// Derived from Inferno utils/6l/l.h and related files.
// http://code.google.com/p/inferno-os/source/browse/utils/6l/l.h
//
//	Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
//	Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
//	Portions Copyright © 1997-1999 Vita Nuova Limited
//	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
//	Portions Copyright © 2004,2006 Bruce Ellis
//	Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
//	Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
//	Portions Copyright © 2009 The Go Authors.  All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

package obj

import (
	"bufio"
	"cmd/internal/sys"
)

// An Addr is an argument to an instruction.
// The general forms and their encodings are:
//
//	sym±offset(symkind)(reg)(index*scale)
//		Memory reference at address &sym(symkind) + offset + reg + index*scale.
//		Any of sym(symkind), ±offset, (reg), (index*scale), and *scale can be omitted.
//		If (reg) and *scale are both omitted, the resulting expression (index) is parsed as (reg).
//		To force a parsing as index*scale, write (index*1).
//		Encoding:
//			type = TYPE_MEM
//			name = symkind (NAME_AUTO, ...) or 0 (NAME_NONE)
//			sym = sym
//			offset = ±offset
//			reg = reg (REG_*)
//			index = index (REG_*)
//			scale = scale (1, 2, 4, 8)
//
//	$<mem>
//		Effective address of memory reference <mem>, defined above.
//		Encoding: same as memory reference, but type = TYPE_ADDR.
//
//	$<±integer value>
//		This is a special case of $<mem>, in which only ±offset is present.
//		It has a separate type for easy recognition.
//		Encoding:
//			type = TYPE_CONST
//			offset = ±integer value
//
//	*<mem>
//		Indirect reference through memory reference <mem>, defined above.
//		Only used on x86 for CALL/JMP *sym(SB), which calls/jumps to a function
//		pointer stored in the data word sym(SB), not a function named sym(SB).
//		Encoding: same as above, but type = TYPE_INDIR.
//
//	$*$<mem>
//		No longer used.
//		On machines with actual SB registers, $*$<mem> forced the
//		instruction encoding to use a full 32-bit constant, never a
//		reference relative to SB.
//
//	$<floating point literal>
//		Floating point constant value.
//		Encoding:
//			type = TYPE_FCONST
//			val = floating point value
//
//	$<string literal, up to 8 chars>
//		String literal value (raw bytes used for DATA instruction).
//		Encoding:
//			type = TYPE_SCONST
//			val = string
//
//	<register name>
//		Any register: integer, floating point, control, segment, and so on.
//		If looking for specific register kind, must check type and reg value range.
//		Encoding:
//			type = TYPE_REG
//			reg = reg (REG_*)
//
//	x(PC)
//		Encoding:
//			type = TYPE_BRANCH
//			val = Prog* reference OR ELSE offset = target pc (branch takes priority)
//
//	$±x-±y
//		Final argument to TEXT, specifying local frame size x and argument size y.
//		In this form, x and y are integer literals only, not arbitrary expressions.
//		This avoids parsing ambiguities due to the use of - as a separator.
//		The ± are optional.
//		If the final argument to TEXT omits the -±y, the encoding should still
//		use TYPE_TEXTSIZE (not TYPE_CONST), with u.argsize = ArgsSizeUnknown.
//		Encoding:
//			type = TYPE_TEXTSIZE
//			offset = x
//			val = int32(y)
//
//	reg<<shift, reg>>shift, reg->shift, reg@>shift
//		Shifted register value, for ARM.
//		In this form, reg must be a register and shift can be a register or an integer constant.
//		Encoding:
//			type = TYPE_SHIFT
//			offset = (reg&15) | shifttype<<5 | count
//			shifttype = 0, 1, 2, 3 for <<, >>, ->, @>
//			count = (reg&15)<<8 | 1<<4 for a register shift count, (n&31)<<7 for an integer constant.
//
//	(reg, reg)
//		A destination register pair. When used as the last argument of an instruction,
//		this form makes clear that both registers are destinations.
//		Encoding:
//			type = TYPE_REGREG
//			reg = first register
//			offset = second register
//
//	[reg, reg, reg-reg]
//		Register list for ARM.
//		Encoding:
//			type = TYPE_REGLIST
//			offset = bit mask of registers in list; R0 is low bit.
//
//	reg, reg
//		Register pair for ARM.
//		TYPE_REGREG2
//
//	(reg+reg)
//		Register pair for PPC64.
//		Encoding:
//			type = TYPE_MEM
//			reg = first register
//			index = second register
//			scale = 1
//
type Addr struct {
	Reg    int16
	Index  int16
	Scale  int16 // Sometimes holds a register.
	Type   AddrType
	Name   int8
	Class  int8
	Etype  uint8
	Offset int64
	Width  int64
	Sym    *LSym
	Gotype *LSym

