diff --git a/m4/ax_cc_maxopt_swift.m4 b/m4/ax_cc_maxopt_swift.m4
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+# ===========================================================================
+#       https://www.gnu.org/software/autoconf-archive/ax_cc_maxopt.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CC_MAXOPT_SWIFT
+#
+# DESCRIPTION
+#
+#   Try to turn on "good" C optimization flags for various compilers and
+#   architectures, for some definition of "good". (In our case, good for
+#   FFTW and hopefully for other scientific codes. Modify as needed.)
+#
+#   The user can override the flags by setting the CFLAGS environment
+#   variable. The user can also specify --enable-portable-binary in order to
+#   disable any optimization flags that might result in a binary that only
+#   runs on the host architecture.
+#
+#   Note also that the flags assume that ANSI C aliasing rules are followed
+#   by the code (e.g. for gcc's -fstrict-aliasing), and that floating-point
+#   computations can be re-ordered as needed.
+#
+#   Requires macros: AX_CHECK_COMPILE_FLAG, AX_COMPILER_VENDOR,
+#   AX_GCC_ARCHFLAG, AX_GCC_X86_CPUID.
+#
+#   SWIFT version does not add the optimization flags to CFLAGS.  This allows
+#   us to use these flags in automake so we have have different levels of
+#   optimization on a library by library basis. It and also means the user
+#   CFLAGS can just extended these rather than needing to copy them all.
+#
+#   The flags are returned in the variable OPT_CFLAGS. This is set regardless
+#   of the CFLAGS value, unlike the original version of this macro.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Steven G. Johnson <stevenj@alum.mit.edu>
+#   Copyright (c) 2008 Matteo Frigo
+#   Copyright (c) 2023 Peter W. Draper
+#
+#   This program is free software: you can redistribute it and/or modify it
+#   under the terms of the GNU General Public License as published by the
+#   Free Software Foundation, either version 3 of the License, or (at your
+#   option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+#   Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program. If not, see <https://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 18
+
+AC_DEFUN([AX_CC_MAXOPT_SWIFT],
+[
+AC_REQUIRE([AC_PROG_CC])
+AC_REQUIRE([AX_COMPILER_VENDOR])
+AC_REQUIRE([AC_CANONICAL_HOST])
+
+AC_ARG_ENABLE(portable-binary, [AS_HELP_STRING([--enable-portable-binary], [disable compiler optimizations that would produce unportable binaries])],
+	acx_maxopt_portable=$enableval, acx_maxopt_portable=no)
+
+# Try to determine "good" native compiler flags.
+case $ax_cv_c_compiler_vendor in
+  dec) OPT_CFLAGS="-newc -w0 -O5 -ansi_alias -ansi_args -fp_reorder -tune host"
+       if test "x$acx_maxopt_portable" = xno; then
+          OPT_CFLAGS="$OPT_CFLAGS -arch host"
+       fi
+     ;;
+
+  sun) OPT_CFLAGS="-native -fast -xO5 -dalign"
+       if test "x$acx_maxopt_portable" = xyes; then
+	  CFLAGS="$OPT_CFLAGS -xarch=generic"
+       fi
+     ;;
+
+  hp)  CFLAGS="+Oall +Optrs_ansi +DSnative"
+       if test "x$acx_maxopt_portable" = xyes; then
+          CFLAGS="$OPT_CFLAGS +DAportable"
+       fi
+     ;;
+
+  ibm) if test "x$acx_maxopt_portable" = xno; then
+          xlc_opt="-qarch=auto -qtune=auto"
+       else
+          xlc_opt="-qtune=auto"
+       fi
+       AX_CHECK_COMPILE_FLAG($xlc_opt,
