go decode 源码
golang decode 代码
文件路径：/src/cmd/vendor/golang.org/x/arch/arm/armasm/decode.go
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package armasm
import (
"encoding/binary"
"fmt"
)
// An instFormat describes the format of an instruction encoding.
// An instruction with 32-bit value x matches the format if x&mask == value
// and the condition matches.
// The condition matches if x>>28 == 0xF && value>>28==0xF
// or if x>>28 != 0xF and value>>28 == 0.
// If x matches the format, then the rest of the fields describe how to interpret x.
// The opBits describe bits that should be extracted from x and added to the opcode.
// For example opBits = 0x1234 means that the value
// (2 bits at offset 1) followed by (4 bits at offset 3)
// should be added to op.
// Finally the args describe how to decode the instruction arguments.
// args is stored as a fixed-size array; if there are fewer than len(args) arguments,
// args[i] == 0 marks the end of the argument list.
type instFormat struct {
mask uint32
value uint32
priority int8
op Op
opBits uint64
args instArgs
}
type instArgs [4]instArg
var (
errMode = fmt.Errorf("unsupported execution mode")
errShort = fmt.Errorf("truncated instruction")
errUnknown = fmt.Errorf("unknown instruction")
)
var decoderCover []bool
// Decode decodes the leading bytes in src as a single instruction.
func Decode(src []byte, mode Mode) (inst Inst, err error) {
if mode != ModeARM {
return Inst{}, errMode
}
if len(src) < 4 {
return Inst{}, errShort
}
if decoderCover == nil {
decoderCover = make([]bool, len(instFormats))
}
x := binary.LittleEndian.Uint32(src)
// The instFormat table contains both conditional and unconditional instructions.
// Considering only the top 4 bits, the conditional instructions use mask=0, value=0,
// while the unconditional instructions use mask=f, value=f.
// Prepare a version of x with the condition cleared to 0 in conditional instructions
// and then assume mask=f during matching.
const condMask = 0xf0000000
xNoCond := x
if x&condMask != condMask {
xNoCond &^= condMask
}
var priority int8
Search:
for i := range instFormats {
f := &instFormats[i]
if xNoCond&(f.mask|condMask) != f.value || f.priority <= priority {
continue
}
delta := uint32(0)
deltaShift := uint(0)
for opBits := f.opBits; opBits != 0; opBits >>= 16 {
n := uint(opBits & 0xFF)
off := uint((opBits >> 8) & 0xFF)
delta |= (x >> off) & (1<<n - 1) << deltaShift
deltaShift += n
}
op := f.op + Op(delta)
// Special case: BKPT encodes with condition but cannot have one.
if op&^15 == BKPT_EQ && op != BKPT {
continue Search
}
var args Args
for j, aop := range f.args {
if aop == 0 {
break
}
arg := decodeArg(aop, x)
if arg == nil { // cannot decode argument
continue Search
}
args[j] = arg
}
decoderCover[i] = true
inst = Inst{
Op: op,
Args: args,
Enc: x,
Len: 4,
}
priority = f.priority
continue Search
}
if inst.Op != 0 {
return inst, nil
}
return Inst{}, errUnknown
}
// An instArg describes the encoding of a single argument.
// In the names used for arguments, _p_ means +, _m_ means -,
// _pm_ means ± (usually keyed by the U bit).
// The _W suffix indicates a general addressing mode based on the P and W bits.
// The _offset and _postindex suffixes force the given addressing mode.
// The rest should be somewhat self-explanatory, at least given
// the decodeArg function.
type instArg uint8
const (
_ instArg = iota
arg_APSR
arg_FPSCR
arg_Dn_half
arg_R1_0
arg_R1_12
arg_R2_0
arg_R2_12
arg_R_0
arg_R_12
arg_R_12_nzcv
arg_R_16
arg_R_16_WB
arg_R_8
arg_R_rotate
arg_R_shift_R
arg_R_shift_imm
arg_SP
arg_Sd
arg_Sd_Dd
arg_Dd_Sd
arg_Sm
arg_Sm_Dm
arg_Sn
arg_Sn_Dn
arg_const
arg_endian
arg_fbits
arg_fp_0
arg_imm24
arg_imm5
arg_imm5_32
arg_imm5_nz
arg_imm_12at8_4at0
arg_imm_4at16_12at0
arg_imm_vfp
arg_label24
arg_label24H
arg_label_m_12
arg_label_p_12
arg_label_pm_12
arg_label_pm_4_4
arg_lsb_width
arg_mem_R
arg_mem_R_pm_R_W
arg_mem_R_pm_R_postindex
arg_mem_R_pm_R_shift_imm_W
arg_mem_R_pm_R_shift_imm_offset
arg_mem_R_pm_R_shift_imm_postindex
arg_mem_R_pm_imm12_W
arg_mem_R_pm_imm12_offset
arg_mem_R_pm_imm12_postindex
arg_mem_R_pm_imm8_W
arg_mem_R_pm_imm8_postindex
arg_mem_R_pm_imm8at0_offset
arg_option
arg_registers
arg_registers1
arg_registers2
arg_satimm4
arg_satimm5
arg_satimm4m1
arg_satimm5m1
arg_widthm1
)
// decodeArg decodes the arg described by aop from the instruction bits x.
