根据http://www.agner.org/optimize/instruction_tables.pdf ，POPCNT 指令(返回 32 位或 64 位寄存器中设置的位数)在现代 Intel 和 AMD 处理器上的吞吐量为每个时钟周期 1 条指令。这比任何需要多条指令的软件实现要快得多 ( How to count the number of set bits in a 32-bit integer? )。
POPCNT 是如何在硬件中如此高效地实现的？
最佳答案
有一项组合 popcnt、位扫描正向/反向的专利:
US8214414 B2 - Combined set bit count and detector logic
Abstract
A merged datapath for PopCount and BitScan is described. A hardware circuit includes a compressor tree utilized for a PopCount function, which is reused by a BitScan function (e.g., bit scan forward (BSF) or bit scan reverse (BSR)). Selector logic enables the compressor tree to operate on an input word for the PopCount or BitScan operation, based on a microprocessor instruction. The input word is encoded if a BitScan operation is selected. The compressor tree receives the input word, operates on the bits as though all bits have same level of significance (e.g., for an N-bit input word, the input word is treated as N one-bit inputs). The result of the compressor tree circuit is a binary value representing a number related to the operation performed (the number of set bits for PopCount, or the bit position of the first set bit encountered by scanning the input word).
关于assembly - POPCNT是如何在硬件中实现的？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/28802692/