>> 
>>>> 
>>>> Thanks Quentin,
>>>> 
>>>> That reduces the slow-down when enabling globalisel at -O0 from 13% (on
>>>> r302453)  to 9.5% (on r302679) in my experiments.
>>>> The code size increase also reduces from just over 3% to 2.8%.
>>>> 
>>>> Kristof
>>>> 
>>>> 
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>>>> 
>>>> 
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