Enable OpenMP support for CRAN Mac binaries

.gitignore

@@ -20,3 +20,7 @@ altdoc/freeze.rds
 _quarto/*
 !_quarto/_freeze/
 .DS_Store
+
+# Configure script outputs
+config.log
+src/Makevars

configure

@@ -0,0 +1,58 @@
+#!/usr/bin/env sh
+
+# Find R compilers
+CC=`${R_HOME}/bin/R CMD config CC`
+CFLAGS=`${R_HOME}/bin/R CMD config CFLAGS`
+CXX=`${R_HOME}/bin/R CMD config CXX`
+CXXFLAGS=`${R_HOME}/bin/R CMD config CXXFLAGS`
+
+# Test if we have OpenMP support
+cat <<EOF > test-omp.cpp
+#include <omp.h>
+int main() {
+  return omp_get_num_threads();
+}
+EOF
+
+detect_openmp () {
+  if [ "$(uname)" = "Darwin" ]; then
+    # Test for macOS OpenMP support
+    printf "%s" "* checking if R installation supports OpenMP with \"-Xclang -fopenmp\" ... "
+    if CPPFLAGS="${CPPFLAGS} -Xclang -fopenmp" PKG_LIBS="-lomp" "${R_HOME}/bin/R" CMD SHLIB test-omp.cpp >> config.log 2>&1; then
+      echo "yes"
+      export R_OPENMP_ENABLED=1
+      return
+    else
+      echo "no"
+    fi
+  fi
+
+  if [ "$(uname)" = "Linux" ]; then
+    # Test for Linux OpenMP support
+    printf "%s" "* checking if R installation supports OpenMP with \"-fopenmp\" ... "
+    if ${CXX} ${CXXFLAGS} -fopenmp test-omp.cpp >> config.log 2>&1; then
+      echo "yes"
+      export R_OPENMP_ENABLED=1
+      return
+    else
+      echo "no"
+    fi
+  fi
+
+  # No OpenMP support found
+  export R_OPENMP_ENABLED=0
+}
+
+detect_openmp
+rm -f test-omp.* a.out
+
+# Generate Makevars from template based on OpenMP detection
+if [ "${R_OPENMP_ENABLED}" = "0" ]; then
+  echo "*** OpenMP not supported! fixest will run single-threaded."
+  sed -e "s/\$(SHLIB_OPENMP_CXXFLAGS)//g" src/Makevars.in > src/Makevars
+else
+  # Keep the template as-is (OpenMP variables will be expanded by R)
+  cp src/Makevars.in src/Makevars
+fi
+
+exit 0

src/Makevars.in

@@ -0,0 +1,4 @@
+
+PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS) -DSTRICT_R_HEADERS
+PKG_LIBS = $(SHLIB_OPENMP_CXXFLAGS)
+