common/opthelpers.h

   1 #ifndef OPTHELPERS_H
   2 #define OPTHELPERS_H
   3
   4 #ifdef __has_builtin
   5 #define HAS_BUILTIN __has_builtin
   6 #else
   7 #define HAS_BUILTIN(x) (0)
   8 #endif
   9
  10 #if defined(__GNUC__) || HAS_BUILTIN(__builtin_expect)
  11 /* likely() optimizes for the case where the condition is true. The condition
  12  * is not required to be true, but it can result in more optimal code for the
  13  * true path at the expense of a less optimal false path.
  14  */
  15 template<typename T>
  16 constexpr bool likely(T&& expr) noexcept
  17 { return __builtin_expect(static_cast<bool>(expr), true); }
  18 /* The opposite of likely(), optimizing for the case where the condition is
  19  * false.
  20  */
  21 template<typename T>
  22 constexpr bool unlikely(T&& expr) noexcept
  23 { return __builtin_expect(static_cast<bool>(expr), false); }
  24
  25 #else
  26
  27 template<typename T>
  28 constexpr bool likely(T&& expr) noexcept { return static_cast<bool>(expr); }
  29 template<typename T>
  30 constexpr bool unlikely(T&& expr) noexcept { return static_cast<bool>(expr); }
  31 #endif
  32 #define LIKELY(x) (likely(x))
  33 #define UNLIKELY(x) (unlikely(x))
  34
  35 #if HAS_BUILTIN(__builtin_assume)
  36 /* Unlike LIKELY, ASSUME requires the condition to be true or else it invokes
  37  * undefined behavior. It's essentially an assert without actually checking the
  38  * condition at run-time, allowing for stronger optimizations than LIKELY.
  39  */
  40 #define ASSUME __builtin_assume
  41 #elif defined(_MSC_VER)
  42 #define ASSUME __assume
  43 #elif defined(__GNUC__)
  44 #define ASSUME(x) do { if(!(x)) __builtin_unreachable(); } while(0)
  45 #else
  46 #define ASSUME(x) ((void)0)
  47 #endif
  48
  49 #if __cplusplus >= 201703L || defined(__cpp_if_constexpr)
  50 #define if_constexpr if constexpr
  51 #else
  52 #define if_constexpr if
  53 #endif
  54
  55 #endif /* OPTHELPERS_H */