gasm-sdk/asm/amd64_x87.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 x87 floating-point family the Go assembler
// carries: the no-operand stack controls, the two-register arithmetic pair,
// the register compares, the memory loads and stores and the FXSAVE pair.
// Every encoding here is pinned byte for byte against go tool asm through
// the corpus lines in amd64_x87_test.go.

// x87NoOperand maps the no-operand x87 instruction to its postfix byte
// inside the D9 escape: D9 <postfix>, no ModR/M, no operand.
var x87NoOperand = map[string]byte{
	"F2XM1":   0xF0,
	"FABS":    0xE1,
	"FCHS":    0xE0,
	"FCOS":    0xFF,
	"FDECSTP": 0xF6,
	"FINCSTP": 0xF7,
	"FLD1":    0xE8,
	"FLDL2E":  0xEA,
	"FLDL2T":  0xE9,
	"FLDLG2":  0xEC,
	"FLDPI":   0xEB,
	"FNOP":    0xD0,
	"FPATAN":  0xF3,
	"FPREM":   0xF8,
	"FPREM1":  0xF5,
	"FPTAN":   0xF2,
	"FRNDINT": 0xFC,
	"FSCALE":  0xFD,
	"FSIN":    0xFE,
	"FSINCOS": 0xFB,
	"FSQRT":   0xFA,
	"FTST":    0xE4,
	"FXAM":    0xE5,
	"FXTRACT": 0xF4,
	"FYL2X":   0xF1,
	"FYL2XP1": 0xF9,
}

// x87ArithSpec describes one member of the D8/DC two-register arithmetic
// pair.  The D8 form reads ST(0) as its second operand (FADDD F2, F0), the
// DC form ST(0) as its first (FADDD F0, F2) or a memory source (FADDD (BX),
// F0).  FDIV is the odd member: with ST(0) as the first operand the toolchain
// assembles the reversed register form (DC F8+i, the FDIVR digit), so the DC
// digit differs from the memory digit there.
type x87ArithSpec struct {
	d8Digit  int // the /digit of the D8 form (dst = ST(0))
	dcDigit  int // the /digit of the DC register form (src = ST(0))
	memDigit int // the /digit of the DC memory form (dst = ST(0))
}

// x87Arith maps the arithmetic mnemonics to their digits.
var x87Arith = map[string]x87ArithSpec{
	"FADDD": {0, 0, 0},
	"FCOMD": {2, 2, 2},
	"FDIVD": {6, 7, 6},
}

// x87FCmov maps the conditional x87 moves to their escape byte and postfix
// base (the C0/C8/D0/D8 group the condition selects); the compared register
// rides the postfix's low three bits.
var x87FCmov = map[string][2]byte{
	"FCMOVB":   {0xDA, 0xC0},
	"FCMOVBE":  {0xDA, 0xD0},
	"FCMOVE":   {0xDA, 0xC8},
	"FCMOVNB":  {0xDB, 0xC0},
	"FCMOVNBE": {0xDB, 0xD0},
	"FCMOVNE":  {0xDB, 0xC8},
	"FCMOVNU":  {0xDB, 0xD8},
	"FCMOVU":   {0xDA, 0xD8},
}

// x87Compare maps the register compare pair to their escape byte; the
// register form is escape F0+i (mod 11, reg 110, rm = the compared stack
// register), ST(0) fixed as the second operand.
var x87Compare = map[string]byte{
	"FCOMI":  0xDB,
	"FCOMIP": 0xDF,
}

// x87MemUnary maps the one-memory-operand x87 controls to their escape byte
// and /digit.
var x87MemUnary = map[string]struct {
	escape byte
	digit  int
}{
	"FBLD":  {0xDF, 4},
	"FBSTP": {0xDF, 6},
	"FLDCW": {0xD9, 5},
}

// x87Fxsav maps the FXSAVE pair to their /digit in the 0F AE group; the 64
// spellings carry REX.W.
var x87Fxsav = map[string]struct {
	digit int
	rexW  bool
}{
	"FXSAVE":    {0, false},
	"FXSAVE64":  {0, true},
	"FXRSTOR":   {1, false},
	"FXRSTOR64": {1, true},
}

