gasm-sdk/asm/arm64_assemble.go

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

package asm

import (
	"fmt"
	"strings"

	"sourcedock.dev/petrbalvin/gasm-devkit/ast"
)

// assembleARM64 assembles an AArch64 (arm64) TEXT function body into machine
// code.  Every instruction is 4 bytes; the MOV pseudo-instruction and the
// immediate-arithmetic forms expand to 2–4 instructions when the immediate
// does not fit, so the layout is computed in two passes (sizes, then encoding
// with resolved branch targets).
//
// The emitted bytes match the Go toolchain's arm64 assembler, which is the
// ground-truth oracle: prologue/epilogue, FP/SP frame mapping, branch
// encodings and the MOV immediate expansions all follow cmd/internal/obj/
// arm64's asmout cases.
func assembleARM64(t *ast.Text) ([]byte, map[string]int, []Reloc, []LineEntry, []SpadjStep, error) {
	fi := arm64ComputeFrame(t)
	prologue := arm64Prologue(fi)
	chain := arm64JumpChain(t)
	resolve := func(name string) string {
		if r, ok := chain[name]; ok {
			return r
		}
		return name
	}

	var relocs []Reloc
	var spadj []SpadjStep

	// The prologue (3 instructions when a small frame, 4 for large)
	// raises the SP delta by autosize.
	if fi.autosize != 0 {
		spadj = append(spadj, SpadjStep{PC: arm64PrologueSpadjPC(fi), Value: fi.autosize})
	}

	// Pass 1: label offsets from the instruction sizes.
	offsets := map[string]int{}
	pos := len(prologue)
	for _, stmt := range t.Body {
		switch s := stmt.(type) {
		case *ast.Label:
			offsets[s.Name.Text] = pos
		case *ast.Instr:
			pos += arm64InstrSize(s, fi)
		}
	}

	// Pass 2: encode.  Relocation offsets are recorded function-relative.
	out := append([]byte(nil), prologue...)
	pc := len(prologue)
	preCount := len(relocs)
	var lines []LineEntry
	for _, stmt := range t.Body {
		in, ok := stmt.(*ast.Instr)
		if !ok {
			continue
		}
		code, err := encodeARM64Instr(in, pc, offsets, fi, &relocs, resolve)
		if err != nil {
			return nil, nil, nil, nil, nil, fmt.Errorf("%s: %w", in.Mnemonic.Text, err)
		}
		for j := preCount; j < len(relocs); j++ {
			relocs[j].Off += pc - len(prologue)
		}
		preCount = len(relocs)
		lines = append(lines, LineEntry{Offset: pc, Line: in.Pos().Line})
		// The RET's epilogue closes the frame: the SP delta returns to zero.
		if strings.ToUpper(in.Mnemonic.Text) == "RET" && fi.autosize != 0 {
			epi := arm64ReturnEpilogueLen(fi)
			spadj = append(spadj, SpadjStep{PC: pc + epi, Value: 0})
		}
		out = append(out, code...)
		pc += len(code)
	}
	return out, offsets, relocs, lines, spadj, nil
}

// arm64JumpChain precomputes jump-to-jump folding: a label whose first
// instruction is an unconditional local jump redirects its own jumpers to
// the ultimate target.  The Go toolchain chases these chains before it
// encodes branches, so matching its bytes requires the same redirection.
func arm64JumpChain(t *ast.Text) map[string]string {
	leadsTo := map[string]string{}
	for i, stmt := range t.Body {
		l, ok := stmt.(*ast.Label)
		if !ok {
			continue
		}
		j := i + 1
		for j < len(t.Body) {
			if _, isLabel := t.Body[j].(*ast.Label); !isLabel {
				break
			}
			j++
		}
		if j >= len(t.Body) {
			continue
		}
		in, ok := t.Body[j].(*ast.Instr)
		if !ok {
			continue
		}
		mnem := strings.ToUpper(in.Mnemonic.Text)
		if (mnem != "JMP" && mnem != "B") || len(in.Operands) != 1 {
			continue
		}
		if name, ok := arm64LabelOK(in.Operands[0]); ok {
			leadsTo[l.Name.Text] = name
		}
	}
	chain := map[string]string{}
	for name := range leadsTo {
		visited := map[string]bool{name: true}
		cur := name
		for {
			next, ok := leadsTo[cur]
			if !ok || visited[next] {
				break
			}
			visited[next] = true
			cur = next
		}
		if cur != name {
			chain[name] = cur
		}
	}
	return chain
}