	// argument value:
	//	for TYPE_SCONST, a string
	//	for TYPE_FCONST, a float64
	//	for TYPE_BRANCH, a *Prog (optional)
	//	for TYPE_TEXTSIZE, an int32 (optional)
	Val interface{}

	Node interface{} // for use by compiler
}

type AddrType uint8

const (
	NAME_NONE = 0 + iota
	NAME_EXTERN
	NAME_STATIC
	NAME_AUTO
	NAME_PARAM
	// A reference to name@GOT(SB) is a reference to the entry in the global offset
	// table for 'name'.
	NAME_GOTREF
)

const (
	TYPE_NONE AddrType = 0

	TYPE_BRANCH AddrType = 5 + iota
	TYPE_TEXTSIZE
	TYPE_MEM
	TYPE_CONST
	TYPE_FCONST
	TYPE_SCONST
	TYPE_REG
	TYPE_ADDR
	TYPE_SHIFT
	TYPE_REGREG
	TYPE_REGREG2
	TYPE_INDIR
	TYPE_REGLIST
)

// TODO(rsc): Describe prog.
// TODO(rsc): Describe TEXT/GLOBL flag in from3
type Prog struct {
	Ctxt   *Link
	Link   *Prog
	From   Addr
	From3  *Addr // optional
	To     Addr
	Opt    interface{}
	Forwd  *Prog
	Pcond  *Prog
	Rel    *Prog // Source of forward jumps on x86; pcrel on arm
	Pc     int64
	Lineno int32
	Spadj  int32
	As     As // Assembler opcode.
	Reg    int16
	RegTo2 int16  // 2nd register output operand
	Mark   uint16 // bitmask of arch-specific items
	Optab  uint16
	Scond  uint8
	Back   uint8
	Ft     uint8
	Tt     uint8
	Isize  uint8 // size of the instruction in bytes (x86 only)
	Mode   int8

	Info ProgInfo
}

// From3Type returns From3.Type, or TYPE_NONE when From3 is nil.
func (p *Prog) From3Type() AddrType {
	if p.From3 == nil {
		return TYPE_NONE
	}
	return p.From3.Type
}

// From3Offset returns From3.Offset, or 0 when From3 is nil.
func (p *Prog) From3Offset() int64 {
	if p.From3 == nil {
		return 0
	}
	return p.From3.Offset
}

// ProgInfo holds information about the instruction for use
// by clients such as the compiler. The exact meaning of this
// data is up to the client and is not interpreted by the cmd/internal/obj/... packages.
type ProgInfo struct {
	_        struct{} // to prevent unkeyed literals. Trailing zero-sized field will take space.
	Flags    uint32   // flag bits
	Reguse   uint64   // registers implicitly used by this instruction
	Regset   uint64   // registers implicitly set by this instruction
	Regindex uint64   // registers used by addressing mode
}

// An As denotes an assembler opcode.
// There are some portable opcodes, declared here in package obj,
// that are common to all architectures.
// However, the majority of opcodes are arch-specific
// and are declared in their respective architecture's subpackage.
type As int16

// These are the portable opcodes.
const (
	AXXX As = iota
	ACALL
	ACHECKNIL
	ADUFFCOPY
	ADUFFZERO
	AEND
	AFUNCDATA
	AGLOBL
	AJMP
	ANOP
	APCDATA
	ARET
	ATEXT
	ATYPE
	AUNDEF
	AUSEFIELD
	AVARDEF
	AVARKILL
	AVARLIVE
	A_ARCHSPECIFIC
)

// Each architecture is allotted a distinct subspace of opcode values
// for declaring its arch-specific opcodes.
// Within this subspace, the first arch-specific opcode should be
// at offset A_ARCHSPECIFIC.
//
// Subspaces are aligned to a power of two so opcodes can be masked
// with AMask and used as compact array indices.
const (
	ABase386 = (1 + iota) << 12
	ABaseARM
	ABaseAMD64
	ABasePPC64
	ABaseARM64
	ABaseMIPS64
	ABaseS390X

