path: root/js/src/jit/arm/disasm/Constants-arm.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 */
// Copyright 2011 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef jit_arm_disasm_Constants_arm_h
#define jit_arm_disasm_Constants_arm_h

#ifdef JS_DISASM_ARM

#include "mozilla/Assertions.h"
#include "mozilla/Types.h"

#include <string.h>

namespace js {
namespace jit {
namespace disasm {

// Constant pool marker.
// Use UDF, the permanently undefined instruction.
const int kConstantPoolMarkerMask = 0xfff000f0;
const int kConstantPoolMarker = 0xe7f000f0;
const int kConstantPoolLengthMaxMask = 0xffff;

inline int
EncodeConstantPoolLength(int length)
{
    MOZ_ASSERT((length & kConstantPoolLengthMaxMask) == length);
    return ((length & 0xfff0) << 4) | (length & 0xf);
}

inline int
DecodeConstantPoolLength(int instr)
{
    MOZ_ASSERT((instr & kConstantPoolMarkerMask) == kConstantPoolMarker);
    return ((instr >> 4) & 0xfff0) | (instr & 0xf);
}

// Used in code age prologue - ldr(pc, MemOperand(pc, -4))
const int kCodeAgeJumpInstruction = 0xe51ff004;

// Number of registers in normal ARM mode.
const int kNumRegisters = 16;

// VFP support.
const int kNumVFPSingleRegisters = 32;
const int kNumVFPDoubleRegisters = 32;
const int kNumVFPRegisters = kNumVFPSingleRegisters + kNumVFPDoubleRegisters;

// PC is register 15.
const int kPCRegister = 15;
const int kNoRegister = -1;

// -----------------------------------------------------------------------------
// Conditions.

// Defines constants and accessor classes to assemble, disassemble and
// simulate ARM instructions.
//
// Section references in the code refer to the "ARM Architecture Reference
// Manual" from July 2005 (available at http://www.arm.com/miscPDFs/14128.pdf)
//
// Constants for specific fields are defined in their respective named enums.
// General constants are in an anonymous enum in class Instr.

// Values for the condition field as defined in section A3.2
enum Condition {
    kNoCondition = -1,

    eq =  0 << 28,                 // Z set            Equal.
    ne =  1 << 28,                 // Z clear          Not equal.
    cs =  2 << 28,                 // C set            Unsigned higher or same.
    cc =  3 << 28,                 // C clear          Unsigned lower.
    mi =  4 << 28,                 // N set            Negative.
    pl =  5 << 28,                 // N clear          Positive or zero.
    vs =  6 << 28,                 // V set            Overflow.
    vc =  7 << 28,                 // V clear          No overflow.
    hi =  8 << 28,                 // C set, Z clear   Unsigned higher.
    ls =  9 << 28,                 // C clear or Z set Unsigned lower or same.
    ge = 10 << 28,                 // N == V           Greater or equal.
    lt = 11 << 28,                 // N != V           Less than.
    gt = 12 << 28,                 // Z clear, N == V  Greater than.
    le = 13 << 28,                 // Z set or N != V  Less then or equal
    al = 14 << 28,                 //                  Always.

    kSpecialCondition = 15 << 28,  // Special condition (refer to section A3.2.1).
    kNumberOfConditions = 16,

    // Aliases.
    hs = cs,                       // C set            Unsigned higher or same.
    lo = cc                        // C clear          Unsigned lower.
};


inline Condition
NegateCondition(Condition cond)
{
    MOZ_ASSERT(cond != al);
    return static_cast<Condition>(cond ^ ne);
}


// Commute a condition such that {a cond b == b cond' a}.
inline Condition
CommuteCondition(Condition cond)
{
    switch (cond) {
      case lo:
        return hi;
      case hi:
        return lo;
      case hs:
        return ls;
      case ls:
        return hs;
      case lt:
        return gt;
      case gt:
        return lt;
      case ge:
        return le;
      case le:
        return ge;
      default:
        return cond;
    }
}


// -----------------------------------------------------------------------------
// Instructions encoding.