// It returns nil if x cannot be decoded according to aop.
func decodeArg(aop instArg, x uint32) Arg {
switch aop {
default:
return nil
case arg_APSR:
return APSR
case arg_FPSCR:
return FPSCR
case arg_R_0:
return Reg(x & (1<<4 - 1))
case arg_R_8:
return Reg((x >> 8) & (1<<4 - 1))
case arg_R_12:
return Reg((x >> 12) & (1<<4 - 1))
case arg_R_16:
return Reg((x >> 16) & (1<<4 - 1))
case arg_R_12_nzcv:
r := Reg((x >> 12) & (1<<4 - 1))
if r == R15 {
return APSR_nzcv
}
return r
case arg_R_16_WB:
mode := AddrLDM
if (x>>21)&1 != 0 {
mode = AddrLDM_WB
}
return Mem{Base: Reg((x >> 16) & (1<<4 - 1)), Mode: mode}
case arg_R_rotate:
Rm := Reg(x & (1<<4 - 1))
typ, count := decodeShift(x)
// ROR #0 here means ROR #0, but decodeShift rewrites to RRX #1.
if typ == RotateRightExt {
return Reg(Rm)
}
return RegShift{Rm, typ, uint8(count)}
case arg_R_shift_R:
Rm := Reg(x & (1<<4 - 1))
Rs := Reg((x >> 8) & (1<<4 - 1))
typ := Shift((x >> 5) & (1<<2 - 1))
return RegShiftReg{Rm, typ, Rs}
case arg_R_shift_imm:
Rm := Reg(x & (1<<4 - 1))
typ, count := decodeShift(x)
if typ == ShiftLeft && count == 0 {
return Reg(Rm)
}
return RegShift{Rm, typ, uint8(count)}
case arg_R1_0:
return Reg((x & (1<<4 - 1)))
case arg_R1_12:
return Reg(((x >> 12) & (1<<4 - 1)))
case arg_R2_0:
return Reg((x & (1<<4 - 1)) | 1)
case arg_R2_12:
return Reg(((x >> 12) & (1<<4 - 1)) | 1)
case arg_SP:
return SP
case arg_Sd_Dd:
v := (x >> 12) & (1<<4 - 1)
vx := (x >> 22) & 1
sz := (x >> 8) & 1
if sz != 0 {
return D0 + Reg(vx<<4+v)
} else {
return S0 + Reg(v<<1+vx)
}
case arg_Dd_Sd:
return decodeArg(arg_Sd_Dd, x^(1<<8))
case arg_Sd:
v := (x >> 12) & (1<<4 - 1)
vx := (x >> 22) & 1
return S0 + Reg(v<<1+vx)
case arg_Sm_Dm:
v := (x >> 0) & (1<<4 - 1)
vx := (x >> 5) & 1
sz := (x >> 8) & 1
if sz != 0 {
return D0 + Reg(vx<<4+v)
} else {
return S0 + Reg(v<<1+vx)
}
case arg_Sm:
v := (x >> 0) & (1<<4 - 1)
vx := (x >> 5) & 1
return S0 + Reg(v<<1+vx)
case arg_Dn_half:
v := (x >> 16) & (1<<4 - 1)
vx := (x >> 7) & 1
return RegX{D0 + Reg(vx<<4+v), int((x >> 21) & 1)}
case arg_Sn_Dn:
v := (x >> 16) & (1<<4 - 1)
vx := (x >> 7) & 1
sz := (x >> 8) & 1
if sz != 0 {
return D0 + Reg(vx<<4+v)
} else {
return S0 + Reg(v<<1+vx)
}
case arg_Sn:
v := (x >> 16) & (1<<4 - 1)
vx := (x >> 7) & 1
return S0 + Reg(v<<1+vx)
case arg_const:
v := x & (1<<8 - 1)
rot := (x >> 8) & (1<<4 - 1) * 2
if rot > 0 && v&3 == 0 {
// could rotate less
return ImmAlt{uint8(v), uint8(rot)}
}
if rot >= 24 && ((v<<(32-rot))&0xFF)>>(32-rot) == v {
// could wrap around to rot==0.
