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

[Eric Christopher via llvm-dev]

On Fri, Jun 16, 2017 at 4:43 PM Quentin Colombet <qcolombet at apple.com>
wrote:

> Hi all,
>
> We had some internal discussions about flipping the default for O0 and we
> concluded that we wanted to postpone it.
>
>
> *** Why Is That? ***
>
> We don’t want to send the wrong message that GlobalISel’s design is set in
> stone and ready for broader adoption.
> In particular,
> 1. The APIs are still evolving and can still possibly change significantly
> 2. The TableGen backend to reuse the existing SD patterns is still at its
> early stage
> 3. We want to investigate closely the performance of global-isel
> (compile-time, runtime, code size, fallbacks)
>
> The rationale behind those items is that we want to minimize the pain of
> moving forward for everybody. We also want the out-of-the-box experience to
> be pleasant (like all/most of the tablegen patterns just work, we have
> documentation on how to target a new backend, etc.) Finally, we want to
> gain confidence we are going to be able to address the performance issues
> we have with the current design and if not, derive a plan for that.
>
> We purposely left out of the conversation what will be the right time and
> requirements to flip the switch. We want to gather more data first. Your
> help would be appreciated!
>
>
> *** Short-Term Proposal ***
>
> What we would like to do instead short-term is:
> A. Repurpose or create an option “-aarch64-enable-global-isel-at-O” to
> enable GISel with fallbacks and warnings enables (i.e., equivalent of
> -global-isel -global-isel-abort=2)
> B. Advertise this option in the next open source release to allow compiler
> enthusiastic to try it and report problems
> C. Have GISel always built so we can push thing in the right place,
> MachineVerifier in mind, and stop doing some weird gymnastic
>
> What do people think?
>
>
How about -fexperimental-global-isel as a flag to clang?

-eric


>
> *** Your Help Is Needed ***
>
> - Please share your experience in using the GISel APIs and how we can make
> them better. Moving forward we’ll have those conversations on open source
> instead of internally/with a narrower audience.
> - Report any performance problem you identify
> - Propose patches!
>
> Cheers,
> -Quentin
>
>
>
> On Jun 16, 2017, at 3:06 PM, Quentin Colombet via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>
> On Jun 14, 2017, at 7:27 AM, Diana Picus <diana.picus at linaro.org> wrote:
>
> On 12 June 2017 at 18:54, Diana Picus <diana.picus at linaro.org> wrote:
>
>> Hi all,
>>
>> I added a buildbot [1] running the test-suite with -O0 -global-isel. It
>> runs into the same 2 timeouts that I reported previously on this thread
>> (paq8p and scimark2). It would be nice to make it green before flipping the
>> switch.
>>
>>
> I did some more investigations on a machine similar to the one running the
> buildbot. For paq8p and scimark2, I get these results for O0:
>
> PAQ8p:
> Fast isel: 666.344
> Global isel: 731.384
>
> SciMark2-C:
> Fast isel: 463.908
> Global isel: 496.22
>
> The current timeout is 500s (so in this particular case we didn't hit it
> for scimark2, and it ran successfully to completion). I don't think the
> difference between FastISel and GlobalISel is too atrocious, so I would
> propose increasing the timeout for these 2 benchmarks. I'm not sure if we
> can do this on a per-bot basis, but I see some precedent for setting custom
> timeout thresholds for various benchmarks on different architectures
> (sometimes with comments that it's done so we can run O0 on that particular
> benchmark).
>
> Something along these lines works:
> https://reviews.llvm.org/differential/diff/102547/
>
> What do you guys think about this approach?
>
>
> Looks reasonable to me.
>
>
> Thanks,
> Diana
>
> PS: The buildbot is using the Makefiles because that's what our other
> AArch64 test-suite bots use. Moving all of them to CMake is a transition
> for another time.
>
>
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