Find First Set - Hardware Support

Hardware Support

Many architectures include instructions to rapidly perform find first set and/or related operations, listed below. The most common operation is count leading zeros (clz), likely because all other operations can be implemented efficiently in terms of it (see Properties and relations).

Platform Mnemonic Name Word sizes Description Result on zero input
Intel 386 and later bsf Bit Scan Forward 16, 32, 64 ctz Undefined, sets zero flag
Intel 386 and later bsr Bit Scan Reverse 16, 32, 64 log base 2 Undefined, sets zero flag
AMD supporting SSE4.2 lzcnt Count Leading Zeros 16, 32, 64 clz input size, sets carry flag
Itanium clz Count Leading Zeros 64 clz 64
ARM 5 or later clz Count Leading Zeros 32 clz 32
IBM POWER/PowerPC cntlz/cntlzw/cntlzd Count Leading Zeros 32, 64 clz input size
MIPS clz Count Leading Zeros in Word 32, 64 clz input size
MIPS clo Count Leading Ones in Word 32, 64 clo input size
DEC Alpha ctlz Count Leading Zero 64 clz 64
DEC Alpha cttz Count Trailing Zero 64 ctz 64
Motorola 68020 and later bfffo Find First One in Bit Field arbitrary log base 2 field offset + field width

Notes: On some Alpha platforms CTLZ and CTTZ are emulated in software.

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