gdb-experiment.go: add DWARF debugging information
Now we can debug compiled Brainfuck!
This commit is contained in:
parent
4efc032827
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def3218714
GPG Key ID:
B715679E3A361BE6
3
.gitignore
vendored
3
.gitignore
vendored
@ -1,6 +1,9 @@
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a.out
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gdb-experiment
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bfc-amd64-*
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ir-dump.txt
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bfc.creator*
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bfc.config
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bfc.files
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12
README.adoc
12
README.adoc
@ -35,6 +35,18 @@ When no input file is specified, standard input is used. Similarly, the default
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output filename is `a.out`. After the compilation, the resulting file can be
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run on the target platform.
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gdb
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---
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You may have noticed the `gdb-experiment.go` file. It is a non-optimizing
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version of the compiler targeting Linux only that adds DWARF debugging
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information mapping code locations onto lines in the `ir-dump.txt` byproduct
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output file. It's been rewritten in Go since managing all those binary buffers
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required to build the symbol table proved to be too painful in C.
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$ go run gdb-experiment.go [INPUT-FILE] [OUTPUT-FILE]
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Use `break *0x4000b7` to get a breakpoint at the first Brainfuck instruction.
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Contributing and Support
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------------------------
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Use this project's GitHub to report any bugs, request features, or submit pull
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@ -1,14 +1,21 @@
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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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// Non-optimizing Brainfuck compiler generating binaries for Linux on x86-64
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// with debugging information mapping instructions onto an IR dump.
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// gofmt has been tried, with disappointing results.
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// codegen{} is also pretty ugly in the way it works but damn convenient.
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package main
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import (
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"encoding/binary"
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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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// Let's not repeat all those constants here onstants
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"debug/dwarf"
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"debug/elf"
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)
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const ( RIGHT = iota; LEFT; INC; DEC; IN; OUT; BEGIN; END )
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@ -124,16 +131,15 @@ 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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b := make([]byte, 8)
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if unsigned, err := strconv.ParseUint(formatted, 10, 64); err == nil {
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v = unsigned
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binary.LittleEndian.PutUint64(b, 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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binary.LittleEndian.PutUint64(b, 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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return b
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}
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func (a *codegen) append(v []byte) { a.buf = append(a.buf, v...) }
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@ -154,8 +160,8 @@ const (
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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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func codegenAmd64(irb []instruction) (code []byte, offsets []int) {
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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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@ -268,7 +274,7 @@ func codegenAmd64(irb []instruction) []byte {
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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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return a.buf, offsets
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}
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// --- Main --------------------------------------------------------------------
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@ -301,55 +307,244 @@ func main() {
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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, offsets := codegenAmd64(irb)
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code := codegenAmd64(irb)
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a := codegen{}
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// - - ELF generation - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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// TODO: also use the constants in package "debug/elf"
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// Now that we know how long the machine code is, we can write the header.
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// Note that for PIE we would need to depend on the dynamic linker, so no.
