diff --git a/.gitignore b/.gitignore
index 5aa6e0df6b..366639ffe5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,6 +14,7 @@ src/config.h.in
 src/fft_tuning.h
 src/flint.h
 src/flint-config.h
+src/gmpcompat.h
 build/
 config/
 config.log
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 795f89035c..145e8848b7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -218,6 +218,14 @@ else()
 	set(FLINT_REENTRANT OFF)
 endif()
 
+# gmpcompat.h configuration
+set(CMAKE_REQUIRED_INCLUDES ${GMP_INCLUDE_DIRS})
+check_c_source_compiles([[#include <gmp.h>
+  #ifndef _LONG_LONG_LIMB
+  # error mp_limb_t != unsigned long long limb
+  #endif
+  void main(){};]] LONG_LONG_LIMB)
+
 # Populate headers
 configure_file(
     ${CMAKE_CURRENT_SOURCE_DIR}/CMake/cmake_config.h.in
@@ -232,6 +240,19 @@ configure_file(
     ${CMAKE_CURRENT_SOURCE_DIR}/src/fmpz/fmpz.c
     COPYONLY
 )
+if(LONG_LONG_LIMB)
+    configure_file(
+        ${CMAKE_CURRENT_SOURCE_DIR}/src/gmpcompat-longlong.h.in
+        ${CMAKE_CURRENT_SOURCE_DIR}/src/gmpcompat.h
+        COPYONLY
+    )
+else()
+    configure_file(
+        ${CMAKE_CURRENT_SOURCE_DIR}/src/gmpcompat.h.in
+        ${CMAKE_CURRENT_SOURCE_DIR}/src/gmpcompat.h
+        COPYONLY
+    )
+endif()
 
 if(CMAKE_SIZEOF_VOID_P EQUAL 8)
     configure_file(
diff --git a/Makefile.in b/Makefile.in
index cb5094d2b8..408cd1e9c9 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -117,7 +117,8 @@ CFG_FILES :=                                                                \
         $(FLINT_DIR)/config.h           $(SRC_DIR)/flint-config.h           \
         $(FLINT_DIR)/config.log         $(SRC_DIR)/flint.h                  \
         $(SRC_DIR)/fft_tuning.h         $(FLINT_DIR)/flint.pc               \
-        $(FLINT_DIR)/Makefile           $(SRC_DIR)/fmpz/fmpz.c
+        $(FLINT_DIR)/Makefile           $(SRC_DIR)/fmpz/fmpz.c              \
+        $(SRC_DIR)/gmpcompat.h
 
 ################################################################################
 # directories
diff --git a/configure.ac b/configure.ac
index e2886034aa..afd6d4cab7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -629,6 +629,19 @@ AC_COMPILE_IFELSE(
     AC_MSG_RESULT([no])
     AC_MSG_ERROR(["FLINT requires GMP version 6.2.1 or later."]))
 
+AC_MSG_CHECKING([if GMP defines mp_limb_t as unsigned long long int])
+AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM(
+        [#include <gmp.h>],
+        [#if !defined(_LONG_LONG_LIMB)
+         # error mp_limb_t != unsigned long long int
+         #endif]
+    )],
+    AC_MSG_RESULT([yes])
+    gmpcompat_h_in="gmpcompat-longlong.h.in",
+    AC_MSG_RESULT([no])
+    gmpcompat_h_in="gmpcompat.h.in")
+
 # Check that MPFR >= 4.1.0
 AC_MSG_CHECKING([if version of MPFR is greater than 4.1.0])
 AC_COMPILE_IFELSE(
@@ -1155,7 +1168,7 @@ fi
 # epilog
 ################################################################################
 
-AC_CONFIG_FILES([Makefile flint.pc src/flint.h])
+AC_CONFIG_FILES([Makefile flint.pc src/flint.h src/gmpcompat.h:src/${gmpcompat_h_in}])
 AC_OUTPUT
 
 dnl Shorten the original help message
diff --git a/src/gmpcompat.h b/src/gmpcompat-longlong.h.in
similarity index 90%
rename from src/gmpcompat.h
rename to src/gmpcompat-longlong.h.in
index d9836981d7..adebefe8b3 100644
--- a/src/gmpcompat.h
+++ b/src/gmpcompat-longlong.h.in
@@ -79,8 +79,6 @@ void flint_mpz_add_signed_uiuiui(mpz_ptr a, mpz_srcptr b,
     mpz_add(a, b, c);
 }
 
-#if WORD_MAX != LONG_MAX
-
 #define FLINT_MOCK_MPZ_UI(xxx, yyy) \
    __mpz_struct (xxx)[1] = {{ 1, 0, NULL }}; \
    do { \
@@ -831,64 +829,6 @@ int flint_mpf_fits_slong_p(mpf_srcptr f)
   return fl <= (fs >= 0 ? (mp_limb_t) WORD_MAX : - (mp_limb_t) WORD_MIN);
 }
 
