popcntd

Population Count Doubleword

popcntd RA, RS

Counts the number of set bits (1s) in a 64-bit register.

Encoding

Binary Layout

31

0:5

RS

6:10

RA

11:15

00000

16:20

506

21:30

/

31

 

Format X-form

Opcode 0x7C0003F4

Extension Base

Operands

• RA
  Target Register
• RS
  Source Register

Example

popcntd r3, r4

// Hamming weight of r4.

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 (population count) in a 64-bit general-purpose register and stores the count in another register. This is a base category instruction that produces the count of 1-bits across the entire doubleword with no condition register or status updates.

Operation

RA ← popcount(RS[0:63])

Programming Note

The popcntd instruction is useful for counting the number of set bits (1s) in a 64-bit value. It operates on each doubleword independently, so if you're working with 128-bit values, ensure that both halves are processed separately. This instruction does not require any special alignment and can be executed at user privilege level. Be cautious when using this instruction in performance-critical sections, as it may have varying execution times depending on the input data.