nmhash32: add minimal test-case for the hash implementation being broken

It's not 100% clear to me if it's UB, CC bug or lack of some kind
of memory barrier, but the very same quite innocent-looking code leads
to different results on macos-arm64 platform and breaks nmhash32

See rurban#294

Hashes.cpp

@@ -1278,8 +1278,43 @@ void crc32c_pclmul_test(const void *key, int len, uint32_t seed, void *out)
 #endif
 
 #include "hash-garage/nmhash.h"
-const char * const nmhash32_desc("nmhash32, le:" MACRO_ITOA(NMHASH_LITTLE_ENDIAN) ", vector:" MACRO_ITOA(NMH_VECTOR) ", align:" MACRO_ITOA(NMH_ACC_ALIGN));
-const char * const nmhash32x_desc("nmhash32x, le:" MACRO_ITOA(NMHASH_LITTLE_ENDIAN) ", vector:" MACRO_ITOA(NMH_VECTOR) ", align:" MACRO_ITOA(NMH_ACC_ALIGN));
+#define NMHASH32_DESC_STR "NMHASH_LITTLE_ENDIAN:" MACRO_ITOA(NMHASH_LITTLE_ENDIAN) ", " \
+                          "NMH_VECTOR:" MACRO_ITOA(NMH_VECTOR) ", " \
+                          "NMH_ACC_ALIGN:" MACRO_ITOA(NMH_ACC_ALIGN) ", " \
+                          "NMH_RESTRICT:" MACRO_ITOA(NMH_RESTRICT)
+const char * const nmhash32_desc("nmhash32, " NMHASH32_DESC_STR);
+const char * const nmhash32x_desc("nmhash32x, " NMHASH32_DESC_STR);
+
+bool nmhash32_broken( void ) {
+  static bool done = false, result;
+  if (done)
+    return result;
+
+  const char entropy[] = "rwgk8M1uxM6XX6c3teQX2yaw8FQWArmcWUSBJ8dcQQJWHYC9Wt2BmpvETxwhYcJTheTbjf49SVRaDJhbEZCq7ki1D6KxpKQSjgwqsiHGSgHLxvPG5kcRnBhjJ1YC8kuh";
+
+  NMH_ALIGN(NMH_ACC_ALIGN) uint32_t accX[sizeof(NMH_ACC_INIT)/sizeof(*NMH_ACC_INIT)];
+  static_assert(sizeof(entropy) >= sizeof(accX), "Need more entropy in entropy[]");
+  memcpy(accX, entropy, sizeof(accX));
+
+  const size_t nbGroups = sizeof(NMH_ACC_INIT) / sizeof(*NMH_ACC_INIT);
+  size_t i;
+
+  for (i = 0; i < nbGroups * 2; ++i) {
+    ((uint16_t*)accX)[i] *= ((uint16_t*)__NMH_M1_V)[i];
+  }
+
+  // XXX: no memory barrier takes place here, just like in NMHASH32_long_round_scalar()
+  // and it affects the `acc` result.
+
+  uint32_t acc = 0;
+  for (i = 0; i < nbGroups; ++i)
+    acc += accX[i];
+
+  result = (acc != UINT32_C(0x249abaee));
+  done = true;
+  return result;
+}
+
 // objsize: 4202f0-420c7d: 2445
 void nmhash32_test ( const void * key, int len, uint32_t seed, void * out ) {
   *(uint32_t*)out = NMHASH32 (key, (const size_t) len, seed);

Hashes.h

@@ -1315,6 +1315,7 @@ extern "C" {
 
 extern const char * const nmhash32_desc;
 extern const char * const nmhash32x_desc;
+bool nmhash32_broken ( void );
 void nmhash32_test ( const void * key, int len, uint32_t seed, void * out );
 void nmhash32x_test ( const void * key, int len, uint32_t seed, void * out );
 

main.cpp

@@ -754,8 +754,8 @@ HashInfo g_hashes[] =
 { rapidhash_test,          64, 0xAF404C4B, "rapidhash", "rapidhash v1", GOOD, {}},
 { rapidhash_unrolled_test,          64, 0xAF404C4B, "rapidhash_unrolled", "rapidhash v1 - unrolled", GOOD, {}},
 #endif
-{ nmhash32_test,        32, 0x12A30553, "nmhash32",       nmhash32_desc, GOOD, {}},
-{ nmhash32x_test,       32, 0xA8580227, "nmhash32x",      nmhash32x_desc, GOOD, {}},
+{ nmhash32_test,        32, nmhash32_broken() ? 0U : 0x12A30553, "nmhash32",  nmhash32_desc,  GOOD, {}},
+{ nmhash32x_test,       32, nmhash32_broken() ? 0U : 0xA8580227, "nmhash32x", nmhash32x_desc, GOOD, {}},
 #ifdef HAVE_KHASHV
 #ifdef __clang__ // also gcc 9.4
 #define KHASHV32_VERIF  0xB69DF8EB