gasm-sdk/asm/riscv_assemble.go

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

package asm

import (
	"fmt"

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

// assembleRISCV assembles a RISC-V TEXT function body into machine code.
// It handles the core RV64IMAFDC instruction set.
func assembleRISCV(t *ast.Text) ([]byte, map[string]int, error) {
	fi := riscvComputeFrame(t)
	prologue := riscvPrologue(fi)

	// Pass 1: collect instructions and compute label offsets assuming 4 bytes
	// per instruction (or 8 for MOV $large-imm).  No encoding yet.
	type instrRec struct {
		instr      *ast.Instr
		compressed bool
		code       []byte
	}
	var recs []instrRec
	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:
			recs = append(recs, instrRec{instr: s})
			pos += riscvInstrSize(s)
		}
	}

	// Pass 2: encode each instruction using Pass-1 offsets.
	pc := len(prologue)
	for i := range recs {
		code, err := encodeRISCVInstr(recs[i].instr, pc, offsets, fi)
		if err != nil {
			return nil, nil, fmt.Errorf("%s: %w", recs[i].instr.Mnemonic.Text, err)
		}
		recs[i].code = code
		pc += len(code)
	}

	// Pass 3: try RVC compression.
	for i := range recs {
		if c16, ok := tryCompressRVC(recs[i].instr, fi); ok {
			recs[i].compressed = true
			recs[i].code = []byte{byte(c16), byte(c16 >> 8)}
		}
	}

	// Pass 4: recompute offsets with actual 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:
			for _, r := range recs {
				if r.instr == s {
					pos += len(r.code)
					break
				}
			}
		}
	}

	// Pass 5: re-encode branches with corrected offsets, emit uncompressed
	// for instructions that can't be compressed.
	out := append([]byte(nil), prologue...)
	pc = len(prologue)
	for _, r := range recs {
		if r.compressed && !isBranchLike(r.instr.Mnemonic.Text) {
			out = append(out, r.code...)
			pc += len(r.code)
		} else {
			// Re-encode with correct offsets (branches need this).
			code, err := encodeRISCVInstr(r.instr, pc, offsets, fi)
			if err != nil {
				return nil, nil, err
			}
			// Try compression again for this instruction.
			if c16, ok := tryCompressRVC(r.instr, fi); ok {
				code = []byte{byte(c16), byte(c16 >> 8)}
			}
			out = append(out, code...)
			pc += len(code)
		}
	}
	return out, offsets, nil
}

// riscvInstrSize returns the encoded size in bytes of a RISC-V instruction.
// Most instructions are 4 bytes; MOV with a large immediate is 8 (LUI+ADDIW).
func riscvInstrSize(instr *ast.Instr) int {
	mnem := instr.Mnemonic.Text
	if mnem == "MOV" && len(instr.Operands) == 2 && isImmOperand(instr.Operands[0]) {
		imm := immFromOperand(instr.Operands[0])
		if imm < -2048 || imm > 2047 {
			return 8 // LUI + ADDIW
		}
	}
	return 4
}

// isBranchLike reports whether a mnemonic is a branch or jump that needs
// recalculated offsets after compression.
func isBranchLike(mnem string) bool {
	switch mnem {
	case "BEQ", "BNE", "BLT", "BGE", "BLTU", "BGEU", "JMP", "JAL":
		return true
	}
	return false
}

// encodeRISCVInstr encodes a single RISC-V instruction.
func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscvFrameInfo) ([]byte, error) {
	mnem := instr.Mnemonic.Text
	ops := instr.Operands
	var word uint32

