[R-pkg-devel] using portable simd instructions

[jesse koops]

I tried that but I found the interface awkward and there was really no
performance bonus. It was in the early phase of experimentation and I
didn't save it, so it could very well be that I got the compiler
settings wrong and simd was not used. But if that was the case, there
would still be the problem of using the correct compiler settings
cross platform.

Op wo 27 mrt 2024 om 14:44 schreef Serguei Sokol <serguei.sokol using gmail.com>:
>
> 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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> >> https://stat.ethz.ch/mailman/listinfo/r-package-devel
> >
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>