gasm-sdk/asm/evex.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

package asm

import "fmt"

// This file implements EVEX (AVX-512) instruction encoding: the four-byte
// EVEX prefix with 5-bit vector register fields (Z0–Z31, X/Y 16–31), the
// compressed disp8×N displacement, and the operand shapes the go-flac
// AVX-512 kernels use.  Masking ({k}) and zeroing ({z}) are not supported —
// the kernels do not use them.  K-register operands (mask destinations,
// KMOVW, KTESTW) are.

// evexSpec describes one EVEX instruction's encoding parameters.  The form
// field reuses the vexForm shapes, which carry over unchanged.
type evexSpec struct {
	mapSel  int // 1 = 0F, 2 = 0F38, 3 = 0F3A
	opcode  byte
	w       int
	pp      int     // 0 = none, 1 = 66, 2 = F3, 3 = F2
	opdigit int     // ModRM.reg /digit, or -1 when reg is a register
	form    vexForm // vexNDS3, vexRM, vexShiftImm, vexNDS3Imm, vexExtract
	n       [3]int  // disp8×N multiplier per vector length (128/256/512)
}

// evexTable maps an upper-case mnemonic to its EVEX encoding.  Mnemonics
// that also have a VEX form (VPADDD, VMOVUPD, …) are dispatched here only
// when an operand demands EVEX (a ZMM or K register); EVEX-only mnemonics
// (VPXORD, VALIGND, …) always encode through this table.  The N multipliers
// are taken from the Go assembler's opcode tables, which are authoritative
// for byte-for-byte agreement.
var evexTable = map[string]evexSpec{
	// EVEX.128/256/512.66.0F — integer arithmetic / logic, NDS form.
	"VPADDD":      {1, 0xFE, 0, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPADDQ":      {1, 0xD4, 1, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPSUBD":      {1, 0xFA, 0, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPSUBQ":      {1, 0xFB, 1, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPUNPCKLDQ":  {1, 0x62, 0, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPUNPCKHDQ":  {1, 0x6A, 0, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPXORD":      {1, 0xEF, 0, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPXORQ":      {1, 0xEF, 1, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPCMPEQD":    {1, 0x76, 0, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VFMADD231PD": {2, 0xB8, 1, 1, -1, vexNDS3, [3]int{16, 32, 64}},

	// EVEX.128/256/512.66.0F.W1 — packed double arithmetic.
	"VADDPD": {1, 0x58, 1, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VMULPD": {1, 0x59, 1, 1, -1, vexNDS3, [3]int{16, 32, 64}},

	// EVEX.512.66.0F3A — align (NDS + imm8).
	"VALIGND": {3, 0x03, 0, 1, -1, vexNDS3Imm, [3]int{16, 32, 64}},

	// EVEX.128/256/512.66.0F — immediate shift (VPSRAD /4).
	"VPSRAD": {1, 0x72, 0, 1, 4, vexShiftImm, [3]int{16, 32, 64}},
	// EVEX.128/256/512.66.0F.W1 — variable shift with an XMM count (VPSRAQ;
	// the W bit distinguishes it from VPSRAD's E2 form).
	"VPSRAQ": {1, 0xE2, 1, 1, -1, vexNDS3, [3]int{16, 32, 64}},

	// EVEX.128/256/512.F3.0F.W1 — signed qword to packed double (reg=dst,
	// rm=src, no vvvv).
	"VCVTQQ2PD": {1, 0xE6, 1, 2, -1, vexRM, [3]int{16, 32, 64}},
	// EVEX.128/256/512.66.0F38.W0 — sign-extend dwords to qwords; the memory
	// operand is the narrow source, so disp8×N follows its size (8/16/32 for
	// the xmm/ymm/zmm destination lengths).
	"VPMOVSXDQ": {2, 0x25, 0, 1, -1, vexRM, [3]int{8, 16, 32}},

	// EVEX.512.66.0F3A.W1 — lane extract (reg=ZMM source, rm=YMM/memory
	// destination, imm8).
	"VEXTRACTI64X4": {3, 0x3B, 1, 1, -1, vexExtract, [3]int{0, 0, 32}},
	"VEXTRACTF64X4": {3, 0x1B, 1, 1, -1, vexExtract, [3]int{0, 0, 32}},

