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Initial commit of gdb-experiment.go

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Přemysl Janouch 3 years ago
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Signed by: Přemysl Janouch <p.janouch@gmail.com> GPG Key ID: B715679E3A361BE6
1 changed files with 355 additions and 0 deletions
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// Non-optimizing Brainfuck compiler generating binaries for Linux on x86-64;
// gofmt has been tried, with disappointing results
package main

import (
"errors"
"fmt"
"io/ioutil"
"log"
"os"
"strconv"
)

const ( RIGHT = iota; LEFT; INC; DEC; IN; OUT; BEGIN; END )

var info = []struct {
grouped bool
name string
}{
{true, "RIGHT"},
{true, "LEFT"},
{true, "INC"},
{true, "DEC"},
{false, "IN"},
{false, "OUT"},
{false, "BEGIN"},
{false, "END"},
}

type instruction struct {
command int
arg int
}

// Dump internal representation to a file for debugging purposes
func dump(filename string, irb []instruction) error {
out, err := os.Create(filename)
if err != nil {
return err
}

indent := 0
for _, x := range irb {
if x.command == END {
indent--
}
for i := 0; i < indent; i++ {
out.WriteString(" ")
}
out.WriteString(info[x.command].name)
if info[x.command].grouped {
fmt.Fprintf(out, " %d", x.arg)
}
out.WriteString("\n")
if x.command == BEGIN {
indent++
}
}
if err = out.Close(); err != nil {
return err
}
return nil
}

// Decode a Brainfuck program into internal representation,
// coalescing identical commands together as the most basic optimization
func decode(program []byte) (irb []instruction) {
for _, c := range program {
var command int
switch c {
case '>': command = RIGHT
case '<': command = LEFT
case '+': command = INC
case '-': command = DEC
case '.': command = OUT
case ',': command = IN
case '[': command = BEGIN
case ']': command = END
default: continue
}

if len(irb) == 0 || !info[command].grouped ||
irb[len(irb)-1].command != command {
irb = append(irb, instruction{command, 1})
} else {
irb[len(irb)-1].arg++
}
}
return
}

// Match loop commands so that we know where to jump
func pairLoops(irb []instruction) error {
nesting := 0
stack := make([]int, len(irb))
for i, x := range irb {
switch x.command {
case BEGIN:
stack[nesting] = i
nesting++
case END:
if nesting <= 0 {
return errors.New("unbalanced loops")
}
nesting--
irb[stack[nesting]].arg = i + 1
irb[i].arg = stack[nesting] + 1
}
}
if nesting != 0 {
return errors.New("unbalanced loops")
}
return nil
}

// --- Code generation ---------------------------------------------------------

type codegen struct {
buf []byte
}

// Convert an arbitrary integral value up to 8 bytes long to little endian
func le(unknown interface{}) []byte {
// Trying hard to avoid reflect.Value.Int/Uint
formatted := fmt.Sprintf("%d", unknown)

var v uint64
if unsigned, err := strconv.ParseUint(formatted, 10, 64); err == nil {
v = unsigned
} else if signed, err := strconv.ParseInt(formatted, 10, 64); err == nil {
v = uint64(signed)
} else {
panic("cannot convert to number")
}
return []byte{byte(v), byte(v >> 8), byte(v >> 16), byte(v >> 24),
byte(v >> 32), byte(v >> 40), byte(v >> 48), byte(v >> 56)}
}

func (a *codegen) append(v []byte) { a.buf = append(a.buf, v...) }
func (a *codegen) code(v string) *codegen { a.append([]byte(v)); return a }
func (a *codegen) db(v interface{}) *codegen { a.append(le(v)[:1]); return a }
func (a *codegen) dw(v interface{}) *codegen { a.append(le(v)[:2]); return a }
func (a *codegen) dd(v interface{}) *codegen { a.append(le(v)[:4]); return a }
func (a *codegen) dq(v interface{}) *codegen { a.append(le(v)[:8]); return a }

const (
ElfCodeAddr = 0x400000 // Where the code is loaded in memory
ElfDataAddr = 0x800000 // Where the tape is placed in memory
)

