Initial commit of gdb-experiment.go
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355
gdb-experiment.go
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355
gdb-experiment.go
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@ -0,0 +1,355 @@
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// Non-optimizing Brainfuck compiler generating binaries for Linux on x86-64;
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// gofmt has been tried, with disappointing results
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package main
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import (
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"errors"
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"fmt"
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"io/ioutil"
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"log"
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"os"
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"strconv"
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)
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const ( RIGHT = iota; LEFT; INC; DEC; IN; OUT; BEGIN; END )
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var info = []struct {
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grouped bool
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name string
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}{
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{true, "RIGHT"},
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{true, "LEFT"},
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{true, "INC"},
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{true, "DEC"},
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{false, "IN"},
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{false, "OUT"},
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{false, "BEGIN"},
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{false, "END"},
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}
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type instruction struct {
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command int
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arg int
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}
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// Dump internal representation to a file for debugging purposes
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func dump(filename string, irb []instruction) error {
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out, err := os.Create(filename)
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if err != nil {
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return err
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}
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indent := 0
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for _, x := range irb {
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if x.command == END {
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indent--
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}
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for i := 0; i < indent; i++ {
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out.WriteString(" ")
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}
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out.WriteString(info[x.command].name)
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if info[x.command].grouped {
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fmt.Fprintf(out, " %d", x.arg)
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}
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out.WriteString("\n")
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if x.command == BEGIN {
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indent++
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}
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}
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if err = out.Close(); err != nil {
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return err
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}
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return nil
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}
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// Decode a Brainfuck program into internal representation,
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// coalescing identical commands together as the most basic optimization
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func decode(program []byte) (irb []instruction) {
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for _, c := range program {
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var command int
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switch c {
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case '>': command = RIGHT
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case '<': command = LEFT
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case '+': command = INC
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case '-': command = DEC
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case '.': command = OUT
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case ',': command = IN
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case '[': command = BEGIN
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case ']': command = END
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default: continue
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}
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if len(irb) == 0 || !info[command].grouped ||
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irb[len(irb)-1].command != command {
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irb = append(irb, instruction{command, 1})
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} else {
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irb[len(irb)-1].arg++
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}
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}
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return
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}
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// Match loop commands so that we know where to jump
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func pairLoops(irb []instruction) error {
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nesting := 0
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stack := make([]int, len(irb))
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for i, x := range irb {
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switch x.command {
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case BEGIN:
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stack[nesting] = i
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nesting++
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case END:
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if nesting <= 0 {
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return errors.New("unbalanced loops")
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}
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nesting--
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irb[stack[nesting]].arg = i + 1
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irb[i].arg = stack[nesting] + 1
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}
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}
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if nesting != 0 {
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return errors.New("unbalanced loops")
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}
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return nil
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}
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// --- Code generation ---------------------------------------------------------
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type codegen struct {
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buf []byte
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}
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// Convert an arbitrary integral value up to 8 bytes long to little endian
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func le(unknown interface{}) []byte {
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// Trying hard to avoid reflect.Value.Int/Uint
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formatted := fmt.Sprintf("%d", unknown)
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var v uint64
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if unsigned, err := strconv.ParseUint(formatted, 10, 64); err == nil {
