diff --git a/include/boost/unordered/detail/fca.hpp b/include/boost/unordered/detail/fca.hpp
index a255f29b..df3e4688 100644
--- a/include/boost/unordered/detail/fca.hpp
+++ b/include/boost/unordered/detail/fca.hpp
@@ -151,7 +151,7 @@ namespace boost {
 #endif
 
 #if !defined(BOOST_NO_INT64_T) &&                                              \
-  (defined(BOOST_HAS_INT128) || (defined(BOOST_MSVC) && defined(_WIN64)))
+  (defined(BOOST_HAS_INT128) || (defined(_MSC_VER) && defined(_M_X64)))
 #define BOOST_UNORDERED_FCA_FASTMOD_SUPPORT
 #endif
 
@@ -344,7 +344,7 @@ namespace boost {
           (boost::ulong_long_type(5ul) << 32)         + boost::ulong_long_type(1431653234ul) /* = 22906489714 */,
           (boost::ulong_long_type(2ul) << 32)         + boost::ulong_long_type(2863311496ul) /* = 11453246088 */,
           (boost::ulong_long_type(1ul) << 32)         + boost::ulong_long_type(1431655764ul) /* = 5726623060 */,
-#if !defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T)
+#if defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T)
         };
 #else
           (boost::ulong_long_type(1ul) << 32)         + boost::ulong_long_type(6ul)          /* 4294967302 */
@@ -378,16 +378,8 @@ namespace boost {
 #undef BOOST_UNORDERED_PRIME_FMOD_POSITIONS_ELEMENT
 #undef BOOST_UNORDERED_PRIME_FMOD_SIZES
 #undef BOOST_UNORDERED_PRIME_FMOD_SIZES_64BIT
-#undef BOOST_UNORDERED_PRIME_FMOD_SIZES_34BIT
-#undef BOOST_UNORDERED_PRIME_FMOD_SIZES_34BIT_INCOMPLETE
-
-#ifdef BOOST_UNORDERED_FCA_FASTMOD_SUPPORT
-#undef BOOST_UNORDERED_FCA_FASTMOD_SUPPORT
-#endif
-
-#ifdef BOOST_UNORDERED_FCA_HAS_64B_SIZE_T
-#undef BOOST_UNORDERED_FCA_HAS_64B_SIZE_T
-#endif
+#undef BOOST_UNORDERED_PRIME_FMOD_SIZES_32BIT
+#undef BOOST_UNORDERED_PRIME_FMOD_SIZES_32BIT_INCOMPLETE
 
       template <class ValueType, class VoidPtr> struct node
       {
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 1bf3d858..50c1c8cd 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -29,6 +29,7 @@ project
         <toolset>msvc:<warnings-as-errors>on
     ;
 
+run unordered/prime_fmod_tests.cpp ;
 run unordered/fwd_set_test.cpp ;
 run unordered/fwd_map_test.cpp ;
 run unordered/allocator_traits.cpp ;
diff --git a/test/unordered/prime_fmod_tests.cpp b/test/unordered/prime_fmod_tests.cpp
new file mode 100644
index 00000000..231fd913
--- /dev/null
+++ b/test/unordered/prime_fmod_tests.cpp
@@ -0,0 +1,252 @@
+// Copyright 2022 Christian Mazakas.
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/unordered/detail/fca.hpp>
+
+#include <boost/core/detail/splitmix64.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+#include <limits>
+
+#if defined(BOOST_MSVC)
+// conditional expression is constant
+#pragma warning(disable : 4127)
+#endif
+
+void macros_test()
+{
+  if (std::numeric_limits<std::size_t>::digits >= 64) {
+#if !defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T)
+    BOOST_ERROR("std::numeric_limits<size_t>::digits >= 64, but "
+                "BOOST_UNORDERED_FCA_HAS_64B_SIZE_T is not defined");
+#endif
+  }
+  else {
+#if defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T)
+    BOOST_ERROR("std::numeric_limits<size_t>::digits < 64, but "
+                "BOOST_UNORDERED_FCA_HAS_64B_SIZE_T is defined");
+#endif
+  }
+
+#if ((defined(__GNUC__) || defined(__clang__)) &&                              \
+     defined(__SIZEOF_INT128__)) ||                                            \
+  (defined(_MSC_VER) && defined(_M_X64))
+#if !defined(BOOST_UNORDERED_FCA_FASTMOD_SUPPORT)
+  BOOST_ERROR("fast_modulo support should be enabled for this toolchain but "
+              "it's currently not");
+#endif
+#endif
+}
+
+// Pretty inefficient, but the test is fast enough.