// Instr is merely used by the Assembler to distinguish 32bit integers
// representing instructions from usual 32 bit values.
// Instruction objects are pointers to 32bit values, and provide methods to
// access the various ISA fields.
typedef int32_t Instr;


// Opcodes for Data-processing instructions (instructions with a type 0 and 1)
// as defined in section A3.4
enum Opcode {
    AND =  0 << 21,  // Logical AND.
    EOR =  1 << 21,  // Logical Exclusive OR.
    SUB =  2 << 21,  // Subtract.
    RSB =  3 << 21,  // Reverse Subtract.
    ADD =  4 << 21,  // Add.
    ADC =  5 << 21,  // Add with Carry.
    SBC =  6 << 21,  // Subtract with Carry.
    RSC =  7 << 21,  // Reverse Subtract with Carry.
    TST =  8 << 21,  // Test.
    TEQ =  9 << 21,  // Test Equivalence.
    CMP = 10 << 21,  // Compare.
    CMN = 11 << 21,  // Compare Negated.
    ORR = 12 << 21,  // Logical (inclusive) OR.
    MOV = 13 << 21,  // Move.
    BIC = 14 << 21,  // Bit Clear.
    MVN = 15 << 21   // Move Not.
};


// The bits for bit 7-4 for some type 0 miscellaneous instructions.
enum MiscInstructionsBits74 {
    // With bits 22-21 01.
    BX   =  1 << 4,
    BXJ  =  2 << 4,
    BLX  =  3 << 4,
    BKPT =  7 << 4,

    // With bits 22-21 11.
    CLZ  =  1 << 4
};

// Load and store exclusive instructions.

// Bit positions.
enum {
    ExclusiveOpHi = 24,         // Hi bit of opcode field
    ExclusiveOpLo = 23,         // Lo bit of opcode field
    ExclusiveSizeHi = 22,       // Hi bit of operand size field
    ExclusiveSizeLo = 21,       // Lo bit of operand size field
    ExclusiveLoad = 20          // Bit indicating load
};

// Opcode bits for exclusive instructions.
enum {
    ExclusiveOpcode = 3
};

// Operand size, Bits(ExclusiveSizeHi,ExclusiveSizeLo).
enum {
    ExclusiveWord = 0,
    ExclusiveDouble = 1,
    ExclusiveByte = 2,
    ExclusiveHalf = 3
};

// Instruction encoding bits and masks.
enum {
    H = 1 << 5,   // Halfword (or byte).
    S6 = 1 << 6,  // Signed (or unsigned).
    L = 1 << 20,  // Load (or store).
    S = 1 << 20,  // Set condition code (or leave unchanged).
    W = 1 << 21,  // Writeback base register (or leave unchanged).
    A = 1 << 21,  // Accumulate in multiply instruction (or not).
    B = 1 << 22,  // Unsigned byte (or word).
    N = 1 << 22,  // Long (or short).
    U = 1 << 23,  // Positive (or negative) offset/index.
    P = 1 << 24,  // Offset/pre-indexed addressing (or post-indexed addressing).
    I = 1 << 25,  // Immediate shifter operand (or not).
    B0 = 1 << 0,
    B4 = 1 << 4,
    B5 = 1 << 5,
    B6 = 1 << 6,
    B7 = 1 << 7,
    B8 = 1 << 8,
    B9 = 1 << 9,
    B12 = 1 << 12,
    B16 = 1 << 16,
    B17 = 1 << 17,
    B18 = 1 << 18,
    B19 = 1 << 19,
    B20 = 1 << 20,
    B21 = 1 << 21,
    B22 = 1 << 22,
    B23 = 1 << 23,
    B24 = 1 << 24,
    B25 = 1 << 25,
    B26 = 1 << 26,
    B27 = 1 << 27,
    B28 = 1 << 28,

