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

[Diana Picus via llvm-dev]

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
>
>
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