Refactor Endian

Summary:
[Folly] Refactor `Endian`.

Define `swap` for all types, instead of only declaring it for all types, but have its implementation call a function that only has a given set of non-template overloads for a given set of supported types.

The effective change is that now `Endian::swap<std::string>` will fail to compile, rather than compile but fail to link.

And support floating-point types.

Reviewed By: nbronson

Differential Revision: D4615706

fbshipit-source-id: 5034e1e4466b8118a258d917ce8f4094460ca01a

folly/Bits.h

@@ -71,6 +71,7 @@
 #include <folly/Likely.h>
 
 #include <cassert>
+#include <cstring>
 #include <cinttypes>
 #include <iterator>
 #include <limits>
@@ -232,43 +233,48 @@ inline typename std::enable_if<
  */
 namespace detail {
 
-template <class T>
-struct EndianIntBase {
- public:
-  static T swap(T x);
-};
+template <size_t Size>
+struct uint_types_by_size;
 
-#define FB_GEN(t, fn)                             \
-  template <>                                     \
-  inline t EndianIntBase<t>::swap(t x) {          \
-    return t(fn(std::make_unsigned<t>::type(x))); \
-  }
+#define FB_GEN(sz, fn)                                      \
+  static inline uint##sz##_t byteswap_gen(uint##sz##_t v) { \
+    return fn(v);                                           \
+  }                                                         \
+  template <>                                               \
+  struct uint_types_by_size<sz> {                           \
+    using type = uint##sz##_t;                              \
+  };
 
-// fn(x) expands to (x) if the second argument is empty, which is exactly
-// what we want for [u]int8_t.
-FB_GEN( int8_t,)
-FB_GEN(uint8_t,)
+FB_GEN(8, uint8_t)
 #ifdef _MSC_VER
-FB_GEN( int64_t, _byteswap_uint64)
-FB_GEN(uint64_t, _byteswap_uint64)
-FB_GEN( int32_t, _byteswap_ulong)
-FB_GEN(uint32_t, _byteswap_ulong)
-FB_GEN( int16_t, _byteswap_ushort)
-FB_GEN(uint16_t, _byteswap_ushort)
+FB_GEN(64, _byteswap_uint64)
+FB_GEN(32, _byteswap_ulong)
+FB_GEN(16, _byteswap_ushort)
 #else
-FB_GEN( int64_t, __builtin_bswap64)
-FB_GEN(uint64_t, __builtin_bswap64)
-FB_GEN( int32_t, __builtin_bswap32)
-FB_GEN(uint32_t, __builtin_bswap32)
-FB_GEN( int16_t, __builtin_bswap16)
-FB_GEN(uint16_t, __builtin_bswap16)
+FB_GEN(64, __builtin_bswap64)
+FB_GEN(32, __builtin_bswap32)
+FB_GEN(16, __builtin_bswap16)
 #endif
 
 #undef FB_GEN
 
 template <class T>
-struct EndianInt : public EndianIntBase<T> {
- public:
+struct EndianInt {
+  static_assert(
+      (std::is_integral<T>::value && !std::is_same<T, bool>::value) ||
+          std::is_floating_point<T>::value,
+      "template type parameter must be non-bool integral or floating point");
+  static T swap(T x) {
+    // we implement this with memcpy because that is defined behavior in C++
+    // we rely on compilers to optimize away the memcpy calls
+    constexpr auto s = sizeof(T);
+    using B = typename uint_types_by_size<8 * s>::type;
+    B b;
+    std::memcpy(&b, &x, s);
+    b = byteswap_gen(b);
+    std::memcpy(&x, &b, s);
+    return x;
+  }
   static T big(T x) {
     return kIsLittleEndian ? EndianInt::swap(x) : x;
   }

folly/test/BitsTest.cpp

@@ -18,6 +18,8 @@
 
 #include <folly/Bits.h>
 
+#include <random>
+
 #include <folly/portability/GTest.h>
 
 using namespace folly;
@@ -165,3 +167,21 @@ TEST(Bits, popcount) {
   EXPECT_EQ(32, popcount(uint32_t(-1)));
   EXPECT_EQ(64, popcount(uint64_t(-1)));
 }
+
+TEST(Bits, Endian_swap_uint) {
+  EXPECT_EQ(uint8_t(0xda), Endian::swap(uint8_t(0xda)));
+  EXPECT_EQ(uint16_t(0x4175), Endian::swap(uint16_t(0x7541)));
+  EXPECT_EQ(uint32_t(0x42efb918), Endian::swap(uint32_t(0x18b9ef42)));
+  EXPECT_EQ(
+      uint64_t(0xa244f5e862c71d8a), Endian::swap(uint64_t(0x8a1dc762e8f544a2)));
+}
+
+TEST(Bits, Endian_swap_real) {
+  std::mt19937_64 rng;
+  auto f = std::uniform_real_distribution<float>()(rng);
+  EXPECT_NE(f, Endian::swap(f));
+  EXPECT_EQ(f, Endian::swap(Endian::swap(f)));
+  auto d = std::uniform_real_distribution<double>()(rng);
+  EXPECT_NE(d, Endian::swap(d));
+  EXPECT_EQ(d, Endian::swap(Endian::swap(d)));
+}