-#else
-
-#define flint_mpz_get_si mpz_get_si
-#define flint_mpz_get_ui mpz_get_ui
-#define flint_mpz_set_si mpz_set_si
-#define flint_mpz_set_ui mpz_set_ui
-#define flint_mpz_init_set_si mpz_init_set_si
-#define flint_mpz_init_set_ui mpz_init_set_ui
-#define flint_mpz_add_ui mpz_add_ui
-#define flint_mpz_sub_ui mpz_sub_ui
-#define flint_mpz_mul_si mpz_mul_si
-#define flint_mpz_mul_ui mpz_mul_ui
-#define flint_mpz_addmul_ui mpz_addmul_ui
-#define flint_mpz_submul_ui mpz_submul_ui
-#define flint_mpz_ui_sub mpz_ui_sub
-#define flint_mpz_ui_pow_ui mpz_ui_pow_ui
-#define flint_mpz_cdiv_q_ui mpz_cdiv_q_ui
-#define flint_mpz_cdiv_r_ui mpz_cdiv_r_ui
-#define flint_mpz_cdiv_qr_ui mpz_cdiv_qr_ui
-#define flint_mpz_cdiv_ui mpz_cdiv_ui
-#define flint_mpz_fdiv_q_ui mpz_fdiv_q_ui
-#define flint_mpz_fdiv_r_ui mpz_fdiv_r_ui
-#define flint_mpz_fdiv_qr_ui mpz_fdiv_qr_ui
-#define flint_mpz_fdiv_ui mpz_fdiv_ui
-#define flint_mpz_tdiv_q_ui mpz_tdiv_q_ui
-#define flint_mpz_tdiv_r_ui mpz_tdiv_r_ui
-#define flint_mpz_tdiv_qr_ui mpz_tdiv_qr_ui
-#define flint_mpz_tdiv_ui mpz_tdiv_ui
-#define flint_mpz_mod_ui mpz_mod_ui
-#define flint_mpz_divexact_ui mpz_divexact_ui
-#define flint_mpz_divisible_ui_p mpz_divisible_ui_p
-#define flint_mpz_congruent_ui_p mpz_congruent_ui_p
-#define flint_mpz_powm_ui mpz_powm_ui
-#define flint_mpz_pow_ui mpz_pow_ui
-#define flint_mpz_fac_ui mpz_fac_ui
-#define flint_mpz_bin_uiui mpz_bin_uiui
-#define flint_mpz_fib_ui mpz_fib_ui
-#define flint_mpz_cmp_si mpz_cmp_si
-#define flint_mpz_cmp_ui mpz_cmp_ui
-#define flint_mpq_cmp_si mpq_cmp_si
-#define flint_mpq_cmp_ui mpq_cmp_ui
-#define flint_mpq_set_si mpq_set_si
-#define flint_mpq_set_ui mpq_set_ui
-
-#define flint_mpf_set_si mpf_set_si
-#define flint_mpf_set_ui mpf_set_ui
-#define flint_mpf_get_si mpf_get_si
-#define flint_mpf_cmp_ui mpf_cmp_ui
-#define flint_mpf_fits_slong_p mpf_fits_slong_p
-
-#endif
-
-#if WORD_MAX == LONG_MAX
-
-#define flint_mpf_get_d_2exp mpf_get_d_2exp
-
-#else
-
 /* defined in mpn_extras.h and used by flint_mpf_get_d_2exp below */
 double
 flint_mpn_get_d(mp_srcptr ptr, mp_size_t size, mp_size_t sign, long exp);
@@ -941,5 +881,3 @@ double flint_mpf_get_d_2exp(slong * exp2, mpf_srcptr src)
 }
 