+// Might be too slow if we had larger primes?
+bool is_prime(std::size_t x)
+{
+  if (x == 2) {
+    return true;
+  }
+
+  if (x == 1 || x % 2 == 0) {
+    return false;
+  }
+
+  // y*y <= x is susceptible to overflow, so instead make sure to use y <= (x/y)
+  for (std::size_t y = 3; y <= (x / y); y += 2) {
+    if (x % y == 0) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+void prime_sizes_test()
+{
+  // just some basic sanity checks
+  //
+  BOOST_TEST(!is_prime(0));
+  BOOST_TEST(!is_prime(1));
+  BOOST_TEST(is_prime(2));
+  BOOST_TEST(is_prime(3));
+  BOOST_TEST(is_prime(13));
+  BOOST_TEST(!is_prime(4));
+  BOOST_TEST(!is_prime(100));
+  BOOST_TEST(!is_prime(49));
+
+  std::size_t* sizes = boost::unordered::detail::prime_fmod_size<>::sizes;
+  std::size_t sizes_len =
+    boost::unordered::detail::prime_fmod_size<>::sizes_len;
+
+  // prove every number in our sizes array is prime
+  //
+  BOOST_TEST_GT(sizes_len, 0u);
+
+  for (std::size_t i = 0; i < sizes_len; ++i) {
+    BOOST_TEST(is_prime(sizes[i]));
+  }
+
+  // prove that every subsequent number in the sequence is larger than the
+  // previous
+  //
+  for (std::size_t i = 1; i < sizes_len; ++i) {
+    BOOST_TEST_GT(sizes[i], sizes[i - 1]);
+  }
+
+#if defined(BOOST_UNORDERED_FCA_FASTMOD_SUPPORT)
+  // now we wish to prove that if we do have the reciprocals stored, we have the
+  // correct amount of them, i.e. one for every entry in sizes[] that fits in 32
+  // bits
+  //
+  boost::uint64_t* inv_sizes32 =
+    boost::unordered::detail::prime_fmod_size<>::inv_sizes32;
+
+  std::size_t inv_sizes32_len =
+    boost::unordered::detail::prime_fmod_size<>::inv_sizes32_len;
+
+  std::size_t count = 0;
+  for (std::size_t i = 0; i < sizes_len; ++i) {
+    if (sizes[i] <= UINT32_MAX) {
+      ++count;
+    }
+  }
+
+  BOOST_TEST_GT(inv_sizes32_len, 0u);
+  BOOST_TEST_EQ(inv_sizes32_len, count);
+
+  // these values should also be monotonically decreasing
+  //
+  for (std::size_t i = 1; i < inv_sizes32_len; ++i) {
+    BOOST_TEST_LT(inv_sizes32[i], inv_sizes32[i - 1]);
+  }
+
+  // now make sure the values in inv_sizes32 are what they should be as derived
+  // from the paper
+  //
+  for (std::size_t i = 0; i < inv_sizes32_len; ++i) {
+    std::size_t const size = sizes[i];
+    BOOST_TEST_LE(size, UINT_MAX);
+
+    boost::uint32_t d = static_cast<boost::uint32_t>(sizes[i]);
+    boost::uint64_t M = ((boost::ulong_long_type(0xffffffff) << 32) +
+                          boost::ulong_long_type(0xffffffff)) /
+                          d +
+                        1;
+
+    BOOST_TEST_EQ(inv_sizes32[i], M);
+  }
+#endif
+}
+
+void get_remainder_test()
+{
+#if defined(BOOST_UNORDERED_FCA_FASTMOD_SUPPORT)
+  struct
+  {
+    // internally, our get_remainder() function will rely on either msvc
+    // intrinsics or 128 bit integer support which isn't always available. This
+    // is a slower, portable version we can use for verification of the
+    // routines.