// arm64LabelOK returns the local label name of a jump operand.
func arm64LabelOK(op *ast.Operand) (string, bool) {
	if op.Kind == ast.OpAddr && op.Addr.Sym != nil && op.Addr.Sym.Pseudo == "" &&
		op.Addr.Base == "" && op.Addr.Sym.Name != "" {
		return op.Addr.Sym.Name, true
	}
	return "", false
}

// arm64InstrSize returns the encoded size of an instruction: 4 bytes for
// most, more for the multi-instruction expansions.
func arm64InstrSize(instr *ast.Instr, fi arm64FrameInfo) int {
	mnem := strings.ToUpper(instr.Mnemonic.Text)
	ops := instr.Operands

	if mnem == "RET" {
		return len(arm64Return(fi))
	}
	switch mnem {
	case "MOV", "MOVD", "MOVW", "MOVWU", "MOVH", "MOVHU", "MOVB", "MOVBU",
		"FMOVS", "FMOVD":
		return arm64MovSize(mnem, ops, fi)
	case "ADD", "ADDW", "SUB", "SUBW", "AND", "ANDW", "ORR", "ORRW", "EOR", "EORW":
		if len(ops) >= 2 && isImmOperand(ops[0]) {
			v := immFromOperand(ops[0])
			// Small immediate (0..4095 or -2048..-1) fits in one instruction.
			if v >= 0 && v <= 0xFFF {
				return 4
			}
			if v >= -2048 && v < 0 {
				return 4
			}
			// Larger immediates need MOV materialisation + op.
			return 8
		}
	}
	return 4
}

// encodeARM64Instr encodes a single AArch64 instruction.
func encodeARM64Instr(instr *ast.Instr, pc int, offsets map[string]int, fi arm64FrameInfo, relocs *[]Reloc, resolve func(string) string) ([]byte, error) {
	mnem := strings.ToUpper(instr.Mnemonic.Text)
	ops := instr.Operands

	// Pseudo-instructions and special cases first.
	switch mnem {
	case "RET":
		return arm64Return(fi), nil
	case "NOP", "NOOP":
		return a64wordLE(a64NOP), nil
	case "UNDEF":
		return a64wordLE(a64BRK(0)), nil
	case "WORD":
		if len(ops) != 1 {
			return nil, fmt.Errorf("WORD expects 1 operand, got %d", len(ops))
		}
		return a64wordLE(uint32(immFromOperand(ops[0]))), nil
	case "B":
		return encodeARM64Branch(mnem, ops, pc, offsets, false, resolve)
	case "BL", "CALL":
		return encodeARM64Branch(mnem, ops, pc, offsets, true, resolve)
	case "MOV", "MOVD", "MOVW", "MOVWU", "MOVH", "MOVHU", "MOVB", "MOVBU",
		"FMOVS", "FMOVD":
		return encodeARM64Mov(instr, mnem, fi, relocs)
	}

	// Conditional branches (BEQ, BNE, BGE, BLT, BGT, BLE, etc.).
	if enc, ok := a64InstrTable[mnem]; ok && enc.format == a64FBranchCond {
		return encodeARM64BranchCond(mnem, enc.op, ops, pc, offsets, resolve)
	}

	// ADD/SUB immediate.
	if mnem == "ADD" || mnem == "ADDW" || mnem == "SUB" || mnem == "SUBW" ||
		mnem == "CMP" || mnem == "CMPW" || mnem == "CMN" || mnem == "CMNW" {
		if len(ops) >= 2 && isImmOperand(ops[0]) {
			return encodeARM64AddSubImm(mnem, ops)
		}
	}

	// Register-register data processing.
	if enc, ok := a64InstrTable[mnem]; ok && enc.format == a64FDPSR {
		return encodeARM64DPSR(mnem, enc.op, ops)
	}

	return nil, fmt.Errorf("unsupported arm64 instruction %q", mnem)
}

// ---- branch encoding ----

// encodeARM64Branch encodes an unconditional branch (B/BL) to a label.
func encodeARM64Branch(mnem string, ops []*ast.Operand, pc int, offsets map[string]int, link bool, resolve func(string) string) ([]byte, error) {
	if len(ops) != 1 {
		return nil, fmt.Errorf("%s expects 1 operand, got %d", mnem, len(ops))
	}
	target := resolve(arm64Label(ops[0]))
	targetOff, ok := offsets[target]
	if !ok {
		return nil, fmt.Errorf("undefined label %q", target)
	}
	// Branch offset in bytes, shifted right by 2 (instructions are 4-byte aligned).
	rel := (targetOff - pc) >> 2
	if rel < -(1<<25) || rel >= (1<<25) {
		return nil, fmt.Errorf("branch to %q too far (26-bit range)", target)
	}
	op := uint32(0) // B
	if link {
		op = 1 // BL
	}
	return a64wordLE(a64Branch(op, int32(rel))), nil
}

