Add modular exponentiation for UNSIGNED.

gcc/fortran/ChangeLog:

	* arith.cc (arith_power): Handle modular arithmetic for
	BT_UNSIGNED.
	(eval_intrinsic):  Error for unsigned exponentiation with
	-pedantic.
	* expr.cc (gfc_type_convert_binary): Use type of first
	argument for unsigned exponentiation.
	* gfortran.texi: Mention arithmetic exponentiation.
	* resolve.cc (resolve_operator): Allow unsigned exponentiation.
	* trans-decl.cc (gfc_build_intrinsic_function_decls): Build
	declarations for unsigned exponentiation.
	* trans-expr.cc (gfc_conv_cst_uint_power): New function.
	(gfc_conv_power_op): Call it.  Handle unsigned exponentiation.
	* trans.h (gfor_fndecl_unsigned_pow_list):  Add declaration.

libgfortran/ChangeLog:

	* Makefile.am: Add files for unsigned exponentiation.
	* Makefile.in: Regenerate.
	* gfortran.map: Add functions for unsigned exponentiation.
	* generated/pow_m16_m1.c: New file.
	* generated/pow_m16_m16.c: New file.
	* generated/pow_m16_m2.c: New file.
	* generated/pow_m16_m4.c: New file.
	* generated/pow_m16_m8.c: New file.
	* generated/pow_m1_m1.c: New file.
	* generated/pow_m1_m16.c: New file.
	* generated/pow_m1_m2.c: New file.
	* generated/pow_m1_m4.c: New file.
	* generated/pow_m1_m8.c: New file.
	* generated/pow_m2_m1.c: New file.
	* generated/pow_m2_m16.c: New file.
	* generated/pow_m2_m2.c: New file.
	* generated/pow_m2_m4.c: New file.
	* generated/pow_m2_m8.c: New file.
	* generated/pow_m4_m1.c: New file.
	* generated/pow_m4_m16.c: New file.
	* generated/pow_m4_m2.c: New file.
	* generated/pow_m4_m4.c: New file.
	* generated/pow_m4_m8.c: New file.
	* generated/pow_m8_m1.c: New file.
	* generated/pow_m8_m16.c: New file.
	* generated/pow_m8_m2.c: New file.
	* generated/pow_m8_m4.c: New file.
	* generated/pow_m8_m8.c: New file.
	* m4/powu.m4: New file.

gcc/testsuite/ChangeLog:

	* gfortran.dg/unsigned_15.f90: Adjust error messages.
	* gfortran.dg/unsigned_43.f90: New test.
	* gfortran.dg/unsigned_44.f90: New test.

1 files changed, 80 insertions, 0 deletions

diff --git a/libgfortran/m4/powu.m4 b/libgfortran/m4/powu.m4
new file mode 100644
index 0000000..d8bc375
--- /dev/null
+++ b/libgfortran/m4/powu.m4
@@ -0,0 +1,80 @@
+`/* Support routines for the intrinsic power (**) operator
+   for UNSIGNED, using modulo arithmetic.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   Contributed by Thomas Koenig.
+
+This file is part of the GNU Fortran 95 runtime library (libgfortran).
+
+Libgfortran 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.
+
+Libgfortran 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.
+
+Under Section 7 of GPL version 3, you are granted additional
+permissions described in the GCC Runtime Library Exception, version
+3.1, as published by the Free Software Foundation.
+
+You should have received a copy of the GNU General Public License and
+a copy of the GCC Runtime Library Exception along with this program;
+see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+<http://www.gnu.org/licenses/>.  */
+
+#include "libgfortran.h"'
+
+include(iparm.m4)dnl
+
+/* Use Binary Method to calculate the powi. This is not an optimal but
+   a simple and reasonable arithmetic. See section 4.6.3, "Evaluation of
+   Powers" of Donald E. Knuth, "Seminumerical Algorithms", Vol. 2, "The Art
+   of Computer Programming", 3rd Edition, 1998.  */
+
+`#if defined (HAVE_'rtype_name`) && defined (HAVE_'atype_name`)'
+
+rtype_name `pow_'rtype_code`_'atype_code` ('rtype_name` x, 'atype_name` n);
+export_proto(pow_'rtype_code`_'atype_code`);
+
+inline static 'rtype_name`
+power_simple_'rtype_code`_'atype_code` ('rtype_name` x, 'atype_name` n)
+{
+  'rtype_name` pow = 1;
+  for (;;)
+    {
+      if (n & 1)
+	pow *= x;
+      n >>= 1;
+      if (n)
+	x *= x;
+      else
+	break;
+    }
+  return pow; 
+}
+
+/* For odd x, Euler''`s theorem tells us that x**(2^(m-1)) = 1 mod 2^m.
+   For even x, we use the fact that (2*x)^m = 0 mod 2^m.  */
+
+'rtype_name`
+pow_'rtype_code`_'atype_code` ('rtype_name` x, 'atype_name` n)
+{
+  const 'rtype_name` mask = ('rtype_name`) (-1) / 2;
+  if (n == 0)
+    return 1;
+
+  if  (x == 0)
+    return 0;
+
+  if (x & 1)
+    return power_simple_'rtype_code`_'atype_code` (x, n & mask);
+
+  if (n > sizeof (x) * 8)
+    return 0;
+
+  return power_simple_'rtype_code`_'atype_code` (x, n);
+}
+
+#endif'