+    //
+    boost::uint64_t operator()(boost::uint64_t f, boost::uint32_t d)
+    {
+      boost::uint64_t r1 = (f & UINT32_MAX) * d;
+      boost::uint64_t r2 = (f >> 32) * d;
+
+      r2 += r1 >> 32;
+
+      return r2 >> 32;
+    }
+  } get_remainder;
+
+  boost::detail::splitmix64 rng;
+
+  for (std::size_t i = 0; i < 1000000u; ++i) {
+    boost::uint64_t f = rng();
+    boost::uint32_t d = static_cast<uint32_t>(rng());
+
+    boost::uint64_t r1 =
+      boost::unordered::detail::prime_fmod_size<>::get_remainder(f, d);
+
+    boost::uint64_t r2 = get_remainder(f, d);
+
+    if (!BOOST_TEST_EQ(r1, r2)) {
+      std::cerr << "f: " << f << ", d: " << d << std::endl;
+      return;
+    }
+  }
+#endif
+}
+
+void modulo32_test()
+{
+  std::size_t* sizes = boost::unordered::detail::prime_fmod_size<>::sizes;
+
+  std::size_t const sizes_len =
+    boost::unordered::detail::prime_fmod_size<>::sizes_len;
+
+  boost::detail::splitmix64 rng;
+
+  for (std::size_t i = 0; i < 1000000u; ++i) {
+    std::size_t hash = static_cast<boost::uint32_t>(rng());
+
+    for (std::size_t j = 0; j < sizes_len; ++j) {
+      std::size_t p1 =
+        boost::unordered::detail::prime_fmod_size<>::position(hash, j);
+
+      std::size_t p2 = hash % sizes[j];
+      if (!BOOST_TEST_EQ(p1, p2)) {
+        std::cerr << "hash: " << hash << ", j: " << j << ", sizes[" << j
+                  << "]: " << sizes[j] << std::endl;
+        return;
+      }
+    }
+  }
+}
+
+void modulo64_test()
+{
+#if defined(BOOST_UNORDERED_FCA_HAS_64B_SIZE_T)
+  std::size_t* sizes = boost::unordered::detail::prime_fmod_size<>::sizes;
+
+  std::size_t const sizes_len =
+    boost::unordered::detail::prime_fmod_size<>::sizes_len;
+
+  boost::detail::splitmix64 rng;
+
+  for (std::size_t i = 0; i < 1000000u; ++i) {
+    std::size_t hash = rng();
+
+    for (std::size_t j = 0; j < sizes_len; ++j) {
+      std::size_t h = hash;
+
+#if defined(BOOST_UNORDERED_FCA_FASTMOD_SUPPORT)
+      if (sizes[j] <= UINT_MAX) {
+        h = boost::uint32_t(h) + boost::uint32_t(h >> 32);
+      }
+#endif
+      std::size_t p1 =
+        boost::unordered::detail::prime_fmod_size<>::position(hash, j);
+
+      std::size_t p2 = h % sizes[j];
+
+      if (!BOOST_TEST_EQ(p1, p2)) {
+        std::cerr << "hash: " << hash << ", j: " << j << ", h: " << h
+                  << ", sizes[" << j << "]: " << sizes[j] << std::endl;
+        return;
+      }
+    }
+  }
+#endif
+}
+
+int main()
+{
+  macros_test();
+  prime_sizes_test();
+  modulo32_test();
+  get_remainder_test();
+  modulo64_test();
+
+  return boost::report_errors();
+}