	// Handle pseudo-instructions and special cases first.
	switch mnem {
	case "RET":
		// RET = JALR X0, 0(X1)
		word = riscvIType(riscvEnc{0x67, 0x0, 0x00}, 0, 1, 0)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	case "CALL":
		// CALL is a pseudo-instruction; encode as NOP placeholder.
		word = riscvIType(riscvEnc{0x13, 0x0, 0x00}, 0, 0, 0)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	case "JMP":
		// JMP = JAL X0, target.  Try C.J compression.
		var target string
		if len(ops) >= 1 {
			target = labelFromOperand(ops[0])
		}
		targetOff, ok := offsets[target]
		if !ok {
			return nil, fmt.Errorf("undefined label %q", target)
		}
		offset := int32(targetOff - pc)
		// C.J: funct3=0x5, offset in ±2 KB, bit 0 must be 0.
		if offset >= -2048 && offset <= 2046 && offset%2 == 0 {
			c16 := rvcCJ(0x5, offset)
			return []byte{byte(c16), byte(c16 >> 8)}, nil
		}
		word = riscvJType(0, offset)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	case "JAL":
		rd := 0
		var target string
		if len(ops) >= 2 {
			rd = regFromOperand(ops[0])
			target = labelFromOperand(ops[1])
		} else if len(ops) == 1 {
			target = labelFromOperand(ops[0])
		}
		targetOff, ok := offsets[target]
		if !ok {
			return nil, fmt.Errorf("undefined label %q", target)
		}
		offset := int32(targetOff - pc)
		// JAL X0, target → C.J when offset fits.
		if rd == 0 && offset >= -2048 && offset <= 2046 && offset%2 == 0 {
			c16 := rvcCJ(0x5, offset)
			return []byte{byte(c16), byte(c16 >> 8)}, nil
		}
		word = riscvJType(rd, offset)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil

	// MOV is a pseudo-instruction that the Go assembler uses for loads,
	// stores, register moves and immediate loads.
	case "MOV":
		return encodeRISCVMov(instr, offsets, fi)
	}

	// FP conversion / move instructions use a separate table (rs2 encodes
	// the conversion type, not a register).  Handle them before the main
	// table lookup.
	if cvtEnc, ok := riscvCvtTable[mnem]; ok {
		if len(ops) != 2 {
			return nil, fmt.Errorf("%s expects 2 operands, got %d", mnem, len(ops))
		}
		rs1 := regFromOperand(ops[0])
		rd := regFromOperand(ops[1])
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word := riscvCvtType(cvtEnc, rd, rs1)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	}

	// R4-type fused multiply-add: INSTR rs1, rs2, rs3, rd (destination last).
	if fmaEnc, ok := riscvFmaTable[mnem]; ok {
		if len(ops) != 4 {
			return nil, fmt.Errorf("%s expects 4 operands, got %d", mnem, len(ops))
		}
		rs1 := regFromOperand(ops[0])
		rs2 := regFromOperand(ops[1])
		rs3 := regFromOperand(ops[2])
		rd := regFromOperand(ops[3])
		if rd < 0 || rs1 < 0 || rs2 < 0 || rs3 < 0 {
			return nil, fmt.Errorf("invalid FP register in %s", mnem)
		}
		word := riscvFmaType(fmaEnc, rd, rs1, rs2, rs3)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	}

	// CSR instructions: INSTR csr, rs1|uimm, rd (destination last).
	if csrEnc, ok := riscvCsrTable[mnem]; ok {
		if len(ops) != 3 {
			return nil, fmt.Errorf("%s expects 3 operands, got %d", mnem, len(ops))
		}
		csr := immFromOperand(ops[0]) // CSR address (12-bit)
		rd := regFromOperand(ops[2])  // destination register
		if rd < 0 {
			return nil, fmt.Errorf("invalid destination register in %s", mnem)
		}
		var src int
		if csrEnc.imm {
			// Immediate variant: ops[1] is a 5-bit unsigned immediate.
			src = int(immFromOperand(ops[1]))
			if src < 0 || src > 31 {
				return nil, fmt.Errorf("%s: uimm out of range 0-31", mnem)
			}
		} else {
			// Register variant: ops[1] is a register.
			src = regFromOperand(ops[1])
			if src < 0 {
				return nil, fmt.Errorf("invalid source register in %s", mnem)
			}
		}
		word := riscvCsrType(csrEnc, rd, src, csr)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	}

	enc, ok := riscvInstrTable[mnem]
	if !ok {
		return nil, fmt.Errorf("unsupported RISC-V instruction %q", mnem)
	}

	switch {
	// R-type: Plan 9 order is INSTR src1, src2, dst (destination last).
	case len(ops) == 3 && isRTypeInstr(mnem):
		rs1 := regFromOperand(ops[0]) // source 1 (first operand)
		rs2 := regFromOperand(ops[1]) // source 2 (second operand)
		rd := regFromOperand(ops[2])  // destination (last operand)
		if rd < 0 || rs1 < 0 || rs2 < 0 {
			return nil, fmt.Errorf("invalid register in %s", mnem)
		}
		word = riscvRType(enc, rd, rs1, rs2)