    // Instruction bit masks.
    kCondMask = 15 << 28,
    kALUMask = 0x6f << 21,
    kRdMask = 15 << 12,  // In str instruction.
    kCoprocessorMask = 15 << 8,
    kOpCodeMask = 15 << 21,  // In data-processing instructions.
    kImm24Mask = (1 << 24) - 1,
    kImm16Mask = (1 << 16) - 1,
    kImm8Mask = (1 << 8) - 1,
    kOff12Mask = (1 << 12) - 1,
    kOff8Mask = (1 << 8) - 1
};


// -----------------------------------------------------------------------------
// Addressing modes and instruction variants.

// Condition code updating mode.
enum SBit {
    SetCC   = 1 << 20,  // Set condition code.
    LeaveCC = 0 << 20   // Leave condition code unchanged.
};


// Status register selection.
enum SRegister {
    CPSR = 0 << 22,
    SPSR = 1 << 22
};


// Shifter types for Data-processing operands as defined in section A5.1.2.
enum ShiftOp {
    LSL = 0 << 5,   // Logical shift left.
    LSR = 1 << 5,   // Logical shift right.
    ASR = 2 << 5,   // Arithmetic shift right.
    ROR = 3 << 5,   // Rotate right.

    // RRX is encoded as ROR with shift_imm == 0.
    // Use a special code to make the distinction. The RRX ShiftOp is only used
    // as an argument, and will never actually be encoded. The Assembler will
    // detect it and emit the correct ROR shift operand with shift_imm == 0.
    RRX = -1,
    kNumberOfShifts = 4
};


// Status register fields.
enum SRegisterField {
    CPSR_c = CPSR | 1 << 16,
    CPSR_x = CPSR | 1 << 17,
    CPSR_s = CPSR | 1 << 18,
    CPSR_f = CPSR | 1 << 19,
    SPSR_c = SPSR | 1 << 16,
    SPSR_x = SPSR | 1 << 17,
    SPSR_s = SPSR | 1 << 18,
    SPSR_f = SPSR | 1 << 19
};

// Status register field mask (or'ed SRegisterField enum values).
typedef uint32_t SRegisterFieldMask;


// Memory operand addressing mode.
enum AddrMode {
    // Bit encoding P U W.
    Offset       = (8|4|0) << 21,  // Offset (without writeback to base).
    PreIndex     = (8|4|1) << 21,  // Pre-indexed addressing with writeback.
    PostIndex    = (0|4|0) << 21,  // Post-indexed addressing with writeback.
    NegOffset    = (8|0|0) << 21,  // Negative offset (without writeback to base).
    NegPreIndex  = (8|0|1) << 21,  // Negative pre-indexed with writeback.
    NegPostIndex = (0|0|0) << 21   // Negative post-indexed with writeback.
};


// Load/store multiple addressing mode.
enum BlockAddrMode {
    // Bit encoding P U W .
    da           = (0|0|0) << 21,  // Decrement after.
    ia           = (0|4|0) << 21,  // Increment after.
    db           = (8|0|0) << 21,  // Decrement before.
    ib           = (8|4|0) << 21,  // Increment before.
    da_w         = (0|0|1) << 21,  // Decrement after with writeback to base.
    ia_w         = (0|4|1) << 21,  // Increment after with writeback to base.
    db_w         = (8|0|1) << 21,  // Decrement before with writeback to base.
    ib_w         = (8|4|1) << 21,  // Increment before with writeback to base.

