Perform stronger mixing on 32-bit platforms and enable the LowEntropyStrings test.

Also avoid the conditional use of intrinsic 128-bit integers in Mix(), which is no longer needed. There is no generated code difference for x86-64 clang.

PiperOrigin-RevId: 780592497
Change-Id: I2e0b05a78521f42e3eebcb7e99e8387c34fe7e33

Author: ezbr

absl/hash/hash_test.cc

@@ -1266,11 +1266,6 @@ TEST(PrecombineLengthMix, ShortStringCollision) {
 // Test that we don't cause excessive collisions on the hash table for
 // doubles in the range [-1024, 1024]. See cl/773069881 for more information.
 TEST(SwisstableCollisions, DoubleRange) {
-#ifdef GOOGLE_UNSUPPORTED_OS_LOONIX
-  // TODO(b/424834054): make this test pass on Loonix.
-  GTEST_SKIP() << "Test fails on Loonix.";
-#endif
-
   absl::flat_hash_set<double> set;
   for (double t = -1024.0; t < 1024.0; t += 1.0) {
     set.insert(t);
@@ -1282,12 +1277,6 @@ TEST(SwisstableCollisions, DoubleRange) {
 // Test that for each pair of adjacent bytes in a string, if there's only
 // entropy in those two bytes, then we don't have excessive collisions.
 TEST(SwisstableCollisions, LowEntropyStrings) {
-  if (sizeof(size_t) < 8) {
-    // TODO(b/424834054): make this test pass on 32-bit platforms. We need to
-    // make 32-bit Mix() stronger.
-    GTEST_SKIP() << "Test fails on 32-bit platforms";
-  }
-
   constexpr char kMinChar = 0;
   constexpr char kMaxChar = 64;
   // These sizes cover the different hashing cases.
@@ -1302,7 +1291,7 @@ TEST(SwisstableCollisions, LowEntropyStrings) {
           set.insert(s);
           ASSERT_LT(HashtableDebugAccess<decltype(set)>::GetNumProbes(set, s),
                     64)
-              << size << " " << b;
+              << "size: " << size << "; bit: " << b;
         }
       }
     }

absl/hash/internal/hash.h

@@ -955,24 +955,9 @@ inline uint64_t PrecombineLengthMix(uint64_t state, size_t len) {
 }
 
 ABSL_ATTRIBUTE_ALWAYS_INLINE inline uint64_t Mix(uint64_t lhs, uint64_t rhs) {
-  // For 32 bit platforms we are trying to use all 64 lower bits.
-  if constexpr (sizeof(size_t) < 8) {
-    uint64_t m = lhs * rhs;
-    return m ^ absl::byteswap(m);
-  }
-  // absl::uint128 is not an alias or a thin wrapper around the intrinsic.
-  // We use the intrinsic when available to improve performance.
-  // TODO(b/399425325): Try to remove MulType since compiler seem to generate
-  // the same code with just absl::uint128.
-  // See https://gcc.godbolt.org/z/s3hGarraG for details.
-#ifdef ABSL_HAVE_INTRINSIC_INT128
-  using MulType = __uint128_t;
-#else   // ABSL_HAVE_INTRINSIC_INT128
-  using MulType = absl::uint128;
-#endif  // ABSL_HAVE_INTRINSIC_INT128
-  // Though the 128-bit product on AArch64 needs two instructions, it is
-  // still a good balance between speed and hash quality.
-  MulType m = lhs;
+  // Though the 128-bit product needs multiple instructions on non-x86-64
+  // platforms, it is still a good balance between speed and hash quality.
+  absl::uint128 m = lhs;
   m *= rhs;
   return Uint128High64(m) ^ Uint128Low64(m);
 }