diff --git a/third_party/gmp/nextprime.c b/third_party/gmp/nextprime.c
new file mode 100644
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--- /dev/null
+++ b/third_party/gmp/nextprime.c
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+/* gmp_nextprime -- generate small primes reasonably efficiently for internal
+   GMP needs.
+
+   Contributed to the GNU project by Torbjorn Granlund.  Miscellaneous
+   improvements by Martin Boij.
+
+   THE FUNCTIONS IN THIS FILE ARE INTERNAL WITH MUTABLE INTERFACES.  IT IS ONLY
+   SAFE TO REACH THEM THROUGH DOCUMENTED INTERFACES.  IN FACT, IT IS ALMOST
+   GUARANTEED THAT THEY WILL CHANGE OR DISAPPEAR IN A FUTURE GNU MP RELEASE.
+
+Copyright 2009 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 either:
+
+  * the GNU Lesser General Public License as published by the Free
+    Software Foundation; either version 3 of the License, or (at your
+    option) any later version.
+
+or
+
+  * the GNU General Public License as published by the Free Software
+    Foundation; either version 2 of the License, or (at your option) any
+    later version.
+
+or both in parallel, as here.
+
+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 General Public License
+for more details.
+
+You should have received copies of the GNU General Public License and the
+GNU Lesser General Public License along with the GNU MP Library.  If not,
+see https://www.gnu.org/licenses/.  */
+
+/*
+  Optimisation ideas:
+
+  1. Unroll the sieving loops.  Should reach 1 write/cycle.  That would be a 2x
+     improvement.
+
+  2. Separate sieving with primes p < SIEVESIZE and p >= SIEVESIZE.  The latter
+     will need at most one write, and thus not need any inner loop.
+
+  3. For primes p >= SIEVESIZE, i.e., typically the majority of primes, we
+     perform more than one division per sieving write.  That might dominate the
+     entire run time for the nextprime function.  A incrementally initialised
+     remainder table of Pi(65536) = 6542 16-bit entries could replace that
+     division.
+*/
+
+#include "gmp-impl.h"
+#include <string.h>		/* for memset */
+
+
+unsigned long int
+gmp_nextprime (gmp_primesieve_t *ps)
+{
+  unsigned long p, d, pi;
+  unsigned char *sp;
+  static unsigned char addtab[] =
+    { 2,4,2,4,6,2,6,4,2,4,6,6,2,6,4,2,6,4,6,8,4,2,4,2,4,8,6,4,6,2,4,6,2,6,6,4,
+      2,4,6,2,6,4,2,4,2,10,2,10 };
+  unsigned char *addp = addtab;
+  unsigned long ai;
+
+  /* Look for already sieved primes.  A sentinel at the end of the sieving
+     area allows us to use a very simple loop here.  */
+  d = ps->d;
+  sp = ps->s + d;
+  while (*sp != 0)
+    sp++;
+  if (sp != ps->s + SIEVESIZE)
+    {
+      d = sp - ps->s;
+      ps->d = d + 1;
+      return ps->s0 + 2 * d;
+    }
+
+  /* Handle the number 2 separately.  */
+  if (ps->s0 < 3)
+    {
+      ps->s0 = 3 - 2 * SIEVESIZE; /* Tricky */
+      return 2;
+    }
+
+  /* Exhausted computed primes.  Resieve, then call ourselves recursively.  */
+
+#if 0
+  for (sp = ps->s; sp < ps->s + SIEVESIZE; sp++)
+    *sp = 0;
+#else
+  memset (ps->s, 0, SIEVESIZE);
+#endif
+
+  ps->s0 += 2 * SIEVESIZE;
+
+  /* Update sqrt_s0 as needed.  */
+  while ((ps->sqrt_s0 + 1) * (ps->sqrt_s0 + 1) <= ps->s0 + 2 * SIEVESIZE - 1)
+    ps->sqrt_s0++;
+
+  pi = ((ps->s0 + 3) / 2) % 3;
+  if (pi > 0)
+    pi = 3 - pi;
+  if (ps->s0 + 2 * pi <= 3)
+    pi += 3;
+  sp = ps->s + pi;
+  while (sp < ps->s + SIEVESIZE)
+    {
+      *sp = 1, sp += 3;
+    }
+
+  pi = ((ps->s0 + 5) / 2) % 5;
+  if (pi > 0)
+    pi = 5 - pi;
+  if (ps->s0 + 2 * pi <= 5)
+    pi += 5;
+  sp = ps->s + pi;
+  while (sp < ps->s + SIEVESIZE)
+    {
+      *sp = 1, sp += 5;
+    }
+
+  pi = ((ps->s0 + 7) / 2) % 7;
+  if (pi > 0)
+    pi = 7 - pi;
+  if (ps->s0 + 2 * pi <= 7)
+    pi += 7;
+  sp = ps->s + pi;
+  while (sp < ps->s + SIEVESIZE)
+    {
+      *sp = 1, sp += 7;
+    }
+
+  p = 11;
+  ai = 0;
+  while (p <= ps->sqrt_s0)
+    {
+      pi = ((ps->s0 + p) / 2) % p;
+      if (pi > 0)
+	pi = p - pi;
+      if (ps->s0 + 2 * pi <= p)
+	  pi += p;
+      sp = ps->s + pi;
+      while (sp < ps->s + SIEVESIZE)
+	{
+	  *sp = 1, sp += p;
+	}
+      p += addp[ai];
+      ai = (ai + 1) % 48;
+    }
+  ps->d = 0;
+  return gmp_nextprime (ps);
+}
+
+void
+gmp_init_primesieve (gmp_primesieve_t *ps)
+{
+  ps->s0 = 0;
+  ps->sqrt_s0 = 0;
+  ps->d = SIEVESIZE;
+  ps->s[SIEVESIZE] = 0;		/* sentinel */
+}