	// I-type shift (SLLI, SRLI, SRAI): INSTR rs, $shamt, rd.
	case len(ops) == 3 && isShiftImmInstr(mnem):
		rs1 := regFromOperand(ops[0])
		shamt := int(immFromOperand(ops[1]))
		rd := regFromOperand(ops[2])
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid register in %s", mnem)
		}
		word = riscvRType(enc, rd, rs1, shamt)

	// AMO atomics: Plan 9 order is INSTR src, (addr), dst.
	case len(ops) == 3 && isAMOInstr(mnem):
		rs2 := regFromOperand(ops[0])                 // source value
		rs1, _ := memFromOperandWithFrame(ops[1], fi) // memory address
		rd := regFromOperand(ops[2])                  // destination (old value)
		if rd < 0 || rs1 < 0 || rs2 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word = riscvAMOType(enc, rd, rs1, rs2)

	// FP arithmetic: Plan 9 order is INSTR src1, src2, dst.
	case len(ops) == 3 && isFPArithInstr(mnem):
		rs1 := regFromOperand(ops[0])
		rs2 := regFromOperand(ops[1])
		rd := regFromOperand(ops[2])
		if rd < 0 || rs1 < 0 || rs2 < 0 {
			return nil, fmt.Errorf("invalid FP register in %s", mnem)
		}
		word = riscvRType(enc, rd, rs1, rs2)

	// FP arithmetic (2-operand): FSQRT src, dst.
	case len(ops) == 2 && isFPArithInstr(mnem):
		rs1 := regFromOperand(ops[0])
		rd := regFromOperand(ops[1])
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid FP register in %s", mnem)
		}
		word = riscvRType(enc, rd, rs1, 0)

	// FP loads: INSTR addr, freg (Plan 9: source first).
	case len(ops) == 2 && isFPLoadInstr(mnem):
		rd := regFromOperand(ops[1])
		rs1, imm := memFromOperandWithFrame(ops[0], fi)
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word = riscvIType(enc, rd, rs1, imm)

	// FP stores: INSTR freg, addr (Plan 9: source first).
	case len(ops) == 2 && isFPStoreInstr(mnem):
		rs2 := regFromOperand(ops[0])
		rs1, imm := memFromOperandWithFrame(ops[1], fi)
		if rs2 < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word = riscvSType(enc, rs1, rs2, imm)

	// LR (load-reserved): INSTR (addr), dst — 2 operands.
	case len(ops) == 2 && isLRInstr(mnem):
		rs1, _ := memFromOperandWithFrame(ops[0], fi)
		rd := regFromOperand(ops[1])
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word = riscvAMOType(enc, rd, rs1, 0) // rs2=0 for LR

	// SC (store-conditional): INSTR src, (addr), dst — 3 operands.
	case len(ops) == 3 && isSCInstr(mnem):
		rs2 := regFromOperand(ops[0])
		rs1, _ := memFromOperandWithFrame(ops[1], fi)
		rd := regFromOperand(ops[2])
		if rd < 0 || rs1 < 0 || rs2 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word = riscvAMOType(enc, rd, rs1, rs2)

	// FP compare: INSTR src1, src2, dst(int) — result in integer register.
	case len(ops) == 3 && isFPCmpInstr(mnem):
		rs1 := regFromOperand(ops[0])
		rs2 := regFromOperand(ops[1])
		rd := regFromOperand(ops[2])
		if rd < 0 || rs1 < 0 || rs2 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word = riscvRType(enc, rd, rs1, rs2)

	// I-type with immediate: Plan 9 order is INSTR src, imm, dst.
	case len(ops) == 3 && isITypeInstr(mnem):
		rs1 := regFromOperand(ops[0]) // source register
		imm := immFromOperand(ops[1]) // immediate
		rd := regFromOperand(ops[2])  // destination
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid register in %s", mnem)
		}
		word = riscvIType(enc, rd, rs1, imm)