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//
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// Recommended reading:
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// http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
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// man 5 elf
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//
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// In case of unexpected gdb problems, also see:
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// DWARF4.pdf
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// https://sourceware.org/elfutils/DwarfLint
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// http://wiki.osdev.org/DWARF
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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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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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)
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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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// - - Program headers - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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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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ph := codegen{}
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phCount := 2
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codeOffset := ElfHeaderSize + phCount*ElfProgramEntrySize
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codeEndOffset := codeOffset + len(code)
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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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ph.dd(elf.PT_LOAD).dd(elf.PF_R | elf.PF_X)
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ph.dq(0) // Offset within the file
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ph.dq(ElfCodeAddr) // Address in virtual memory
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ph.dq(ElfCodeAddr) // Address in physical memory
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ph.dq(codeEndOffset) // Length within the file
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ph.dq(codeEndOffset) // Length within memory
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ph.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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ph.dd(elf.PT_LOAD).dd(elf.PF_R | elf.PF_W)
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ph.dq(0) // Offset within the file
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ph.dq(ElfDataAddr) // Address in virtual memory
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ph.dq(ElfDataAddr) // Address in physical memory
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ph.dq(0) // Length within the file
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ph.dq(1 << 20) // One megabyte of memory
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ph.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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// Now that the rigid part has been generated, we can append sections
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pieces := [][]byte{ph.buf, code}
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position := codeEndOffset
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// - - Sections - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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sh := codegen{}
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shCount := 0
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// This section is created on the go as we need to name other sections
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stringTable := codegen{}
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// - - Text - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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sh.dd(len(stringTable.buf)) // Index for the name of the section
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stringTable.code(".text\x00")
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sh.dd(elf.SHT_PROGBITS)
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sh.dq(elf.SHF_ALLOC | elf.SHF_EXECINSTR)
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sh.dq(ElfCodeAddr + codeOffset) // Memory address
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sh.dq(codeOffset) // Byte offset
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sh.dq(len(code) - codeOffset) // Byte size
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sh.dd(0).dd(0) // No link, no info
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sh.dq(0).dq(0) // No alignment, no entry size
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shCount++
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// - - Debug line - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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const (
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opcodeBase = 13 // Offset by DWARF4 standard opcodes
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lineBase = 0 // We don't need negative line indexes
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lineRange = 2 // Either we advance a line or not (we always do)
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)
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// FIXME: we use db() a lot instead of a proper un/signed LEB128 encoder;
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// that means that values > 127/63 or < 0 would break it;
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// see Appendix C to DWARF4.pdf for an algorithm
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lineProgram := codegen{}
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// Extended opcode DW_LNE_set_address to reset the PC to the start of code
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lineProgram.db(0).db(1 + 8).db(2).dq(ElfCodeAddr + codeOffset)
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if len(irb) > 0 {
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lineProgram.db(opcodeBase + offsets[0] * lineRange)
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}
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// The epilog, which is at the very end of the offset array, is included
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for i := 1; i <= len(irb); i++ {
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size := offsets[i] - offsets[i - 1]
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lineProgram.db(opcodeBase + (1 - lineBase) + size * lineRange)
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}
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// Extended opcode DW_LNE_end_sequence is mandatory at the end
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lineProgram.db(0).db(1).db(1)
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lineHeader := codegen{}
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lineHeader.db(1) // Minimum instruction length
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lineHeader.db(1) // Maximum operations per instruction
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lineHeader.db(1) // default_is_stmt
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lineHeader.db(lineBase)
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lineHeader.db(lineRange)
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lineHeader.db(opcodeBase)
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// Number of operands for all standard opcodes (1..opcodeBase-1)
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opcodeLengths := []byte{0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 1}
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lineHeader.buf = append(lineHeader.buf, opcodeLengths...)
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// include_directories []string \x00
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lineHeader.db(0)
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// file_names []struct{base string; dir u8; modified u8; length u8} \x00
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lineHeader.code("ir-dump.txt\x00").db(0).db(0).db(0).db(0)
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lineEntry := codegen{}
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lineEntry.dw(4) // .debug_line version number
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lineEntry.dd(len(lineHeader.buf))
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lineEntry.buf = append(lineEntry.buf, lineHeader.buf...)
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lineEntry.buf = append(lineEntry.buf, lineProgram.buf...)
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debugLine := codegen{}
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debugLine.dd(len(lineEntry.buf))
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debugLine.buf = append(debugLine.buf, lineEntry.buf...)
