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

Hi Quentin,

I actually did a run with -mllvm -optimize-regalloc -mllvm
-regalloc=greedy over the weekend and the test does pass with that.
Haven't measured the compile time though.

Cheers,
Diana

On 19 May 2017 at 19:06, Quentin Colombet <qcolombet at apple.com>
wrote:
> 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>
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
>
>
> _______________________________________________
> 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 22 May 2017, at 09:09, Diana Picus <diana.picus at linaro.org>
wrote:
> 
> Hi Quentin,
> 
> I actually did a run with -mllvm -optimize-regalloc -mllvm
> -regalloc=greedy over the weekend and the test does pass with that.
> Haven't measured the compile time though.
> 
> Cheers,
> Diana
I also did my usual benchmarking run with the same options as Diana did above:
- Comparing against -O0 without globalisel: 2.5% performance drop, 0.8% code
size improvement.
- Comparing against -O0 without globalisel but with the above regalloc options:
5.6% performance drop, 1% code size drop.

In summary, the measurements indicate some good improvements.
I also haven't measure the impact on compile time.

Doing a few quick spot checks on the generated code, I still see some constants
being created in the entry BB and stored on the stack.
I haven't tried to investigate if the entry BB seems like a good place to
materialize those remaining constants.

Thanks,

Kristof
> 
> On 19 May 2017 at 19:06, Quentin Colombet <qcolombet at apple.com>
wrote:
>> 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> 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
>> 
>> 
>> _______________________________________________
>> 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
>> 
>>

Great!
I thought I had to look at our pipeline at O0 to make sure optimized regalloc
was supported (https://bugs.llvm.org/show_bug.cgi?id=33022
<https://bugs.llvm.org/show_bug.cgi?id=33022> in mind). Glad I was wrong,
it saves me some time.
> On May 22, 2017, at 12:51 AM, Kristof Beyls <kristof.beyls at
arm.com> wrote:
> 
> 
>> On 22 May 2017, at 09:09, Diana Picus <diana.picus at linaro.org>
wrote:
>> 
>> Hi Quentin,
>> 
>> I actually did a run with -mllvm -optimize-regalloc -mllvm
>> -regalloc=greedy over the weekend and the test does pass with that.
>> Haven't measured the compile time though.
>> 
>> Cheers,
>> Diana
> 
> I also did my usual benchmarking run with the same options as Diana did
above:
> - Comparing against -O0 without globalisel: 2.5% performance drop, 0.8%
code size improvement.
That’s compared to 9.5% performance drop and 2.8% code size regression, without
that regalloc scheme, right?
> - Comparing against -O0 without globalisel but with the above regalloc
options: 5.6% performance drop, 1% code size drop.
> 
> In summary, the measurements indicate some good improvements.
> I also haven't measure the impact on compile time.
Do you have a mean to make this measurement?
Ahmed did a bunch of compile time measurements on our side and I wanted to see
if I need to put him on the hook again :).
> 
> Doing a few quick spot checks on the generated code, I still see some
constants being created in the entry BB and stored on the stack.
Feel free to file PRs if you think that shouldn’t happen.

@Eric, how does it look on your side?

Cheers,
-Quentin
> I haven't tried to investigate if the entry BB seems like a good place
to materialize those remaining constants.
> 
> Thanks,
> 
> Kristof
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