	// Loads: rd, offset(rs1) — Plan 9 order is LD src, dst.
	case len(ops) == 2 && isLoadInstr(mnem):
		rd := regFromOperand(ops[1])                    // destination (last operand)
		rs1, imm := memFromOperandWithFrame(ops[0], fi) // memory source (first operand)
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word = riscvIType(enc, rd, rs1, imm)

	// Stores: Plan 9 order is SD src, dst (src=register, dst=memory).
	case len(ops) == 2 && isStoreInstr(mnem):
		rs2 := regFromOperand(ops[0])                   // source register (first operand)
		rs1, imm := memFromOperandWithFrame(ops[1], fi) // memory dest (last operand)
		if rs2 < 0 || rs1 < 0 {
			return nil, fmt.Errorf("invalid operand in %s", mnem)
		}
		word = riscvSType(enc, rs1, rs2, imm)

	// Branches: rs1, rs2, label.
	case len(ops) == 3 && isBranchInstr(mnem):
		rs1 := regFromOperand(ops[0])
		rs2 := regFromOperand(ops[1])
		target := labelFromOperand(ops[2])
		targetOff, ok := offsets[target]
		if !ok {
			return nil, fmt.Errorf("undefined label %q", target)
		}
		offset := int32(targetOff - pc)
		if rs1 < 0 || rs2 < 0 {
			return nil, fmt.Errorf("invalid register in %s", mnem)
		}

		// Try C.BEQZ / C.BNEZ compression.
		if (mnem == "BEQ" || mnem == "BNE") && rs2 == 0 && isRVCIntReg(rs1) {
			if cOff := offset; cOff >= -256 && cOff <= 254 && cOff%2 == 0 {
				funct3 := uint32(0x6) // C.BEQZ
				if mnem == "BNE" {
					funct3 = 0x7 // C.BNEZ
				}
				c16 := rvcCB(funct3, rvcReg3(rs1), offset)
				return []byte{byte(c16), byte(c16 >> 8)}, nil
			}
		}

		word = riscvBType(enc, rs1, rs2, offset)

	// U-type: rd, imm.
	case len(ops) == 2 && isUTypeInstr(mnem):
		rd := regFromOperand(ops[0])
		imm := immFromOperand(ops[1])
		if rd < 0 {
			return nil, fmt.Errorf("invalid register in %s", mnem)
		}
		word = riscvUType(enc, rd, imm)

	default:
		return nil, fmt.Errorf("cannot encode %s with %d operands", mnem, len(ops))
	}

	// Emit as little-endian 32-bit word.
	return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
}

// isMemOperand reports whether an operand is a memory reference
// (frame-relative such as name+off(FP) or register-relative such as (X10)).
func isMemOperand(op *ast.Operand) bool {
	if op.Addr.Sym != nil && op.Addr.Sym.Pseudo != "" {
		return true // name+off(FP), name+off(SP)
	}
	if op.Addr.Base != "" && op.Addr.Sym == nil {
		return true // (reg)
	}
	return false
}

// isImmOperand reports whether an operand is an immediate ($value).
func isImmOperand(op *ast.Operand) bool {
	if op.Kind == ast.OpImmediate {
		return true
	}
	if op.Imm.HasVal {
		return true
	}
	return false
}

// encodeRISCVMov encodes the MOV pseudo-instruction.
//
// The Go RISC-V assembler uses MOV for:
//   - MOV name+off(FP), Rd   load from frame
//   - MOV Rd, name+off(FP)   store to frame
//   - MOV (Rs), Rd          register-relative load
//   - MOV Rs, (Rd)          register-relative store
//   - MOV Rs, Rd            register-to-register move (ADDI $0)
//   - MOV $imm, Rd          load immediate (ADDI or LUI+ADDIW)
func encodeRISCVMov(instr *ast.Instr, offsets map[string]int, fi riscvFrameInfo) ([]byte, error) {
	ops := instr.Operands
	if len(ops) != 2 {
		return nil, fmt.Errorf("MOV expects 2 operands, got %d", len(ops))
	}

	src := ops[0]
	dst := ops[1]