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sh.dd(len(stringTable.buf)) // Index for the name of the section
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stringTable.code(".debug_line\x00")
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sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address
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sh.dq(position) // Byte offset
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sh.dq(len(debugLine.buf)) // Byte size
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sh.dd(0).dd(0) // No link, no info
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sh.dq(0).dq(0) // No alignment, no entry size
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shCount++
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pieces = append(pieces, debugLine.buf)
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position += len(debugLine.buf)
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// - - Debug abbreviations - - - - - - - - - - - - - - - - - - - - - - - - - - -
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const (
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formAddr = 0x01 // Pointer size
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formSecOffset = 0x17 // DWARF size
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)
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debugAbbrev := codegen{}
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debugAbbrev.db(1) // Our abbreviation code
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debugAbbrev.db(dwarf.TagCompileUnit)
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debugAbbrev.db(0) // DW_CHILDREN_no
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debugAbbrev.db(dwarf.AttrLowpc).db(formAddr)
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debugAbbrev.db(dwarf.AttrHighpc).db(formAddr)
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debugAbbrev.db(dwarf.AttrStmtList).db(formSecOffset)
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debugAbbrev.db(0).db(0) // End of attributes
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debugAbbrev.db(0) // End of abbreviations
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sh.dd(len(stringTable.buf)) // Index for the name of the section
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stringTable.code(".debug_abbrev\x00")
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sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address
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sh.dq(position) // Byte offset
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sh.dq(len(debugAbbrev.buf)) // Byte size
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sh.dd(0).dd(0) // No link, no info
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sh.dq(0).dq(0) // No alignment, no entry size
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shCount++
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pieces = append(pieces, debugAbbrev.buf)
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position += len(debugAbbrev.buf)
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// - - Debug info - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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cuEntry := codegen{}
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cuEntry.dw(4) // .debug_info version number
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cuEntry.dd(0) // Offset into .debug_abbrev
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cuEntry.db(8) // Pointer size
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// Single compile unit as per .debug_abbrev
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cuEntry.db(1)
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cuEntry.dq(ElfCodeAddr + codeOffset)
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cuEntry.dq(ElfCodeAddr + codeEndOffset)
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cuEntry.dd(0)
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debugInfo := codegen{}
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debugInfo.dd(len(cuEntry.buf))
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debugInfo.buf = append(debugInfo.buf, cuEntry.buf...)
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sh.dd(len(stringTable.buf)) // Index for the name of the section
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stringTable.code(".debug_info\x00")
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sh.dd(elf.SHT_PROGBITS).dq(0).dq(0) // Type, no flags, no memory address
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sh.dq(position) // Byte offset
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sh.dq(len(debugInfo.buf)) // Byte size
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sh.dd(0).dd(0) // No link, no info
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sh.dq(0).dq(0) // No alignment, no entry size
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shCount++
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pieces = append(pieces, debugInfo.buf)
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position += len(debugInfo.buf)
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// - - Section names and section table - - - - - - - - - - - - - - - - - - - - -
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sh.dd(len(stringTable.buf)) // Index for the name of the section
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stringTable.code(".shstrtab\x00")
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sh.dd(elf.SHT_STRTAB).dq(0).dq(0) // Type, no flags, no memory address
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sh.dq(position) // Byte offset
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sh.dq(len(stringTable.buf)) // Byte size
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sh.dd(0).dd(0) // No link, no info
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sh.dq(0).dq(0) // No alignment, no entry size
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shCount++
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pieces = append(pieces, stringTable.buf)
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position += len(stringTable.buf)
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pieces = append(pieces, sh.buf)
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// Don't increment the position, we want to know where section headers start
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// - - Final assembly of parts - - - - - - - - - - - - - - - - - - - - - - - - -
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bin := codegen{}
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// ELF header
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bin.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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bin.code("\x00\x00" + "\x00\x00\x00\x00\x00\x00\x00")
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bin.dw(elf.ET_EXEC).dw(elf.EM_X86_64).dd(elf.EV_CURRENT)
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bin.dq(ElfCodeAddr + codeOffset) // Entry point address
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bin.dq(ElfHeaderSize) // Program header offset
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bin.dq(position) // Section header offset
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bin.dd(0) // No processor-specific flags
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bin.dw(ElfHeaderSize) // ELF header size
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bin.dw(ElfProgramEntrySize) // Program header table entry size
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bin.dw(phCount) // Program header table entry count
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bin.dw(ElfSectionEntrySize) // Section header table entry size
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bin.dw(shCount) // Section header table entry count
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bin.dw(shCount - 1) // Section index for strings
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for _, x := range pieces {
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bin.buf = append(bin.buf, x...)
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}
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if err = ioutil.WriteFile(outputPath, bin.buf, 0777); err != nil {
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log.Fatalf("%s", err)
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}
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}
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