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

[jesse koops]

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?

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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> > https://stat.ethz.ch/mailman/listinfo/r-package-devel