	// EVEX.66.0F38 — more integer NDS forms (W distinguishes D/Q).
	"VPMULLD": {2, 0x40, 0, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPMULLQ": {2, 0x40, 1, 1, -1, vexNDS3, [3]int{16, 32, 64}},
	"VPERMD":  {2, 0x36, 0, 1, -1, vexNDS3, [3]int{0, 32, 64}},

	// EVEX.66.0F — immediate shift (VPSLLD /6).
	"VPSLLD": {1, 0x72, 0, 1, 6, vexShiftImm, [3]int{16, 32, 64}},

	// EVEX.F3.0F38.W0 — narrowing stores: reg = wide source, rm = narrow
	// destination (VPMOVDW dword→word, VPMOVQD qword→dword).
	"VPMOVDW": {2, 0x33, 0, 2, -1, vexRMRev, [3]int{8, 16, 32}},
	"VPMOVQD": {2, 0x35, 0, 2, -1, vexRMRev, [3]int{8, 16, 32}},
}

// evexBcastSpec describes an EVEX broadcast (VPBROADCASTD/Q): the opcode
// depends on the source kind — a GPR source uses opReg, a memory source uses
// opMem with a disp8×N of n.
type evexBcastSpec struct {
	mapSel int
	opReg  byte
	opMem  byte
	w      int
	n      int
}

var evexBcastTable = map[string]evexBcastSpec{
	// EVEX.128/256/512.66.0F38 — broadcast a dword/qword to all lanes.
	"VPBROADCASTD": {2, 0x7C, 0x58, 0, 4},
	"VPBROADCASTQ": {2, 0x7C, 0x59, 1, 8},
}

// evexMoveSpec describes an EVEX move (load and store opcodes, like the VEX
// move table).
type evexMoveSpec struct {
	mapSel int
	pp     int
	load   byte // r/m → vector
	store  byte // vector → r/m
	w      int
	n      [3]int
}

// evexMoveTable maps an upper-case EVEX move mnemonic to its encoding.
var evexMoveTable = map[string]evexMoveSpec{
	// EVEX.128/256/512.F3.0F.W0 — unaligned integer move.
	"VMOVDQU32": {1, 2, 0x6F, 0x7F, 0, [3]int{16, 32, 64}},
	// EVEX.128/256/512.F3.0F.W1 — unaligned qword move.
	"VMOVDQU64": {1, 2, 0x6F, 0x7F, 1, [3]int{16, 32, 64}},
	// EVEX.128/256/512.66.0F.W1 — unaligned packed double move.
	"VMOVUPD": {1, 1, 0x10, 0x11, 1, [3]int{16, 32, 64}},
}

// isEvex reports whether the mnemonic has an EVEX encoding we handle.
func isEvex(mnemUpper string) bool {
	if _, ok := evexTable[mnemUpper]; ok {
		return true
	}
	if _, ok := evexBcastTable[mnemUpper]; ok {
		return true
	}
	_, ok := evexMoveTable[mnemUpper]
	return ok
}

// evexRequired reports whether the operands force the EVEX encoding of a
// mnemonic that also has a VEX form: ZMM and K registers do, and so do
// register indices 16–31, which only EVEX can represent (X16–Y31 exist
// solely under AVX-512).
func evexRequired(upper string, ops []Operand) bool {
	_, inVex := vexTable[upper]
	_, inVexMove := vexMoveTable[upper]
	if !inVex && !inVexMove {
		return true // EVEX-only mnemonic
	}
	for _, op := range ops {
		if r, ok := op.(Reg); ok && (r.size == 64 || r.mask || (r.isVec() && r.idx >= 16)) {
			return true
		}
	}
	return false
}

