OpenMP Support for Standalone Scripts in 7.900.0 about rcpparmadillo HOT 10 CLOSED

I think it would be better to be contained within armadillo. I noodled around with the Makevars approach I outlined above. It didn't work out well since I do not believe Makevars is even used by R/Attributes.R after browsing the file.

So, I would propose an approach similar to RQuantlib for setting the flag.

inlineCxxPlugin <- function(...) {
  plugin <- Rcpp:::Rcpp.plugin.maker(include.before = "#include <RcppArmadillo.h>", 
                                     libs           = "$(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS) $(SHLIB_OPENMP_CFLAGS)", 
                                     package        = "RcppArmadillo")
  settings <- plugin()
  settings$env$PKG_CPPFLAGS <- paste0("$(SHLIB_OPENMP_CXXFLAGS)")
  settings
}

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But ... as I recall this gist of getting OpenMP was to both compile and link with -fopenmp so we would need to get it into CXXFLAGS, or PKG_CXXFLAGS, or ... And make sure we cover the various C++ dialect options we now have.

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Ie here is what Rcpp does in its plugin for openmp:

## built-in OpenMP++11 plugin
.plugins[["openmp"]] <- function() {
    list(env = list(PKG_CXXFLAGS="-fopenmp",
                    PKG_LIBS="-fopenmp"))
}

(I suppose OpenMP++11 is a cut-and-pasto. Should be OpenMP.)

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@eddelbuettel You are correct. The functionality of Rcpp.plugin.maker needs to be extended...

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Or, what can happen is an injection into the Makevars slot in a similar way as the libs line is defined. See R/inline.R#L44-L54

Rcpp:::Rcpp.plugin.maker(
	include.before = "#include <RcppArmadillo.h>", 
	libs           = "$(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS) $(SHLIB_OPENMP_CFLAGS)", 
        Makevars       = list( PKG_CXXFLAGS = "$(SHLIB_OPENMP_CXXFLAGS)" ) ,
        Makevars.win   = list( PKG_CXXFLAGS = "$(SHLIB_OPENMP_CXXFLAGS)" ),
	package        = "RcppArmadillo")

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Or ... we just punt and tell folks to rely on the existing plugin for OpenMP which reliably turns on OpenMP?

Ie recommend to turn on RcppArmadillo (via Rcpp::depends()) and OpenMP (via Rcpp::plugins()) ?

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RcppGSL does something similar. The crux, as you say, is that this once aimed the inline package, and Rcpp sort of just picked it up.

We have to figure put the different use case and try to make sure we hit most of them home...

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Per 70a27b7 and 150899f this can be closed out.

However, note that Makevars uses PKG_CXXFLAGS while Makevars.win uses PKG_CPPFLAGS.
Though, setting the flags through either appears to matriculate the correct flags for the compile.

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Yes, testing this on Metra this am. Seems to work. Users may still have to supply the C++11 or C++14 (Rcpp) plugin in addition to the the depends getting them this from the pluginMaker.

Thanks for the reminder on the Makevars. I'll clean this up. Writing R Extension is a little rambling there but I PKG_CXXFLAGS may be better.

One thought I had was to ... maybe adding comments that all this is optional may help? Then again adding -fopenmp without actual OpenMP directive is just a null-op so no harm done.

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I think this one can be closed now.

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