gasm-sdk/asm/amd64_sse.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 the amd64 SIMD pieces the main tables lack: the MMX
// bank glue (EMMS, the masked stores, the MMX shifts and shuffle, the
// byte-mask extract), the MOVQ bank crossings' odd spellings and the leaf
// aliases.  Every encoding here is pinned byte for byte against go tool asm
// through the corpus lines in amd64_sse_test.go.

// sseMaskmov maps the masked cache-line stores to their prefix and opcode:
// OP src, dst with the second operand in the reg field, no memory operand.
var sseMaskmov = map[string]sseBin{
	"MASKMOVQ":  {0, 0xF7, false},
	"MASKMOVOU": {0x66, 0xF7, false},
}

// sseMoreBin maps the packed and scalar legacy binaries the main table
// lacks, reg = destination and r/m = source: the float comparisons, square
// roots and reciprocal estimates, the SSE3 horizontal arithmetic, the SSE4.1
// packed integers and the SSSE3 sign and horizontal ops.  The MMX twins of
// the 0x66-prefixed members drop the prefix in the shared encoder.
var sseMoreBin = map[string]sseBin{
	"COMISS":     {0, 0x2F, false},
	"UCOMISS":    {0, 0x2E, false},
	"UCOMISD":    {0x66, 0x2E, false},
	"SQRTPS":     {0, 0x51, false},
	"SQRTPD":     {0x66, 0x51, false},
	"SQRTSS":     {0xF3, 0x51, false},
	"RCPPS":      {0, 0x53, false},
	"RCPSS":      {0xF3, 0x53, false},
	"RSQRTPS":    {0, 0x52, false},
	"RSQRTSS":    {0xF3, 0x52, false},
	"ADDSUBPD":   {0x66, 0xD0, false},
	"ADDSUBPS":   {0xF2, 0xD0, false},
	"HADDPD":     {0x66, 0x7C, false},
	"HADDPS":     {0xF2, 0x7C, false},
	"HSUBPD":     {0x66, 0x7D, false},
	"HSUBPS":     {0xF2, 0x7D, false},
	"MOVDDUP":    {0xF2, 0x12, false},
	"MOVSHDUP":   {0xF3, 0x16, false},
	"MOVSLDUP":   {0xF3, 0x12, false},
	"LDDQU":      {0xF2, 0xF0, false},
	"MOVNTDQA":   {0x66, 0x2A, true},
	"PTEST":      {0x66, 0x17, true},
	"PABSB":      {0x66, 0x1C, true},
	"PABSW":      {0x66, 0x1D, true},
	"PABSD":      {0x66, 0x1E, true},
	"PACKSSWB":   {0x66, 0x63, false},
	"PACKUSWB":   {0x66, 0x67, false},
	"PACKSSLW":   {0x66, 0x6B, false},
	"PACKUSDW":   {0x66, 0x2B, true},
	"PADDSB":     {0x66, 0xEC, false},
	"PADDSW":     {0x66, 0xED, false},
	"PADDUSB":    {0x66, 0xDC, false},
	"PADDUSW":    {0x66, 0xDD, false},
	"PAVGB":      {0x66, 0xE0, false},
	"PAVGW":      {0x66, 0xE3, false},
	"PCMPEQQ":    {0x66, 0x29, true},
