common/opthelpers.h

   1 #ifndef OPTHELPERS_H
   2 #define OPTHELPERS_H
   3
   4 #include <cstdint>
   5 #include <utility>
   6 #include <memory>
   7
   8 #ifdef __has_builtin
   9 #define HAS_BUILTIN __has_builtin
  10 #else
  11 #define HAS_BUILTIN(x) (0)
  12 #endif
  13
  14 #ifdef __has_cpp_attribute
  15 #define HAS_ATTRIBUTE __has_cpp_attribute
  16 #else
  17 #define HAS_ATTRIBUTE(x) (0)
  18 #endif
  19
  20 #ifdef __GNUC__
  21 #define force_inline [[gnu::always_inline]] inline
  22 #elif defined(_MSC_VER)
  23 #define force_inline __forceinline
  24 #else
  25 #define force_inline inline
  26 #endif
  27
  28 /* Unlike the likely attribute, ASSUME requires the condition to be true or
  29  * else it invokes undefined behavior. It's essentially an assert without
  30  * actually checking the condition at run-time, allowing for stronger
  31  * optimizations than the likely attribute.
  32  */
  33 #if HAS_BUILTIN(__builtin_assume)
  34 #define ASSUME __builtin_assume
  35 #elif defined(_MSC_VER)
  36 #define ASSUME __assume
  37 #elif __has_attribute(assume)
  38 #define ASSUME(x) [[assume(x)]]
  39 #elif HAS_BUILTIN(__builtin_unreachable)
  40 #define ASSUME(x) do { if(x) break; __builtin_unreachable(); } while(0)
  41 #else
  42 #define ASSUME(x) ((void)0)
  43 #endif
  44
  45 /* This shouldn't be needed since unknown attributes are ignored, but older
  46  * versions of GCC choke on the attribute syntax in certain situations.
  47  */
  48 #if HAS_ATTRIBUTE(likely)
  49 #define LIKELY [[likely]]
  50 #define UNLIKELY [[unlikely]]
  51 #else
  52 #define LIKELY
  53 #define UNLIKELY
  54 #endif
  55
  56 namespace al {
  57
  58 template<typename T>
  59 constexpr std::underlying_type_t<T> to_underlying(T e) noexcept
  60 { return static_cast<std::underlying_type_t<T>>(e); }
  61
  62 [[noreturn]] inline void unreachable()
  63 {
  64 #if HAS_BUILTIN(__builtin_unreachable)
  65     __builtin_unreachable();
  66 #else
  67     ASSUME(false);
  68 #endif
  69 }
  70
  71 template<std::size_t alignment, typename T>
  72 force_inline constexpr auto assume_aligned(T *ptr) noexcept
  73 {
  74 #ifdef __cpp_lib_assume_aligned
  75     return std::assume_aligned<alignment,T>(ptr);
  76 #elif HAS_BUILTIN(__builtin_assume_aligned)
  77     return static_cast<T*>(__builtin_assume_aligned(ptr, alignment));
  78 #elif defined(_MSC_VER)
  79     constexpr std::size_t alignment_mask{(1<<alignment) - 1};
  80     if((reinterpret_cast<std::uintptr_t>(ptr)&alignment_mask) == 0)
  81         return ptr;
  82     __assume(0);
  83 #elif defined(__ICC)
  84     __assume_aligned(ptr, alignment);
  85     return ptr;
  86 #else
  87     return ptr;
  88 #endif
  89 }
  90
  91 } // namespace al
  92
  93 #endif /* OPTHELPERS_H */