Make hasher<float> avalanching

Summary: It currently only sets the lower 32 bits.

Reviewed By: nbronson, philippv

Differential Revision: D7216798

fbshipit-source-id: 090f6dba990eeb7f1a9cc56e8615161c11eea026

folly/hash/Hash.h

@@ -401,22 +401,12 @@ struct float_hasher {
       return 0;
     }
 
-    /* constexpr */ if (sizeof(F) <= 4) {
-      uint32_t u32 = 0;
-      memcpy(&u32, &f, sizeof(F));
-      return static_cast<size_t>(hash::jenkins_rev_mix32(u32));
-    } else {
-      uint64_t u64 = 0;
-      memcpy(&u64, &f, sizeof(F));
-      return static_cast<size_t>(hash::twang_mix64(u64));
-    }
+    uint64_t u64 = 0;
+    memcpy(&u64, &f, sizeof(F));
+    return static_cast<size_t>(hash::twang_mix64(u64));
   }
 };
 
-template <typename F>
-using float_hasher_avalanches =
-    std::integral_constant<bool, sizeof(F) == 8 || sizeof(size_t) == 4>;
-
 } // namespace detail
 
 template <class Key, class Enable = void>
@@ -467,10 +457,7 @@ struct IsAvalanchingHasher<hasher<T, E>, K>
     : std::conditional<
           std::is_enum<T>::value || std::is_integral<T>::value,
           detail::integral_hasher_avalanches<T>,
-          typename std::conditional<
-              std::is_floating_point<T>::value,
-              detail::float_hasher_avalanches<T>,
-              std::false_type>::type>::type {};
+          std::is_floating_point<T>>::type {};
 
 template <typename K>
 struct IsAvalanchingHasher<Hash, K> : IsAvalanchingHasher<hasher<K>, K> {};

folly/hash/test/HashTest.cpp

@@ -597,7 +597,7 @@ static_assert(
     folly::IsAvalanchingHasher<folly::hasher<int64_t>, int64_t>::value,
     "");
 static_assert(
-    !folly::IsAvalanchingHasher<folly::hasher<float>, float>::value,
+    folly::IsAvalanchingHasher<folly::hasher<float>, float>::value,
     "");
 static_assert(
     folly::IsAvalanchingHasher<folly::hasher<double>, double>::value,
@@ -736,6 +736,10 @@ TEST(Traits, follyHasherInt64Avalances) {
       int64_t{0}, [](int64_t& v) { v++; });
 }
 
+TEST(Traits, follyHasherFloatAvalanches) {
+  verifyAvalanching<folly::hasher<float>>(0.0f, [](float& v) { v += 1; });
+}
+
 TEST(Traits, follyHasherDoubleAvalanches) {
   verifyAvalanching<folly::hasher<double>>(0.0, [](double& v) { v += 1; });
 }