	"PCMPGTQ":    {0x66, 0x37, true},
	"PHADDW":     {0x66, 0x01, true},
	"PHADDD":     {0x66, 0x02, true},
	"PHADDSW":    {0x66, 0x03, true},
	"PHSUBW":     {0x66, 0x05, true},
	"PHSUBD":     {0x66, 0x06, true},
	"PHSUBSW":    {0x66, 0x07, true},
	"PHMINPOSUW": {0x66, 0x41, true},
	"PMADDUBSW":  {0x66, 0x04, true},
	"PMADDWL":    {0x66, 0xF5, false},
	"PMAXSB":     {0x66, 0x3C, true},
	"PMAXSD":     {0x66, 0x3D, true},
	"PMAXSW":     {0x66, 0xEE, false},
	"PMAXUB":     {0x66, 0xDE, false},
	"PMAXUD":     {0x66, 0x3F, true},
	"PMAXUW":     {0x66, 0x3E, true},
	"PMINSB":     {0x66, 0x38, true},
	"PMINSD":     {0x66, 0x39, true},
	"PMINSW":     {0x66, 0xEA, false},
	"PMINUB":     {0x66, 0xDA, false},
	"PMINUD":     {0x66, 0x3B, true},
	"PMINUW":     {0x66, 0x3A, true},
	"PMULDQ":     {0x66, 0x28, true},
	"PMULLD":     {0x66, 0x40, true},
	"PMULHRSW":   {0x66, 0x0B, true},
	"PMULHUW":    {0x66, 0xE4, false},
	"PMULHW":     {0x66, 0xE5, false},
	"PMULLW":     {0x66, 0xD5, false},
	"PMULULQ":    {0x66, 0xF4, false},
	"PCMPGTL":    {0x66, 0x66, false},
	"PMOVSXBD":   {0x66, 0x21, true},
	"PMOVSXBQ":   {0x66, 0x22, true},
	"PMOVSXBW":   {0x66, 0x20, true},
	"PMOVSXDQ":   {0x66, 0x25, true},
	"PMOVSXWD":   {0x66, 0x23, true},
	"PMOVSXWQ":   {0x66, 0x24, true},
	"PMOVZXBD":   {0x66, 0x31, true},
	"PMOVZXBQ":   {0x66, 0x32, true},
	"PMOVZXBW":   {0x66, 0x30, true},
	"PMOVZXDQ":   {0x66, 0x35, true},
	"PMOVZXWD":   {0x66, 0x33, true},
	"PMOVZXWQ":   {0x66, 0x34, true},
	// The conversion aliases the Plan 9 table spells with an L: the dword
	// sources and destinations of the packed integer/float converts.
	"CVTPL2PD":   {0xF3, 0xE6, false},
	"CVTPL2PS":   {0, 0x5B, false},
	"CVTPD2PL":   {0xF2, 0xE6, false},
	"CVTPS2PL":   {0x66, 0x5B, false},
	"CVTTPD2PL":  {0x66, 0xE6, false},
	"CVTTPS2PL":  {0xF3, 0x5B, false},
	"PSADBW":     {0x66, 0xF6, false},
	"PSUBSB":     {0x66, 0xE8, false},
	"PSUBSW":     {0x66, 0xE9, false},
	"PSUBUSB":    {0x66, 0xD8, false},
	"PSUBUSW":    {0x66, 0xD9, false},
	"PSIGNB":     {0x66, 0x08, true},
	"PSIGNW":     {0x66, 0x09, true},
	"PSIGND":     {0x66, 0x0A, true},
	"PUNPCKHBW":  {0x66, 0x68, false},
	"PUNPCKHLQ":  {0x66, 0x6A, false},
	"PUNPCKHQDQ": {0x66, 0x6D, false},
	"PUNPCKHWL":  {0x66, 0x69, false},
	"PUNPCKLLQ":  {0x66, 0x62, false},
	"PUNPCKLQDQ": {0x66, 0x6C, false},
	"PUNPCKLWL":  {0x66, 0x61, false},
}

