llvm dev - May 2017 - [GlobalISel][AArch64] Toward flipping the switch for O0: Please give it a try!

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? Have you been working on
it since then? Should I investigate the segfault more and send you a
reproducer? Is this patch the way forward, or do you have other ideas
for reducing the stack usage?

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
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>

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?
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 :).
> 
> 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
>> 
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> 
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> 
>>

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
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>