Just use a volatile static in Malloc.h

[folly.git] / folly / Bits.h

diff --git a/folly/Bits.h b/folly/Bits.h
index cb4234c48f18c62a2b350e8370057adb59e02966..4b71c18963600ebdd74b4eadc076d98cbf5fbb96 100644 (file)
--- a/folly/Bits.h
+++ b/folly/Bits.h
@@ -54,29 +54,28 @@
 
 #pragma once
 
-#if !defined(__clang__) && !(defined(_MSC_VER) && (_MSC_VER < 1900))
-#define FOLLY_INTRINSIC_CONSTEXPR constexpr
-#else
-// GCC and MSVC 2015+ are the only compilers with
-// intrinsics constexpr.
+// MSVC does not support intrinsics constexpr
+#if defined(_MSC_VER)
 #define FOLLY_INTRINSIC_CONSTEXPR const
+#else
+#define FOLLY_INTRINSIC_CONSTEXPR constexpr
 #endif
 
-#include <folly/Portability.h>
-#include <folly/portability/Builtins.h>
-
-#include <folly/Assume.h>
-#include <folly/detail/BitsDetail.h>
-#include <folly/detail/BitIteratorDetail.h>
-#include <folly/Likely.h>
-
 #include <cassert>
 #include <cinttypes>
+#include <cstdint>
+#include <cstring>
 #include <iterator>
 #include <limits>
 #include <type_traits>
+
 #include <boost/iterator/iterator_adaptor.hpp>
-#include <stdint.h>
+
+#include <folly/Assume.h>
+#include <folly/Likely.h>
+#include <folly/Portability.h>
+#include <folly/detail/BitIteratorDetail.h>
+#include <folly/portability/Builtins.h>
 
 namespace folly {
 
@@ -213,7 +212,7 @@ inline typename std::enable_if<
    sizeof(T) <= sizeof(unsigned int)),
   size_t>::type
   popcount(T x) {
-  return size_t(detail::popcount(x));
+  return size_t(__builtin_popcount(x));
 }
 
 template <class T>
@@ -224,7 +223,7 @@ inline typename std::enable_if<
    sizeof(T) <= sizeof(unsigned long long)),
   size_t>::type
   popcount(T x) {
-  return size_t(detail::popcountll(x));
+  return size_t(__builtin_popcountll(x));
 }
 
 /**
@@ -232,43 +231,48 @@ 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 / 8> {                       \
+    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<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;
   }
@@ -277,7 +281,7 @@ struct EndianInt : public EndianIntBase<T> {
   }
 };
 
-}  // namespace detail
+} // namespace detail
 
 // big* convert between native and big-endian representations
 // little* convert between native and little-endian representations
@@ -545,4 +549,4 @@ inline void storeUnaligned(void* p, T value) {
   }
 }
 
-}  // namespace folly
+} // namespace folly