// sseMoreImm3 maps the imm8-controlled three-operand instructions the main
// table lacks: OP $imm, src, dst.
var sseMoreImm3 = map[string]sseImm3{
	"ROUNDPS":   {0x66, 0x08, true},
	"ROUNDPD":   {0x66, 0x09, true},
	"ROUNDSS":   {0x66, 0x0A, true},
	"ROUNDSD":   {0x66, 0x0B, true},
	"DPPS":      {0x66, 0x40, true},
	"DPPD":      {0x66, 0x41, true},
	"BLENDPS":   {0x66, 0x0C, true},
	"BLENDPD":   {0x66, 0x0D, true},
	"INSERTPS":  {0x66, 0x21, true},
	"MPSADBW":   {0x66, 0x42, true},
	"PCMPESTRM": {0x66, 0x60, true},
	"PCMPESTRI": {0x66, 0x61, true},
	"PCMPISTRM": {0x66, 0x62, true},
	"PCMPISTRI": {0x66, 0x63, true},
}

// sseBlendv maps the variable blends whose implicit mask is X0: the first
// operand must be the literal X0, the register the hardware reads.
var sseBlendv = map[string]sseBin{
	"BLENDVPS": {0x66, 0x14, true},
	"BLENDVPD": {0x66, 0x15, true},
	"PBLENDVB": {0x66, 0x10, true},
}

// sseHighLow maps the high/low half moves to their load/store opcode pair.
// A memory source loads (reg = destination), a memory destination stores
// (reg = the register source).
var sseHighLow = map[string]sseMove{
	"MOVHPD": {0x66, 0x16, 0x17},
	"MOVHPS": {0, 0x16, 0x17},
	"MOVLPD": {0x66, 0x12, 0x13},
	"MOVLPS": {0, 0x12, 0x13},
}

// sseRegReg maps the register-to-register half moves, register destination
// and register source alone: MOVHLPS and MOVLHPS.
var sseRegReg = map[string]sseBin{
	"MOVHLPS": {0, 0x12, false},
	"MOVLHPS": {0, 0x16, false},
}

// sseMovmsk maps the sign-mask extractions to a GPR: OP vec, gpr.
var sseMovmsk = map[string]sseBin{
	"MOVMSKPS": {0, 0x50, false},
	"MOVMSKPD": {0x66, 0x50, false},
}

// sseMovnt maps the non-temporal stores, OP reg, mem, plus MOVNTDQA's load
// (which rides sseMoreBin).
var sseMovnt = map[string]sseBin{
	"MOVNTPS": {0, 0x2B, false},
	"MOVNTPD": {0x66, 0x2B, false},
	"MOVNTQ":  {0, 0xE7, false},
	"MOVNTO":  {0x66, 0xE7, false},
	"MOVNTIL": {0, 0xC3, false},
	"MOVNTIQ": {0, 0xC3, false},
}

// mmxShiftImm lists the packed integer shifts whose immediate form the MMX
// bank spells without the 0x66 prefix; the digit rides the 0F 71/72/73
// group, the same /digits the XMM forms carry.
var mmxShiftImm = map[string]sseShift{
	"PSLLW": {0x71, 6},
	"PSRLW": {0x71, 2},
	"PSRAW": {0x71, 4},
	"PSLLL": {0x72, 6},
	"PSRLL": {0x72, 2},
	"PSRAL": {0x72, 4},
	"PSLLQ": {0x73, 6},
	"PSRLQ": {0x73, 2},
}

// mmxShiftVar lists the variable-count forms over the MMX bank, the same
// opcodes the XMM variable shifts ride, prefix dropped.
var mmxShiftVar = map[string]byte{
	"PSLLW": 0xF1,
	"PSRLW": 0xD1,
	"PSRAW": 0xE1,
	"PSLLL": 0xF2,
	"PSRLL": 0xD2,
	"PSRAL": 0xE2,
	"PSLLQ": 0xF3,
	"PSRLQ": 0xD3,
}

// sseOctaShift lists the octa byte shifts' extra spellings: PSLLO and PSRLO
// are the Plan 9 names of PSLLDQ/PSRLDQ, XMM only.
var sseOctaShift = map[string]sseShift{
	"PSLLO": {0x73, 7},
	"PSRLO": {0x73, 3},
}

// isMmx reports whether the operand is an MMX register.
func isMmx(op Operand) bool {
	r, ok := op.(Reg)
	return ok && r.mmx
}

