Fix a discrepancy between absl::Hash and absl::HashOf for some negative signed…

Fix a discrepancy between absl::Hash and absl::HashOf for some negative signed integral types and improve the performance of absl::Hash.

PiperOrigin-RevId: 507598042
Change-Id: I96a7bd6b9c360f435f216b2671ae84d9768a46e8

absl/hash/hash_test.cc

@@ -17,6 +17,7 @@
 #include <algorithm>
 #include <array>
 #include <bitset>
+#include <cstdint>
 #include <cstring>
 #include <deque>
 #include <forward_list>
@@ -1241,14 +1242,24 @@ TEST(HashTest, DoesNotUseImplicitConversionsToBool) {
 
 TEST(HashOf, MatchesHashForSingleArgument) {
   std::string s = "forty two";
-  int i = 42;
   double d = 42.0;
   std::tuple<int, int> t{4, 2};
+  int i = 42;
+  int neg_i = -42;
+  int16_t i16 = 42;
+  int16_t neg_i16 = -42;
+  int8_t i8 = 42;
+  int8_t neg_i8 = -42;
 
   EXPECT_EQ(absl::HashOf(s), absl::Hash<std::string>{}(s));
-  EXPECT_EQ(absl::HashOf(i), absl::Hash<int>{}(i));
   EXPECT_EQ(absl::HashOf(d), absl::Hash<double>{}(d));
   EXPECT_EQ(absl::HashOf(t), (absl::Hash<std::tuple<int, int>>{}(t)));
+  EXPECT_EQ(absl::HashOf(i), absl::Hash<int>{}(i));
+  EXPECT_EQ(absl::HashOf(neg_i), absl::Hash<int>{}(neg_i));
+  EXPECT_EQ(absl::HashOf(i16), absl::Hash<int16_t>{}(i16));
+  EXPECT_EQ(absl::HashOf(neg_i16), absl::Hash<int16_t>{}(neg_i16));
+  EXPECT_EQ(absl::HashOf(i8), absl::Hash<int8_t>{}(i8));
+  EXPECT_EQ(absl::HashOf(neg_i8), absl::Hash<int8_t>{}(neg_i8));
 }
 
 TEST(HashOf, MatchesHashOfTupleForMultipleArguments) {

absl/hash/internal/hash.h

@@ -969,7 +969,8 @@ class ABSL_DLL MixingHashState : public HashStateBase<MixingHashState> {
   // The result should be the same as running the whole algorithm, but faster.
   template <typename T, absl::enable_if_t<IntegralFastPath<T>::value, int> = 0>
   static size_t hash(T value) {
-    return static_cast<size_t>(Mix(Seed(), static_cast<uint64_t>(value)));
+    return static_cast<size_t>(
+        Mix(Seed(), static_cast<std::make_unsigned_t<T>>(value)));
   }
 
   // Overload of MixingHashState::hash()