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express reverse_bytes in an easy to optimize way

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optimizes to single bswap instruction on gcc and clang
This commit is contained in:
Aaron Bishop 2017-11-11 15:54:36 -05:00 committed by Howard Hinnant
parent 4832ea0ddb
commit c9ef0a8f05
1 changed files with 33 additions and 11 deletions
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44
src/tz.cpp
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@ -1691,17 +1691,32 @@ enum class endian
big = __ORDER_BIG_ENDIAN__
};
template <class T>
static
inline
void
reverse_bytes(T& t)
std::uint32_t
reverse_bytes(std::uint32_t i)
{
unsigned char* bytes = static_cast<unsigned char*>(std::memmove(std::addressof(t),
std::addressof(t),
sizeof(T)));
for (unsigned i = 0; i < sizeof(T)/2; ++i)
std::swap(bytes[i], bytes[sizeof(T)-1-i]);
return
(i & 0xff000000u) >> 24 |
(i & 0x00ff0000u) >> 8 |
(i & 0x0000ff00u) << 8 |
(i & 0x000000ffu) << 24;
}
static
inline
std::uint64_t
reverse_bytes(std::uint64_t i)
{
return
(i & 0xff00000000000000ull) >> 56 |
(i & 0x00ff000000000000ull) >> 40 |
(i & 0x0000ff0000000000ull) >> 24 |
(i & 0x000000ff00000000ull) >> 8 |
(i & 0x00000000ff000000ull) << 8 |
(i & 0x0000000000ff0000ull) << 24 |
(i & 0x000000000000ff00ull) << 40 |
(i & 0x00000000000000ffull) << 56;
}
template <class T>
@ -1712,13 +1727,20 @@ maybe_reverse_bytes(T&, std::false_type)
{
}
template <class T>
static
inline
void
maybe_reverse_bytes(T& t, std::true_type)
maybe_reverse_bytes(std::int32_t& t, std::true_type)
{
reverse_bytes(t);
t = static_cast<std::int32_t>(reverse_bytes(static_cast<std::uint32_t>(t)));
}
static
inline
void
maybe_reverse_bytes(std::int64_t& t, std::true_type)
{
t = static_cast<std::int64_t>(reverse_bytes(static_cast<std::uint64_t>(t)));
}
template <class T>