// encodeARM64BranchCond encodes a conditional branch (B.cond) to a label.
func encodeARM64BranchCond(mnem string, baseOp uint32, ops []*ast.Operand, pc int, offsets map[string]int, resolve func(string) string) ([]byte, error) {
	if len(ops) != 1 {
		return nil, fmt.Errorf("%s expects 1 operand, got %d", mnem, len(ops))
	}
	target := resolve(arm64Label(ops[0]))
	targetOff, ok := offsets[target]
	if !ok {
		return nil, fmt.Errorf("undefined label %q", target)
	}
	rel := (targetOff - pc) >> 2
	if rel < -(1<<18) || rel >= (1<<18) {
		return nil, fmt.Errorf("branch to %q too far (19-bit range)", target)
	}
	// The condition code is in the low 4 bits of baseOp.
	cond := baseOp & 0xF
	return a64wordLE(a64BranchCond(int32(rel), cond)), nil
}

// ---- data-processing (shifted register) ----

// encodeARM64DPSR encodes a data-processing (shifted register) instruction.
// For most instructions: OP Rm, Rn, Rd (3 operands) or OP Rm, Rd (2 operands, Rn=Rd).
// For CMP/CMN/TST: CMP Rm, Rn (Rd=ZR).
// For NEG: NEG Rm, Rd (Rn=ZR).
func encodeARM64DPSR(mnem string, baseOp uint32, ops []*ast.Operand) ([]byte, error) {
	isCmp := mnem == "CMP" || mnem == "CMPW" || mnem == "CMN" || mnem == "CMNW" || mnem == "TST" || mnem == "TSTW"
	isNeg := mnem == "NEG" || mnem == "NEGW" || mnem == "MVN" || mnem == "MVNW"

	switch len(ops) {
	case 3:
		// OP Rm, Rn, Rd
		rm := arm64RegNum(operandRegName(ops[0]))
		rn := arm64RegNum(operandRegName(ops[1]))
		rd := arm64RegNum(operandRegName(ops[2]))
		if rm < 0 || rn < 0 || rd < 0 {
			return nil, fmt.Errorf("invalid register operand in %s", mnem)
		}
		return a64wordLE(baseOp | uint32(rm)<<16 | uint32(rn)<<5 | uint32(rd)), nil
	case 2:
		if isCmp {
			// CMP Rm, Rn → SUBS XZR, Rn, Rm
			rm := arm64RegNum(operandRegName(ops[0]))
			rn := arm64RegNum(operandRegName(ops[1]))
			if rm < 0 || rn < 0 {
				return nil, fmt.Errorf("invalid register operand in %s", mnem)
			}
			return a64wordLE(baseOp | uint32(rm)<<16 | uint32(rn)<<5 | 31), nil
		}
		if isNeg {
			// NEG Rm, Rd → SUB Rd, ZR, Rm
			rm := arm64RegNum(operandRegName(ops[0]))
			rd := arm64RegNum(operandRegName(ops[1]))
			if rm < 0 || rd < 0 {
				return nil, fmt.Errorf("invalid register operand in %s", mnem)
			}
			return a64wordLE(baseOp | uint32(rm)<<16 | 31<<5 | uint32(rd)), nil
		}
		// OP Rm, Rd → OP Rm, Rd, Rd
		rm := arm64RegNum(operandRegName(ops[0]))
		rd := arm64RegNum(operandRegName(ops[1]))
		if rm < 0 || rd < 0 {
			return nil, fmt.Errorf("invalid register operand in %s", mnem)
		}
		return a64wordLE(baseOp | uint32(rm)<<16 | uint32(rd)<<5 | uint32(rd)), nil
	}
	return nil, fmt.Errorf("%s expects 2 or 3 operands, got %d", mnem, len(ops))
}

