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

Thu Jun 1 13:23:01 PDT 2017

Hi, Kristof.

It sounds like a good plan, but one thing is not clear to me from your 
post.  Which pipeline model will be used for AArch64, A53's (i.e., none)?

Thank you,

-- 
Evandro Menezes

On 06/01/2017 01:37 AM, Kristof Beyls wrote:
> Thanks for everyone giving their feedback!
> I saw pretty unanimous support for making -mcpu=generic the default 
> and making -mcpu=generic schedule for an in-order CPU (Cortex-A8 in 
> this case).
> I'll be making those changes shortly.
>
> I think the comments also make clear that it's less obvious whether 
> we'd want -mcpu=native to become a default. It's probably good for 
> some use cases, but really not good for other use cases. I won't be 
> making that change, nor advocate for it.
>
> Thanks!
>
> Kristof
>
>
>> On 31 May 2017, at 17:57, Stephen Hines <srhines at google.com 
>> <mailto:srhines at google.com>> wrote:
>>
>> 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 
>> <mailto: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
>>     <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
>>
>>
>