[llvm-dev] Performance degradation on ARMv7 (cortex-a9)

Bradley Smith via llvm-dev
Wed, 24 Feb 2016 10:42:21 -0000

Hi,

The idea behind that change was to make ARM.td clearer, that is, adding
architecture features to new architecture subtarget features, and to have the
CPUs inherit from this. ProcA9 (and similar) from what I could tell were only
being used for their enum value in making codegen decisions, hence I moved all
of the features they inherit over to the actual CPUs for clarity, the idea being
that all features a given target uses come from a combination of the
architecture it inherits from and the target itself, not any intermediary
features like ProcA9.

I’m not aware of any place where ProcA9 is getting used to get subtarget
features like this, and after a quick look I still can’t find anything. Where
exactly are you seeing ProcA9 being used to get features? Even so, the cortex-a9
processer model itself inherits FeatureFP16 now so I would expect it to use
FP16, unless you’re not using cortex-a9 directly? (In which case all CPUs that
used to inherit ProcA9 now need to inherit all of the features ProcA9 used to
inherit as well as ProcA9, which is what I did in the change you mention).

Regards,
Bradley Smith

From: Grang, Mandeep Singh [mailto:mgrang at codeaurora.org]
Sent: 24 February 2016 03:16
To: Bradley Smith
Cc: llvm-dev at lists.llvm.org
Subject: Performance degradation on ARMv7 (cortex-a9)

 Hi Bradley,

I was doing some performance analysis for ARMv7 (cortex-a9) and I noticed that
one of my benchmarks degraded by 93%. I have tracked the regression down to the
following commit by you:

commit 7c1b77248baaeafec5d6433c3d1da9a2e2b69595
Author: Bradley Smith <bradley.smith at arm.com><mailto:bradley.smith
at arm.com>
Date:   Mon Nov 16 11:10:19 2015 +0000
    [ARM] Introduce subtarget features per ARM architecture.
    This allows for accurate architecture targeting as well as removing
    duplicate information (hardcoded feature strings) from MCTargetDesc.

I see that in lib/Target/ARM/ARM.td all the features have been removed from Proc
definition (e.g.: ProcA9) and added to ProcessorModel definition (e.g.:
ProcessorModel<"cortex-a9").
But I find that the features from Proc are still being read and set in
MCSubtargetInfo through the ARMFeatureKV table. So if the Proc is empty the
corresponding feature is not being set.
In my case, if I add FeatureFP16 back to the ProcA9 definition in ARM.td I get
back all the lost performance.

Could you please give me some insight on how, after your change, do the Proc
features get correctly set in MCSubtargetInfo and other places which access
Proc?

Thanks,
Mandeep
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