// encodeSSEMore encodes the MMX glue and the leaf SIMD spellings.  It
// reports whether the mnemonic belongs to the layer; a false result hands
// the mnemonic back to the caller.
func (e *enc) encodeSSEMore(upper string, ops []Operand) (bool, error) {
	if upper == "EMMS" {
		if len(ops) != 0 {
			return true, fmt.Errorf("EMMS takes no operands, got %d", len(ops))
		}
		return true, e.emit(&instr{opcode: []byte{0x0F, 0x77}, modrm: -1, sib: -1})
	}
	if m, ok := sseMaskmov[upper]; ok {
		if len(ops) != 2 {
			return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
		}
		srcReg, ok1 := ops[0].(Reg)
		dstReg, ok2 := ops[1].(Reg)
		if !ok1 || !ok2 || !srcReg.isVec() && !srcReg.mmx || !dstReg.isVec() && !dstReg.mmx {
			return true, fmt.Errorf("%s takes two vector or MMX registers", upper)
		}
		i := &instr{prefix: m.prefix, opcode: []byte{0x0F, m.op}, modrm: -1, sib: -1}
		if err := setRM(i, dstReg, srcReg, 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	// The packed shifts over the MMX bank drop the 0x66 prefix the XMM forms
	// carry; only a shift name enters the MMX path, the XMM spellings of the
	// shifts and every other packed binary fall through to the main tables.
	if _, isShift := mmxShiftImm[upper]; isShift {
		if e.encodeMmxShiftGate(ops) {
			return e.encodeMmxShift(upper, ops)
		}
	} else if _, isVar := mmxShiftVar[upper]; isVar {
		if e.encodeMmxShiftGate(ops) {
			return e.encodeMmxShift(upper, ops)
		}
	}
	// PSHUFW is the MMX word shuffle, 0F 70 with no prefix: the XMM twins
	// (PSHUFD and friends) dispatch through the main shuffle table.
	if upper == "PSHUFW" {
		if len(ops) != 3 {
			return true, fmt.Errorf("PSHUFW expects 3 operands ($imm, src, dst), got %d", len(ops))
		}
		imm, ok := ops[0].(Imm)
		if !ok {
			return true, fmt.Errorf("PSHUFW needs an imm8 first operand")
		}
		immByte, err := imm8(int64(imm))
		if err != nil {
			return true, err
		}
		dstReg, ok2 := ops[2].(Reg)
		if !ok2 || !dstReg.mmx {
			return true, fmt.Errorf("PSHUFW destination must be an MMX register")
		}
		i := &instr{opcode: []byte{0x0F, 0x70}, modrm: -1, sib: -1}
		if err := setRM(i, dstReg, ops[1], 8); err != nil {
			return true, err
		}
		i.imm = []byte{immByte}
		return true, e.emit(i)
	}
	if spec, ok := sseOctaShift[upper]; ok {
		return e.encodeMmxShiftForm(upper, spec, 0x66, ops)
	}
	// The byte-mask extract over the MMX bank rides the same 0F D7 opcode
	// without the prefix; the XMM spelling falls through.
	if upper == "PMOVMSKB" && len(ops) == 2 {
		if srcReg, ok := ops[0].(Reg); ok && srcReg.mmx {
			dstReg, ok2 := ops[1].(Reg)
			if !ok2 || dstReg.isVec() || dstReg.mmx {
				return true, fmt.Errorf("%s destination must be a general register", upper)
			}
			i := newInstr(4, []byte{0x0F, 0xD7})
			if err := setRM(i, dstReg, srcReg, 4); err != nil {
				return true, err
			}
			return true, e.emit(i)
		}
	}
	// MOVQOZX is the octa-to-quad zero-extend load, F3 0F D6: an MMX or
	// memory source into an XMM destination, the MOVQ2DQ opcode.
	if upper == "MOVQOZX" {
		if len(ops) != 2 {
			return true, fmt.Errorf("MOVQOZX expects 2 operands, got %d", len(ops))
		}
		dstReg, ok := ops[1].(Reg)
		if !ok || !dstReg.isVec() {
			return true, fmt.Errorf("MOVQOZX destination must be an XMM register")
		}
		switch ops[0].(type) {
		case Reg, Mem, sbMem:
		default:
			return true, fmt.Errorf("MOVQOZX source must be an MMX register or memory")
		}
		i := &instr{prefix: 0xF3, opcode: []byte{0x0F, 0xD6}, modrm: -1, sib: -1}
		if err := setRM(i, dstReg, ops[0], 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	// MOVZWW and MOVSWW are the word zero/sign-extend moves under aliases:
	// the MOVWLZX and MOVWLSX opcodes carrying the word width's 0x66 prefix.
	switch upper {
	case "MOVZWW", "MOVSWW":
		op := []byte{0x0F, 0xB7}
		if upper == "MOVSWW" {
			op[1] = 0xBF
		}
		if len(ops) != 2 {
			return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
		}
		dstReg, ok := ops[1].(Reg)
		if !ok {
			return true, fmt.Errorf("%s destination must be a register", upper)
		}
		i := newInstr(2, op)
		if err := setRM(i, dstReg, ops[0], 2); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	// The packed and scalar binaries: reg = destination, r/m = source, the
	// shared encoder carrying the MMX prefix drop.
	if m, ok := sseMoreBin[upper]; ok {
		return true, e.encodeSSEBin(m, ops)
	}
