[R-pkg-devel] using portable simd instructions

[Josiah Parry]

For the Rust curious, there is a portable SIMD module that is part of the
nightly build of Rust. The docs can be found at
https://rust-lang.github.io/portable-simd/core_simd/simd/index.html. I've
use it before with extendr to create R bindings to  a SIMD dot product
function using the examples
https://github.com/rust-lang/portable-simd/blob/master/crates/core_simd/examples/dot_product.rs

It works out of the box quite nicely. It could be a fun way to explore Rust
and SIMD :)

On Wed, Mar 27, 2024 at 9:44 AM Serguei Sokol <serguei.sokol using gmail.com>
wrote:

> 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
> >>
> >> ______________________________________________
> >> R-package-devel using r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-package-devel
> >
> > ______________________________________________
> > R-package-devel using r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-package-devel
>
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