[R-pkg-devel] using portable simd instructions

[Serguei Sokol]

Le 26/03/2024 à 15:51, Tomas Kalibera a écrit :
> 
> 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.
Talking about libraries, may be the 
https://en.cppreference.com/w/cpp/experimental/simd will do the job?

Best,
Serguei.

  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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> 
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