[R-pkg-devel] Fwd: using portable simd instructions

[Tomas Kalibera]

On 3/27/24 08:39, jesse koops wrote:
> Of course you are correct about the portability. But since at ;least
> one other CRAN package by a renowned author does it succesfully, I
> figured I'd experiment first on my machine and learn about portability
> later. Thank you for the links and the warning about the bug. I was
> aware of that, however I am careful to only use the "loadu" and
> "storeu" variants, so I thought this would not bite me. Do you know if
> my assumption is in error?

My advice is please do not publish any packages doing this low level 
stuff unless you fully understand the details yourself. If you don't, 
please work at a higher level abstraction and use existing code for the 
low-level things, to avoid adding to the maintenance costs. These things 
can take very long to debug.

The GCC bug on Windows I've ran into only affects instructions that 
require aligned operands (on the stack), aligned at 32-byte boundary.

Tomas

>
> Op di 26 mrt 2024 om 15:51 schreef Tomas Kalibera <tomas.kalibera using gmail.com>:
>>
>> On 3/26/24 10:53, jesse koops wrote:
>>> Hello R-package-devel,
>>>
>>> I recently got inspired by the rcppsimdjson package to try out simd
>>> registers. It works fantastic on my computer but I struggle to find
>>> information on how to make it portable. It doesn't help in this case
>>> that R and Rcpp make including Cpp code so easy that I have never had
>>> to learn about cmake and compiler flags. I would appreciate any help,
>>> including of the type: "go read instructions at ...".
>>>
>>> I use RcppArmadillo and Rcpp. I currenlty include the following header:
>>>
>>> #include <immintrin.h>
>>>
>>> The functions in immintrin that I use are:
>>>
>>> _mm256_loadu_pd
>>> _mm256_set1_pd
>>> _mm256_mul_pd
>>> _mm256_fmadd_pd
>>> _mm256_storeu_pd
>>>
>>> and I define up to four __m256d registers. From information found
>>> online (not sure where anymore) I constructed the following makevars
>>> file:
>>>
>>> CXX_STD = CXX14
>>>
>>> PKG_CPPFLAGS = -I../inst/include -mfma -msse4.2 -mavx
>>>
>>> PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS)
>>> PKG_LIBS = $(SHLIB_OPENMP_CXXFLAGS) $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)
>>>
>>> (I also use openmp, that has always worked fine, I just included all
>>> lines for completeness) Rcheck gives me two notes:
>>>
>>> ─  using R version 4.3.2 (2023-10-31 ucrt)
>>> ─  using platform: x86_64-w64-mingw32 (64-bit)
>>> ─  R was compiled by
>>>          gcc.exe (GCC) 12.3.0
>>>          GNU Fortran (GCC) 12.3.0
>>>
>>> ❯ checking compilation flags used ... NOTE
>>>     Compilation used the following non-portable flag(s):
>>>       '-mavx' '-mfma' '-msse4.2'
>>>
>>> ❯ checking C++ specification ... NOTE
>>>       Specified C++14: please drop specification unless essential
>>>
>>> But as far as I understand, the flags are necessary, at least in GCC.
>>> How can I make this portable and CRAN-acceptable?
>> I think it the best way for portability is to use a higher-level library
>> that already has done the low-level business of maintaining multiple
>> versions of the code (with multiple instruction sets) and choosing one
>> appropriate for the current CPU. It could be say LAPACK, BLAS, openmp,
>> depending of the problem at hand. In some cases, code can be rewritten
>> so that the compiler can vectorize it better, using the level of
>> vectorized instructions that have been enabled.
>>
>> Unconditionally using GCC-specific or architecture-specific options in
>> packages would certainly not be portable. Even on Windows, R is now used
>> also with clang and on aarch64, so one should not assume a concrete
>> compiler and architecture.
>>
>> Please note also that GCC on Windows has a bug due to which AVX2
>> instructions cannot be used reliably - the compiler doesn't always
>> properly align local variables on the stack when emitting these. See
>> [1,2] for more information.
>>
>> Best
>> Tomas
>>
>> [1] https://stat.ethz.ch/pipermail/r-sig-windows/2024q1/000113.html
>> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54412
>>
>>> kind regards,
>>> Jesse
>>>
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