// encodeEvex encodes an EVEX instruction with operands in Plan 9 order.
func (e *enc) encodeEvex(mnemUpper string, ops []Operand) error {
	if bs, ok := evexBcastTable[mnemUpper]; ok {
		return e.encodeEvexBcast(bs, ops)
	}
	if ms, ok := evexMoveTable[mnemUpper]; ok {
		return e.encodeEvexMove(mnemUpper, ms, ops)
	}
	spec, ok := evexTable[mnemUpper]
	if !ok {
		return fmt.Errorf("unsupported instruction %q for ZMM/K operands", mnemUpper)
	}
	switch spec.form {
	case vexNDS3:
		return e.encodeEvexNDS3(spec, ops)
	case vexRM:
		return e.encodeEvexRM(spec, ops)
	case vexRMRev:
		return e.encodeEvexRMRev(spec, ops)
	case vexShiftImm:
		return e.encodeEvexShiftImm(spec, ops)
	case vexNDS3Imm:
		return e.encodeEvexNDS3Imm(spec, ops)
	case vexExtract:
		return e.encodeEvexExtract(spec, ops)
	}
	return fmt.Errorf("unhandled EVEX form for %s", mnemUpper)
}

// encodeEvexNDS3 encodes the three-operand NDS form: OP src2, src1, dst.  The
// destination may be an opmask register (VPCMPEQD), in which case the vector
// length comes from the sources.
func (e *enc) encodeEvexNDS3(spec evexSpec, ops []Operand) error {
	if len(ops) != 3 {
		return fmt.Errorf("EVEX NDS instruction expects 3 operands, got %d", len(ops))
	}
	src2, src1, dst := ops[0], ops[1], ops[2]
	dstReg, ok := dst.(Reg)
	if !ok || (!dstReg.isVec() && !dstReg.mask) {
		return fmt.Errorf("EVEX destination must be a vector or mask register")
	}
	vvvvReg, ok := src1.(Reg)
	if !ok || !vvvvReg.isVec() {
		return fmt.Errorf("EVEX vvvv operand must be a vector register")
	}
	ll := dstReg.vecLenBit()
	if dstReg.mask {
		ll = vvvvReg.vecLenBit()
		if r, ok := src2.(Reg); ok && r.isVec() {
			ll = r.vecLenBit()
		}
	}
	return e.emitEvexFields(spec, ll, dstReg.idx, vvvvReg.idx, src2)
}

// encodeEvexRM encodes the two-operand form: OP src, dst (reg=dst, rm=src,
// no vvvv), e.g. VCVTQQ2PD.
func (e *enc) encodeEvexRM(spec evexSpec, ops []Operand) error {
	if len(ops) != 2 {
		return fmt.Errorf("EVEX two-operand instruction expects 2 operands, got %d", len(ops))
	}
	src, dst := ops[0], ops[1]
	dstReg, ok := dst.(Reg)
	if !ok || !dstReg.isVec() {
		return fmt.Errorf("EVEX destination must be a vector register")
	}
	return e.emitEvexFields(spec, dstReg.vecLenBit(), dstReg.idx, -1, src)
}

// encodeEvexShiftImm encodes an immediate shift: OP $imm, src, dst
// (ModRM.reg = /digit, vvvv = dst, rm = src, imm8), e.g. VPSRAD $31, Z3, Z5.
func (e *enc) encodeEvexShiftImm(spec evexSpec, ops []Operand) error {
	if len(ops) != 3 {
		return fmt.Errorf("EVEX shift expects 3 operands ($imm, src, dst), got %d", len(ops))
	}
	imm, src, dst := ops[0], ops[1], ops[2]
	immVal, ok := imm.(Imm)
	if !ok {
		return fmt.Errorf("shift count must be an immediate")
	}
	srcReg, ok := src.(Reg)
	if !ok || !srcReg.isVec() {
		return fmt.Errorf("shift source must be a vector register")
	}
	dstReg, ok := dst.(Reg)
	if !ok || !dstReg.isVec() {
		return fmt.Errorf("shift destination must be a vector register")
	}
	immByte, err := imm8(int64(immVal))
	if err != nil {
		return err
	}
	if err := e.emitEvexFields(spec, dstReg.vecLenBit(), spec.opdigit, dstReg.idx, srcReg); err != nil {
		return err
	}
	e.out = append(e.out, immByte)
	return nil
}