	AMask = 1<<12 - 1 // AND with this to use the opcode as an array index.
)

// An LSym is the sort of symbol that is written to an object file.
type LSym struct {
	Name      string
	Type      int16
	Version   int16
	Dupok     bool
	Cfunc     bool
	Nosplit   bool
	Leaf      bool
	Seenglobl bool
	Onlist    bool

	// ReflectMethod means the function may call reflect.Type.Method or
	// reflect.Type.MethodByName. Matching is imprecise (as reflect.Type
	// can be used through a custom interface), so ReflectMethod may be
	// set in some cases when the reflect package is not called.
	//
	// Used by the linker to determine what methods can be pruned.
	ReflectMethod bool

	// Local means make the symbol local even when compiling Go code to reference Go
	// symbols in other shared libraries, as in this mode symbols are global by
	// default. "local" here means in the sense of the dynamic linker, i.e. not
	// visible outside of the module (shared library or executable) that contains its
	// definition. (When not compiling to support Go shared libraries, all symbols are
	// local in this sense unless there is a cgo_export_* directive).
	Local bool

	RefIdx int // Index of this symbol in the symbol reference list.
	Args   int32
	Locals int32
	Size   int64
	Gotype *LSym
	Autom  *Auto
	Text   *Prog
	Pcln   *Pcln
	P      []byte
	R      []Reloc
}

// The compiler needs LSym to satisfy fmt.Stringer, because it stores
// an LSym in ssa.ExternSymbol.
func (s *LSym) String() string {
	return s.Name
}

type Pcln struct {
	Pcsp        Pcdata
	Pcfile      Pcdata
	Pcline      Pcdata
	Pcdata      []Pcdata
	Funcdata    []*LSym
	Funcdataoff []int64
	File        []*LSym
	Lastfile    *LSym
	Lastindex   int
}

// LSym.type
const (
	Sxxx = iota
	STEXT
	SELFRXSECT

	STYPE
	SSTRING
	SGOSTRING
	SGOSTRINGHDR
	SGOFUNC
	SGCBITS
	SRODATA
	SFUNCTAB

	// Types STYPE-SFUNCTAB above are written to the .rodata section by default.
	// When linking a shared object, some conceptually "read only" types need to
	// be written to by relocations and putting them in a section called
	// ".rodata" interacts poorly with the system linkers. The GNU linkers
	// support this situation by arranging for sections of the name
	// ".data.rel.ro.XXX" to be mprotected read only by the dynamic linker after
	// relocations have applied, so when the Go linker is creating a shared
	// object it checks all objects of the above types and bumps any object that
	// has a relocation to it to the corresponding type below, which are then
	// written to sections with appropriate magic names.
	STYPERELRO
	SSTRINGRELRO
	SGOSTRINGRELRO
	SGOSTRINGHDRRELRO
	SGOFUNCRELRO
	SGCBITSRELRO
	SRODATARELRO
	SFUNCTABRELRO

	STYPELINK
	SITABLINK
	SSYMTAB
	SPCLNTAB
	SELFROSECT
	SMACHOPLT
	SELFSECT
	SMACHO
	SMACHOGOT
	SWINDOWS
	SELFGOT
	SNOPTRDATA
	SINITARR
	SDATA
	SBSS
	SNOPTRBSS
	STLSBSS
	SXREF
	SMACHOSYMSTR
	SMACHOSYMTAB
	SMACHOINDIRECTPLT
	SMACHOINDIRECTGOT
	SFILE
	SFILEPATH
	SCONST
	SDYNIMPORT
	SHOSTOBJ
	SDWARFSECT
	SDWARFINFO
	SSUB       = 1 << 8
	SMASK      = SSUB - 1
	SHIDDEN    = 1 << 9
	SCONTAINER = 1 << 10 // has a sub-symbol
)

type Reloc struct {
	Off  int32
	Siz  uint8
	Type int32
	Add  int64
	Sym  *LSym
}