// encodeX87 encodes the x87 family.  It reports whether the mnemonic belongs
// to the family; a false result hands the mnemonic back to the caller, an
// error result a failed attempt to encode it.
func (e *enc) encodeX87(upper string, ops []Operand) (bool, error) {
	if post, ok := x87NoOperand[upper]; ok {
		if len(ops) != 0 {
			return true, fmt.Errorf("%s takes no operands, got %d", upper, len(ops))
		}
		return true, e.emit(&instr{opcode: []byte{0xD9, post}, modrm: -1, sib: -1})
	}
	if spec, ok := x87Arith[upper]; ok {
		return true, e.encodeX87Arith(upper, spec, ops)
	}
	if esc, ok := x87FCmov[upper]; ok {
		src, _, err := x87PairOperands(upper, ops)
		if err != nil {
			return true, err
		}
		// The condition applies between the named register and ST(0), so the
		// second operand is always F0; the first rides the postfix's low bits.
		return true, e.emit(&instr{opcode: []byte{esc[0], esc[1] | byte(src.idx&7)}, modrm: -1, sib: -1})
	}
	if escape, ok := x87Compare[upper]; ok {
		if _, _, err := x87PairOperands(upper, ops); err != nil {
			return true, err
		}
		src, _ := ops[0].(Reg)
		return true, e.emit(&instr{opcode: []byte{escape, 0xF0 | byte(src.idx&7)}, modrm: -1, sib: -1})
	}
	if upper == "FADDDP" {
		if len(ops) != 2 {
			return true, fmt.Errorf("FADDDP expects 2 operands, got %d", len(ops))
		}
		first, ok1 := ops[0].(Reg)
		second, ok2 := ops[1].(Reg)
		if !ok1 || !ok2 || !first.fp || !second.fp {
			return true, fmt.Errorf("FADDDP takes two x87 stack registers")
		}
		if first.idx != 0 {
			return true, fmt.Errorf("FADDDP: the first operand must be F0")
		}
		return true, e.emit(&instr{opcode: []byte{0xDE, 0xC0 | byte(second.idx&7)}, modrm: -1, sib: -1})
	}
	if m, ok := x87MemUnary[upper]; ok {
		if len(ops) != 1 {
			return true, fmt.Errorf("%s expects 1 memory operand, got %d", upper, len(ops))
		}
		if !isX86Mem(ops[0]) {
			return true, fmt.Errorf("%s requires a memory operand", upper)
		}
		i := &instr{opcode: []byte{m.escape}, modrm: -1, sib: -1}
		if err := setRMDigit(i, m.digit, ops[0], 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	if m, ok := x87Fxsav[upper]; ok {
		if len(ops) != 1 {
			return true, fmt.Errorf("%s expects 1 memory operand, got %d", upper, len(ops))
		}
		if !isX86Mem(ops[0]) {
			return true, fmt.Errorf("%s requires a memory operand", upper)
		}
		i := newInstr(0, []byte{0x0F, 0xAE})
		i.rexW = m.rexW
		if err := setRMDigit(i, m.digit, ops[0], 8); err != nil {
			return true, err
		}
		return true, e.emit(i)
	}
	return false, nil
}

// encodeX87Arith encodes one member of the D8/DC arithmetic pair.  The tool-
// chain's shape set: (Fn, F0) rides D8, (F0, Fn) rides DC, ((m), F0) rides
// the DC memory digit; every other pairing is an error.
func (e *enc) encodeX87Arith(mnem string, spec x87ArithSpec, ops []Operand) error {
	if len(ops) != 2 {
		return fmt.Errorf("%s expects 2 operands, got %d", mnem, len(ops))
	}
	src, dst := ops[0], ops[1]
	dstReg, dstIsF := dst.(Reg)
	if !dstIsF || !dstReg.fp {
		return fmt.Errorf("%s: the destination must be an x87 stack register", mnem)
	}
	switch s := src.(type) {
	case Reg:
		if !s.fp {
			return fmt.Errorf("%s: the source must be an x87 stack register", mnem)
		}
		switch {
		case dstReg.idx == 0:
			return e.emit(&instr{opcode: []byte{0xD8, 0xC0 | byte(spec.d8Digit)<<3 | byte(s.idx&7)}, modrm: -1, sib: -1})
		case s.idx == 0:
			return e.emit(&instr{opcode: []byte{0xDC, 0xC0 | byte(spec.dcDigit)<<3 | byte(dstReg.idx&7)}, modrm: -1, sib: -1})
		default:
			return fmt.Errorf("%s: one operand must be F0", mnem)
		}
	default:
		if !isX86Mem(src) {
			return fmt.Errorf("%s: the source must be an x87 stack register or memory", mnem)
		}
		if dstReg.idx != 0 {
			return fmt.Errorf("%s: the destination must be F0 with a memory source", mnem)
		}
		i := &instr{opcode: []byte{0xDC}, modrm: -1, sib: -1}
		if err := setRMDigit(i, spec.memDigit, src, 8); err != nil {
			return err
		}
		return e.emit(i)
	}
}

// x87PairOperands validates the (register, F0) shape the conditional moves
// and compares take and returns the two registers.
func x87PairOperands(mnem string, ops []Operand) (Reg, Reg, error) {
	if len(ops) != 2 {
		return Reg{}, Reg{}, fmt.Errorf("%s expects 2 operands, got %d", mnem, len(ops))
	}
	src, ok1 := ops[0].(Reg)
	dst, ok2 := ops[1].(Reg)
	if !ok1 || !ok2 || !src.fp || !dst.fp {
		return Reg{}, Reg{}, fmt.Errorf("%s takes two x87 stack registers", mnem)
	}
	if dst.idx != 0 {
		return Reg{}, Reg{}, fmt.Errorf("%s: the second operand must be F0", mnem)
	}
	return src, dst, nil
}