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Add constexpr to bit operations #749

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8c7930f
Make bit operations constexpr
AndrewSasmito Jul 29, 2026
68545ca
Fix bug
AndrewSasmito Aug 13, 2026
5c8179a
Merge branch 'main' into new-bit-windows
AndrewSasmito Aug 13, 2026
6c04e0e
Update nanosecond bug + Switch to one branch for bit operations
AndrewSasmito Aug 13, 2026
2bf995a
Merge main
AndrewSasmito Aug 13, 2026
06dc231
Remove nanosecond change + Fix bug
AndrewSasmito Aug 14, 2026
2262c06
Merge branch 'main' into new-bit-windows
AndrewSasmito Aug 14, 2026
8d971ad
Remove unnecessary comment
AndrewSasmito Aug 16, 2026
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11 changes: 2 additions & 9 deletions cpp/csp/core/DynamicBitSet.h
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Original file line number Diff line number Diff line change
Expand Up @@ -155,19 +155,12 @@ class DynamicBitSet
private:
using nbit_type = uint8_t;

#ifndef WIN32
#define CLZ_CONSTEXPR constexpr
#else
#define CLZ_CONSTEXPR
#endif

template<typename value_type,
std::enable_if_t<std::is_unsigned<value_type>::value, bool> = true>
static constexpr nbit_type nbits() { return sizeof( value_type ) * 8; }

template<typename U, std::enable_if_t<std::is_unsigned<U>::value, bool> = true>
static CLZ_CONSTEXPR nbit_type log2( U n ) { return nbits<uint32_t>() - clz(static_cast<uint32_t>( n )) - 1; } //upcast to 32 bit to avoid truncation for log2
static CLZ_CONSTEXPR nbit_type log2(uint64_t n) { return nbits<uint64_t>() - clz(n) - 1; }
static constexpr nbit_type log2( U n ) { return nbits<U>() - clz( n ) - 1; }

static constexpr node_type mask( nbit_type bitIndex ) { return ( node_type )1 << bitIndex; }

Expand All @@ -182,7 +175,7 @@ class DynamicBitSet
}

static constexpr nbit_type _bits = nbits<NodeT>();
static inline CLZ_CONSTEXPR nbit_type _logBits = log2( _bits );
static inline constexpr nbit_type _logBits = log2( _bits );

node_type * m_nodes;
index_type m_size;
Expand Down
50 changes: 5 additions & 45 deletions cpp/csp/core/Platform.h
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
#ifndef _IN_CSP_CORE_PLATFORM_H
#define _IN_CSP_CORE_PLATFORM_H
#include <bit>
#include <type_traits>
#include <stdint.h>
#include <time.h>
Expand Down Expand Up @@ -52,42 +53,6 @@ inline int nanosleep(const timespec* req, timespec* rem)
return 0;
}

inline uint8_t clz(uint64_t n)
{
unsigned long index = 0;
if (_BitScanReverse64(&index, n))
return 64 - index - 1;
return 0;
}

inline uint8_t clz(uint32_t n)
{
unsigned long index = 0;
if (_BitScanReverse(&index, n))
return 32 - index - 1;
return 0;
}

inline uint8_t clz(uint16_t n) { return clz(static_cast<uint32_t>(n)) - 16; }
inline uint8_t clz(uint8_t n) { return clz(static_cast<uint32_t>(n)) - 24; }

template<typename U, std::enable_if_t<std::is_unsigned<U>::value, bool> = true>
inline uint8_t ffs(U n)
{
unsigned long index = 0;
if (_BitScanForward(&index, n))
return index + 1;
return 0;
}

inline uint8_t ffs(uint64_t n)
{
unsigned long index = 0;
if (_BitScanForward64(&index, n))
return index + 1;
return 0;
}

#else

#define CSPIMPL_EXPORT
Expand All @@ -100,19 +65,14 @@ inline uint8_t ffs(uint64_t n)

#define NO_INLINE __attribute__ ((noinline))

inline constexpr uint8_t clz(uint32_t n) { return __builtin_clz(n); }
inline constexpr uint8_t clz(uint64_t n) { return __builtin_clzl(n); }
#endif

// clz (count leading zeros) returns number of leading zeros before MSB (i.e. clz(00110..) = 2 )
// __builtin_clz auto-promotes to 32-bits: need to subtract off extra leading zeros
inline constexpr uint8_t clz(uint16_t n) { return clz(static_cast<uint32_t>(n)) - 16; }
inline constexpr uint8_t clz(uint8_t n) { return clz(static_cast<uint32_t>(n)) - 24; }
template<typename U, std::enable_if_t<std::is_unsigned<U>::value, bool> = true>
inline constexpr uint8_t clz( U n ) { return std::countl_zero(n); }

// ffs (find first set) returns offset of first set bit (i.e. ffs(..0110) = 2 ), with ffs(0) = 0
template<typename U, std::enable_if_t<std::is_unsigned<U>::value, bool> = true>
inline constexpr uint8_t ffs( U n ) { return __builtin_ffs(n); }
inline constexpr uint8_t ffs( uint64_t n ) { return __builtin_ffsl(n); }

#endif
inline constexpr uint8_t ffs( U n ) { return n ? std::countr_zero(n) + 1 : 0; }

#endif
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