bfc/gdb-experiment.go

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// Non-optimizing Brainfuck compiler generating binaries for Linux on x86-64
// with debugging information mapping instructions onto an IR dump.
// gofmt has been tried, with disappointing results.
// codegen{} is also pretty ugly in the way it works but damn convenient.
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package main
import (
"encoding/binary"
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"errors"
"fmt"
"io/ioutil"
"log"
"os"
"strconv"
// Let's not repeat all those constants here onstants
"debug/dwarf"
"debug/elf"
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)
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)
b := make([]byte, 8)
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if unsigned, err := strconv.ParseUint(formatted, 10, 64); err == nil {
binary.LittleEndian.PutUint64(b, unsigned)
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} else if signed, err := strconv.ParseInt(formatted, 10, 64); err == nil {
binary.LittleEndian.PutUint64(b, uint64(signed))
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} else {
panic("cannot convert to number")
}
return b
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}
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) (code []byte, offsets []int) {
offsets = make([]int, len(irb)+1)
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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, offsets
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}
// --- 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, offsets := codegenAmd64(irb)
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// - - ELF generation - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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// Now that we know how long the machine code is, we can write the header.
// Note that for PIE we would need to depend on the dynamic linker, so no.
//
// Recommended reading:
// http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
// man 5 elf
//
// In case of unexpected gdb problems, also see:
// DWARF4.pdf
// https://sourceware.org/elfutils/DwarfLint
// http://wiki.osdev.org/DWARF
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const (
ElfHeaderSize = 64 // Size of the ELF header
ElfProgramEntrySize = 56 // Size of a program header
ElfSectionEntrySize = 64 // Size of a section header
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)
// - - Program headers - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
ph := codegen{}
phCount := 2
codeOffset := ElfHeaderSize + phCount*ElfProgramEntrySize
codeEndOffset := codeOffset + len(code)
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// Program header for code
// The entry point address seems to require alignment, so map start of file
ph.dd(elf.PT_LOAD).dd(elf.PF_R | elf.PF_X)
ph.dq(0) // Offset within the file
ph.dq(ElfCodeAddr) // Address in virtual memory
ph.dq(ElfCodeAddr) // Address in physical memory
ph.dq(codeEndOffset) // Length within the file
ph.dq(codeEndOffset) // Length within memory
ph.dq(4096) // Segment alignment
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// Program header for the tape
ph.dd(elf.PT_LOAD).dd(elf.PF_R | elf.PF_W)
ph.dq(0) // Offset within the file
ph.dq(ElfDataAddr) // Address in virtual memory
ph.dq(ElfDataAddr) // Address in physical memory
ph.dq(0) // Length within the file
ph.dq(1 << 20) // One megabyte of memory
ph.dq(4096) // Segment alignment
// Now that the rigid part has been generated, we can append sections
pieces := [][]byte{ph.buf, code}
position := codeEndOffset
// - - Sections - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
sh := codegen{}
shCount := 0
// This section is created on the go as we need to name other sections
stringTable := codegen{}
// - - Text - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
sh.dd(len(stringTable.buf)) // Index for the name of the section
stringTable.code(".text\x00")
sh.dd(elf.SHT_PROGBITS)
sh.dq(elf.SHF_ALLOC | elf.SHF_EXECINSTR)
sh.dq(ElfCodeAddr + codeOffset) // Memory address
sh.dq(codeOffset) // Byte offset
sh.dq(len(code) - codeOffset) // Byte size
sh.dd(0).dd(0) // No link, no info
sh.dq(0).dq(0) // No alignment, no entry size
shCount++
// - - Debug line - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
const (
opcodeBase = 13 // Offset by DWARF4 standard opcodes
lineBase = 0 // We don't need negative line indexes
lineRange = 2 // Either we advance a line or not (we always do)
)
// FIXME: we use db() a lot instead of a proper un/signed LEB128 encoder;
// that means that values > 127/63 or < 0 would break it;
// see Appendix C to DWARF4.pdf for an algorithm
lineProgram := codegen{}
// Extended opcode DW_LNE_set_address to reset the PC to the start of code
lineProgram.db(0).db(1 + 8).db(2).dq(ElfCodeAddr + codeOffset)
if len(irb) > 0 {
lineProgram.db(opcodeBase + offsets[0] * lineRange)
}
// The epilog, which is at the very end of the offset array, is included
for i := 1; i <= len(irb); i++ {
size := offsets[i] - offsets[i - 1]
lineProgram.db(opcodeBase + (1 - lineBase) + size * lineRange)
}
// Extended opcode DW_LNE_end_sequence is mandatory at the end
lineProgram.db(0).db(1).db(1)
lineHeader := codegen{}
lineHeader.db(1) // Minimum instruction length
lineHeader.db(1) // Maximum operations per instruction
lineHeader.db(1) // default_is_stmt
lineHeader.db(lineBase)
lineHeader.db(lineRange)
lineHeader.db(opcodeBase)
// Number of operands for all standard opcodes (1..opcodeBase-1)
opcodeLengths := []byte{0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 1}
lineHeader.buf = append(lineHeader.buf, opcodeLengths...)