// ---- ADD/SUB immediate ----

// encodeARM64AddSubImm encodes an ADD/SUB immediate instruction.
func encodeARM64AddSubImm(mnem string, ops []*ast.Operand) ([]byte, error) {
	if len(ops) != 2 && len(ops) != 3 {
		return nil, fmt.Errorf("%s expects 2 or 3 operands, got %d", mnem, len(ops))
	}
	v := int32(immFromOperand(ops[0]))
	rd := arm64RegNum(operandRegName(ops[len(ops)-1]))
	rn := rd
	if len(ops) == 3 {
		rn = arm64RegNum(operandRegName(ops[1]))
	}
	if rn < 0 || rd < 0 {
		return nil, fmt.Errorf("invalid register operand in %s", mnem)
	}

	isSub := mnem == "SUB" || mnem == "SUBW" || mnem == "CMP" || mnem == "CMPW"
	isS := mnem == "CMP" || mnem == "CMPW" || mnem == "CMN" || mnem == "CMNW"
	sf := uint32(1) // 64-bit
	if mnem == "ADDW" || mnem == "SUBW" || mnem == "CMPW" || mnem == "CMNW" {
		sf = 0 // 32-bit
	}
	if mnem == "CMP" || mnem == "CMPW" {
		rd = 31 // ZR
	}
	if mnem == "CMN" || mnem == "CMNW" {
		rd = 31 // ZR
	}

	op := uint32(0) // ADD
	S := uint32(0)
	if isSub {
		op = 1
	}
	if isS {
		S = 1
	}

	if v >= 0 && v <= 0xFFF {
		return a64wordLE(a64AddSub(sf, op, S, 0, uint32(v), uint32(rn), uint32(rd))), nil
	}
	if v >= -2048 && v < 0 {
		// Encode as the opposite operation with positive immediate.
		opp := op ^ 1
		return a64wordLE(a64AddSub(sf, opp, S, 0, uint32(-v), uint32(rn), uint32(rd))), nil
	}
	// Try with shift by 12.
	if v >= 0 && v <= 0xFFF000 && v&0xFFF == 0 {
		return a64wordLE(a64AddSub(sf, op, S, 1, uint32(v>>12), uint32(rn), uint32(rd))), nil
	}
	return nil, fmt.Errorf("%s: immediate %d out of range for single instruction", mnem, v)
}

// ---- MOV pseudo-instruction ----

// encodeARM64Mov encodes the MOV family — the load/store/immediate workhorse
// of Go's arm64 assembly.  MOV is an alias of MOVD (the width mnemonics
// select the access width).  The forms, mirroring the toolchain:
//
//	MOVx $imm, rd              load immediate (MOVZ/MOVN/MOVK)
//	MOVx mem, rd               load from memory
//	MOVx rd, mem               store to memory
//	MOVx rs, rd                register move (ORR Rd, ZR, Rs)
//	MOVx $sym(SB), rd          address of a static symbol (ADRP+ADD)
//	MOVx sym(SB), rd           load from a static symbol (ADRP+LDR)
//	MOVx rd, sym(SB)           store to a static symbol (ADRP+STR)
func encodeARM64Mov(instr *ast.Instr, mnem string, fi arm64FrameInfo, relocs *[]Reloc) ([]byte, error) {
	ops := instr.Operands
	if len(ops) != 2 {
		return nil, fmt.Errorf("%s expects 2 operands, got %d", mnem, len(ops))
	}
	src, dst := ops[0], ops[1]

	// Immediate → register (including $sym(SB)).
	if isImmOperand(src) && !isMemOperand(src) {
		if src.Imm.Sym != nil && src.Imm.Sym.Pseudo == "SB" {
			rd := arm64RegNum(operandRegName(dst))
			if rd < 0 {
				return nil, fmt.Errorf("%s $sym(SB): invalid destination register", mnem)
			}
			return encodeARM64SBAddr(src.Imm.Sym, rd, relocs), nil
		}
		rd := arm64RegNum(operandRegName(dst))
		if rd < 0 {
			return nil, fmt.Errorf("%s $imm: invalid destination register", mnem)
		}
		return encodeARM64LoadImm(rd, immFromOperand(src), mnem)
	}