const (
SYS_READ = 0
SYS_WRITE = 1
SYS_EXIT = 60
)

func codegenAmd64(irb []instruction) []byte {
offsets := make([]int, len(irb)+1)
a := codegen{}

a.code("\xB8").dd(ElfDataAddr) // mov rax, "ElfCodeAddr"
a.code("\x30\xDB") // xor bl, bl

for i, x := range irb {
offsets[i] = len(a.buf)
if x.command == LEFT || x.command == RIGHT {
a.code("\x88\x18") // mov [rax], bl
}
switch x.command {
case RIGHT: a.code("\x48\x05").dd(x.arg) // add rax, "arg"
case LEFT: a.code("\x48\x2D").dd(x.arg) // sub rax, "arg"
case INC: a.code("\x80\xC3").db(x.arg) // add bl, "arg"
case DEC: a.code("\x80\xEB").db(x.arg) // sub bl, "arg"
case OUT: a.code("\xE8").dd(0) // call "write"
case IN: a.code("\xE8").dd(0) // call "read"
case BEGIN:
// test bl, bl; jz "offsets[arg]"
a.code("\x84\xDB" + "\x0F\x84").dd(0)
case END:
// test bl, bl; jnz "offsets[arg]"
a.code("\x84\xDB" + "\x0F\x85").dd(0)
}
if x.command == LEFT || x.command == RIGHT {
a.code("\x8A\x18") // mov bl, [rax]
}
}
// When there is a loop at the end we need to be able to jump past it
offsets[len(irb)] = len(a.buf)

// Write an epilog which handles all the OS interfacing
//
// System V x86-64 ABI:
// rax <-> both syscall number and return value
// args -> rdi, rsi, rdx, r10, r8, r9
// trashed <- rcx, r11

a.code("\xB8").dd(SYS_EXIT) // mov eax, 0x3c
a.code("\x48\x31\xFF") // xor rdi, rdi
a.code("\x0F\x05") // syscall

fatal := len(a.buf)
a.code("\x48\x89\xF7") // mov rdi, rsi -- use the string in rsi
a.code("\x30\xC0") // xor al, al -- look for the nil byte
a.code("\x48\x31\xC9") // xor rcx, rcx
a.code("\x48\xF7\xD1") // not rcx -- start from -1
a.code("\xFC" + "\xF2\xAE") // cld; repne scasb -- decrement until found
a.code("\x48\xF7\xD1") // not rcx
a.code("\x48\x8D\x51\xFF") // lea rdx, [rcx-1] -- save length in rdx
a.code("\xB8").dd(SYS_WRITE) // mov eax, "SYS_WRITE"
a.code("\xBF").dd(2) // mov edi, "STDERR_FILENO"
a.code("\x0F\x05") // syscall

a.code("\xB8").dd(SYS_EXIT) // mov eax, "SYS_EXIT"
a.code("\xBF").dd(1) // mov edi, "EXIT_FAILURE"
a.code("\x0F\x05") // syscall

read := len(a.buf)
a.code("\x50") // push rax -- save tape position
a.code("\xB8").dd(SYS_READ) // mov eax, "SYS_READ"
a.code("\x48\x89\xC7") // mov rdi, rax -- STDIN_FILENO
a.code("\x66\x6A\x00") // push word 0 -- the default value for EOF
a.code("\x48\x89\xE6") // mov rsi, rsp -- the char starts at rsp
a.code("\xBA").dd(1) // mov edx, 1 -- count
a.code("\x0F\x05") // syscall
a.code("\x66\x5B") // pop bx

a.code("\x48\x83\xF8\x00") // cmp rax, 0
a.code("\x48\x8D\x35").dd(4) // lea rsi, [rel read_message]
a.code("\x7C") // jl "fatal_offset" -- write failure message
a.db(fatal - len(a.buf) - 1)
a.code("\x58") // pop rax -- restore tape position
a.code("\xC3") // ret
a.code("fatal: read failed\n\x00")