// include_directories []string \x00
lineHeader.db(0)
// file_names []struct{base string; dir u8; modified u8; length u8} \x00
lineHeader.code("ir-dump.txt\x00").db(0).db(0).db(0).db(0)
lineEntry := codegen{}
lineEntry.dw(4) // .debug_line version number
lineEntry.dd(len(lineHeader.buf))
lineEntry.buf = append(lineEntry.buf, lineHeader.buf...)
lineEntry.buf = append(lineEntry.buf, lineProgram.buf...)
debugLine := codegen{}
debugLine.dd(len(lineEntry.buf))
debugLine.buf = append(debugLine.buf, lineEntry.buf...)
sh.dd(len(stringTable.buf)) // Index for the name of the section
stringTable.code(".debug_line\x00")
sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address
sh.dq(position) // Byte offset
sh.dq(len(debugLine.buf)) // Byte size
sh.dd(0).dd(0) // No link, no info
sh.dq(0).dq(0) // No alignment, no entry size
shCount++
pieces = append(pieces, debugLine.buf)
position += len(debugLine.buf)
// - - Debug abbreviations - - - - - - - - - - - - - - - - - - - - - - - - - - -
const (
formAddr = 0x01 // Pointer size
formSecOffset = 0x17 // DWARF size
)
debugAbbrev := codegen{}
debugAbbrev.db(1) // Our abbreviation code
debugAbbrev.db(dwarf.TagCompileUnit)
debugAbbrev.db(0) // DW_CHILDREN_no
debugAbbrev.db(dwarf.AttrLowpc).db(formAddr)
debugAbbrev.db(dwarf.AttrHighpc).db(formAddr)
debugAbbrev.db(dwarf.AttrStmtList).db(formSecOffset)
debugAbbrev.db(0).db(0) // End of attributes
debugAbbrev.db(0) // End of abbreviations
sh.dd(len(stringTable.buf)) // Index for the name of the section
stringTable.code(".debug_abbrev\x00")
sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address
sh.dq(position) // Byte offset
sh.dq(len(debugAbbrev.buf)) // Byte size
sh.dd(0).dd(0) // No link, no info
sh.dq(0).dq(0) // No alignment, no entry size
shCount++
pieces = append(pieces, debugAbbrev.buf)
position += len(debugAbbrev.buf)
// - - Debug info - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
cuEntry := codegen{}
cuEntry.dw(4) // .debug_info version number
cuEntry.dd(0) // Offset into .debug_abbrev
cuEntry.db(8) // Pointer size
// Single compile unit as per .debug_abbrev
cuEntry.db(1)
cuEntry.dq(ElfCodeAddr + codeOffset)
cuEntry.dq(ElfCodeAddr + codeEndOffset)
cuEntry.dd(0)
debugInfo := codegen{}
debugInfo.dd(len(cuEntry.buf))
debugInfo.buf = append(debugInfo.buf, cuEntry.buf...)
sh.dd(len(stringTable.buf)) // Index for the name of the section
stringTable.code(".debug_info\x00")
sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address
sh.dq(position) // Byte offset
sh.dq(len(debugInfo.buf)) // Byte size
sh.dd(0).dd(0) // No link, no info
sh.dq(0).dq(0) // No alignment, no entry size
shCount++
pieces = append(pieces, debugInfo.buf)
position += len(debugInfo.buf)
// - - Section names and section table - - - - - - - - - - - - - - - - - - - - -
sh.dd(len(stringTable.buf)) // Index for the name of the section
stringTable.code(".shstrtab\x00")
sh.dd(elf.SHT_STRTAB).dq(0).dq(0) // Type, no flags, no memory address
sh.dq(position) // Byte offset
sh.dq(len(stringTable.buf)) // Byte size
sh.dd(0).dd(0) // No link, no info
sh.dq(0).dq(0) // No alignment, no entry size
shCount++
pieces = append(pieces, stringTable.buf)
position += len(stringTable.buf)
pieces = append(pieces, sh.buf)
// Don't increment the position, we want to know where section headers start
// - - Final assembly of parts - - - - - - - - - - - - - - - - - - - - - - - - -
bin := codegen{}
// ELF header
bin.code("\x7FELF\x02\x01\x01") // ELF, 64-bit, little endian, v1
// Unix System V ABI, v0, padding
bin.code("\x00\x00" + "\x00\x00\x00\x00\x00\x00\x00")
bin.dw(elf.ET_EXEC).dw(elf.EM_X86_64).dd(elf.EV_CURRENT)
bin.dq(ElfCodeAddr + codeOffset) // Entry point address
bin.dq(ElfHeaderSize) // Program header offset
bin.dq(position) // Section header offset
bin.dd(0) // No processor-specific flags
bin.dw(ElfHeaderSize) // ELF header size
bin.dw(ElfProgramEntrySize) // Program header table entry size
bin.dw(phCount) // Program header table entry count
bin.dw(ElfSectionEntrySize) // Section header table entry size
bin.dw(shCount) // Section header table entry count
bin.dw(shCount - 1) // Section index for strings
for _, x := range pieces {
bin.buf = append(bin.buf, x...)
}
if err = ioutil.WriteFile(outputPath, bin.buf, 0777); err != nil {
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log.Fatalf("%s", err)
}
}