	// Static symbol load/store via ADRP.
	if src.Addr.Sym != nil && src.Addr.Sym.Pseudo == "SB" && isMemOperand(src) {
		rd := arm64RegNum(operandRegName(dst))
		if rd < 0 {
			return nil, fmt.Errorf("%s sym(SB): invalid destination register", mnem)
		}
		return encodeARM64SBLoad(src.Addr.Sym, rd, mnem, relocs)
	}
	if dst.Addr.Sym != nil && dst.Addr.Sym.Pseudo == "SB" && isMemOperand(dst) {
		rs := arm64RegNum(operandRegName(src))
		if rs < 0 {
			return nil, fmt.Errorf("%s rd, sym(SB): invalid source register", mnem)
		}
		return encodeARM64SBStore(dst.Addr.Sym, rs, mnem, relocs)
	}

	// Memory load/store with offset.
	if isMemOperand(src) && !isMemOperand(dst) {
		rd := arm64RegNum(operandRegName(dst))
		if rd < 0 {
			return nil, fmt.Errorf("%s: invalid destination register", mnem)
		}
		return encodeARM64MemOp(mnem, src, rd, true, fi)
	}
	if !isMemOperand(src) && isMemOperand(dst) {
		rs := arm64RegNum(operandRegName(src))
		if rs < 0 {
			return nil, fmt.Errorf("%s: invalid source register", mnem)
		}
		return encodeARM64MemOp(mnem, dst, rs, false, fi)
	}

	// Register → register.
	return encodeARM64RegMove(mnem, src, dst)
}

// arm64MovSize returns the encoded size of a MOV instruction.
func arm64MovSize(mnem string, ops []*ast.Operand, fi arm64FrameInfo) int {
	if len(ops) != 2 {
		return 4
	}
	src, dst := ops[0], ops[1]
	switch {
	case isImmOperand(src):
		if src.Imm.Sym != nil && src.Imm.Sym.Pseudo == "SB" {
			return 8 // ADRP + ADD
		}
		v := immFromOperand(src)
		if v == 0 {
			return 4
		}
		if arm64Movcon(int64(v)) >= 0 || arm64Movcon(^int64(v)) >= 0 {
			return 4
		}
		return 8 // MOVZ + MOVK
	case src.Addr.Sym != nil && src.Addr.Sym.Pseudo == "SB":
		return 8 // ADRP + LDR
	case dst.Addr.Sym != nil && dst.Addr.Sym.Pseudo == "SB":
		return 8 // ADRP + STR
	case isMemOperand(src) || isMemOperand(dst):
		mem := src
		if !isMemOperand(src) {
			mem = dst
		}
		_, off := arm64MemWithFrame(mem, fi)
		// Scaled unsigned offset fits if aligned and in range.
		lt := a64LoadTable[mnem]
		if lt.size == 0 {
			lt.size = 3 // default to64-bit for MOV
		}
		scale := int32(1) << uint(lt.size)
		if off >= 0 && off%scale == 0 && off/scale < 4096 {
			return 4
		}
		if off >= -256 && off <= 255 {
			return 4 // unscaled
		}
		return 12 // materialise offset + LDR/STR
	default:
		return 4 // register move
	}
}

// encodeARM64LoadImm loads an immediate into a register, matching the
// toolchain's MOVZ/MOVN/MOVK sequence.
func encodeARM64LoadImm(rd int, v int32, mnem string) ([]byte, error) {
	d := int64(v)
	// For 32-bit MOVW, zero-extend.
	if mnem == "MOVW" || mnem == "MOVWU" {
		d = int64(uint32(v))
	}

	if d == 0 {
		// ORR Rd, ZR, ZR (MOV $0, Rd)
		op := uint32(1<<31 | 1<<29 | 0x0a<<24) // ORR 64-bit
		if mnem == "MOVW" || mnem == "MOVWU" {
			op = 0<<31 | 1<<29 | 0x0a<<24 // ORR 32-bit
		}
		return a64wordLE(op | 31<<16 | 31<<5 | uint32(rd)), nil
	}

	sf := uint32(1) // 64-bit
	if mnem == "MOVW" || mnem == "MOVWU" {
		sf = 0
	}

	// Try logical immediate (bitmask) encoding.  The Go toolchain uses ORR
	// with a bitmask immediate for constants like $1, $-2, $0xFF, etc.
	// that can be represented as a repeating pattern of contiguous 1s.
	N, immr, imms, ok := arm64Bitmask(uint64(d), int(sf))
	if ok {
		// ORR Rd, XZR, #bitmask (logical immediate)
		return a64wordLE(sf<<31 | 1<<29 | 0x24<<23 | N<<22 | immr<<16 | imms<<10 | 31<<5 | uint32(rd)), nil
	}

