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

[Kristof Beyls via llvm-dev]

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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