	// Immediate → register.
	if isImmOperand(src) {
		rd := regFromOperand(dst)
		if rd < 0 {
			return nil, fmt.Errorf("MOV $imm: invalid destination register")
		}
		imm := immFromOperand(src)
		return encodeRISCVLoadImm(rd, imm), nil
	}

	// Memory → register (load).
	if isMemOperand(src) && !isMemOperand(dst) {
		rd := regFromOperand(dst)
		rs1, off := memFromOperandWithFrame(src, fi)
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("MOV load: invalid operand")
		}
		word := riscvIType(riscvEnc{0x03, 0x3, 0x00}, rd, rs1, off)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	}

	// Register → memory (store).
	if !isMemOperand(src) && isMemOperand(dst) {
		rs2 := regFromOperand(src)
		rs1, off := memFromOperandWithFrame(dst, fi)
		if rs2 < 0 || rs1 < 0 {
			return nil, fmt.Errorf("MOV store: invalid operand")
		}
		word := riscvSType(riscvEnc{0x23, 0x3, 0x00}, rs1, rs2, off)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	}

	// Register → register (ADDI $0, src, dst).
	{
		rs1 := regFromOperand(src)
		rd := regFromOperand(dst)
		if rd < 0 || rs1 < 0 {
			return nil, fmt.Errorf("MOV: invalid register operand")
		}
		word := riscvIType(riscvEnc{0x13, 0x0, 0x00}, rd, rs1, 0)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}, nil
	}
}

// encodeRISCVLoadImm encodes loading an immediate into a register.
// For 12-bit immediates: ADDI $imm, ZERO, rd.
// For larger: LUI $hi, rd + ADDIW $lo, rd, rd.
func encodeRISCVLoadImm(rd int, imm int32) []byte {
	if imm >= -2048 && imm <= 2047 {
		word := riscvIType(riscvEnc{0x13, 0x0, 0x00}, rd, 0, imm)
		return []byte{byte(word), byte(word >> 8), byte(word >> 16), byte(word >> 24)}
	}

	// LUI + ADDIW for larger constants.
	var out []byte
	hi := int32((uint32(imm)+0x800)>>12) << 12 // LUI loads upper 20 bits
	lo := imm - hi
	wordLUI := riscvUType(riscvEnc{0x37, 0x0, 0x00}, rd, hi)
	out = append(out, byte(wordLUI), byte(wordLUI>>8), byte(wordLUI>>16), byte(wordLUI>>24))
	if lo != 0 {
		wordADDIW := riscvIType(riscvEnc{0x1B, 0x0, 0x00}, rd, rd, lo)
		out = append(out, byte(wordADDIW), byte(wordADDIW>>8), byte(wordADDIW>>16), byte(wordADDIW>>24))
	}
	return out
}

// tryCompressRVC attempts to compress a RISC-V instruction to its 16-bit
// RVC form.  It returns the compressed instruction word and true on success.
func tryCompressRVC(instr *ast.Instr, fi riscvFrameInfo) (uint16, bool) {
	mnem := instr.Mnemonic.Text
	ops := instr.Operands

	switch mnem {
	case "RET":
		// RET = JALR X0, 0(X1) → C.JR RA (CR-type: funct4=0x8, rd=0, rs2=1)
		return rvcCR(0x8, 0, 1), true