+                OPT_CFLAGS="-O3 -qansialias -w $xlc_opt",
+               [OPT_CFLAGS="-O3 -qansialias -w"
+                echo "******************************************************"
+                echo "*  You seem to have the IBM  C compiler.  It is      *"
+                echo "*  recommended for best performance that you use:    *"
+                echo "*                                                    *"
+                echo "*    CFLAGS=-O3 -qarch=xxx -qtune=xxx -qansialias -w *"
+                echo "*                      ^^^        ^^^                *"
+                echo "*  where xxx is pwr2, pwr3, 604, or whatever kind of *"
+                echo "*  CPU you have.  (Set the CFLAGS environment var.   *"
+                echo "*  and re-run configure.)  For more info, man cc.    *"
+                echo "******************************************************"])
+     ;;
+
+  intel) OPT_CFLAGS="-O3 -ansi-alias"
+        if test "x$acx_maxopt_portable" = xno; then
+          icc_archflag=unknown
+          icc_flags=""
+          case $host_cpu in
+            i686*|x86_64*)
+              # icc accepts gcc assembly syntax, so these should work:
+              AX_GCC_X86_CPUID(0)
+              AX_GCC_X86_CPUID(1)
+              case $ax_cv_gcc_x86_cpuid_0 in # see AX_GCC_ARCHFLAG
+                *:756e6547:6c65746e:49656e69) # Intel
+                  case $ax_cv_gcc_x86_cpuid_1 in
+                    *0?6[[78ab]]?:*:*:*|?6[[78ab]]?:*:*:*|6[[78ab]]?:*:*:*) icc_flags="-xK" ;;
+                    *0?6[[9d]]?:*:*:*|?6[[9d]]?:*:*:*|6[[9d]]?:*:*:*|*1?65?:*:*:*) icc_flags="-xSSE2 -xB -xK" ;;
+                    *0?6e?:*:*:*|?6e?:*:*:*|6e?:*:*:*) icc_flags="-xSSE3 -xP -xO -xB -xK" ;;
+                    *0?6f?:*:*:*|?6f?:*:*:*|6f?:*:*:*|*1?66?:*:*:*) icc_flags="-xSSSE3 -xT -xB -xK" ;;
+                    *1?6[[7d]]?:*:*:*) icc_flags="-xSSE4.1 -xS -xT -xB -xK" ;;
+                    *1?6[[aef]]?:*:*:*|*2?6[[5cef]]?:*:*:*) icc_flags="-xSSE4.2 -xS -xT -xB -xK" ;;
+                    *2?6[[ad]]?:*:*:*) icc_flags="-xAVX -SSE4.2 -xS -xT -xB -xK" ;; # Sandy-bridge
+                    *3?6[[ae]]?:*:*:*) icc_flags="-xCORE-AVX-I -xAVX -SSE4.2 -xS -xT -xB -xK" ;; #Ivy-bridge
+                    *3?6[[cf]]?:*:*:*|*4?6[[56]]?:*:*:*|*4?6[[ef]]?:*:*:*) icc_flags="-xCORE-AVX2 -xCORE-AVX-I -xAVX -SSE4.2 -xS -xT -xB -xK" ;; # Haswell
+                    *3?6d?:*:*:*|*4?6[[7f]]?:*:*:*|*5?66?:*:*:*) icc_flags="-xCORE-AVX2 -xCORE-AVX-I -xAVX -SSE4.2 -xS -xT -xB -xK" ;; # Broadwell
+                    *4?6[[de]]?:*:*:*) icc_flags="-xCORE-AVX2 -xCORE-AVX-I -xAVX -SSE4.2 -xS -xT -xB -xK" ;; # Skylake
+                    *5?6[[56]]?:*:*:*) icc_flags="-xCORE-AVX512 -xCORE-AVX2 -xCORE-AVX-I -xAVX -SSE4.2 -xS -xT -xB -xK" ;; # Skylake-AVX512
+                    *5?67?:*:*:*) icc_flags="-xMIC-AVX512 -xCORE-AVX2 -xCORE-AVX-I -xAVX -SSE4.2 -xS -xT -xB -xK" ;; # Knights-Landing
+                    *8?6[[de]]?:*:*:*|*9?6[[de]]?:*:*:*) icc_flags="-xCORE-AVX2 -xCORE-AVX-I -xAVX -SSE4.2 -xS -xT -xB -xK" ;;# Kabylake
+                    *000?f[[346]]?:*:*:*|?f[[346]]?:*:*:*|f[[346]]?:*:*:*) icc_flags="-xSSE3 -xP -xO -xN -xW -xK" ;;
+                    *00??f??:*:*:*|??f??:*:*:*|?f??:*:*:*|f??:*:*:*) icc_flags="-xSSE2 -xN -xW -xK" ;;
+                  esac ;;
+                *:68747541:444d4163:69746e65) # AMDs with AVX2 support.
