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Use _BitScanReverse

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This commit is contained in:
Peter Dimov
2020-12-29 01:53:30 +02:00
parent aca46fae91
commit c838ebc003
1 changed files with 42 additions and 0 deletions
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42
include/boost/core/bit.hpp
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@ -21,6 +21,14 @@
#include <limits>
#include <cstring>
#if defined(_MSC_VER)
# include <intrin.h>
# pragma intrinsic(_BitScanReverse)
# if defined(_M_X64)
# pragma intrinsic(_BitScanReverse64)
# endif
#endif // defined(_MSC_VER)
namespace boost
{
namespace core
@ -85,6 +93,21 @@ namespace detail
inline int countl_impl( boost::uint32_t x ) BOOST_NOEXCEPT
{
#if defined(_MSC_VER)
unsigned long r;
if( _BitScanReverse( &r, x ) )
{
return 31 - static_cast<int>( r );
}
else
{
return 32;
}
#else
static unsigned char const mod37[ 37 ] = { 32, 31, 6, 30, 9, 5, 0, 29, 16, 8, 2, 4, 21, 0, 19, 28, 25, 15, 0, 7, 10, 1, 17, 3, 22, 20, 26, 0, 11, 18, 23, 27, 12, 24, 13, 14, 0 };
x |= x >> 1;
@ -94,13 +117,32 @@ inline int countl_impl( boost::uint32_t x ) BOOST_NOEXCEPT
x |= x >> 16;
return mod37[ x % 37 ];
#endif
}
inline int countl_impl( boost::uint64_t x ) BOOST_NOEXCEPT
{
#if defined(_MSC_VER) && defined(_M_X64)
unsigned long r;
if( _BitScanReverse64( &r, x ) )
{
return 63 - static_cast<int>( r );
}
else
{
return 64;
}
#else
return static_cast<boost::uint32_t>( x >> 32 ) != 0?
boost::core::detail::countl_impl( static_cast<boost::uint32_t>( x >> 32 ) ):
boost::core::detail::countl_impl( static_cast<boost::uint32_t>( x ) ) + 32;
#endif
}
inline int countl_impl( boost::uint8_t x ) BOOST_NOEXCEPT