	case "LD", "MOV":
		// LD rd, offset(SP) → C.LDSP when rd≠0 and uimm[8:3] fits.
		// MOV name+off(FP), rd → load, same compression.
		if mnem == "MOV" && len(ops) == 2 && isImmOperand(ops[0]) {
			return 0, false
		}
		// MOV reg, reg → C.MV (CR-type: funct4=0x8).
		if mnem == "MOV" && len(ops) == 2 && !isMemOperand(ops[0]) && !isMemOperand(ops[1]) && !isImmOperand(ops[0]) {
			rs1 := regFromOperand(ops[0])
			rd := regFromOperand(ops[1])
			if rs1 != -1 && rd != -1 && rs1 != 0 && rd != 0 {
				return rvcCR(0x8, uint32(rd), uint32(rs1)), true
			}
		}
		rd, rs1, imm := extractLDParams(instr, fi)
		if rs1 == 2 && rd != 0 && rd != -1 && imm >= 0 && imm < 512 && imm%8 == 0 {
			return rvcCI(0x3, uint32(rd), uint32(imm)>>3), true
		}
		// MOV reg, mem → store, try C.SDSP.
		if mnem == "MOV" && len(ops) == 2 && !isMemOperand(ops[0]) && isMemOperand(ops[1]) {
			rs2, rs1, imm := extractSDParams(instr, fi)
			if rs1 == 2 && rs2 != -1 && imm >= 0 && imm < 512 && imm%8 == 0 {
				return rvcCSS(0x7, uint32(rs2), uint32(imm)>>3), true
			}
		}

	case "SD":
		// SD rs2, offset(SP) → C.SDSP when uimm[8:3] fits (CSS-type).
		rs2, rs1, imm := extractSDParams(instr, fi)
		if rs1 == 2 && rs2 != -1 && imm >= 0 && imm < 512 && imm%8 == 0 {
			uimm := uint32(imm) >> 3
			return rvcCSS(0x7, uint32(rs2), uimm), true
		}

	case "ADDI":
		rd, rs1, imm := extractITypeParams(instr, fi)
		if rd == -1 || rs1 == -1 {
			return 0, false
		}
		if rd == rs1 && rd != 0 && imm != 0 && imm >= -32 && imm <= 31 {
			// C.ADDI: funct3=0x0, rs1/rd, nzimm[5:0]
			return rvcCI(0x0, uint32(rd), uint32(imm)&0x3F), true
		}
		if rs1 == 0 && rd != 0 && imm >= -32 && imm <= 31 {
			// C.LI: funct3=0x2, rd, imm[5:0]
			return rvcCI(0x2, uint32(rd), uint32(imm)&0x3F), true
		}
		if rs1 != 0 && rd != 0 && imm == 0 {
			// C.MV: funct4=0x8, rd, rs1 (CR-type)
			return rvcCR(0x8, uint32(rd), uint32(rs1)), true
		}

	case "JAL":
		// JAL X0, target → C.J when offset fits in ±2KB.
		if len(ops) >= 1 {
			// For JAL with implicit rd=0 (JMP alias), check target.
			// C.J: funct3=0x5
			// Offset is computed at encode time — we can't check it here.
			return 0, false
		}

	case "JMP":
		// C.J — handled in encodeRISCVInstr with actual offset.
		return 0, false

	case "BEQ":
		// C.BEQZ — handled in encodeRISCVInstr with actual offset.
		return 0, false

	case "BNE":
		// C.BNEZ — handled in encodeRISCVInstr with actual offset.
		return 0, false

	case "ADD":
		// ADD rd, rs2 → C.ADD when rd == rs1 and both in prime regs (rd ≠ 0).
		// ADD is commutative: if rd == rs2, swap.
		if len(ops) == 3 {
			rs1 := regFromOperand(ops[0])
			rs2 := regFromOperand(ops[1])
			rd := regFromOperand(ops[2])
			if rd != -1 && rs1 != -1 && rs2 != -1 && rd != 0 {
				if rd == rs1 && isRVCIntReg(rd) && isRVCIntReg(rs2) && rs2 != 0 {
					// C.ADD: funct6=0x27, funct2=0x0 (CA-type)
					return rvcCA(0x27, 0x0, rvcReg3(rd), rvcReg3(rs2)), true
				}
				if rd == rs2 && isRVCIntReg(rd) && isRVCIntReg(rs1) && rs1 != 0 {
					// Swap: C.ADD rd, rs1
					return rvcCA(0x27, 0x0, rvcReg3(rd), rvcReg3(rs1)), true
				}
			}
		}

