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

Fri May 19 01:33:40 PDT 2017

On 18 May 2017 at 19:09, Quentin Colombet <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 :)

>
>>
>> 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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