Find First Set - Tool and Library Support

Tool and Library Support

A number of compiler and library vendors supply compiler intrinsics or library functions to perform find first set and/or related operations, which are frequently implemented in terms of the hardware instructions above:

Tool/library Name Type Input type(s) Notes Result for zero input
POSIX.1 compliant libc
4.3BSD libc
OS X 10.3 libc		 ffs Library function int Includes glibc.
POSIX does not supply the complementary log base 2 / clz.		 0
FreeBSD 5.3 libc
OS X 10.4 libc		 ffsl
fls, flsl		 Library function int, long fls ("find last set") computes (log base 2) + 1. 0
FreeBSD 7.1 libc ffsll, flsll Library function long long 0
GCC 3.2.2 __builtin_ffs Built-in functions unsigned int,
unsigned long,
unsigned long long		 Possibly available before 3.2.2. 0
__builtin_ctz undefined
__builtin_clz undefined
Visual Studio 2005 _BitScanForward
_BitScanReverse		 Compiler intrinsics unsigned long,
unsigned __int64		 Returns zero on zero input, unlike x86 bsf/bsr 0
Visual Studio 2008 __lzcnt Compiler intrinsic unsigned short,
unsigned int,
unsigned __int64		 Relies on AMD-only lzcnt instruction Input size in bits
Intel C++ Compiler _bit_scan_forward
_bit_scan_reverse		 Compiler intrinsics int undefined
NVIDIA CUDA __clz Functions 32-bit, 64-bit Compiles to fewer instructions on the GeForce 400 Series 32
__ffs 0
LLVM llvm.ctlz.*
llvm.cttz.*		 Intrinsic 8, 16, 32, 64, 256 LLVM assembly language Input size if arg 2
is 0, else undefined

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