return ImmAlt{uint8(v), uint8(rot)}
}
return Imm(v>>rot | v<<(32-rot))
case arg_endian:
return Endian((x >> 9) & 1)
case arg_fbits:
return Imm((16 << ((x >> 7) & 1)) - ((x&(1<<4-1))<<1 | (x>>5)&1))
case arg_fp_0:
return Imm(0)
case arg_imm24:
return Imm(x & (1<<24 - 1))
case arg_imm5:
return Imm((x >> 7) & (1<<5 - 1))
case arg_imm5_32:
x = (x >> 7) & (1<<5 - 1)
if x == 0 {
x = 32
}
return Imm(x)
case arg_imm5_nz:
x = (x >> 7) & (1<<5 - 1)
if x == 0 {
return nil
}
return Imm(x)
case arg_imm_4at16_12at0:
return Imm((x>>16)&(1<<4-1)<<12 | x&(1<<12-1))
case arg_imm_12at8_4at0:
return Imm((x>>8)&(1<<12-1)<<4 | x&(1<<4-1))
case arg_imm_vfp:
x = (x>>16)&(1<<4-1)<<4 | x&(1<<4-1)
return Imm(x)
case arg_label24:
imm := (x & (1<<24 - 1)) << 2
return PCRel(int32(imm<<6) >> 6)
case arg_label24H:
h := (x >> 24) & 1
imm := (x&(1<<24-1))<<2 | h<<1
return PCRel(int32(imm<<6) >> 6)
case arg_label_m_12:
d := int32(x & (1<<12 - 1))
return Mem{Base: PC, Mode: AddrOffset, Offset: int16(-d)}
case arg_label_p_12:
d := int32(x & (1<<12 - 1))
return Mem{Base: PC, Mode: AddrOffset, Offset: int16(d)}
case arg_label_pm_12:
d := int32(x & (1<<12 - 1))
u := (x >> 23) & 1
if u == 0 {
d = -d
}
return Mem{Base: PC, Mode: AddrOffset, Offset: int16(d)}
case arg_label_pm_4_4:
d := int32((x>>8)&(1<<4-1)<<4 | x&(1<<4-1))
u := (x >> 23) & 1
if u == 0 {
d = -d
}
return PCRel(d)
case arg_lsb_width:
lsb := (x >> 7) & (1<<5 - 1)
msb := (x >> 16) & (1<<5 - 1)
if msb < lsb || msb >= 32 {
return nil
}
return Imm(msb + 1 - lsb)
case arg_mem_R:
Rn := Reg((x >> 16) & (1<<4 - 1))
return Mem{Base: Rn, Mode: AddrOffset}
case arg_mem_R_pm_R_postindex:
// Treat [<Rn>],+/-<Rm> like [<Rn>,+/-<Rm>{,<shift>}]{!}
// by forcing shift bits to <<0 and P=0, W=0 (postindex=true).
return decodeArg(arg_mem_R_pm_R_shift_imm_W, x&^((1<<7-1)<<5|1<<24|1<<21))
case arg_mem_R_pm_R_W:
// Treat [<Rn>,+/-<Rm>]{!} like [<Rn>,+/-<Rm>{,<shift>}]{!}
// by forcing shift bits to <<0.
return decodeArg(arg_mem_R_pm_R_shift_imm_W, x&^((1<<7-1)<<5))
case arg_mem_R_pm_R_shift_imm_offset:
// Treat [<Rn>],+/-<Rm>{,<shift>} like [<Rn>,+/-<Rm>{,<shift>}]{!}
// by forcing P=1, W=0 (index=false, wback=false).
return decodeArg(arg_mem_R_pm_R_shift_imm_W, x&^(1<<21)|1<<24)
case arg_mem_R_pm_R_shift_imm_postindex:
// Treat [<Rn>],+/-<Rm>{,<shift>} like [<Rn>,+/-<Rm>{,<shift>}]{!}
// by forcing P=0, W=0 (postindex=true).