write := len(a.buf)
a.code("\x50") // push rax -- save tape position
a.code("\xB8").dd(SYS_WRITE) // mov eax, "SYS_WRITE"
a.code("\x48\x89\xC7") // mov rdi, rax -- STDOUT_FILENO
a.code("\x66\x53") // push bx
a.code("\x48\x89\xE6") // mov rsi, rsp -- the char starts at rsp
a.code("\xBA").dd(1) // mov edx, 1 -- count
a.code("\x0F\x05") // syscall
a.code("\x66\x5B") // pop bx

a.code("\x48\x83\xF8\x00") // cmp rax, 0
a.code("\x48\x8D\x35").dd(4) // lea rsi, [rel write_message]
a.code("\x7C") // jl "fatal_offset" -- write failure message
a.db(fatal - len(a.buf) - 1)
a.code("\x58") // pop rax -- restore tape position
a.code("\xC3") // ret
a.code("fatal: write failed\n\x00")

// Now that we know where each instruction is, fill in relative jumps
for i, x := range irb {
// This must accurately reflect the code generators
target, fixup := 0, offsets[i]
if x.command == BEGIN || x.command == END {
fixup += 4
target = offsets[x.arg]
} else if x.command == IN {
fixup += 1
target = read
} else if x.command == OUT {
fixup += 1
target = write
} else {
continue
}
copy(a.buf[fixup:], le(target - fixup - 4)[:4])
}
return a.buf
}

// --- Main --------------------------------------------------------------------

func main() {
var err error
if len(os.Args) > 3 {
log.Fatalf("usage: %s [INPUT-FILE] [OUTPUT-FILE]", os.Args[0])
}

input := os.Stdin
if len(os.Args) > 1 {
if input, err = os.Open(os.Args[1]); err != nil {
log.Fatalf("%s", err)
}
}

outputPath := "a.out"
if len(os.Args) > 2 {
outputPath = os.Args[2]
}

program, err := ioutil.ReadAll(input)
input.Close()
if err != nil {
log.Fatalf("can't read program: %s", err)
}

irb := decode(program)
// ... various optimizations could be performed here if we give up brevity
pairLoops(irb)
dump("ir-dump.txt", irb)

code := codegenAmd64(irb)
a := codegen{}

// TODO: also use the constants in package "debug/elf"

const (
ElfHeaderSize = 64 // size of the ELF header
ElfProgramEntrySize = 56 // size of a program header
ElfSectionEntrySize = 64 // size of a section header
ElfPrologSize = ElfHeaderSize + 2*ElfProgramEntrySize
)

// ELF header
a.code("\x7FELF\x02\x01\x01") // ELF, 64-bit, little endian, v1
// Unix System V ABI, v0, padding
a.code("\x00\x00" + "\x00\x00\x00\x00\x00\x00\x00")
a.dw(2).dw(62).dd(1) // executable, x86-64, v1
a.dq(ElfCodeAddr + ElfPrologSize) // entry point address

// We only append section headers with debugging info with DEBUG
a.dq(ElfHeaderSize).dq(0) // program, section header offset
a.dd(0) // no processor-specific flags
a.dw(ElfHeaderSize) // ELF header size
a.dw(ElfProgramEntrySize).dw(2) // program hdr tbl entry size, count
a.dw(ElfSectionEntrySize).dw(0) // section hdr tbl entry size, count
a.dw(0) // no section index for strings

// Program header for code
// The entry point address seems to require alignment, so map start of file
a.dd(1).dd(5) // PT_LOAD, PF_R | PF_X
a.dq(0) // offset within the file
a.dq(ElfCodeAddr) // address in virtual memory
a.dq(ElfCodeAddr) // address in physical memory
a.dq(ElfPrologSize + len(code)) // length within the file
a.dq(ElfPrologSize + len(code)) // length within memory
a.dq(4096) // segment alignment

// Program header for the tape
a.dd(1).dd(6) // PT_LOAD, PF_R | PF_W
a.dq(0) // offset within the file
a.dq(ElfDataAddr) // address in virtual memory
a.dq(ElfDataAddr) // address in physical memory
a.dq(0) // length within the file
a.dq(1 << 20) // one megabyte of memory
a.dq(4096) // segment alignment

a.buf = append(a.buf, code...)
if err = ioutil.WriteFile(outputPath, a.buf, 0777); err != nil {
log.Fatalf("%s", err)
}
}

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