// encodeEvexNDS3Imm encodes OP $imm, src2, src1, dst (reg=dst, vvvv=src1,
// rm=src2, imm8), e.g. VALIGND.
func (e *enc) encodeEvexNDS3Imm(spec evexSpec, ops []Operand) error {
	if len(ops) != 4 {
		return fmt.Errorf("instruction expects 4 operands ($imm, src2, src1, dst), got %d", len(ops))
	}
	imm, src2, src1, dst := ops[0], ops[1], ops[2], ops[3]
	immVal, ok := imm.(Imm)
	if !ok {
		return fmt.Errorf("shuffle control must be an immediate")
	}
	dstReg, ok := dst.(Reg)
	if !ok || !dstReg.isVec() {
		return fmt.Errorf("destination must be a vector register")
	}
	vvvvReg, ok := src1.(Reg)
	if !ok || !vvvvReg.isVec() {
		return fmt.Errorf("second source must be a vector register")
	}
	immByte, err := imm8(int64(immVal))
	if err != nil {
		return err
	}
	if err := e.emitEvexFields(spec, dstReg.vecLenBit(), dstReg.idx, vvvvReg.idx, src2); err != nil {
		return err
	}
	e.out = append(e.out, immByte)
	return nil
}

// encodeEvexExtract encodes OP $imm, zsrc, ydst (reg=ZMM source, rm=YMM/memory
// destination, imm8), e.g. VEXTRACTI64X4.
func (e *enc) encodeEvexExtract(spec evexSpec, ops []Operand) error {
	if len(ops) != 3 {
		return fmt.Errorf("extract expects 3 operands ($imm, zsrc, ydst), got %d", len(ops))
	}
	imm, src, dst := ops[0], ops[1], ops[2]
	immVal, ok := imm.(Imm)
	if !ok {
		return fmt.Errorf("extract lane must be an immediate")
	}
	srcReg, ok := src.(Reg)
	if !ok || !srcReg.isVec() {
		return fmt.Errorf("extract source must be a vector register")
	}
	immByte, err := imm8(int64(immVal))
	if err != nil {
		return err
	}
	if err := e.emitEvexFields(spec, srcReg.vecLenBit(), srcReg.idx, -1, dst); err != nil {
		return err
	}
	e.out = append(e.out, immByte)
	return nil
}

// encodeEvexMove encodes a two-operand EVEX move; a vector→vector move uses
// the store-form opcode (reg = source, rm = destination), matching the Go
// assembler.
func (e *enc) encodeEvexMove(mnem string, ms evexMoveSpec, ops []Operand) error {
	if len(ops) != 2 {
		return fmt.Errorf("EVEX move expects 2 operands, got %d", len(ops))
	}
	src, dst := ops[0], ops[1]
	srcReg, srcIsVec := vecReg(src)
	dstReg, dstIsVec := vecReg(dst)

	op := ms.store
	var reg Reg
	var rm Operand
	switch {
	case srcIsVec && dstIsVec:
		reg, rm = srcReg, dst
	case srcIsVec:
		if !memOperand(dst) {
			return fmt.Errorf("%s: invalid destination operand", mnem)
		}
		reg, rm = srcReg, dst
	case dstIsVec:
		if !memOperand(src) {
			return fmt.Errorf("%s: invalid source operand", mnem)
		}
		op = ms.load
		reg, rm = dstReg, src
	default:
		return fmt.Errorf("%s needs a vector register operand", mnem)
	}
	spec := evexSpec{mapSel: ms.mapSel, opcode: op, w: ms.w, pp: ms.pp, opdigit: -1, n: ms.n}
	return e.emitEvexFields(spec, reg.vecLenBit(), reg.idx, -1, rm)
}

// memOperand reports whether op is a memory reference (including a
// static-symbol reference).
func memOperand(op Operand) bool {
	switch op.(type) {
	case Mem, sbMem:
		return true
	}
	return false
}

// encodeEvexRMRev encodes the narrowing-store form: OP src, dst with the wide
// source in the reg field and the narrow destination in r/m (VPMOVDW/QD).
func (e *enc) encodeEvexRMRev(spec evexSpec, ops []Operand) error {
	if len(ops) != 2 {
		return fmt.Errorf("EVEX store instruction expects 2 operands, got %d", len(ops))
	}
	src, dst := ops[0], ops[1]
	srcReg, ok := src.(Reg)
	if !ok || !srcReg.isVec() {
		return fmt.Errorf("EVEX source must be a vector register")
	}
	return e.emitEvexFields(spec, srcReg.vecLenBit(), srcReg.idx, -1, dst)
}

