Further macOS OpenMP tweaks

[eddelbuettel]

As discussed with @coatless in #493 -- marking it draft for now.

Edit: Draft status now removed.

[Copilot]

Pull Request Overview

This PR improves OpenMP support on macOS by removing the previous blanket special-casing and introducing a more flexible configuration approach that respects compiler capabilities while still allowing opt-out.

Key changes:

• Introduced macOS-specific OpenMP detection that enables OpenMP by default when the compiler supports it (_OPENMP is defined), with an opt-out mechanism via a new configuration macro
• Removed the blanket exclusion of OpenMP flags for macOS in the inline plugin function
• Conditionally restricted ARMA_CRIPPLED_LAPACK definition to only legacy Armadillo versions on Windows

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
inst/include/RcppArmadillo/config/RcppArmadilloConfigGenerated.h.in	Adds macOS-specific OpenMP logic with conditional enabling based on compiler support and new opt-out macro; updates copyright years
inst/include/RcppArmadillo/config/RcppArmadilloConfig.h	Restricts ARMA_CRIPPLED_LAPACK to legacy Armadillo versions only; updates copyright years
R/inline.R	Removes macOS special case for OpenMP flags, now consistently uses $(SHLIB_OPENMP_CFLAGS) across all platforms; updates copyright year
ChangeLog	Documents the R/inline.R change

---

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[coatless]

In short, this is solid; however, we need to tweak Rcpp::plugins(openmp).

Specifics:

1. We now get the appropriate number of cores if the local computer configuration has OpenMP.
   • under the apple compiler setting we're still setting the #define ARMA_DONT_USE_OPENMP 1.
2. Users must explicitly set in their ~/.R/Makevars the required OpenMP flags to opt-in:

CPPFLAGS += -Xclang -fopenmp
LDFLAGS += -lomp

3. If we switch over to trying to compile with OpenMP enabled via // [[Rcpp::plugins(openmp)]] we're still getting a compile error on a locally configured mac due to only -fopenmp being set instead of -Xclang -fopenmp.

Failed compilation using `Rcpp::plugins(openmp)`

#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::plugins(openmp)]]

// [[Rcpp::export]]
arma::ivec openmp_limits() {
  arma::ivec limit_data(3);

#ifdef _OPENMP
  limit_data[0] = omp_get_num_procs();
  limit_data[1] = omp_get_max_threads();
  limit_data[2] = omp_get_thread_limit();
#endif
  return limit_data;
}

// [[Rcpp::export]]
bool openmp_enabled() {
  bool enabled = false;

#ifdef _OPENMP
  enabled = true;
#endif

  return enabled;
}

// [[Rcpp::export]]
arma::ivec timesTwo(arma::ivec x) {

#pragma omp parallel for
  for (size_t i = 0; i < x.size(); ++i) {
    x[i] *= 2;
  }

  return x ;
}

/*** R
openmp_enabled()
openmp_limits()
timesTwo(42)
*/

using C++ compiler: ‘Apple clang version 17.0.0 (clang-1700.4.4.1)’
using SDK: ‘MacOSX26.1.sdk’
clang++ -arch arm64 -std=gnu++17 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I../inst/include   -I"/Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library/Rcpp/include" -I"/Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library/RcppArmadillo/include" -I"/Users/ronin/Documents/GitHub/rcppcore/RcppArmadillo" -I/opt/R/arm64/include -I/opt/R/arm64/include   -fopenmp -fPIC  -falign-functions=64 -Wall -g -O2   -c test.cpp -o test.o
clang++: error: unsupported option '-fopenmp'
make: *** [test.o] Error 1
Error in Rcpp::sourceCpp("test.cpp") : 
  Error 1 occurred building shared library.

Standalone compilation via `sourceCpp()` with explicit `~/.R/Makevars` set

Set in ~/.R/Makevars:

CPPFLAGS += -Xclang -fopenmp
LDFLAGS += -lomp

Same script without plugins(openmp).

#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]

// [[Rcpp::export]]
arma::ivec openmp_limits() {
  arma::ivec limit_data(3);

#ifdef _OPENMP
  limit_data[0] = omp_get_num_procs();
  limit_data[1] = omp_get_max_threads();
  limit_data[2] = omp_get_thread_limit();
#endif
  return limit_data;
}

// [[Rcpp::export]]
bool openmp_enabled() {
  bool enabled = false;

#ifdef _OPENMP
  enabled = true;
#endif

  return enabled;
}

// [[Rcpp::export]]
arma::ivec timesTwo(arma::ivec x) {

#pragma omp parallel for
  for (size_t i = 0; i < x.size(); ++i) {
    x[i] *= 2;
  }

  return x ;
}

/*** R
openmp_enabled()
openmp_limits()
timesTwo(42)
*/

Compilation trail

/Library/Frameworks/R.framework/Resources/bin/R CMD SHLIB --preclean -o 'sourceCpp_2.so' 'test.cpp' 
using C++ compiler: ‘Apple clang version 17.0.0 (clang-1700.4.4.1)’
using SDK: ‘MacOSX26.1.sdk’
clang++ -arch arm64 -std=gnu++17 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG -I../inst/include   -I"/Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library/Rcpp/include" -I"/Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library/RcppArmadillo/include" -I"/Users/ronin/Documents/GitHub/rcppcore/RcppArmadillo" -I/opt/R/arm64/include -I/opt/R/arm64/include -Xclang -fopenmp    -fPIC  -falign-functions=64 -Wall -g -O2   -c test.cpp -o test.o
clang++ -arch arm64 -std=gnu++17 -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -L/Library/Frameworks/R.framework/Resources/lib -L/opt/R/arm64/lib -L/opt/R/arm64/lib -lomp -o sourceCpp_2.so test.o -L/Library/Frameworks/R.framework/Resources/lib -lRlapack -L/Library/Frameworks/R.framework/Resources/lib -lRblas -L/opt/gfortran/lib/gcc/aarch64-apple-darwin20.0/14.2.0 -L/opt/gfortran/lib -lemutls_w -lheapt_w -lgfortran -lquadmath -F/Library/Frameworks/R.framework/.. -framework R

Output

> openmp_enabled()
[1] TRUE

> openmp_limits()
           [,1]
[1,]         12
[2,]         12
[3,] 2147483647

> timesTwo(42)
     [,1]
[1,]   84

[coatless]

Shouldn't this be SHLIB_OPENMP_CXXFLAGS instead?

Suggested change

	openmpflag <- "$(SHLIB_OPENMP_CFLAGS)"
	openmpflag <- "$(SHLIB_OPENMP_CXXFLAGS)"

[coatless]

Is the standard prefix RCPPARMA? Or should we go RCPPARMADILLO ?