    // Alias modes for comparison when writeback does not matter.
    da_x         = (0|0|0) << 21,  // Decrement after.
    ia_x         = (0|4|0) << 21,  // Increment after.
    db_x         = (8|0|0) << 21,  // Decrement before.
    ib_x         = (8|4|0) << 21,  // Increment before.

    kBlockAddrModeMask = (8|4|1) << 21
};


// Coprocessor load/store operand size.
enum LFlag {
    Long  = 1 << 22,  // Long load/store coprocessor.
    Short = 0 << 22   // Short load/store coprocessor.
};


// NEON data type
enum NeonDataType {
    NeonS8 = 0x1,   // U = 0, imm3 = 0b001
    NeonS16 = 0x2,  // U = 0, imm3 = 0b010
    NeonS32 = 0x4,  // U = 0, imm3 = 0b100
    NeonU8 = 1 << 24 | 0x1,   // U = 1, imm3 = 0b001
    NeonU16 = 1 << 24 | 0x2,  // U = 1, imm3 = 0b010
    NeonU32 = 1 << 24 | 0x4,   // U = 1, imm3 = 0b100
    NeonDataTypeSizeMask = 0x7,
    NeonDataTypeUMask = 1 << 24
};

enum NeonListType {
    nlt_1 = 0x7,
    nlt_2 = 0xA,
    nlt_3 = 0x6,
    nlt_4 = 0x2
};

enum NeonSize {
    Neon8 = 0x0,
    Neon16 = 0x1,
    Neon32 = 0x2,
    Neon64 = 0x3
};

// -----------------------------------------------------------------------------
// Supervisor Call (svc) specific support.

// Special Software Interrupt codes when used in the presence of the ARM
// simulator.
// svc (formerly swi) provides a 24bit immediate value. Use bits 22:0 for
// standard SoftwareInterrupCode. Bit 23 is reserved for the stop feature.
enum SoftwareInterruptCodes {
    // transition to C code
    kCallRtRedirected = 0x10,
    // break point
    kBreakpoint = 0x20,
    // stop
    kStopCode = 1 << 23
};
const uint32_t kStopCodeMask = kStopCode - 1;
const uint32_t kMaxStopCode = kStopCode - 1;
const int32_t  kDefaultStopCode = -1;


// Type of VFP register. Determines register encoding.
enum VFPRegPrecision {
    kSinglePrecision = 0,
    kDoublePrecision = 1
};


// VFP FPSCR constants.
enum VFPConversionMode {
    kFPSCRRounding = 0,
    kDefaultRoundToZero = 1
};

// This mask does not include the "inexact" or "input denormal" cumulative
// exceptions flags, because we usually don't want to check for it.
const uint32_t kVFPExceptionMask = 0xf;
const uint32_t kVFPInvalidOpExceptionBit = 1 << 0;
const uint32_t kVFPOverflowExceptionBit = 1 << 2;
const uint32_t kVFPUnderflowExceptionBit = 1 << 3;
const uint32_t kVFPInexactExceptionBit = 1 << 4;
const uint32_t kVFPFlushToZeroMask = 1 << 24;
const uint32_t kVFPDefaultNaNModeControlBit = 1 << 25;

const uint32_t kVFPNConditionFlagBit = 1 << 31;
const uint32_t kVFPZConditionFlagBit = 1 << 30;
const uint32_t kVFPCConditionFlagBit = 1 << 29;
const uint32_t kVFPVConditionFlagBit = 1 << 28;


// VFP rounding modes. See ARM DDI 0406B Page A2-29.
enum VFPRoundingMode {
    RN = 0 << 22,   // Round to Nearest.
    RP = 1 << 22,   // Round towards Plus Infinity.
    RM = 2 << 22,   // Round towards Minus Infinity.
    RZ = 3 << 22,   // Round towards zero.

    // Aliases.
    kRoundToNearest = RN,
    kRoundToPlusInf = RP,
    kRoundToMinusInf = RM,
    kRoundToZero = RZ
};

const uint32_t kVFPRoundingModeMask = 3 << 22;

enum CheckForInexactConversion {
    kCheckForInexactConversion,
    kDontCheckForInexactConversion
};

// -----------------------------------------------------------------------------
// Hints.

