llvm dev - May 2012 - [LLVMdev] Request for Help: Teach ARM target to auto-detect cpu / subtarget features

Hi all,

I've just filed PR12794: Add ARM cpu / subtarget features auto-detection.
And I would very much appreciate the community's help to implement this.

What motivated this? Well this:
http://www.phoronix.com/scan.php?page=news_item&px=MTA5OTM

I believe one of the reason the benchmark numbers are totally bogus is that the
compilation are done on ARM hosts. Given the benchmarks are apparently compiled
without -mcpu=cortex-a9, I suspect LLVM ended up generating code for
"generic" ARMv4 cpu. This article makes me sick in my stomach.

Thanks,

Evan

> I've just filed PR12794: Add ARM cpu / subtarget features
auto-detection. And I would very much appreciate the community's help to
implement this.
> 
> What motivated this? Well this:
> http://www.phoronix.com/scan.php?page=news_item&px=MTA5OTM
> 
> I believe one of the reason the benchmark numbers are totally bogus is that
the compilation are done on ARM hosts. Given the benchmarks are apparently
compiled without -mcpu=cortex-a9, I suspect LLVM ended up generating code for
"generic" ARMv4 cpu. This article makes me sick in my stomach.
  I skip through MCTargetDesc/ARMAsmBackend.cpp, it seems
llvm::createARMAsmBackend
only pickup different ARM ISA for Darwin. As for Linux, I guess we need to tweak
ELFARMAsmBackend? Do we need to modify Clang as well?

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj

On 11/05/12 04:56, 陳韋任 wrote:
>> I've just filed PR12794: Add ARM cpu / subtarget features
auto-detection. And I would very much appreciate the community's help to
implement this.
>>
>> What motivated this? Well this:
>> http://www.phoronix.com/scan.php?page=news_item&px=MTA5OTM
>>
>> I believe one of the reason the benchmark numbers are totally bogus is
that the compilation are done on ARM hosts. Given the benchmarks are apparently
compiled without -mcpu=cortex-a9, I suspect LLVM ended up generating code for
"generic" ARMv4 cpu. This article makes me sick in my stomach.
>    I skip through MCTargetDesc/ARMAsmBackend.cpp, it seems
llvm::createARMAsmBackend
> only pickup different ARM ISA for Darwin. As for Linux, I guess we need to
tweak
> ELFARMAsmBackend? Do we need to modify Clang as well?
>
> Regards,
> chenwj
>
The backend sounds like the wrong place to implement this feature.

I'd have thought the Clang driver would be the ideal place?