	// The imm8-controlled instructions: OP $imm, src, dst.
	if m, ok := sseMoreImm3[upper]; ok {
		return true, e.encodeSSEImm3(m, ops)
	}
	// The variable blends with their implicit X0 mask: the first operand is
	// the literal X0, the register the encoding leaves out.
	if m, ok := sseBlendv[upper]; ok {
		if len(ops) != 3 {
			return true, fmt.Errorf("%s expects 3 operands (X0, src, dst), got %d", upper, len(ops))
		}
		x0, ok := ops[0].(Reg)
		if !ok || !x0.isVec() || x0.idx != 0 || x0.size != 16 {
			return true, fmt.Errorf("%s first operand must be X0", upper)
		}
		return true, e.encodeSSEBin(m, ops[1:])
	}
	// The high/low half moves split by direction: a memory source loads, a
	// memory destination stores, both two-operand.
	if m, ok := sseHighLow[upper]; ok {
		if len(ops) != 2 {
			return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
		}
		srcReg, srcVec := vecReg(ops[0])
		dstReg, dstVec := vecReg(ops[1])
		var op byte
		var reg Reg
		var rm Operand
		switch {
		case srcVec && isX86Mem(ops[1]):
			op, reg, rm = m.store, srcReg, ops[1]
		case dstVec && isX86Mem(ops[0]):
			op, reg, rm = m.load, dstReg, ops[0]
		default:
			return true, fmt.Errorf("%s takes one vector register and one memory operand", upper)
		}
		i := &instr{prefix: m.prefix, opcode: []byte{0x0F, op}, modrm: -1, sib: -1}
		if err := setRM(i, reg, rm, 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	// The register-to-register half moves.
	if m, ok := sseRegReg[upper]; ok {
		if len(ops) != 2 {
			return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
		}
		srcReg, srcVec := vecReg(ops[0])
		dstReg, dstVec := vecReg(ops[1])
		if !srcVec || !dstVec {
			return true, fmt.Errorf("%s takes two XMM registers", upper)
		}
		i := &instr{opcode: []byte{0x0F, m.op}, modrm: -1, sib: -1}
		if err := setRM(i, dstReg, srcReg, 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	// The sign-mask extractions: the vector source's sign bits pack into a
	// general register.
	if m, ok := sseMovmsk[upper]; ok {
		if len(ops) != 2 {
			return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
		}
		srcReg, srcVec := vecReg(ops[0])
		dstReg, ok := ops[1].(Reg)
		if !srcVec || !ok || dstReg.isVec() || dstReg.mmx {
			return true, fmt.Errorf("%s takes a vector register and a general register", upper)
		}
		i := &instr{prefix: m.prefix, opcode: []byte{0x0F, m.op}, modrm: -1, sib: -1}
		if err := setRM(i, dstReg, srcReg, 4); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	// The non-temporal stores: the register source rides reg, the memory
	// destination r/m; MOVNTIL/IQ store from a general register and MOVNTIQ
	// carries REX.W.
	if m, ok := sseMovnt[upper]; ok {
		if len(ops) != 2 {
			return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
		}
		var srcReg Reg
		switch r := ops[0].(type) {
		case Reg:
			if upper == "MOVNTIL" || upper == "MOVNTIQ" {
				if r.isVec() || r.mmx {
					return true, fmt.Errorf("%s source must be a general register", upper)
				}
				srcReg = r
			} else {
				if !r.isVec() && !r.mmx {
					return true, fmt.Errorf("%s source must be a vector or MMX register", upper)
				}
				srcReg = r
			}
		default:
			return true, fmt.Errorf("%s source must be a register", upper)
		}
		if !isX86Mem(ops[1]) {
			return true, fmt.Errorf("%s destination must be memory", upper)
		}
		i := &instr{prefix: m.prefix, opcode: []byte{0x0F, m.op}, modrm: -1, sib: -1, rexW: upper == "MOVNTIQ"}
		if err := setRM(i, srcReg, ops[1], 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	// EXTRACTPS is the lane extract to a GPR or memory, the PEXTR layout.
	if upper == "EXTRACTPS" {
		return true, e.encodeSSEExtract(sseExtract{op: []byte{0x0F, 0x3A, 0x17}}, ops)
	}
	// CVTSL2SS and CVTSQ2SS are the integer-to-scalar-single converts, the
	// CVTSL2SD pair's F3 twin: F3 0F 2A with reg = XMM destination, REX.W
	// on the quad source spelling.
	if upper == "CVTSL2SS" || upper == "CVTSQ2SS" {
		if len(ops) != 2 {
			return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
		}
		dstReg, ok := ops[1].(Reg)
		if !ok || !dstReg.isVec() {
			return true, fmt.Errorf("%s destination must be a vector register", upper)
		}
		i := newInstr(0, []byte{0x0F, 0x2A})
		i.rexW = upper == "CVTSQ2SS"
		i.prefix = 0xF3
		if err := setRM(i, dstReg, ops[0], 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	return false, nil
}