// Branch hints are not used on the ARM.  They are defined so that they can
// appear in shared function signatures, but will be ignored in ARM
// implementations.
enum Hint { no_hint };

// Hints are not used on the arm.  Negating is trivial.
inline Hint
NegateHint(Hint ignored)
{
    return no_hint;
}


// -----------------------------------------------------------------------------
// Instruction abstraction.

// The class Instruction enables access to individual fields defined in the ARM
// architecture instruction set encoding as described in figure A3-1.
// Note that the Assembler uses typedef int32_t Instr.
//
// Example: Test whether the instruction at ptr does set the condition code
// bits.
//
// bool InstructionSetsConditionCodes(byte* ptr) {
//   Instruction* instr = Instruction::At(ptr);
//   int type = instr->TypeValue();
//   return ((type == 0) || (type == 1)) && instr->HasS();
// }
//
class Instruction {
  public:
    enum {
        kInstrSize = 4,
        kInstrSizeLog2 = 2,
        kPCReadOffset = 8
    };

    // Helper macro to define static accessors.
    // We use the cast to char* trick to bypass the strict anti-aliasing rules.
#define DECLARE_STATIC_TYPED_ACCESSOR(return_type, Name)        \
    static inline return_type Name(Instr instr) {               \
        char* temp = reinterpret_cast<char*>(&instr);           \
        return reinterpret_cast<Instruction*>(temp)->Name();    \
    }

#define DECLARE_STATIC_ACCESSOR(Name) DECLARE_STATIC_TYPED_ACCESSOR(int, Name)

    // Get the raw instruction bits.
    inline Instr InstructionBits() const {
        return *reinterpret_cast<const Instr*>(this);
    }

    // Set the raw instruction bits to value.
    inline void SetInstructionBits(Instr value) {
        *reinterpret_cast<Instr*>(this) = value;
    }

    // Read one particular bit out of the instruction bits.
    inline int Bit(int nr) const {
        return (InstructionBits() >> nr) & 1;
    }

    // Read a bit field's value out of the instruction bits.
    inline int Bits(int hi, int lo) const {
        return (InstructionBits() >> lo) & ((2 << (hi - lo)) - 1);
    }

    // Read a bit field out of the instruction bits.
    inline int BitField(int hi, int lo) const {
        return InstructionBits() & (((2 << (hi - lo)) - 1) << lo);
    }

    // Static support.

    // Read one particular bit out of the instruction bits.
    static inline int Bit(Instr instr, int nr) {
        return (instr >> nr) & 1;
    }

    // Read the value of a bit field out of the instruction bits.
    static inline int Bits(Instr instr, int hi, int lo) {
        return (instr >> lo) & ((2 << (hi - lo)) - 1);
    }


    // Read a bit field out of the instruction bits.
    static inline int BitField(Instr instr, int hi, int lo) {
        return instr & (((2 << (hi - lo)) - 1) << lo);
    }


    // Accessors for the different named fields used in the ARM encoding.
    // The naming of these accessor corresponds to figure A3-1.
    //
    // Two kind of accessors are declared:
    // - <Name>Field() will return the raw field, i.e. the field's bits at their
    //   original place in the instruction encoding.
    //   e.g. if instr is the 'addgt r0, r1, r2' instruction, encoded as
    //   0xC0810002 ConditionField(instr) will return 0xC0000000.
    // - <Name>Value() will return the field value, shifted back to bit 0.
    //   e.g. if instr is the 'addgt r0, r1, r2' instruction, encoded as
    //   0xC0810002 ConditionField(instr) will return 0xC.


