popcntw

Population Count Word

popcntw RA, RS

Counts the number of set bits (1s) in the lower 32 bits of a register.

Encoding

Binary Layout

31

0:5

RS

6:10

RA

11:15

00000

16:20

378

21:30

/

31

 

Format X-form

Opcode 0x7C0002F4

Extension Base

Operands

• RA
  Target Register
• RS
  Source Register

Example

popcntw r3, r4

// Hamming weight of 32-bit word.

Related

Across architectures

Population Count : how x86, ARM, RISC-V, and PowerISA each do this.

More in Base

Reference

View in PowerISA v3.1C Specification ↗ OPF Power ISA v3.1C

Description

Counts the number of set bits in the lower 32 bits of a general-purpose register and stores the count in another register. This is a base category instruction that produces the count of 1-bits in the word, with the upper 32 bits of the result cleared and no condition register or status updates.

Operation

RA ← popcount(RS[32:63])

Programming Note

The popcntw instruction is useful for counting the number of set bits (1s) in a 32-bit word. It operates on each 32-bit segment of the source register, making it ideal for bit manipulation tasks where population count is needed. Ensure that the source and destination registers are properly aligned to avoid unexpected behavior. This instruction does not require any special privileges and will execute without exceptions if the operands are valid.