[llvm-dev] [RFC] Making -mcpu=generic the default for ARM armv7a and arm8a rather than -mcpu=cortex-a8 or -mcpu=cortex-a53

[Stephen Hines via llvm-dev]

Wow, these are some fantastic results! Android is definitely in favor of
fixing the defaults, so this proposal looks great from our perspective.

Thanks,
Steve

On Wed, May 31, 2017 at 5:57 AM, Kristof Beyls <Kristof.Beyls at arm.com>
wrote:

> *Motivation*
>
> At the moment, when targeting armv7a, clang defaults to generate code as
> if -mcpu=cortex-a8 was specified.
> When targeting armv8a, it defaults to generate code as if -mcpu=cortex-a53
> was specified.
>
> This leads to surprising code generation, by the compiler optimizing for a
> specific micro-architecture, whereas the intent from the user was probably
> to generate code that is "blended" for all the cores implementing the
> requested architecture. One example of a user being surprised like this is
> at https://bugs.llvm.org//show_bug.cgi?id=27219, where vmla's are not
> produced to optimize for a Cortex-A8-specific micro-architectural
> behaviour, even though the user didn't request to optimize specifically for
> Cortex-A8.
>
> It would be much cleaner conceptually if clang would default to
> -mcpu=generic when no specific cpu is specified.
>
> *What is the impact of this change on execution speed?*
>
> I think the main reason to be hesitant to change the default CPU for ARM
> to -mcpu=generic is the potential impact on performance of generated code.
>
> I've measured quite a wide selection of benchmarks with this change, on
> the following cores: Cortex-A9, Cortex-A53, Cortex-A57, Cortex-A72.
>
> Impact on execution speed, for each core, when using -march=armv7a, after
> changing the default cpu from cortex-a8 to generic is as follows.
> A positive numbers means speedup, a negative number means slow-down. These
> are the geomean results over 350 programs coming from benchmark suites such
> as the test-suite, SPEC2000, SPEC2006 and a range of proprietary suites.
>
> Cortex-A9: 0.96%
> Cortex-A53: -0.64%
> Cortex-A57: 1.04%
> Cortex-A72: 1.17%
>
> Impact on execution speed, for each core, when using -march=armv8a, after
> changing the default cpu from cortex-a53 to generic:
>
> (Cortex-A9 is an armv7a core, so can't execute armv8a binaries)
> Cortex-A53: -0.09%
> Cortex-A57: -0.12%
> Cortex-A72: 0.03%
>
> *Should we enable scheduling for an in-order core even for -mcpu=generic?*
>
> In the above measurements it shows that the biggest negative impact seen
> is with -march=armv7a on Cortex-A53: -0.64%.
> It seems that the in-order Cortex-A53 core is losing quite a bit of
> performance when the instructions aren't scheduled - which is to be
> expected.
> Therefore, I also experimented with letting instructions be scheduled
> according to the Cortex-A8 pipeline model, even for -mcpu=generic, trying
> to figure out if it's beneficial to schedule instructions for an in-order
> core rather than not trying to schedule them at all, for -mcpu=generic.
>
> Measurement results:
>
> -march=armv7a
>
> Cortex-A9: 1.57% (up from 0.96%)
> Cortex-A53: 0.47% (up from -0.64%)
> Cortex-A57: 1.74% (up from 1.04%)
> Cortex-A72: 1.72% (up from 1.17%)
>
> -march=armv8a (Note that there isn't a pipeline model for Cortex-A53 in
> the 32-bit ARM backend):
>
> (Cortex-A9 is an armv7a core, so can't execute armv8a binaries)
> Cortex-A53: 0.49% (up from -0.09%)
> Cortex-A57: 0.09% (up from -0.12%)
> Cortex-A72: 0.20% (up from 0.03%)
>
> Conclusion: for all the in-order and out-of-order cores I measured, it's
> beneficial to get the instructions scheduled using the Cortex-A8 pipeline
> model in combination with -mcpu=generic.
>
>
> Taking into account the above measurements, my conclusions are:
> 1. We should make -mcpu=generic the default cpu, not Cortex-A8 or
> Cortex-A53 for march=armv7a and march=armv8a.
> 2. We probably want to let the compiler schedule instructions using the
> Cortex-A8 pipeline model for -mcpu=generic, since it gives a bit of speedup
> on all cores tested.
>
> Do people agree with these conclusions?
> Any objections against implementing this?
> Any other potential impact this may have that I forgot to consider above?
>
> Thanks,
>
> Kristof
>
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