Initial commit of gdb-experiment.go

1 files changed, 355 insertions, 0 deletions

diff --git a/gdb-experiment.go b/gdb-experiment.go
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+++ b/gdb-experiment.go
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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)
+	}
+}