popcnt

Population Count

POPCNT r, r/m

Counts number of bits set to 1.

Pseudocode Operation

dest ← count_set_bits(src)

Example

POPCNT rax, rbx

Encoding

Binary Layout

Format VEX

Opcode F3 0F B8

Extension SSE4.2

Operands

dest
General-purpose register
src
Register or memory operand

Across architectures

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

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	Description
F3 0F B8 /r	POPCNT r16, r/m16	RM	Valid Valid	POPCNT on r/m16
F3 0F B8 /r	POPCNT r32, r/m32	RM	Valid Valid	POPCNT on r/m32
F3 REX.W 0F B8 /r	POPCNT r64, r/m64	RM	Valid N.E.	POPCNT on r/m64

Description

This instruction calculates the number of bits set to 1 in the second operand (source) and returns the count in the first operand (a destination register).

Operation

Count = 0;
For (i=0; i < OperandSize; i++)
{        IF (SRC[ i] = 1) // i’th bit
THEN Count++; FI;
}
DEST := Count;

Intel C/C++ Compiler Intrinsic Equivalent

POPCNT int _mm_popcnt_u32(unsigned int a);
POPCNT int64_t _mm_popcnt_u64(unsigned __int64 a);

Flags Affected

OF, SF, ZF, AF, CF, PF are all cleared. ZF is set if SRC = 0, otherwise ZF is cleared.

Exceptions

Protected Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS or GS segments. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF (fault-code) For a page fault. #AC(0) If an unaligned memory reference is made while the current privilege level is 3 and alignment checking is enabled. #UD If CPUID.01H:ECX.POPCNT[23] = 0. If LOCK prefix is used.

Real-Address Mode Exceptions

#GP(0) If any part of the operand lies outside of the effective address space from 0 to 0FFFFH. #SS(0) If a memory operand effective address is outside the SS segment limit. #UD If CPUID.01H:ECX.POPCNT[23] = 0. If LOCK prefix is used. POPCNT-Return the Count of Number of Bits Set to 1 Vol. 2B 4-405 Virtual 8086 Mode Exceptions #GP(0) If any part of the operand lies outside of the effective address space from 0 to 0FFFFH. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF (fault-code) For a page fault. #AC(0) If an unaligned memory reference is made while alignment checking is enabled. #UD If CPUID.01H:ECX.POPCNT[23] = 0. If LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in Protected Mode.

64-Bit Mode Exceptions

#GP(0) If the memory address is in a non-canonical form. #SS(0) If a memory address referencing the SS segment is in a non-canonical form. #PF (fault-code) For a page fault. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If CPUID.01H:ECX.POPCNT[23] = 0. If LOCK prefix is used. POPCNT-Return the Count of Number of Bits Set to 1 Vol. 2B 4-406