return decodeArg(arg_mem_R_pm_R_shift_imm_W, x&^(1<<24|1<<21))
case arg_mem_R_pm_R_shift_imm_W:
Rn := Reg((x >> 16) & (1<<4 - 1))
Rm := Reg(x & (1<<4 - 1))
typ, count := decodeShift(x)
u := (x >> 23) & 1
w := (x >> 21) & 1
p := (x >> 24) & 1
if p == 0 && w == 1 {
return nil
}
sign := int8(+1)
if u == 0 {
sign = -1
}
mode := AddrMode(uint8(p<<1) | uint8(w^1))
return Mem{Base: Rn, Mode: mode, Sign: sign, Index: Rm, Shift: typ, Count: count}
case arg_mem_R_pm_imm12_offset:
// Treat [<Rn>,#+/-<imm12>] like [<Rn>{,#+/-<imm12>}]{!}
// by forcing P=1, W=0 (index=false, wback=false).
return decodeArg(arg_mem_R_pm_imm12_W, x&^(1<<21)|1<<24)
case arg_mem_R_pm_imm12_postindex:
// Treat [<Rn>],#+/-<imm12> like [<Rn>{,#+/-<imm12>}]{!}
// by forcing P=0, W=0 (postindex=true).
return decodeArg(arg_mem_R_pm_imm12_W, x&^(1<<24|1<<21))
case arg_mem_R_pm_imm12_W:
Rn := Reg((x >> 16) & (1<<4 - 1))
u := (x >> 23) & 1
w := (x >> 21) & 1
p := (x >> 24) & 1
if p == 0 && w == 1 {
return nil
}
sign := int8(+1)
if u == 0 {
sign = -1
}
imm := int16(x & (1<<12 - 1))
mode := AddrMode(uint8(p<<1) | uint8(w^1))
return Mem{Base: Rn, Mode: mode, Offset: int16(sign) * imm}
case arg_mem_R_pm_imm8_postindex:
// Treat [<Rn>],#+/-<imm8> like [<Rn>{,#+/-<imm8>}]{!}
// by forcing P=0, W=0 (postindex=true).
return decodeArg(arg_mem_R_pm_imm8_W, x&^(1<<24|1<<21))
case arg_mem_R_pm_imm8_W:
Rn := Reg((x >> 16) & (1<<4 - 1))
u := (x >> 23) & 1
w := (x >> 21) & 1
p := (x >> 24) & 1
if p == 0 && w == 1 {
return nil
}
sign := int8(+1)
if u == 0 {
sign = -1
}
imm := int16((x>>8)&(1<<4-1)<<4 | x&(1<<4-1))
mode := AddrMode(uint8(p<<1) | uint8(w^1))
return Mem{Base: Rn, Mode: mode, Offset: int16(sign) * imm}
case arg_mem_R_pm_imm8at0_offset:
Rn := Reg((x >> 16) & (1<<4 - 1))
u := (x >> 23) & 1
sign := int8(+1)
if u == 0 {
sign = -1
}
imm := int16(x&(1<<8-1)) << 2
return Mem{Base: Rn, Mode: AddrOffset, Offset: int16(sign) * imm}
case arg_option:
return Imm(x & (1<<4 - 1))
case arg_registers:
return RegList(x & (1<<16 - 1))
case arg_registers2:
x &= 1<<16 - 1
n := 0
for i := 0; i < 16; i++ {
if x>>uint(i)&1 != 0 {
n++
}
}
if n < 2 {
return nil
}
return RegList(x)
case arg_registers1:
Rt := (x >> 12) & (1<<4 - 1)
return RegList(1 << Rt)
case arg_satimm4:
return Imm((x >> 16) & (1<<4 - 1))
case arg_satimm5:
return Imm((x >> 16) & (1<<5 - 1))
case arg_satimm4m1:
return Imm((x>>16)&(1<<4-1) + 1)
case arg_satimm5m1:
return Imm((x>>16)&(1<<5-1) + 1)
case arg_widthm1:
return Imm((x>>16)&(1<<5-1) + 1)
}
}
// decodeShift decodes the shift-by-immediate encoded in x.
func decodeShift(x uint32) (Shift, uint8) {
count := (x >> 7) & (1<<5 - 1)
typ := Shift((x >> 5) & (1<<2 - 1))
switch typ {
case ShiftRight, ShiftRightSigned:
if count == 0 {
count = 32
}
case RotateRight:
if count == 0 {
typ = RotateRightExt
count = 1
}
}
return typ, uint8(count)
}
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