    // Generally applicable fields
    inline Condition ConditionValue() const {
        return static_cast<Condition>(Bits(31, 28));
    }
    inline Condition ConditionField() const {
        return static_cast<Condition>(BitField(31, 28));
    }
    DECLARE_STATIC_TYPED_ACCESSOR(Condition, ConditionValue);
    DECLARE_STATIC_TYPED_ACCESSOR(Condition, ConditionField);

    inline int TypeValue() const { return Bits(27, 25); }
    inline int SpecialValue() const { return Bits(27, 23); }

    inline int RnValue() const { return Bits(19, 16); }
    DECLARE_STATIC_ACCESSOR(RnValue);
    inline int RdValue() const { return Bits(15, 12); }
    DECLARE_STATIC_ACCESSOR(RdValue);

    inline int CoprocessorValue() const { return Bits(11, 8); }
    // Support for VFP.
    // Vn(19-16) | Vd(15-12) |  Vm(3-0)
    inline int VnValue() const { return Bits(19, 16); }
    inline int VmValue() const { return Bits(3, 0); }
    inline int VdValue() const { return Bits(15, 12); }
    inline int NValue() const { return Bit(7); }
    inline int MValue() const { return Bit(5); }
    inline int DValue() const { return Bit(22); }
    inline int RtValue() const { return Bits(15, 12); }
    inline int PValue() const { return Bit(24); }
    inline int UValue() const { return Bit(23); }
    inline int Opc1Value() const { return (Bit(23) << 2) | Bits(21, 20); }
    inline int Opc2Value() const { return Bits(19, 16); }
    inline int Opc3Value() const { return Bits(7, 6); }
    inline int SzValue() const { return Bit(8); }
    inline int VLValue() const { return Bit(20); }
    inline int VCValue() const { return Bit(8); }
    inline int VAValue() const { return Bits(23, 21); }
    inline int VBValue() const { return Bits(6, 5); }
    inline int VFPNRegValue(VFPRegPrecision pre) {
        return VFPGlueRegValue(pre, 16, 7);
    }
    inline int VFPMRegValue(VFPRegPrecision pre) {
        return VFPGlueRegValue(pre, 0, 5);
    }
    inline int VFPDRegValue(VFPRegPrecision pre) {
        return VFPGlueRegValue(pre, 12, 22);
    }

    // Fields used in Data processing instructions
    inline int OpcodeValue() const {
        return static_cast<Opcode>(Bits(24, 21));
    }
    inline Opcode OpcodeField() const {
        return static_cast<Opcode>(BitField(24, 21));
    }
    inline int SValue() const { return Bit(20); }
    // with register
    inline int RmValue() const { return Bits(3, 0); }
    DECLARE_STATIC_ACCESSOR(RmValue);
    inline int ShiftValue() const { return static_cast<ShiftOp>(Bits(6, 5)); }
    inline ShiftOp ShiftField() const {
        return static_cast<ShiftOp>(BitField(6, 5));
    }
    inline int RegShiftValue() const { return Bit(4); }
    inline int RsValue() const { return Bits(11, 8); }
    inline int ShiftAmountValue() const { return Bits(11, 7); }
    // with immediate
    inline int RotateValue() const { return Bits(11, 8); }
    DECLARE_STATIC_ACCESSOR(RotateValue);
    inline int Immed8Value() const { return Bits(7, 0); }
    DECLARE_STATIC_ACCESSOR(Immed8Value);
    inline int Immed4Value() const { return Bits(19, 16); }
    inline int ImmedMovwMovtValue() const {
        return Immed4Value() << 12 | Offset12Value(); }
    DECLARE_STATIC_ACCESSOR(ImmedMovwMovtValue);

    // Fields used in Load/Store instructions
    inline int PUValue() const { return Bits(24, 23); }
    inline int PUField() const { return BitField(24, 23); }
    inline int  BValue() const { return Bit(22); }
    inline int  WValue() const { return Bit(21); }
    inline int  LValue() const { return Bit(20); }
    // with register uses same fields as Data processing instructions above
    // with immediate
    inline int Offset12Value() const { return Bits(11, 0); }
    // multiple
    inline int RlistValue() const { return Bits(15, 0); }
    // extra loads and stores
    inline int SignValue() const { return Bit(6); }
    inline int HValue() const { return Bit(5); }
    inline int ImmedHValue() const { return Bits(11, 8); }
    inline int ImmedLValue() const { return Bits(3, 0); }

