diff --git a/third_party/gmp/compat.c b/third_party/gmp/compat.c
new file mode 100644
index 0000000..b4b44ce
--- /dev/null
+++ b/third_party/gmp/compat.c
@@ -0,0 +1,59 @@
+/* Old function entrypoints retained for binary compatibility.
+
+Copyright 2000, 2001 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/.  */
+
+#include <stdio.h>
+#include "gmp-impl.h"
+
+
+/* mpn_divexact_by3 was a function in gmp 3.0.1, but as of gmp 3.1 it's a
+   macro calling mpn_divexact_by3c.  */
+mp_limb_t
+__MPN (divexact_by3) (mp_ptr dst, mp_srcptr src, mp_size_t size)
+{
+  return mpn_divexact_by3 (dst, src, size);
+}
+
+
+/* mpn_divmod_1 was a function in gmp 3.0.1 and earlier, but marked obsolete
+   in both gmp 2 and 3.  As of gmp 3.1 it's a macro calling mpn_divrem_1. */
+mp_limb_t
+__MPN (divmod_1) (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t divisor)
+{
+  return mpn_divmod_1 (dst, src, size, divisor);
+}
+
+
+/* mpz_legendre was a separate function in gmp 3.1.1 and earlier, but as of
+   4.0 it's a #define alias for mpz_jacobi.  */
+int
+__gmpz_legendre (mpz_srcptr a, mpz_srcptr b)
+{
+  return mpz_jacobi (a, b);
+}