Fix bug where the high bits of `__int128_t`/`__uint128_t` might go unused in the hash function.

This fix increases the hash quality of these types.

Also, make sure that absl::(u)int128 have the same hash expansion as their intrinsic counterparts.

PiperOrigin-RevId: 685706878
Change-Id: Ib8e2e2b7a8ce24cf08f1e8d18094188a6eedbb3a

absl/hash/hash_test.cc

@@ -35,6 +35,7 @@
 #include <utility>
 #include <vector>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
 #include "absl/container/flat_hash_set.h"
@@ -61,6 +62,7 @@ namespace {
 using ::absl::hash_test_internal::is_hashable;
 using ::absl::hash_test_internal::TypeErasedContainer;
 using ::absl::hash_test_internal::TypeErasedValue;
+using ::testing::SizeIs;
 
 template <typename T>
 using TypeErasedVector = TypeErasedContainer<std::vector<T>>;
@@ -381,6 +383,36 @@ absl::Cord FragmentedCord(absl::string_view sv) {
   return absl::MakeFragmentedCord(parts);
 }
 
+#if ABSL_HAVE_INTRINSIC_INT128
+TEST(HashValueTest, TestIntrinsicInt128) {
+  EXPECT_TRUE((is_hashable<__int128_t>::value));
+  EXPECT_TRUE((is_hashable<__uint128_t>::value));
+
+  absl::flat_hash_set<size_t> hashes;
+  std::vector<__uint128_t> values;
+  for (int i = 0; i < 128; ++i) {
+    // Some arbitrary pattern to check if changing each bit changes the hash.
+    static constexpr __uint128_t kPattern =
+        __uint128_t{0x0123456789abcdef} |
+        (__uint128_t{0x0123456789abcdef} << 64);
+    const __uint128_t value = kPattern ^ (__uint128_t{1} << i);
+    const __int128_t as_signed = static_cast<__int128_t>(value);
+
+    values.push_back(value);
+    hashes.insert(absl::Hash<__uint128_t>{}(value));
+
+    // Verify that the fast-path for MixingHashState does not break the hash.
+    EXPECT_EQ(absl::HashOf(value), absl::Hash<__uint128_t>{}(value));
+    EXPECT_EQ(absl::HashOf(as_signed), absl::Hash<__int128_t>{}(as_signed));
+  }
+  EXPECT_THAT(hashes, SizeIs(128));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(values));
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(
+      std::vector<__int128_t>(values.begin(), values.end())));
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
 TEST(HashValueTest, Strings) {
   EXPECT_TRUE((is_hashable<std::string>::value));
 

absl/hash/internal/hash.h

@@ -340,6 +340,14 @@ struct is_uniquely_represented<
 template <>
 struct is_uniquely_represented<bool> : std::false_type {};
 
+#if ABSL_HAVE_INTRINSIC_INT128
+// Specialize the trait for GNU extension types.
+template <>
+struct is_uniquely_represented<__int128> : std::true_type {};
+template <>
+struct is_uniquely_represented<unsigned __int128> : std::true_type {};
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
 // hash_bytes()
 //
 // Convenience function that combines `hash_state` with the byte representation
@@ -1016,8 +1024,12 @@ class ABSL_DLL MixingHashState : public HashStateBase<MixingHashState> {
                       : uint64_t{0x9ddfea08eb382d69};
 
   template <typename T>
+  struct FitsIn64Bits : std::integral_constant<bool, sizeof(T) <= 8> {};
+
+  template <typename T>
   using IntegralFastPath =
-      conjunction<std::is_integral<T>, is_uniquely_represented<T>>;
+      conjunction<std::is_integral<T>, is_uniquely_represented<T>,
+                  FitsIn64Bits<T>>;
 
  public:
   // Move only

absl/numeric/int128.h

@@ -216,7 +216,11 @@ class
   // Support for absl::Hash.
   template <typename H>
   friend H AbslHashValue(H h, uint128 v) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+    return H::combine(std::move(h), static_cast<unsigned __int128>(v));
+#else
     return H::combine(std::move(h), Uint128High64(v), Uint128Low64(v));
+#endif
   }
 
   // Support for absl::StrCat() etc.
@@ -458,7 +462,11 @@ class int128 {
   // Support for absl::Hash.
   template <typename H>
   friend H AbslHashValue(H h, int128 v) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+    return H::combine(std::move(h), v.v_);
+#else
     return H::combine(std::move(h), Int128High64(v), Int128Low64(v));
+#endif
   }
 
   // Support for absl::StrCat() etc.

absl/numeric/int128_test.cc

@@ -17,6 +17,7 @@
 #include <algorithm>
 #include <limits>
 #include <random>
+#include <tuple>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -474,29 +475,51 @@ TEST(Uint128, NumericLimitsTest) {
   EXPECT_EQ(absl::Uint128Max(), std::numeric_limits<absl::uint128>::max());
 }
 
+// Some arbitrary constant to test hashing. The first hex digits of pi.
+constexpr absl::uint128 kPi = (absl::uint128(0x3243f6a8885a308d) << 64) |
+                              absl::uint128(0x313198a2e0370734);
+
 TEST(Uint128, Hash) {
-  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  using Ext128 = unsigned __int128;
+#endif
+  // Make the tuple outside the EXPECT_TRUE because putting the #if inside the
+  // macro argument is not ok.
+  const auto values = std::make_tuple(
       // Some simple values
-      absl::uint128{0},
-      absl::uint128{1},
-      ~absl::uint128{},
+      absl::uint128{0}, absl::uint128{1}, ~absl::uint128{},
       // 64 bit limits
       absl::uint128{std::numeric_limits<int64_t>::max()},
       absl::uint128{std::numeric_limits<uint64_t>::max()} + 0,
       absl::uint128{std::numeric_limits<uint64_t>::max()} + 1,
       absl::uint128{std::numeric_limits<uint64_t>::max()} + 2,
       // Keeping high same
-      absl::uint128{1} << 62,
-      absl::uint128{1} << 63,
+      absl::uint128{1} << 62, absl::uint128{1} << 63,
       // Keeping low same
-      absl::uint128{1} << 64,
-      absl::uint128{1} << 65,
+      absl::uint128{1} << 64, absl::uint128{1} << 65,
       // 128 bit limits
       std::numeric_limits<absl::uint128>::max(),
       std::numeric_limits<absl::uint128>::max() - 1,
       std::numeric_limits<absl::uint128>::min() + 1,
       std::numeric_limits<absl::uint128>::min(),
-  }));
+      // arbitrary constant
+      kPi
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+      // Same but with the intrinsic to verify that they match
+      ,
+      Ext128{0}, Ext128{1}, ~Ext128{},
+      Ext128{std::numeric_limits<int64_t>::max()},
+      Ext128{std::numeric_limits<uint64_t>::max()} + 0,
+      Ext128{std::numeric_limits<uint64_t>::max()} + 1,
+      Ext128{std::numeric_limits<uint64_t>::max()} + 2, Ext128{1} << 62,
+      Ext128{1} << 63, Ext128{1} << 64, Ext128{1} << 65,
+      std::numeric_limits<Ext128>::max(),
+      std::numeric_limits<Ext128>::max() - 1,
+      std::numeric_limits<Ext128>::min() + 1,
+      std::numeric_limits<Ext128>::min(), static_cast<Ext128>(kPi)
+#endif
+  );
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(values));
 }