    // Fields used in Branch instructions
    inline int LinkValue() const { return Bit(24); }
    inline int SImmed24Value() const { return ((InstructionBits() << 8) >> 8); }

    // Fields used in Software interrupt instructions
    inline SoftwareInterruptCodes SvcValue() const {
        return static_cast<SoftwareInterruptCodes>(Bits(23, 0));
    }

    // Test for special encodings of type 0 instructions (extra loads and stores,
    // as well as multiplications).
    inline bool IsSpecialType0() const { return (Bit(7) == 1) && (Bit(4) == 1); }

    // Test for miscellaneous instructions encodings of type 0 instructions.
    inline bool IsMiscType0() const { return (Bit(24) == 1)
            && (Bit(23) == 0)
            && (Bit(20) == 0)
            && ((Bit(7) == 0)); }

    // Test for a nop instruction, which falls under type 1.
    inline bool IsNopType1() const { return Bits(24, 0) == 0x0120F000; }

    // Test for a stop instruction.
    inline bool IsStop() const {
        return (TypeValue() == 7) && (Bit(24) == 1) && (SvcValue() >= kStopCode);
    }

    // Special accessors that test for existence of a value.
    inline bool HasS()    const { return SValue() == 1; }
    inline bool HasB()    const { return BValue() == 1; }
    inline bool HasW()    const { return WValue() == 1; }
    inline bool HasL()    const { return LValue() == 1; }
    inline bool HasU()    const { return UValue() == 1; }
    inline bool HasSign() const { return SignValue() == 1; }
    inline bool HasH()    const { return HValue() == 1; }
    inline bool HasLink() const { return LinkValue() == 1; }

    // Decoding the double immediate in the vmov instruction.
    double DoubleImmedVmov() const;

    // Instructions are read of out a code stream. The only way to get a
    // reference to an instruction is to convert a pointer. There is no way
    // to allocate or create instances of class Instruction.
    // Use the At(pc) function to create references to Instruction.
    static Instruction* At(uint8_t* pc) {
        return reinterpret_cast<Instruction*>(pc);
    }


  private:
    // Join split register codes, depending on single or double precision.
    // four_bit is the position of the least-significant bit of the four
    // bit specifier. one_bit is the position of the additional single bit
    // specifier.
    inline int VFPGlueRegValue(VFPRegPrecision pre, int four_bit, int one_bit) {
        if (pre == kSinglePrecision) {
            return (Bits(four_bit + 3, four_bit) << 1) | Bit(one_bit);
        }
        return (Bit(one_bit) << 4) | Bits(four_bit + 3, four_bit);
    }

    // We need to prevent the creation of instances of class Instruction.
    Instruction() = delete;
    Instruction(const Instruction&) = delete;
    void operator=(const Instruction&) = delete;
};


// Helper functions for converting between register numbers and names.
class Registers {
  public:
    // Return the name of the register.
    static const char* Name(int reg);

    // Lookup the register number for the name provided.
    static int Number(const char* name);

    struct RegisterAlias {
        int reg;
        const char* name;
    };

  private:
    static const char* names_[kNumRegisters];
    static const RegisterAlias aliases_[];
};

// Helper functions for converting between VFP register numbers and names.
class VFPRegisters {
  public:
    // Return the name of the register.
    static const char* Name(int reg, bool is_double);

    // Lookup the register number for the name provided.
    // Set flag pointed by is_double to true if register
    // is double-precision.
    static int Number(const char* name, bool* is_double);

  private:
    static const char* names_[kNumVFPRegisters];
};


}  // namespace disasm
}  // namespace jit
}  // namespace js

#endif // JS_DISASM_ARM

#endif  // jit_arm_disasm_Constants_arm_h