// encodeMmxShiftGate reports whether the shift's destination operand is an
// MMX register, the case the bank's own prefix-free forms cover.
func (e *enc) encodeMmxShiftGate(ops []Operand) bool {
	if len(ops) != 2 {
		return false
	}
	dstReg, ok := ops[1].(Reg)
	return ok && dstReg.mmx
}

// encodeMmxShift routes the MMX shift between its immediate form
// (OP $imm, dst, the 0F 71/72/73 /digit group) and its variable-count form
// (OP count, dst, the 0F D1-F3 row).
func (e *enc) encodeMmxShift(upper string, ops []Operand) (bool, error) {
	if spec, ok := mmxShiftImm[upper]; ok {
		if _, isImm := ops[0].(Imm); isImm {
			if len(ops) != 2 {
				return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
			}
			immByte, err := imm8(int64(ops[0].(Imm)))
			if err != nil {
				return true, err
			}
			i := &instr{opcode: []byte{0x0F, spec.op}, modrm: -1, sib: -1}
			if err := setRMDigit(i, spec.digit, ops[1], 8); err != nil {
				return true, err
			}
			i.imm = []byte{immByte}
			return true, e.emit(i)
		}
	}
	if op, ok := mmxShiftVar[upper]; ok {
		if len(ops) != 2 {
			return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
		}
		if !vecOrMem(ops[0]) && !isMmx(ops[0]) {
			return true, fmt.Errorf("%s count must be an immediate, an MMX register or memory", upper)
		}
		dstReg, ok := ops[1].(Reg)
		if !ok || !dstReg.mmx {
			return true, fmt.Errorf("%s destination must be an MMX register", upper)
		}
		i := &instr{opcode: []byte{0x0F, op}, modrm: -1, sib: -1}
		if err := setRM(i, dstReg, ops[0], 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	return true, fmt.Errorf("unsupported instruction %q", upper)
}

// encodeMmxShiftForm emits one XMM octa shift: OP $imm, dst, the 0x66
// prefix carried.
func (e *enc) encodeMmxShiftForm(upper string, spec sseShift, prefix byte, ops []Operand) (bool, error) {
	if len(ops) != 2 {
		return true, fmt.Errorf("%s expects 2 operands, got %d", upper, len(ops))
	}
	imm, ok := ops[0].(Imm)
	if !ok {
		return true, fmt.Errorf("%s needs an immediate count", upper)
	}
	immByte, err := imm8(int64(imm))
	if err != nil {
		return true, err
	}
	dstReg, ok2 := ops[1].(Reg)
	if !ok2 || !dstReg.isVec() {
		return true, fmt.Errorf("%s destination must be the second, vector operand", upper)
	}
	i := &instr{prefix: prefix, opcode: []byte{0x0F, spec.op}, modrm: -1, sib: -1}
	if err := setRMDigit(i, spec.digit, dstReg, 8); err != nil {
		return true, err
	}
	i.imm = []byte{immByte}
	return true, e.emit(i)
}