Update of configure and documentation for new AVX2 features

INSTALL

@@ -108,6 +108,9 @@ The configure installation is best done in four steps
    if the hardware does. If you already included "-mAVX" in the
    flags, you can skip "-march=native".
 
+   If you want to use the newer AVX2 instructions, assuming they are supported on
+   your hardware, please set CFLAGS="-march=avx2 -mfma" and CXXFLAGS="-march=avx2 -mfma".
+
 
    Setting the optimization flags for the AVX kernels can be a hassle. If AVX
    kernels are build for your system, you can set the configure option
@@ -119,6 +122,9 @@ The configure installation is best done in four steps
 
    ./configure CFLAGS="-O2" CXXFLAGS="-O2" FCFLAGS="-O2"
 
+   An istallation with AVX2 and best-optimizations could thus look like this:
+
+   ./configure  CFLAGS="-O2 -mavx2 -mfma" CXXFLAGS="-O2 -mavx2 -mfma" FCFLAGS="-O2" --with-avx-optimization
 
 1.4 Installation location
 
@@ -161,6 +167,10 @@ The configure installation is best done in four steps
    new feature. With the same thought in mind, a binary "elpa2_print_kernels"
    is provided, which is rather self-explanatory.
 
+   Also some of the above mentioned tests are provided as C source files.
+   These should demonstrate, how to call ELPA from a C program (i.e. which headers to include
+   and call the ELPA functions). They are NOT intended as a copy and paste solution!
+
 
 4) run "make install"
 

configure.ac

@@ -231,6 +231,43 @@ if test "${can_compile_avx}" = "yes" ; then
   fi
 fi
 
+AC_MSG_CHECKING([whether we can compile AVX2 intrinsics in C])
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+ #include <x86intrin.h>
+ int main(int argc, char **argv){
+ double* q;
+ __m256d q1 = _mm256_load_pd(q);
+ __m256d y1 = _mm256_fmadd_pd(q1, q1, q1);
+ return 0;
+ }
+ ])],
+ [can_compile_avx2=yes],
+ [can_compile_avx2=no]
+)
+AC_MSG_RESULT([${can_compile_avx2}])
+if test "${can_compile_avx2}" = "yes" ; then
+  AC_MSG_CHECKING([whether we can compile AVX2 intrinsics in C++])
+  AC_LANG_PUSH([C++])
+  AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+   #include <x86intrin.h>
+   int main(int argc, char **argv){
+   double* q;
+   __m256d q1 = _mm256_load_pd(q);
+   __m256d y1 = _mm256_fmadd_pd(q1, q1, q1);
+   return 0;
+   }
+   ])],
+   [can_compile_avx2=yes],
+   [can_compile_avx2=no]
+  )
+  AC_LANG_POP([C++])
+  AC_MSG_RESULT([${can_compile_avx2}])
+  if test "${can_compile_avx2}" = "no" ; then
+    AC_MSG_WARN([Cannot compile C++ with AVX2!])
+  fi
+fi
+
+
 if test "${can_compile_avx}" = "yes" ; then
   install_real_avx_block2=yes
   install_real_avx_block4=yes
@@ -687,4 +724,9 @@ if test "${can_compile_avx}" = "no" ; then
     AC_MSG_WARN([Could not compile AVX instructions])
   fi
 fi
+if test "${can_compile_avx2}" = "no" ; then
+  if test x"${want_avx}" = x"yes" ; then
+    AC_MSG_WARN([Could not compile AVX2 instructions])
+  fi
+fi