[petsc-dev] AVX kernels, old gcc, still broken

[Jed Brown]

"Smith, Barry F." <bsmith at mcs.anl.gov> writes:

>> On Oct 26, 2019, at 9:09 AM, Jed Brown <jed at jedbrown.org> wrote:
>> 
>> "Smith, Barry F." <bsmith at mcs.anl.gov> writes:
>> 
>>>  The proposed fix is #if defined(PETSC_USE_AVX512_KERNELS)   && && && && && in https://gitlab.com/petsc/petsc/merge_requests/2213/diffs
>> 
>> Looks fine; approved.
>> 
>>> but note that PETSC_USE_AVX512_KERNELS does not even do a configure check to make sure it is valid. The user has to guess that passing that flag will work. Of course a proper configure test is needed and since a proper test is needed it can handle all the issues in one place instead of having one issue in  configure and n - 1 in the source code. 
>> 
>> What are "all the issues"?  32-bit indices, precision=double,
>> scalar=real?  So we'll need 8 CPP macros that test each of those
>> combinations?
>
>    No, if suddenly there is support for single precision for example, the developer would modify the configure test to turn on PETSC_USE_AVX512_KERNELS for that additional  case and not touch the source code at all; 


1. We still need to distinguish code paths for single and double.  They
definitely have to touch the source code because double-precision
intrinsics don't work on single-precision data.

2. The work is inevitably done incrementally so the developer needs a
#if test to work one kernel at a time.  It's error-prone to create a new
preprocessor macro for testing and change it before submitting the MR.
It also forces all single-precision support into a single commit that
implements several kernels instead of just one.

3. "git pull && make libs" will fail for every user of master because
they need to reconfigure.  Testing the branch before merging becomes a
many-minutes job instead of 10 seconds to recompile a few files and run
a test.  Switching back to 'master' after commenting/approval might
require getting back the old macros, thus another reconfigure.