[LLVMdev] LLVM ARM VMLA instruction

[Renato Golin]

On 20 December 2013 13:00, Tim Northover <t.p.northover at gmail.com> wrote:

> Worth investigating more, but as the others have said nowhere near
> enough data on its own. Especially since Evan clearly did some
> benchmarking himself before specifically disabling the vmla formation.
>

Indeed. Not just specific micro benchmarks. I also did some testing and
found similar results.


Probably most relevant is the combination of features given to each
> processor in lib/Target/ARM/ARM.td. This vmul/vmla difference comes
> from "FeatureHasSlowFPVMLx", via ARMSubtarget.h's useFPVMLx and
> ARMInstrInfo.td's UseFPVMLx.
>

Yes, there's no way to turn that on/off from the command line, but I think
this is a good thing, not a bad one.

Ultimately, using the -mcpu flag to chose the right CPU is the best thing
you can do, and LLVM should get it right.

Another thing that comes to my mind is that maybe it's time to set
Cortex-A9 as the default ARMv7 target... no?

cheers,
--renato
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