	// Try MOVZ (single non-zero16-bit chunk).
	s := arm64Movcon(d)
	if s >= 0 {
		return a64wordLE(a64MoveWide(sf, 2, uint32(s>>4), uint32((d>>uint(s))&0xFFFF), uint32(rd))), nil
	}
	// Try MOVN (single non-0xFFFF16-bit chunk of ^d).
	sn := arm64Movcon(^d)
	if sn >= 0 {
		return a64wordLE(a64MoveWide(sf, 0, uint32(sn>>4), uint32((^d>>uint(sn))&0xFFFF), uint32(rd))), nil
	}

	// Multi-instruction: MOVZ + MOVK for each non-zero16-bit chunk.
	var ws []uint32
	first := true
	for i := 0; i < 4; i++ {
		chunk := (d >> uint(i*16)) & 0xFFFF
		if chunk == 0 {
			continue
		}
		if first {
			ws = append(ws, a64MoveWide(sf, 2, uint32(i), uint32(chunk), uint32(rd))) // MOVZ
			first = false
		} else {
			ws = append(ws, a64MoveWide(sf, 3, uint32(i), uint32(chunk), uint32(rd))) // MOVK
		}
	}
	if len(ws) == 0 {
		op := uint32(1<<31 | 1<<29 | 0x0a<<24)
		return a64wordLE(op | 31<<16 | 31<<5 | uint32(rd)), nil
	}
	return a64WordsLE(ws...), nil
}

// arm64Bitmask checks whether a value can be encoded as an AArch64 logical
// immediate (bitmask).  Returns the N, immr, imms fields and true if
// representable.  sf is 0 for 32-bit or 1 for 64-bit.
func arm64Bitmask(v uint64, sf int) (N, immr, imms uint32, ok bool) {
	if v == 0 {
		return
	}
	maxElem := uint(6) // 2^6 = 64
	if sf == 0 {
		maxElem = 5 // 2^5 = 32
		v &= 0xFFFFFFFF
	}

	for e := uint(0); e < maxElem; e++ {
		esize := uint(1) << (e + 1) // 2, 4, 8, 16, 32, 64
		emask := uint64(1<<esize) - 1
		pattern := v & emask
		if pattern == 0 {
			continue
		}

		// Check each rotation: is the rotated pattern a contiguous block of 1s at the LSB?
		for r := uint(0); r < esize; r++ {
			rotated := (pattern >> r) | ((pattern << (esize - r)) & emask)
			if rotated == 0 {
				continue
			}
			// Count trailing 1s (contiguous block of 1s from bit 0).
			tz := uint(0)
			tmp := ^rotated
			for tmp&1 == 0 && tz < esize {
				tz++
				tmp >>= 1
			}
			if tz == 0 || tz >= esize {
				continue
			}
			mask := uint64(1<<tz) - 1
			if rotated != mask {
				continue
			}
			ones := tz

			// Verify the pattern repeats to fill the register.
			full := uint64(0)
			for i := uint(0); i < 64/esize; i++ {
				full |= pattern << (i * esize)
			}
			if sf == 0 {
				full &= 0xFFFFFFFF
			}
			if full != v {
				continue
			}

			// Encode N, immr, imms.
			if esize == 64 && sf == 1 {
				N = 1
			} else {
				N = 0
			}
			imms = uint32((^(esize - 1))&0x3F) | uint32(ones-1)
			immr = uint32(r)
			return N, immr, imms, true
		}
	}
	return
}

// encodeARM64RegMove encodes a register-to-register move as ORR Rd, ZR, Rs.
func encodeARM64RegMove(mnem string, src, dst *ast.Operand) ([]byte, error) {
	rs := arm64RegNum(operandRegName(src))
	rd := arm64RegNum(operandRegName(dst))
	if rs < 0 || rd < 0 {
		return nil, fmt.Errorf("invalid register operand in %s", mnem)
	}
	sc := arm64RegClassOf(operandRegName(src))
	dc := arm64RegClassOf(operandRegName(dst))