// Reloc.type
const (
	R_ADDR = 1 + iota
	// R_ADDRPOWER relocates a pair of "D-form" instructions (instructions with 16-bit
	// immediates in the low half of the instruction word), usually addis followed by
	// another add or a load, inserting the "high adjusted" 16 bits of the address of
	// the referenced symbol into the immediate field of the first instruction and the
	// low 16 bits into that of the second instruction.
	R_ADDRPOWER
	// R_ADDRARM64 relocates an adrp, add pair to compute the address of the
	// referenced symbol.
	R_ADDRARM64
	// R_ADDRMIPS (only used on mips64) resolves to a 32-bit external address,
	// by loading the address into a register with two instructions (lui, ori).
	R_ADDRMIPS
	R_SIZE
	R_CALL
	R_CALLARM
	R_CALLARM64
	R_CALLIND
	R_CALLPOWER
	// R_CALLMIPS (only used on mips64) resolves to non-PC-relative target address
	// of a CALL (JAL) instruction, by encoding the address into the instruction.
	R_CALLMIPS
	R_CONST
	R_PCREL
	// R_TLS_LE, used on 386, amd64, and ARM, resolves to the offset of the
	// thread-local symbol from the thread local base and is used to implement the
	// "local exec" model for tls access (r.Sym is not set on intel platforms but is
	// set to a TLS symbol -- runtime.tlsg -- in the linker when externally linking).
	R_TLS_LE
	// R_TLS_IE, used 386, amd64, and ARM resolves to the PC-relative offset to a GOT
	// slot containing the offset from the thread-local symbol from the thread local
	// base and is used to implemented the "initial exec" model for tls access (r.Sym
	// is not set on intel platforms but is set to a TLS symbol -- runtime.tlsg -- in
	// the linker when externally linking).
	R_TLS_IE
	R_GOTOFF
	R_PLT0
	R_PLT1
	R_PLT2
	R_USEFIELD
	// R_USETYPE resolves to an *rtype, but no relocation is created. The
	// linker uses this as a signal that the pointed-to type information
	// should be linked into the final binary, even if there are no other
	// direct references. (This is used for types reachable by reflection.)
	R_USETYPE
	// R_METHOD resolves to an *rtype for a method.
	// It is used when linking from the uncommonType of another *rtype, and
	// may be set to zero by the linker if it determines the method text is
	// unreachable by the linked program.
	R_METHOD
	R_POWER_TOC
	R_GOTPCREL
	// R_JMPMIPS (only used on mips64) resolves to non-PC-relative target address
	// of a JMP instruction, by encoding the address into the instruction.
	// The stack nosplit check ignores this since it is not a function call.
	R_JMPMIPS
	// R_DWARFREF resolves to the offset of the symbol from its section.
	R_DWARFREF

	// Platform dependent relocations. Architectures with fixed width instructions
	// have the inherent issue that a 32-bit (or 64-bit!) displacement cannot be
	// stuffed into a 32-bit instruction, so an address needs to be spread across
	// several instructions, and in turn this requires a sequence of relocations, each
	// updating a part of an instruction. This leads to relocation codes that are
	// inherently processor specific.

	// Arm64.

	// Set a MOV[NZ] immediate field to bits [15:0] of the offset from the thread
	// local base to the thread local variable defined by the referenced (thread
	// local) symbol. Error if the offset does not fit into 16 bits.
	R_ARM64_TLS_LE

	// Relocates an ADRP; LD64 instruction sequence to load the offset between
	// the thread local base and the thread local variable defined by the
	// referenced (thread local) symbol from the GOT.
	R_ARM64_TLS_IE

	// R_ARM64_GOTPCREL relocates an adrp, ld64 pair to compute the address of the GOT
	// slot of the referenced symbol.
	R_ARM64_GOTPCREL

	// PPC64.

	// R_POWER_TLS_LE is used to implement the "local exec" model for tls
	// access. It resolves to the offset of the thread-local symbol from the
	// thread pointer (R13) and inserts this value into the low 16 bits of an
	// instruction word.
	R_POWER_TLS_LE

	// R_POWER_TLS_IE is used to implement the "initial exec" model for tls access. It
	// relocates a D-form, DS-form instruction sequence like R_ADDRPOWER_DS. It
	// inserts to the offset of GOT slot for the thread-local symbol from the TOC (the
	// GOT slot is filled by the dynamic linker with the offset of the thread-local
	// symbol from the thread pointer (R13)).
	R_POWER_TLS_IE

	// R_POWER_TLS marks an X-form instruction such as "MOVD 0(R13)(R31*1), g" as
	// accessing a particular thread-local symbol. It does not affect code generation
	// but is used by the system linker when relaxing "initial exec" model code to
	// "local exec" model code.
	R_POWER_TLS

