llvm dev - Mar 2015 - [LLVMdev] Removing TargetMachine CPU auto-detection for PowerPC and SystemZ?

Hi Hal,

I only just noticed that about a year ago, Jim removed CPU auto-detection
for the X86 target:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-April/071991.html
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140407/212676.html
>Currently the X86 backend does CPU auto-detection and subtarget feature
>detection when the TargetMachine is created if no explicit CPU was
>specified. It's counterintuitive for low level tools like ‘llc’ to do
>this, as it means the same .ll file compiled on heterogenous machines
>generates different results from the same ‘llc’ command line. It is still
>useful to be able to opt-in to such behavior to, for example, replicate
>clang’s behavior when -mcpu=native is supplied to clang. My thought is
>to do something similar here and teach ‘llc’ to recognize -mcpu=native
>and probe the host CPU if that is given. The subtarget features will
>then be filled in according to the feature string for that CPU. This
>(a) changes the auto-detection from opt-out to opt-in and (b) moves
>the logic out of the core target backend and into the tools drivers.
>Attached are draft patches that do this for X86. Similar but smaller
>cleanups can also be done for SystemZ and PowerPC if it’s agreed this
>is a good idea.
However, this was then never implemented for SystemZ and PowerPC.  Should
we do so as well?

Note that mesa has in the meantime changed to do the auto-detection before
setting up the JIT context, and I assume other JIT users have done so as
well by now.  For clang, this was never done automatically anyway.


Mit freundlichen Gruessen / Best Regards

Ulrich Weigand

--
  Dr. Ulrich Weigand | Phone: +49-7031/16-3727
  STSM, GNU/Linux compilers and toolchain
  IBM Deutschland Research & Development GmbH
  Vorsitzende des Aufsichtsrats: Martina Koederitz | Geschäftsführung: Dirk
Wittkopp
  Sitz der Gesellschaft: Böblingen | Registergericht: Amtsgericht
Stuttgart, HRB 243294

----- Original Message -----
> From: "Ulrich Weigand" <Ulrich.Weigand at de.ibm.com>
> To: "Hal J. Finkel" <hfinkel at anl.gov>
> Cc: llvmdev at cs.uiuc.edu, grosbach at apple.com
> Sent: Monday, March 23, 2015 12:21:17 PM
> Subject: Removing TargetMachine CPU auto-detection for PowerPC and SystemZ?
> 
> 
> Hi Hal,
> 
> I only just noticed that about a year ago, Jim removed CPU
> auto-detection
> for the X86 target:
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-April/071991.html
>
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140407/212676.html
> 
> >Currently the X86 backend does CPU auto-detection and subtarget
> >feature
> >detection when the TargetMachine is created if no explicit CPU was
> >specified. It's counterintuitive for low level tools like ‘llc’ to
> >do
> >this, as it means the same .ll file compiled on heterogenous
> >machines
> >generates different results from the same ‘llc’ command line. It is
> >still
> >useful to be able to opt-in to such behavior to, for example,
> >replicate
> >clang’s behavior when -mcpu=native is supplied to clang. My thought
> >is
> >to do something similar here and teach ‘llc’ to recognize
> >-mcpu=native
> >and probe the host CPU if that is given. The subtarget features will
> >then be filled in according to the feature string for that CPU. This
> >(a) changes the auto-detection from opt-out to opt-in and (b) moves
> >the logic out of the core target backend and into the tools drivers.
> 
> >Attached are draft patches that do this for X86. Similar but smaller
> >cleanups can also be done for SystemZ and PowerPC if it’s agreed
> >this
> >is a good idea.
> 
> However, this was then never implemented for SystemZ and PowerPC.
>  Should
> we do so as well?
Yes, I think we should.

 -Hal
> 
> Note that mesa has in the meantime changed to do the auto-detection
> before
> setting up the JIT context, and I assume other JIT users have done so
> as
> well by now.  For clang, this was never done automatically anyway.
> 
> 
> Mit freundlichen Gruessen / Best Regards
> 
> Ulrich Weigand
> 
> --
>   Dr. Ulrich Weigand | Phone: +49-7031/16-3727
>   STSM, GNU/Linux compilers and toolchain
>   IBM Deutschland Research & Development GmbH
>   Vorsitzende des Aufsichtsrats: Martina Koederitz |
>   Geschäftsführung: Dirk
> Wittkopp
>   Sitz der Gesellschaft: Böblingen | Registergericht: Amtsgericht
> Stuttgart, HRB 243294
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

Hal Finkel <hfinkel at anl.gov> wrote on 23.03.2015 19:42:06:
> > >Attached are draft patches that do this for X86. Similar but
smaller
> > >cleanups can also be done for SystemZ and PowerPC if it’s agreed
> > >this is a good idea.
> >
> > However, this was then never implemented for SystemZ and PowerPC.
> > Should we do so as well?
>
> Yes, I think we should.
Checked in for SystemZ as r233540.  Do you want me to do PowerPC too?

Bye,
Ulrich