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v = unsigned
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} else if signed, err := strconv.ParseInt(formatted, 10, 64); err == nil {
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v = uint64(signed)
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} else {
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panic("cannot convert to number")
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}
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return []byte{byte(v), byte(v >> 8), byte(v >> 16), byte(v >> 24),
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byte(v >> 32), byte(v >> 40), byte(v >> 48), byte(v >> 56)}
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}
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func (a *codegen) append(v []byte) { a.buf = append(a.buf, v...) }
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func (a *codegen) code(v string) *codegen { a.append([]byte(v)); return a }
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func (a *codegen) db(v interface{}) *codegen { a.append(le(v)[:1]); return a }
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func (a *codegen) dw(v interface{}) *codegen { a.append(le(v)[:2]); return a }
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func (a *codegen) dd(v interface{}) *codegen { a.append(le(v)[:4]); return a }
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func (a *codegen) dq(v interface{}) *codegen { a.append(le(v)[:8]); return a }
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const (
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ElfCodeAddr = 0x400000 // Where the code is loaded in memory
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ElfDataAddr = 0x800000 // Where the tape is placed in memory
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)
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const (
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SYS_READ = 0
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SYS_WRITE = 1
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SYS_EXIT = 60
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)
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func codegenAmd64(irb []instruction) []byte {
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offsets := make([]int, len(irb)+1)
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a := codegen{}
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a.code("\xB8").dd(ElfDataAddr) // mov rax, "ElfCodeAddr"
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a.code("\x30\xDB") // xor bl, bl
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for i, x := range irb {
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offsets[i] = len(a.buf)
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if x.command == LEFT || x.command == RIGHT {
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a.code("\x88\x18") // mov [rax], bl
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}
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switch x.command {
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case RIGHT: a.code("\x48\x05").dd(x.arg) // add rax, "arg"
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case LEFT: a.code("\x48\x2D").dd(x.arg) // sub rax, "arg"
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case INC: a.code("\x80\xC3").db(x.arg) // add bl, "arg"
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case DEC: a.code("\x80\xEB").db(x.arg) // sub bl, "arg"
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case OUT: a.code("\xE8").dd(0) // call "write"
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case IN: a.code("\xE8").dd(0) // call "read"
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case BEGIN:
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// test bl, bl; jz "offsets[arg]"
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a.code("\x84\xDB" + "\x0F\x84").dd(0)
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case END:
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// test bl, bl; jnz "offsets[arg]"
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a.code("\x84\xDB" + "\x0F\x85").dd(0)
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}
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if x.command == LEFT || x.command == RIGHT {
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a.code("\x8A\x18") // mov bl, [rax]
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}
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}
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// When there is a loop at the end we need to be able to jump past it
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offsets[len(irb)] = len(a.buf)
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// Write an epilog which handles all the OS interfacing
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//
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// System V x86-64 ABI:
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// rax <-> both syscall number and return value
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// args -> rdi, rsi, rdx, r10, r8, r9
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// trashed <- rcx, r11
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a.code("\xB8").dd(SYS_EXIT) // mov eax, 0x3c
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a.code("\x48\x31\xFF") // xor rdi, rdi
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a.code("\x0F\x05") // syscall
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fatal := len(a.buf)
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a.code("\x48\x89\xF7") // mov rdi, rsi -- use the string in rsi
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a.code("\x30\xC0") // xor al, al -- look for the nil byte
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a.code("\x48\x31\xC9") // xor rcx, rcx
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a.code("\x48\xF7\xD1") // not rcx -- start from -1
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a.code("\xFC" + "\xF2\xAE") // cld; repne scasb -- decrement until found
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a.code("\x48\xF7\xD1") // not rcx
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a.code("\x48\x8D\x51\xFF") // lea rdx, [rcx-1] -- save length in rdx
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a.code("\xB8").dd(SYS_WRITE) // mov eax, "SYS_WRITE"
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a.code("\xBF").dd(2) // mov edi, "STDERR_FILENO"
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a.code("\x0F\x05") // syscall
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a.code("\xB8").dd(SYS_EXIT) // mov eax, "SYS_EXIT"
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a.code("\xBF").dd(1) // mov edi, "EXIT_FAILURE"
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a.code("\x0F\x05") // syscall
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read := len(a.buf)
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a.code("\x50") // push rax -- save tape position
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a.code("\xB8").dd(SYS_READ) // mov eax, "SYS_READ"
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a.code("\x48\x89\xC7") // mov rdi, rax -- STDIN_FILENO
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a.code("\x66\x6A\x00") // push word 0 -- the default value for EOF
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a.code("\x48\x89\xE6") // mov rsi, rsp -- the char starts at rsp
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a.code("\xBA").dd(1) // mov edx, 1 -- count
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a.code("\x0F\x05") // syscall
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a.code("\x66\x5B") // pop bx
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a.code("\x48\x83\xF8\x00") // cmp rax, 0
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a.code("\x48\x8D\x35").dd(4) // lea rsi, [rel read_message]
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a.code("\x7C") // jl "fatal_offset" -- write failure message
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a.db(fatal - len(a.buf) - 1)
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a.code("\x58") // pop rax -- restore tape position
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a.code("\xC3") // ret
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a.code("fatal: read failed\n\x00")
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write := len(a.buf)
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a.code("\x50") // push rax -- save tape position
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a.code("\xB8").dd(SYS_WRITE) // mov eax, "SYS_WRITE"