	// FP → FP: FMOV Rd, Rs
	if sc == arm64ClsFP && dc == arm64ClsFP {
		sf := uint32(1) // 64-bit
		if mnem == "FMOVS" {
			sf = 0
		}
		// FMOV: 0x1E<<24 | type<<22 | 1<<21 | 0x10<<10 | Rm<<5 | Rd
		typ := uint32(1) // 64-bit double
		if mnem == "FMOVS" {
			typ = 0 // 32-bit float
		}
		return a64wordLE(sf<<31 | 0x1E<<24 | typ<<22 | 1<<21 | 0x10<<10 | uint32(rs)<<5 | uint32(rd)), nil
	}

	// Integer → integer: ORR Rd, ZR, Rs
	sf := uint32(1) // 64-bit
	if mnem == "MOVW" || mnem == "MOVWU" || mnem == "MOVB" || mnem == "MOVBU" ||
		mnem == "MOVH" || mnem == "MOVHU" {
		sf = 0
	}
	op := uint32(1<<29 | 0x0a<<24) // ORR
	return a64wordLE(sf<<31 | op | uint32(rs)<<16 | 31<<5 | uint32(rd)), nil
}

// encodeARM64MemOp encodes a memory load or store with offset.
func encodeARM64MemOp(mnem string, mem *ast.Operand, reg int, load bool, fi arm64FrameInfo) ([]byte, error) {
	rn, off := arm64MemWithFrame(mem, fi)
	if rn < 0 {
		return nil, fmt.Errorf("invalid memory operand")
	}
	lt, ok := a64LoadTable[mnem]
	if !ok {
		// MOV defaults to MOVD (64-bit load/store).
		lt = a64LoadTable["MOVD"]
	}

	scale := int32(1) << uint(lt.size)
	if load {
		// Try scaled unsigned offset first.
		if off >= 0 && off%scale == 0 {
			imm12 := uint32(off / scale)
			if imm12 < 4096 {
				return a64wordLE(a64LSU(uint32(lt.size), uint32(lt.V), uint32(lt.opc), imm12, uint32(rn), uint32(reg))), nil
			}
		}
		// Try unscaled (9-bit signed).
		if off >= -256 && off <= 255 {
			return a64wordLE(a64LSUnscaled(lt.size, lt.V, lt.opc, off, rn, reg)), nil
		}
		// Large offset: materialise in R20 (TMP) and use register-offset.
		return nil, fmt.Errorf("%s: offset %d out of range", mnem, off)
	}
	// Store: same encoding but opc bits indicate store.
	storeOpc := a64StoreOpc(lt)
	if off >= 0 && off%scale == 0 {
		imm12 := uint32(off / scale)
		if imm12 < 4096 {
			return a64wordLE(a64LSU(uint32(lt.size), uint32(lt.V), uint32(storeOpc), imm12, uint32(rn), uint32(reg))), nil
		}
	}
	if off >= -256 && off <= 255 {
		return a64wordLE(a64LSUnscaled(lt.size, lt.V, storeOpc, off, rn, reg)), nil
	}
	return nil, fmt.Errorf("%s: offset %d out of range", mnem, off)
}

// ---- static symbol references (ADRP + offset) ----

// encodeARM64SBAddr emits ADRP Rd, 0; ADD Rd, Rd, 0 with the
// R_ADDRARM64 relocation pair, loading a symbol's address.
func encodeARM64SBAddr(sym *ast.Symbol, rd int, relocs *[]Reloc) []byte {
	if relocs != nil {
		*relocs = append(*relocs,
			Reloc{Off: 0, After: 0, Name: sym.Name, Kind: RelArm64Addr, Addend: sym.Offset},
			Reloc{Off: 4, After: 4, Name: sym.Name, Kind: RelArm64Addr, Addend: sym.Offset},
		)
	}
	return a64WordsLE(
		a64ADR(1, 0, 0, uint32(rd)),                      // ADRP Rd, 0
		a64AddSub(1, 0, 0, 0, 0, uint32(rd), uint32(rd)), // ADD $0, Rd, Rd
	)
}

// encodeARM64SBLoad emits ADRP R20, 0; LDR Rd, [R20, 0] with relocations.
func encodeARM64SBLoad(sym *ast.Symbol, rd int, mnem string, relocs *[]Reloc) ([]byte, error) {
	lt, ok := a64LoadTable[mnem]
	if !ok {
		lt = a64LoadTable["MOVD"]
	}
	if relocs != nil {
		*relocs = append(*relocs,
			Reloc{Off: 0, After: 0, Name: sym.Name, Kind: RelArm64Addr, Addend: sym.Offset},
			Reloc{Off: 4, After: 4, Name: sym.Name, Kind: RelArm64Addr, Addend: sym.Offset},
		)
	}
	return a64WordsLE(
		a64ADR(1, 0, 0, 20), // ADRP R20, 0
		a64LSU(uint32(lt.size), uint32(lt.V), uint32(lt.opc), 0, 20, uint32(rd)), // LDR Rd, [R20, #0]
	), nil
}