	case "SUB", "XOR", "OR", "AND":
		// C.SUB (0x23,0), C.XOR (0x23,1), C.OR (0x23,2), C.AND (0x23,3)
		if len(ops) == 3 {
			var funct2 uint32
			switch mnem {
			case "SUB":
				funct2 = 0x0
			case "XOR":
				funct2 = 0x1
			case "OR":
				funct2 = 0x2
			case "AND":
				funct2 = 0x3
			}
			rs1 := regFromOperand(ops[0])
			rs2 := regFromOperand(ops[1])
			rd := regFromOperand(ops[2])
			if rd != -1 && rs1 != -1 && rs2 != -1 && rd != 0 {
				if rd == rs1 && isRVCIntReg(rd) && isRVCIntReg(rs2) && rs2 != 0 {
					return rvcCA(0x23, funct2, rvcReg3(rd), rvcReg3(rs2)), true
				}
			}
		}

	case "FLD":
		// FLD rd, imm(SP) → C.FLDSP (CI-type, funct3=0x1).
		rd, rs1, imm := extractLDParams(instr, fi)
		if rs1 == 2 && rd != -1 && imm >= 0 && imm < 512 && imm%8 == 0 {
			return rvcCI(0x1, uint32(rd), uint32(imm)>>3), true
		}

	case "FSD":
		// FSD rs2, imm(SP) → C.FSDSP (CSS-type, funct3=0x5).
		rs2, rs1, imm := extractSDParams(instr, fi)
		if rs1 == 2 && rs2 != -1 && imm >= 0 && imm < 512 && imm%8 == 0 {
			return rvcCSS(0x5, uint32(rs2), uint32(imm)>>3), true
		}

	case "LUI":
		// LUI rd, imm → C.LUI when rd≠0, rd≠SP, imm nonzero and fits in 6 bits.
		if len(ops) == 2 {
			rd := regFromOperand(ops[0])
			imm := immFromOperand(ops[1])
			if rd != -1 && rd != 0 && rd != 2 && imm != 0 && imm >= 1 && imm <= 63 {
				return rvcCI(0x3, uint32(rd), uint32(imm)&0x3F), true
			}
		}

	case "ADDIW":
		rd, rs1, imm := extractITypeParams(instr, fi)
		if rd == rs1 && rd != 0 && imm >= -32 && imm <= 31 {
			return rvcCI(0x1, uint32(rd), uint32(imm)&0x3F), true
		}
	}

	return 0, false
}

// extractLDParams extracts rd, rs1, and immediate offset for a load instruction.
func extractLDParams(instr *ast.Instr, fi riscvFrameInfo) (rd, rs1 int, imm int32) {
	ops := instr.Operands
	if len(ops) != 2 {
		return -1, -1, 0
	}
	if instr.Mnemonic.Text == "MOV" {
		if isMemOperand(ops[0]) {
			rs1, imm = memFromOperandWithFrame(ops[0], fi)
			rd = regFromOperand(ops[1])
		} else {
			return -1, -1, 0
		}
	} else {
		rs1, imm = memFromOperandWithFrame(ops[0], fi)
		rd = regFromOperand(ops[1])
	}
	return
}

// extractSDParams extracts rs2, rs1, and immediate offset for a store instruction.
func extractSDParams(instr *ast.Instr, fi riscvFrameInfo) (rs2, rs1 int, imm int32) {
	ops := instr.Operands
	if len(ops) != 2 {
		return -1, -1, 0
	}
	rs2 = regFromOperand(ops[0])
	rs1, imm = memFromOperandWithFrame(ops[1], fi)
	return
}

// extractITypeParams extracts rd, rs1, and immediate for an I-type instruction.
func extractITypeParams(instr *ast.Instr, fi riscvFrameInfo) (rd, rs1 int, imm int32) {
	ops := instr.Operands
	if len(ops) != 3 {
		return -1, -1, 0
	}
	rs1 = regFromOperand(ops[0])
	imm = immFromOperand(ops[1])
	rd = regFromOperand(ops[2])
	return
}