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a.code("\x48\x89\xC7") // mov rdi, rax -- STDOUT_FILENO
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a.code("\x66\x53") // push bx
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a.code("\x48\x89\xE6") // mov rsi, rsp -- the char starts at rsp
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a.code("\xBA").dd(1) // mov edx, 1 -- count
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a.code("\x0F\x05") // syscall
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a.code("\x66\x5B") // pop bx
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a.code("\x48\x83\xF8\x00") // cmp rax, 0
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a.code("\x48\x8D\x35").dd(4) // lea rsi, [rel write_message]
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a.code("\x7C") // jl "fatal_offset" -- write failure message
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a.db(fatal - len(a.buf) - 1)
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a.code("\x58") // pop rax -- restore tape position
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a.code("\xC3") // ret
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a.code("fatal: write failed\n\x00")
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// Now that we know where each instruction is, fill in relative jumps
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for i, x := range irb {
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// This must accurately reflect the code generators
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target, fixup := 0, offsets[i]
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if x.command == BEGIN || x.command == END {
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fixup += 4
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target = offsets[x.arg]
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} else if x.command == IN {
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fixup += 1
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target = read
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} else if x.command == OUT {
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fixup += 1
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target = write
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} else {
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continue
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}
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copy(a.buf[fixup:], le(target - fixup - 4)[:4])
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}
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return a.buf
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}
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// --- Main --------------------------------------------------------------------
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func main() {
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var err error
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if len(os.Args) > 3 {
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log.Fatalf("usage: %s [INPUT-FILE] [OUTPUT-FILE]", os.Args[0])
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}
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input := os.Stdin
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if len(os.Args) > 1 {
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if input, err = os.Open(os.Args[1]); err != nil {
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log.Fatalf("%s", err)
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}
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}
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outputPath := "a.out"
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if len(os.Args) > 2 {
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outputPath = os.Args[2]
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}
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program, err := ioutil.ReadAll(input)
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input.Close()
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if err != nil {
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log.Fatalf("can't read program: %s", err)
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}
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irb := decode(program)
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// ... various optimizations could be performed here if we give up brevity
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pairLoops(irb)
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dump("ir-dump.txt", irb)
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code := codegenAmd64(irb)
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a := codegen{}
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// TODO: also use the constants in package "debug/elf"
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const (
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ElfHeaderSize = 64 // size of the ELF header
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ElfProgramEntrySize = 56 // size of a program header
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ElfSectionEntrySize = 64 // size of a section header
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ElfPrologSize = ElfHeaderSize + 2*ElfProgramEntrySize
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)
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// ELF header
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a.code("\x7FELF\x02\x01\x01") // ELF, 64-bit, little endian, v1
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// Unix System V ABI, v0, padding
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a.code("\x00\x00" + "\x00\x00\x00\x00\x00\x00\x00")
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a.dw(2).dw(62).dd(1) // executable, x86-64, v1
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a.dq(ElfCodeAddr + ElfPrologSize) // entry point address
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// We only append section headers with debugging info with DEBUG
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a.dq(ElfHeaderSize).dq(0) // program, section header offset
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a.dd(0) // no processor-specific flags
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a.dw(ElfHeaderSize) // ELF header size
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a.dw(ElfProgramEntrySize).dw(2) // program hdr tbl entry size, count
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a.dw(ElfSectionEntrySize).dw(0) // section hdr tbl entry size, count
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a.dw(0) // no section index for strings
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// Program header for code
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// The entry point address seems to require alignment, so map start of file
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a.dd(1).dd(5) // PT_LOAD, PF_R | PF_X
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a.dq(0) // offset within the file
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a.dq(ElfCodeAddr) // address in virtual memory
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a.dq(ElfCodeAddr) // address in physical memory
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a.dq(ElfPrologSize + len(code)) // length within the file
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a.dq(ElfPrologSize + len(code)) // length within memory
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a.dq(4096) // segment alignment
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// Program header for the tape
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a.dd(1).dd(6) // PT_LOAD, PF_R | PF_W
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a.dq(0) // offset within the file
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a.dq(ElfDataAddr) // address in virtual memory
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a.dq(ElfDataAddr) // address in physical memory
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a.dq(0) // length within the file
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a.dq(1 << 20) // one megabyte of memory
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a.dq(4096) // segment alignment
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a.buf = append(a.buf, code...)
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if err = ioutil.WriteFile(outputPath, a.buf, 0777); err != nil {
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log.Fatalf("%s", err)
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}
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}
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