x86-64.net

Instruction

KXNORB

Computes the bitwise XNOR of two 8-bit masks.

Extensions
Forms
1 (1 VEX)
Data
Intel XED v2026.08.23

KXNORB XORs the low 8 bits of two mask registers, inverts the result and writes it to a third, clearing bits 63:8: a bit is 1 where the two sources agree. It is KXNORW for eight-element masks, and that page explains its main use, all ones from any register for a gather or scatter; KXNORB needs AVX512DQ.

Recorded on Zen 5

  • All ones. kxnorb k7, k7, k7 turned A3353C7A61B6A3B3h into FFh, and kxnorb k1, k2, k3 gave FFh from two masks whose low bytes were both zero, whatever their upper bits held.
  • Only the low byte. FFFFFFFFFFFFFF55h against AAh gave 0, and FFFFFFFFFFFFFFBEh against 57h gave 16h: bits 8 and up of the sources play no part.
  • K0 is a register here. kxnorb k0, k4, k5 wrote FFh to K0, and kxnorb k6, k7, k0 read K0’s low byte, 00h, against FFh to give 0. Only as a write mask does K0 mean “no mask”.

In compiled code

With -march=x86-64-v4, GCC 14 and Clang 23 at -O2 set the mask of an eight-element gather such as _mm512_i32gather_pd with kxnorb k1, k1, k1 and kxnorb k1, k0, k0; with -march=znver5 GCC loads the byte FFh from memory with KMOVB instead. For _kxnor_mask8 of two compares of doubles, GCC keeps KXNORB, while Clang inverts the second predicate (VCMPNLTPD for “less than”) and uses KXORB; written as ~(m1 ^ m2) and used as a write mask, it is KXNORB in GCC and, in Clang, a KXORB that selects the other way round.

AMD’s Zen 5 latency spreadsheet gives KXNORB one cycle and two per cycle on pipes FP0 and FP3, as KXNORW.

Forms

Every encoding of KXNORB that Intel XED knows, one row per XED instruction form (iform). How to read the notation.
InstructionEncodingRequires
KXNORB k, k, k KXNORB_MASKmskw_MASKmskw_MASKmskw_AVX512VEX.L1.66.0F.W0 46 /rAVX10.1 or AVX512DQ

Latency and throughput

What the scheduling models of LLVM 23.1.2 give for each form, not measurements: latency / reciprocal throughput in cycles, the ports under them. A row is a model, or models that agree; a dash, a form that its processors lack. Memory forms: latency from the address, ports without the load and store units. Which processors each model covers.
Modelk, k, k
Skylake-SP, Ice Lake, Sapphire Rapids1 / 1p0
Zen 41 / 0.5FP2 FP3

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

k1.b[0] = ~(k2.b[0] ^ k3.b[0])   # 1 where the bits agree: all ones
                                 # when k2 and k3 are one register
# bits 63:8 of k1 are cleared

Run on AMD Zen 5 (EPYC 9645): 11 cases.

Operands

What each form reads and writes, as Intel XED records it: the operands of its syntax, in order, and the implicit ones that the syntax does not name. How to read.
OperandKindAction
KXNORB k, k, k
kexplicitwrite
kexplicitread
kexplicitread

Flags

None: XED records no flag that the forms test or change.

Exceptions

The exception class is XED's name for the set of exception conditions that a form follows. How to read.

All forms
AVX512-K20

Intrinsics

The C intrinsics of Clang 23.1.2 that compile to KXNORB, each with the form it compiles to and the Clang options it needs. The forms come from compiling every intrinsic and decoding the result with Intel XED; all intrinsics A–Z explains the method.

IntrinsicCompiles to
_kxnor_mask8 __mmask8 _kxnor_mask8(__mmask8 __A, __mmask8 __B)

KXNORB k, k, k VEX.L1 -mavx512dq

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

CPUID requirements and processors of the XED ISA sets above. Alternatives are separated by “or”; processors follow XED’s chip model, whose AMD processors end with Zen 2, and the CPU data of this site for Zen 3 to Zen 6.
ISA setCPUIDProcessors
AVX512DQ_KOPB

AVX10.1
CPUID.(EAX=07H,ECX=1):EDX[19] AVX10
CPUID.(EAX=24H,ECX=0):EBX[7:0] ≥ 1 AVX10 version

or

AVX512DQ
CPUID.(EAX=07H,ECX=0):EBX[17] AVX512DQ

Cannon Lake, Ice Lake (client), Tiger Lake, Nova Lake, Skylake-SP, Cascade Lake, Cooper Lake, Ice Lake (server), Sapphire Rapids, Emerald Rapids, Granite Rapids, Diamond Rapids, Zen 4, Zen 5, Zen 6

More AVX10.1 instructions

All 686 AVX10.1 instructions

Sources

  1. Intel XED v2026.08.23 (commit 0bcb6237345c): forms, encodings, operands, flags, exception classes, attributes, ISA sets, CPUID bits and chip model.
  2. The CPU data of x86-64.net: Zen 3 to Zen 6, from GCC’s -march feature sets and AMD’s Processor Programming References, which the page of each processor cites.
  3. Clang's x86 intrinsic headers, LLVM 23.1.2 (commit 85ac56026243): the intrinsics, their signatures and doc comments, and the Clang of that release, which compiled each of them.
  4. LLVM 23.1.2's x86 scheduling models (commit 85ac56026243): the latency, reciprocal throughput and ports of each form, as llvm-mca reports them.
  5. Intel Advanced Vector Extensions 10.2 Architecture Specification, revision 7.0 (361050-007, April 2026)
  6. Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 2 (325383-093, September 2026): Volume 2A, page 3-530 (KXNORW/KXNORB/KXNORQ/KXNORD)
  7. Software Optimization Guide for the AMD Zen5 Microarchitecture (58455, rev. 1.00, August 2024), Wayback Machine copy of AMD's ZIP

The tables are derived from Intel XED, Copyright Intel Corporation, licensed under the Apache License 2.0; x86-64.net converted and reformatted them. The intrinsics and their signatures come from Clang and the timings from LLVM's scheduling models, both part of the LLVM Project, licensed under the Apache License 2.0 with LLVM Exceptions. The text is our own.