	// R_ADDRPOWER_DS is similar to R_ADDRPOWER above, but assumes the second
	// instruction is a "DS-form" instruction, which has an immediate field occupying
	// bits [15:2] of the instruction word. Bits [15:2] of the address of the
	// relocated symbol are inserted into this field; it is an error if the last two
	// bits of the address are not 0.
	R_ADDRPOWER_DS

	// R_ADDRPOWER_PCREL relocates a D-form, DS-form instruction sequence like
	// R_ADDRPOWER_DS but inserts the offset of the GOT slot for the referenced symbol
	// from the TOC rather than the symbol's address.
	R_ADDRPOWER_GOT

	// R_ADDRPOWER_PCREL relocates two D-form instructions like R_ADDRPOWER, but
	// inserts the displacement from the place being relocated to the address of the
	// the relocated symbol instead of just its address.
	R_ADDRPOWER_PCREL

	// R_ADDRPOWER_TOCREL relocates two D-form instructions like R_ADDRPOWER, but
	// inserts the offset from the TOC to the address of the the relocated symbol
	// rather than the symbol's address.
	R_ADDRPOWER_TOCREL

	// R_ADDRPOWER_TOCREL relocates a D-form, DS-form instruction sequence like
	// R_ADDRPOWER_DS but inserts the offset from the TOC to the address of the the
	// relocated symbol rather than the symbol's address.
	R_ADDRPOWER_TOCREL_DS

	// R_PCRELDBL relocates s390x 2-byte aligned PC-relative addresses.
	// TODO(mundaym): remove once variants can be serialized - see issue 14218.
	R_PCRELDBL
)

type Auto struct {
	Asym    *LSym
	Link    *Auto
	Aoffset int32
	Name    int16
	Gotype  *LSym
}

// Auto.name
const (
	A_AUTO = 1 + iota
	A_PARAM
)

type Pcdata struct {
	P []byte
}

// Pcdata iterator.
//      for(pciterinit(ctxt, &it, &pcd); !it.done; pciternext(&it)) { it.value holds in [it.pc, it.nextpc) }
type Pciter struct {
	d       Pcdata
	p       []byte
	pc      uint32
	nextpc  uint32
	pcscale uint32
	value   int32
	start   int
	done    int
}

// symbol version, incremented each time a file is loaded.
// version==1 is reserved for savehist.
const (
	HistVersion = 1
)

// Link holds the context for writing object code from a compiler
// to be linker input or for reading that input into the linker.
type Link struct {
	Goarm         int32
	Headtype      int
	Arch          *LinkArch
	Debugasm      int32
	Debugvlog     int32
	Debugdivmod   int32
	Debugpcln     int32
	Flag_shared   int32
	Flag_dynlink  bool
	Flag_optimize bool
	Bso           *bufio.Writer
	Pathname      string
	Goroot        string
	Goroot_final  string
	Hash          map[SymVer]*LSym
	LineHist      LineHist
	Imports       []string
	Plist         *Plist
	Plast         *Plist
	Sym_div       *LSym
	Sym_divu      *LSym
	Sym_mod       *LSym
	Sym_modu      *LSym
	Plan9privates *LSym
	Curp          *Prog
	Printp        *Prog
	Blitrl        *Prog
	Elitrl        *Prog
	Rexflag       int
	Vexflag       int
	Rep           int
	Repn          int
	Lock          int
	Asmode        int
	AsmBuf        AsmBuf // instruction buffer for x86
	Instoffset    int64
	Autosize      int32
	Armsize       int32
	Pc            int64
	DiagFunc      func(string, ...interface{})
	Mode          int
	Cursym        *LSym
	Version       int
	Textp         *LSym
	Etextp        *LSym
	Errors        int

	// state for writing objects
	Text []*LSym
	Data []*LSym

	// Cache of Progs
	allocIdx int
	progs    [10000]Prog
}

func (ctxt *Link) Diag(format string, args ...interface{}) {
	ctxt.Errors++
	ctxt.DiagFunc(format, args...)
}

// The smallest possible offset from the hardware stack pointer to a local
// variable on the stack. Architectures that use a link register save its value
// on the stack in the function prologue and so always have a pointer between
// the hardware stack pointer and the local variable area.
func (ctxt *Link) FixedFrameSize() int64 {
	switch ctxt.Arch.Family {
	case sys.AMD64, sys.I386:
		return 0
	case sys.PPC64:
		// PIC code on ppc64le requires 32 bytes of stack, and it's easier to
		// just use that much stack always on ppc64x.
		return int64(4 * ctxt.Arch.PtrSize)
	default:
		return int64(ctxt.Arch.PtrSize)
	}
}

type SymVer struct {
	Name    string
	Version int // TODO: make int16 to match LSym.Version?
}

