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diff --git a/rts/gmp/mpz/tstbit.c b/rts/gmp/mpz/tstbit.c
new file mode 100644
index 0000000..b0a8b0b
--- /dev/null
+++ b/rts/gmp/mpz/tstbit.c
@@ -0,0 +1,70 @@
+/* mpz_tstbit -- test a specified bit.  Simulate 2's complement representation.
+
+Copyright (C) 1997 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library.
+
+The GNU MP Library is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or (at your
+option) any later version.
+
+The GNU MP Library 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 Lesser General Public
+License for more details.
+
+You should have received a copy of the GNU Lesser General Public License
+along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
+the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+MA 02111-1307, USA. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+
+int
+#if __STDC__
+mpz_tstbit (mpz_srcptr d, unsigned long int bit_index)
+#else
+mpz_tstbit (d, bit_index)
+     mpz_srcptr d;
+     unsigned long int bit_index;
+#endif
+{
+  mp_size_t dsize = d->_mp_size;
+  mp_ptr dp = d->_mp_d;
+  mp_size_t limb_index;
+
+  limb_index = bit_index / BITS_PER_MP_LIMB;
+  if (dsize >= 0)
+    {
+      if (limb_index < dsize)
+	return (dp[limb_index] >> (bit_index % BITS_PER_MP_LIMB)) & 1;
+      else
+	/* Testing a bit outside of a positive number.  */
+	return 0;
+    }
+  else
+    {
+      mp_size_t zero_bound;
+
+      dsize = -dsize;
+
+      /* Locate the least significant non-zero limb.  */
+      for (zero_bound = 0; dp[zero_bound] == 0; zero_bound++)
+	;
+
+      if (limb_index > zero_bound)
+	{
+	  if (limb_index < dsize)
+	    return (~dp[limb_index] >> (bit_index % BITS_PER_MP_LIMB)) & 1;
+	  else
+	    /* Testing a bit outside of a negative number.  */
+	    return 1;
+	}
+      else if (limb_index == zero_bound)
+	return (-dp[limb_index] >> (bit_index % BITS_PER_MP_LIMB)) & 1;
+      else
+	return 0;
+    }
+}