// encodeARM64SBStore emits ADRP R20, 0; STR Rs, [R20, 0] with relocations.
func encodeARM64SBStore(sym *ast.Symbol, rs int, mnem string, relocs *[]Reloc) ([]byte, error) {
	lt, ok := a64LoadTable[mnem]
	if !ok {
		lt = a64LoadTable["MOVD"]
	}
	storeOpc := a64StoreOpc(lt)
	if relocs != nil {
		*relocs = append(*relocs,
			Reloc{Off: 0, After: 0, Name: sym.Name, Kind: RelArm64Addr, Addend: sym.Offset},
			Reloc{Off: 4, After: 4, Name: sym.Name, Kind: RelArm64Addr, Addend: sym.Offset},
		)
	}
	return a64WordsLE(
		a64ADR(1, 0, 0, 20), // ADRP R20, 0
		a64LSU(uint32(lt.size), uint32(lt.V), uint32(storeOpc), 0, 20, uint32(rs)), // STR Rs, [R20, #0]
	), nil
}

// ---- operand helpers ----

// arm64Reg returns the register number of an operand, or -1.
func arm64Reg(op *ast.Operand) int {
	return arm64RegNum(operandRegName(op))
}

// arm64MemWithFrame resolves a memory operand, translating FP/SP pseudo-
// registers via the frame mapping.
func arm64MemWithFrame(op *ast.Operand, fi arm64FrameInfo) (rn int, off int32) {
	if op.Addr.Sym != nil && op.Addr.Sym.Pseudo != "" {
		return arm64ResolvePseudo(op.Addr.Sym, fi)
	}
	return arm64RegNum(op.Addr.Base), int32(op.Addr.Offset)
}

// arm64Label returns the label name of an operand.
func arm64Label(op *ast.Operand) string {
	if op.Addr.Sym != nil {
		return op.Addr.Sym.Name
	}
	return op.Raw
}

// AssembleFileARM64 assembles every TEXT function of a parsed arm64 file
// and lays out its static symbols (GLOBL/DATA) in a data section behind the
// code.  SB references in the code are encoded as ADRP pairs with zero
// immediates; the object-file emitters record R_ADDRARM64 relocations for
// the linker.
func AssembleFileARM64(f *ast.File) (*Image, error) {
	dataSyms, err := collectData(f)
	if err != nil {
		return nil, err
	}

	img := &Image{Symbols: map[string]int{}}
	for _, d := range f.Decls {
		t, ok := d.(*ast.Text)
		if !ok {
			continue
		}
		code, labels, relocs, lines, spadj, err := assembleARM64(t)
		if err != nil {
			return nil, fmt.Errorf("%s: %w", t.Name.Name, err)
		}
		fl := FuncLayout{
			Name:   t.Name.Name,
			Pkg:    t.Name.Pkg,
			Static: t.Name.Static,
			Offset: len(img.Code),
			Size:   len(code),
			Frame:  frameSize(t),
			Args:   argsSize(t),
			Line:   t.Pos().Line,
			Labels: labels,
			Lines:  lines,
			Spadj:  spadj,
			Relocs: relocs,
		}
		for _, f := range t.Flags {
			switch f {
			case "NOSPLIT":
				fl.NoSplit = true
			case "SPWRITE":
				fl.SPWrite = true
			}
		}
		img.Funcs = append(img.Funcs, fl)
		img.Code = append(img.Code, code...)
	}

	// Lay out the data section behind the code, 16-aligned.
	dataStart := len(img.Code)
	for _, d := range dataSyms {
		pos := dataStart + len(img.Data)
		for pos%16 != 0 {
			img.Data = append(img.Data, 0)
			pos++
		}
		img.Symbols[d.name] = pos
		img.Data = append(img.Data, d.buf...)
		img.DataSyms = append(img.DataSyms, DataSymbol{
			Name:   d.name,
			Pkg:    d.pkg,
			Offset: len(img.Data) - len(d.buf),
			Size:   d.size,
			Static: d.static,
			Rodata: d.rodata,
			Dupok:  d.dupok,
		})
	}

	return img, nil
}