// encodeEvexBcast encodes VPBROADCASTD/Q: OP src, dst with the GPR or memory
// source broadcast to every lane of the vector destination.
func (e *enc) encodeEvexBcast(bs evexBcastSpec, ops []Operand) error {
	if len(ops) != 2 {
		return fmt.Errorf("broadcast expects 2 operands, got %d", len(ops))
	}
	src, dst := ops[0], ops[1]
	dstReg, ok := dst.(Reg)
	if !ok || !dstReg.isVec() {
		return fmt.Errorf("broadcast destination must be a vector register")
	}
	spec := evexSpec{mapSel: bs.mapSel, w: bs.w, pp: 1, opdigit: -1}
	switch src.(type) {
	case Mem, sbMem:
		spec.opcode = bs.opMem
		spec.n = [3]int{bs.n, bs.n, bs.n}
	case Reg:
		spec.opcode = bs.opReg
	default:
		return fmt.Errorf("broadcast source must be a register or memory")
	}
	return e.emitEvexFields(spec, dstReg.vecLenBit(), dstReg.idx, -1, src)
}

// emitEvexFields emits the EVEX prefix, opcode, ModR/M, SIB and displacement
// (disp8×N compressed) for the given precomputed fields.  regIdx is the
// unextended reg-field register index, or a /digit (0–7); vvvvIdx is the
// vvvv register index, or -1 when unused.
func (e *enc) emitEvexFields(spec evexSpec, ll, regIdx, vvvvIdx int, rm Operand) error {
	if ll > 2 {
		return fmt.Errorf("invalid vector length")
	}
	// reg-field extension bits (R̄, R'̄), inverted.
	rBar, rPrimeBar := 1, 1
	if regIdx&8 != 0 {
		rBar = 0
	}
	if regIdx&16 != 0 {
		rPrimeBar = 0
	}
	// vvvv (inverted) and its extension bit V'̄.
	vBar, vPrimeBar := 15, 1
	if vvvvIdx >= 0 {
		vBar = 15 - (vvvvIdx & 15)
		if vvvvIdx&16 != 0 {
			vPrimeBar = 0
		}
	}

	var modrm, sib int
	var disp []byte
	xBar, bBar := 1, 1
	var sb *sbRef
	switch r := rm.(type) {
	case Reg:
		// ModRM.mod = 11: rm[3] extends via B̄, rm[4] via X̄.
		modrm = 0xC0 | (regIdx&7)<<3 | (r.idx & 7)
		sib = -1
		if r.idx&8 != 0 {
			bBar = 0
		}
		if r.idx&16 != 0 {
			xBar = 0
		}
	case Mem:
		var err error
		modrm, sib, disp, xBar, bBar, err = memComponentsEvex(regIdx&7, r, spec.n[ll])
		if err != nil {
			return err
		}
		// An indexed memory operand carries index[4] in V'̄ (Go folds it
		// together with vvvv[4] into the same bit).
		if r.HasIndex && r.Index.idx&16 != 0 {
			vPrimeBar = 0
		}
	case sbMem:
		// RIP-relative static-symbol reference; disp32 patched at link time
		// (no disp8 scaling for RIP-relative addressing).
		modrm = (regIdx&7)<<3 | 0x05
		sib = -1
		disp = le32(0)
		sb = &sbRef{name: r.name, addend: r.addend}
	default:
		return fmt.Errorf("invalid EVEX r/m operand")
	}

	p0 := byte(rBar<<7 | xBar<<6 | bBar<<5 | rPrimeBar<<4 | spec.mapSel)
	p1 := byte(spec.w<<7 | vBar<<3 | 1<<2 | spec.pp)
	p2 := byte(ll<<5 | vPrimeBar<<3) // z = 0, b = 0, aaa = 0
	e.out = append(e.out, 0x62, p0, p1, p2, spec.opcode, byte(modrm))
	if sib >= 0 {
		e.out = append(e.out, byte(sib))
	}
	if sb != nil {
		e.patches = append(e.patches, encPatch{off: len(e.out), name: sb.name, addend: sb.addend})
	}
	e.out = append(e.out, disp...)
	return nil
}