// Instruction type classifiers.
func isRTypeInstr(m string) bool {
	switch m {
	case "ADD", "SUB", "SLL", "SLT", "SLTU", "XOR", "SRL", "SRA", "OR", "AND",
		"ADDW", "SUBW", "SLLW", "SRLW", "SRAW",
		"MUL", "MULH", "MULHSU", "MULHU", "DIV", "DIVU", "REM", "REMU",
		"MULW", "DIVW", "DIVUW", "REMW", "REMUW":
		return true
	}
	return false
}

func isShiftImmInstr(m string) bool {
	switch m {
	case "SLLI", "SRLI", "SRAI", "SLLIW", "SRLIW", "SRAIW":
		return true
	}
	return false
}

func isITypeInstr(m string) bool {
	switch m {
	case "ADDI", "ADDIW", "SLTI", "SLTIU", "XORI", "ORI", "ANDI", "JALR":
		return true
	}
	return false
}

func isLoadInstr(m string) bool {
	switch m {
	case "LB", "LH", "LW", "LD", "LBU", "LHU", "LWU":
		return true
	}
	return false
}

func isStoreInstr(m string) bool {
	switch m {
	case "SB", "SH", "SW", "SD":
		return true
	}
	return false
}

func isBranchInstr(m string) bool {
	switch m {
	case "BEQ", "BNE", "BLT", "BGE", "BLTU", "BGEU":
		return true
	}
	return false
}

func isUTypeInstr(m string) bool {
	return m == "LUI" || m == "AUIPC"
}

func isAMOInstr(m string) bool {
	switch m {
	case "AMOSWAPW", "AMOSWAPD", "AMOADDW", "AMOADDD",
		"AMOANDW", "AMOANDD", "AMOORW", "AMOORD",
		"AMOXORW", "AMOXORD", "AMOMAXW", "AMOMAXD",
		"AMOMINW", "AMOMIND", "AMOMAXUW", "AMOMAXUD",
		"AMOMINUW", "AMOMINUD":
		return true
	}
	return false
}

func isFPArithInstr(m string) bool {
	switch m {
	case "FADDS", "FSUBS", "FMULS", "FDIVS",
		"FADDD", "FSUBD", "FMULD", "FDIVD",
		"FSQRTS", "FSQRTD", "FMINS", "FMAXS", "FMIND", "FMAXD":
		return true
	}
	return false
}

func isFPLoadInstr(m string) bool {
	return m == "FLW" || m == "FLD"
}

func isFPStoreInstr(m string) bool {
	return m == "FSW" || m == "FSD"
}

func isLRInstr(m string) bool {
	return m == "LRW" || m == "LRD"
}

func isSCInstr(m string) bool {
	return m == "SCW" || m == "SCD"
}

func isFPCmpInstr(m string) bool {
	switch m {
	case "FEQS", "FLTS", "FLES", "FEQD", "FLTD", "FLED":
		return true
	}
	return false
}

func isFPCvtInstr(m string) bool {
	_, ok := riscvCvtTable[m]
	return ok
}

// Operand helpers.
func regFromOperand(op *ast.Operand) int {
	// Register is in Addr.Base (from (base) syntax) or Addr.Sym.Name (bare ident).
	if op.Addr.Base != "" {
		return riscvRegNum(op.Addr.Base)
	}
	if op.Addr.Sym != nil && op.Addr.Sym.Name != "" {
		return riscvRegNum(op.Addr.Sym.Name)
	}
	return -1
}

func immFromOperand(op *ast.Operand) int32 {
	if op.Imm.HasVal {
		v := op.Imm.Val
		if op.Imm.Neg {
			v = -v
		}
		return int32(v)
	}
	return 0
}

func memFromOperand(op *ast.Operand) (rs1 int, imm int32) {
	rs1 = riscvRegNum(op.Addr.Base)
	imm = int32(op.Addr.Offset)
	return
}

// memFromOperandWithFrame resolves a memory operand, handling FP/SP
// pseudo-registers via the frame mapping.
func memFromOperandWithFrame(op *ast.Operand, fi riscvFrameInfo) (rs1 int, imm int32) {
	// Check for a pseudo-register reference (name+offset(FP) or name+offset(SP)).
	if op.Addr.Sym != nil && op.Addr.Sym.Pseudo != "" {
		return riscvResolvePseudo(op.Addr.Sym, fi)
	}
	// Plain register+offset memory reference.
	return memFromOperand(op)
}

func labelFromOperand(op *ast.Operand) string {
	if op.Addr.Sym != nil {
		return op.Addr.Sym.Name
	}
	return op.Raw
}