// LinkArch is the definition of a single architecture.
type LinkArch struct {
	*sys.Arch
	Preprocess func(*Link, *LSym)
	Assemble   func(*Link, *LSym)
	Follow     func(*Link, *LSym)
	Progedit   func(*Link, *Prog)
	UnaryDst   map[As]bool // Instruction takes one operand, a destination.
}

/* executable header types */
const (
	Hunknown = 0 + iota
	Hdarwin
	Hdragonfly
	Hfreebsd
	Hlinux
	Hnacl
	Hnetbsd
	Hopenbsd
	Hplan9
	Hsolaris
	Hwindows
)

type Plist struct {
	Name    *LSym
	Firstpc *Prog
	Recur   int
	Link    *Plist
}

/*
 * start a new Prog list.
 */
func Linknewplist(ctxt *Link) *Plist {
	pl := new(Plist)
	if ctxt.Plist == nil {
		ctxt.Plist = pl
	} else {
		ctxt.Plast.Link = pl
	}
	ctxt.Plast = pl
	return pl
}

// AsmBuf is a simple buffer to assemble variable-length x86 instructions into.
type AsmBuf struct {
	buf [100]byte
	off int
}

// Put1 appends one byte to the end of the buffer.
func (a *AsmBuf) Put1(x byte) {
	a.buf[a.off] = x
	a.off++
}

// Put2 appends two bytes to the end of the buffer.
func (a *AsmBuf) Put2(x, y byte) {
	a.buf[a.off+0] = x
	a.buf[a.off+1] = y
	a.off += 2
}

// Put3 appends three bytes to the end of the buffer.
func (a *AsmBuf) Put3(x, y, z byte) {
	a.buf[a.off+0] = x
	a.buf[a.off+1] = y
	a.buf[a.off+2] = z
	a.off += 3
}

// Put4 appends four bytes to the end of the buffer.
func (a *AsmBuf) Put4(x, y, z, w byte) {
	a.buf[a.off+0] = x
	a.buf[a.off+1] = y
	a.buf[a.off+2] = z
	a.buf[a.off+3] = w
	a.off += 4
}

// PutInt16 writes v into the buffer using little-endian encoding.
func (a *AsmBuf) PutInt16(v int16) {
	a.buf[a.off+0] = byte(v)
	a.buf[a.off+1] = byte(v >> 8)
	a.off += 2
}

// PutInt32 writes v into the buffer using little-endian encoding.
func (a *AsmBuf) PutInt32(v int32) {
	a.buf[a.off+0] = byte(v)
	a.buf[a.off+1] = byte(v >> 8)
	a.buf[a.off+2] = byte(v >> 16)
	a.buf[a.off+3] = byte(v >> 24)
	a.off += 4
}

// PutInt64 writes v into the buffer using little-endian encoding.
func (a *AsmBuf) PutInt64(v int64) {
	a.buf[a.off+0] = byte(v)
	a.buf[a.off+1] = byte(v >> 8)
	a.buf[a.off+2] = byte(v >> 16)
	a.buf[a.off+3] = byte(v >> 24)
	a.buf[a.off+4] = byte(v >> 32)
	a.buf[a.off+5] = byte(v >> 40)
	a.buf[a.off+6] = byte(v >> 48)
	a.buf[a.off+7] = byte(v >> 56)
	a.off += 8
}

// Put copies b into the buffer.
func (a *AsmBuf) Put(b []byte) {
	copy(a.buf[a.off:], b)
	a.off += len(b)
}

// Insert inserts b at offset i.
func (a *AsmBuf) Insert(i int, b byte) {
	a.off++
	copy(a.buf[i+1:a.off], a.buf[i:a.off-1])
	a.buf[i] = b
}

// Last returns the byte at the end of the buffer.
func (a *AsmBuf) Last() byte { return a.buf[a.off-1] }

// Len returns the length of the buffer.
func (a *AsmBuf) Len() int { return a.off }

// Bytes returns the contents of the buffer.
func (a *AsmBuf) Bytes() []byte { return a.buf[:a.off] }

// Reset empties the buffer.
func (a *AsmBuf) Reset() { a.off = 0 }

// Peek returns the byte at offset i.
func (a *AsmBuf) Peek(i int) byte { return a.buf[i] }