 #endif
-
-#endif
diff --git a/src/gmpcompat.h.in b/src/gmpcompat.h.in
new file mode 100644
index 0000000000..7ecfe3bc33
--- /dev/null
+++ b/src/gmpcompat.h.in
@@ -0,0 +1,131 @@
+/*
+    Copyright (C) 2013 William Hart
+
+    This file is part of FLINT.
+
+    FLINT is free software: you can redistribute it and/or modify it under
+    the terms of the GNU Lesser General Public License (LGPL) as published
+    by the Free Software Foundation; either version 2.1 of the License, or
+    (at your option) any later version.  See <https://www.gnu.org/licenses/>.
+*/
+
+#ifndef GMP_COMPAT_H
+#define GMP_COMPAT_H
+
+#include "flint.h"
+
+#define FLINT_MPZ_REALLOC(z, len)       \
+    ((len) > ((z)->_mp_alloc)           \
+        ? (mp_ptr) _mpz_realloc(z, len) \
+        : ((z)->_mp_d))
+
+static __inline__
+void flint_mpz_add_uiui(mpz_ptr a, mpz_srcptr b, ulong c1, ulong c0)
+{
+    ulong d[2];
+    mpz_t c;
+    d[0] = c0;
+    d[1] = c1;
+    c->_mp_d = d;
+    c->_mp_alloc = 2;
+    c->_mp_size = d[1] != 0 ? 2 : d[0] != 0;
+    mpz_add(a, b, c);
+}
+
+static __inline__
+void flint_mpz_add_signed_uiui(mpz_ptr a, mpz_srcptr b, ulong c1, ulong c0)
+{
+    ulong d[2];
+    ulong c2 = FLINT_SIGN_EXT(c1);
+    mpz_t c;
+    sub_ddmmss(d[1], d[0], c2^c1, c2^c0, c2, c2);
+    c->_mp_d = d;
+    c->_mp_alloc = 2;
+    c->_mp_size = d[1] != 0 ? 2 : d[0] != 0;
+    if (c2 != 0)
+        c->_mp_size = -c->_mp_size;
+    mpz_add(a, b, c);
+}
+
+static __inline__
+void flint_mpz_add_uiuiui(mpz_ptr a, mpz_srcptr b, ulong c2, ulong c1, ulong c0)
+{
+    ulong d[3];
+    mpz_t c;
+    d[0] = c0;
+    d[1] = c1;
+    d[2] = c2;
+    c->_mp_d = d;
+    c->_mp_alloc = 3;
+    c->_mp_size = d[2] != 0 ? 3 : d[1] != 0 ? 2 : d[0] != 0;
+    mpz_add(a, b, c);
+}
+
+static __inline__
+void flint_mpz_add_signed_uiuiui(mpz_ptr a, mpz_srcptr b,
+                                                 ulong c2, ulong c1, ulong c0)
+{
+    ulong cs, d[3];
+    mpz_t c;
+    c->_mp_d = d;
+    c->_mp_alloc = 3;
+    cs = FLINT_SIGN_EXT(c2);
+    sub_dddmmmsss(d[2], d[1], d[0], cs^c2, cs^c1, cs^c0, cs, cs, cs);
+    c->_mp_size = d[2] != 0 ? 3 :
+                  d[1] != 0 ? 2 :
+                  d[0] != 0;
+    if (cs != 0)
+        c->_mp_size = -c->_mp_size;
+    mpz_add(a, b, c);
+}
+
+#define flint_mpz_get_si mpz_get_si
+#define flint_mpz_get_ui mpz_get_ui
+#define flint_mpz_set_si mpz_set_si
+#define flint_mpz_set_ui mpz_set_ui
+#define flint_mpz_init_set_si mpz_init_set_si
+#define flint_mpz_init_set_ui mpz_init_set_ui
+#define flint_mpz_add_ui mpz_add_ui
+#define flint_mpz_sub_ui mpz_sub_ui
+#define flint_mpz_mul_si mpz_mul_si
+#define flint_mpz_mul_ui mpz_mul_ui
+#define flint_mpz_addmul_ui mpz_addmul_ui
+#define flint_mpz_submul_ui mpz_submul_ui
+#define flint_mpz_ui_sub mpz_ui_sub
+#define flint_mpz_ui_pow_ui mpz_ui_pow_ui
+#define flint_mpz_cdiv_q_ui mpz_cdiv_q_ui
+#define flint_mpz_cdiv_r_ui mpz_cdiv_r_ui
+#define flint_mpz_cdiv_qr_ui mpz_cdiv_qr_ui
+#define flint_mpz_cdiv_ui mpz_cdiv_ui
+#define flint_mpz_fdiv_q_ui mpz_fdiv_q_ui
+#define flint_mpz_fdiv_r_ui mpz_fdiv_r_ui
+#define flint_mpz_fdiv_qr_ui mpz_fdiv_qr_ui
+#define flint_mpz_fdiv_ui mpz_fdiv_ui
+#define flint_mpz_tdiv_q_ui mpz_tdiv_q_ui
+#define flint_mpz_tdiv_r_ui mpz_tdiv_r_ui
+#define flint_mpz_tdiv_qr_ui mpz_tdiv_qr_ui
+#define flint_mpz_tdiv_ui mpz_tdiv_ui
+#define flint_mpz_mod_ui mpz_mod_ui
+#define flint_mpz_divexact_ui mpz_divexact_ui
+#define flint_mpz_divisible_ui_p mpz_divisible_ui_p
+#define flint_mpz_congruent_ui_p mpz_congruent_ui_p
+#define flint_mpz_powm_ui mpz_powm_ui
+#define flint_mpz_pow_ui mpz_pow_ui
+#define flint_mpz_fac_ui mpz_fac_ui
+#define flint_mpz_bin_uiui mpz_bin_uiui
+#define flint_mpz_fib_ui mpz_fib_ui
+#define flint_mpz_cmp_si mpz_cmp_si
+#define flint_mpz_cmp_ui mpz_cmp_ui
+#define flint_mpq_cmp_si mpq_cmp_si
+#define flint_mpq_cmp_ui mpq_cmp_ui
+#define flint_mpq_set_si mpq_set_si
+#define flint_mpq_set_ui mpq_set_ui
+
+#define flint_mpf_set_si mpf_set_si
+#define flint_mpf_set_ui mpf_set_ui
+#define flint_mpf_get_si mpf_get_si
+#define flint_mpf_cmp_ui mpf_cmp_ui
+#define flint_mpf_fits_slong_p mpf_fits_slong_p
+#define flint_mpf_get_d_2exp mpf_get_d_2exp
+
+#endif