// memComponentsEvex computes the ModR/M byte (with the given reg field), the
// SIB byte (-1 if none), the displacement bytes and the (inverted sense)
// index/base extension bits for an EVEX memory operand.  The displacement is
// compressed to disp8×N when it is a multiple of n and the quotient fits a
// signed byte; otherwise a full disp32 is used.
func memComponentsEvex(regField int, m Mem, n int) (modrm, sib int, disp []byte, xBar, bBar int, err error) {
	sib = -1
	xBar, bBar = 1, 1 // inverted bits: 1 = no extension
	if !m.HasBase && !m.HasIndex {
		return regField<<3 | 0x05, -1, le32(m.Disp), 1, 1, nil // RIP-relative
	}

	needSIB := m.HasIndex || (m.HasBase && m.Base.idx&7 == 4)

	var mod int
	switch {
	case !m.HasBase:
		mod = 0
		disp = le32(m.Disp)
	case m.Base.idx&7 == 5 && m.Disp == 0:
		mod = 1
		disp = []byte{0}
	case m.Disp == 0:
		mod = 0
	case n > 0 && m.Disp%int64(n) == 0 && m.Disp/int64(n) >= -128 && m.Disp/int64(n) <= 127:
		mod = 1
		disp = []byte{byte(int8(m.Disp / int64(n)))}
	default:
		mod = 2
		disp = le32(m.Disp)
	}

	if needSIB {
		idxField := 4 // 100 = no index
		if m.HasIndex {
			idxField = m.Index.idx & 7
			if m.Index.idx&8 != 0 {
				xBar = 0
			}
		}
		baseField := 5 // 101 = no base (with mod=00 → disp32)
		if m.HasBase {
			baseField = m.Base.idx & 7
			if m.Base.idx&8 != 0 {
				bBar = 0
			}
		}
		return mod<<6 | regField<<3 | 0x04, scaleBits(m.Scale)<<6 | idxField<<3 | baseField, disp, xBar, bBar, nil
	}

	if m.Base.idx&8 != 0 {
		bBar = 0
	}
	return mod<<6 | regField<<3 | (m.Base.idx & 7), -1, disp, 1, bBar, nil
}

// encodeKmovw encodes KMOVW, whose opcode depends on the operand direction:
// 90 (k/mem → K), 91 (K → mem), 92 (GPR → K), 93 (K → GPR); k → k uses 90.
func (e *enc) encodeKmovw(ops []Operand) error {
	if len(ops) != 2 {
		return fmt.Errorf("KMOVW expects 2 operands, got %d", len(ops))
	}
	src, dst := ops[0], ops[1]
	srcReg, srcIsReg := src.(Reg)
	dstReg, dstIsReg := dst.(Reg)
	srcK := srcIsReg && srcReg.mask
	dstK := dstIsReg && dstReg.mask
	spec := vexSpec{mapSel: 1, w: 0, pp: 0, opdigit: -1}
	switch {
	case srcK && dstK:
		spec.opcode = 0x90 // k ← k: reg = dst, rm = src
		return e.emitVexFields(spec, 0, dstReg.idx&7, 0, 15, src)
	case srcK && dstIsReg:
		spec.opcode = 0x93 // GPR ← k: reg = dst, rm = src
		rBit := 0
		if dstReg.idx >= 8 {
			rBit = 1
		}
		return e.emitVexFields(spec, 0, dstReg.idx&7, rBit, 15, src)
	case srcK:
		if _, ok := dst.(Mem); !ok {
			return fmt.Errorf("KMOVW: invalid destination operand")
		}
		spec.opcode = 0x91 // mem ← k: reg = src, rm = dst
		return e.emitVexFields(spec, 0, srcReg.idx&7, 0, 15, dst)
	case dstK:
		spec.opcode = 0x92 // k ← GPR/mem: reg = dst, rm = src
		return e.emitVexFields(spec, 0, dstReg.idx&7, 0, 15, src)
	}
	return fmt.Errorf("KMOVW requires a K register operand")
}