+                  case $ax_cv_gcc_x86_cpuid_1 in
+                    *061?f??:*:*:*|61?f??:*:*:*) icc_flags="-march=core-avx2" ;;
+                    *06??f??:*:*:*|6??f??:*:*:*) icc_flags="-march=core-avx2" ;;
+                    *070?f??:*:*:*|70?f??:*:*:*) icc_flags="-march=core-avx2" ;;
+                                   83?f??:*:*:*) icc_flags="-march=core-avx2"
+                                                 OPT_CFLAGS="$OPT_CFLAGS -fma -ftz -fomit-frame-pointer";; # EPYC ROME
+                                   a0?f??:*:*:*) icc_flags="-march=core-avx2"
+                                                 OPT_CFLAGS="$OPT_CFLAGS -fma -ftz -fomit-frame-pointer";; # MILAN
+
+                  esac ;;
+              esac ;;
+          esac
+          if test "x$icc_flags" != x; then
+            for flag in $icc_flags; do
+              AX_CHECK_COMPILE_FLAG($flag, [icc_archflag=$flag; break])
+            done
+          fi
+          AC_MSG_CHECKING([for icc architecture flag])
+          AC_MSG_RESULT($icc_archflag)
+          if test "x$icc_archflag" != xunknown; then
+            OPT_CFLAGS="$OPT_CFLAGS $icc_archflag"
+          fi
+        fi
+     ;;
+
+  clang)
+     # default optimization flags for clang on all systems
+     OPT_CFLAGS="-O3 -fomit-frame-pointer"
+
+     # Always good optimisation to have
+     AX_CHECK_COMPILE_FLAG(-fstrict-aliasing, [OPT_CFLAGS="$OPT_CFLAGS -fstrict-aliasing"])
+
+     # note that we enable "unsafe" fp optimization with other compilers, too
+     AX_CHECK_COMPILE_FLAG(-ffast-math, [OPT_CFLAGS="$OPT_CFLAGS -ffast-math"])
+
+     # not all codes will benefit from this.
+     AX_CHECK_COMPILE_FLAG(-funroll-loops, [OPT_CFLAGS="$OPT_CFLAGS -funroll-loops"])
+
+     AX_GCC_ARCHFLAG($acx_maxopt_portable, [OPT_CFLAGS="$OPT_CFLAGS $ax_cv_gcc_archflag"])
+     ;;
+
+  gnu)
+     # default optimization flags for gcc on all systems
+     OPT_CFLAGS="-O3 -fomit-frame-pointer"
+
+     # -malign-double for x86 systems
+     AX_CHECK_COMPILE_FLAG(-malign-double, [OPT_CFLAGS="$OPT_CFLAGS -malign-double"])
+
+     #  -fstrict-aliasing for gcc-2.95+
+     AX_CHECK_COMPILE_FLAG(-fstrict-aliasing, [OPT_CFLAGS="$OPT_CFLAGS -fstrict-aliasing"])
+
+     # note that we enable "unsafe" fp optimization with other compilers, too
+     AX_CHECK_COMPILE_FLAG(-ffast-math, [OPT_CFLAGS="$OPT_CFLAGS -ffast-math"])
+
+     # not all codes will benefit from this.
+     AX_CHECK_COMPILE_FLAG(-funroll-loops, [OPT_CFLAGS="$OPT_CFLAGS -funroll-loops"])
+
+     AX_GCC_ARCHFLAG($acx_maxopt_portable, [OPT_CFLAGS="$OPT_CFLAGS $ax_cv_gcc_archflag"])
+     ;;
+
+  microsoft)
+     # default optimization flags for MSVC opt builds
+     OPT_CFLAGS="-O2"
+     ;;
+esac
+
+if test -z "$OPT_CFLAGS"; then
+   echo ""
+   echo "********************************************************"
+   echo "* WARNING: Don't know the best optimizing flags for    *"
+   echo "* this system a default of -O3 will be used.           *"
+   echo "* Use ./configure --disable-optimization CFLAGS=...    *"
+   echo "* to add your own flags.                               *"
+   echo "********************************************************"
+   echo ""
+   OPT_CFLAGS="-O3"
+fi
+
+AX_CHECK_COMPILE_FLAG($OPT_CFLAGS, [], [
+   echo ""
+   echo "********************************************************************"
+   echo "* WARNING: The guessed optimization flags don't seem to work with  *"
+   echo "* your compiler.                                       *"
+   echo "* Use ./configure CFLAGS=... to specify your own flags *"
+   echo "********************************************************"
+   echo ""
+   OPT_CFLAGS=""
+])
+
+])