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

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?

On May 11, 2012, at 12:25 AM, James Molloy wrote:
> 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?
The right place to implement this is in lib/Support/Host.cpp.  X86 has an
implementation of sys::getHostCPUName(), but everything else just uses the:

std::string sys::getHostCPUName() {
  return "generic";
}

implementation.

-Chris

Hi James,
> > The backend sounds like the wrong place to implement this feature.
> > 
> > I'd have thought the Clang driver would be the ideal place?
> 
> The right place to implement this is in lib/Support/Host.cpp.  X86 has an
implementation of sys::getHostCPUName(), but everything else just uses the:
> 
> std::string sys::getHostCPUName() {
>   return "generic";
> }
> 
  Do you happened to know examples show how to get cpu model like X86 does in
Host.cpp?

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 May 11, 2012, at 9:25 AM, Chris Lattner wrote:
> 
> On May 11, 2012, at 12:25 AM, James Molloy wrote:
> 
>> 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?
> 
> The right place to implement this is in lib/Support/Host.cpp.  X86 has an
implementation of sys::getHostCPUName(), but everything else just uses the:
> 
> std::string sys::getHostCPUName() {
>  return "generic";
> }
> 
> implementation.
Right. There are many advantages to doing this in the backend. It would work for
everything, such as llc or arbitrary clients of ARM backend. Also the support
library is being used by Clang frontend to handle -march=native.

Evan
> 
> -Chris
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi Chris,
> The right place to implement this is in lib/Support/Host.cpp.  X86 has an
implementation of sys::getHostCPUName(), but everything else just uses the:
> 
> std::string sys::getHostCPUName() {
>   return "generic";
> }
> 
> implementation.
  I tried to let it return "armv7l" or "cortex-a9" on
pandaboard, but the
bitcode output by clang still has 

  target triple = "armv4t-unknown-linux-gnueabi"

not what I expect 

  target triple = "armv7l-unknown-linux-gnueabi"

Do I miss something? Thanks.

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