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Diffstat (limited to 'subx/035labels.cc')
| -rw-r--r-- | subx/035labels.cc | 161 |
1 files changed, 161 insertions, 0 deletions
diff --git a/subx/035labels.cc b/subx/035labels.cc new file mode 100644 index 00000000..1bbf9b5b --- /dev/null +++ b/subx/035labels.cc @@ -0,0 +1,161 @@ +//: Labels are defined by ending names with a ':'. This layer will compute +//: addresses for labels, and compute the offset for instructions using them. + +:(scenarios transform) +:(scenario map_label) +== 0x1 + # instruction effective address operand displacement immediate + # op subop mod rm32 base index scale r32 + # 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes +loop: + 05 0x0d0c0b0a/imm32 # add to EAX ++transform: label 'loop' is at address 1 + +:(before "End Transforms") +Transform.push_back(rewrite_labels); + +:(code) +void rewrite_labels(program& p) { + trace(99, "transform") << "-- rewrite labels" << end(); + if (p.segments.empty()) return; + segment& code = p.segments.at(0); + map<string, int32_t> address; // values are unsigned, but we're going to do subtractions on them so they need to fit in 31 bits + compute_addresses_for_labels(code, address); + if (trace_contains_errors()) return; + drop_labels(code); + if (trace_contains_errors()) return; + replace_labels_with_addresses(code, address); +} + +void compute_addresses_for_labels(const segment& code, map<string, int32_t>& address) { + int current_byte = 0; + for (int i = 0; i < SIZE(code.lines); ++i) { + const line& inst = code.lines.at(i); + for (int j = 0; j < SIZE(inst.words); ++j) { + const word& curr = inst.words.at(j); + // hack: if we have any operand metadata left after previous transforms, + // deduce its size + // Maybe we should just move this transform to before instruction + // packing, and deduce the size of *all* operands. But then we'll also + // have to deal with bitfields. + if (has_metadata(curr, "disp32") || has_metadata(curr, "imm32")) { + if (*curr.data.rbegin() == ':') + raise << "'" << to_string(inst) << "': don't use ':' when jumping to labels\n" << end(); + current_byte += 4; + } + // automatically handle /disp8 and /imm8 here + else if (*curr.data.rbegin() != ':') { + ++current_byte; + } + else { + if (contains_any_operand_metadata(curr)) + raise << "'" << to_string(inst) << "': label definition (':') not allowed in operand\n" << end(); + if (j > 0) + raise << "'" << to_string(inst) << "': labels can only be the first word in a line.\n" << end(); + string label = curr.data.substr(0, SIZE(curr.data)-1); + put(address, label, current_byte); + trace(99, "transform") << "label '" << label << "' is at address " << (current_byte+code.start) << end(); + // no modifying current_byte; label definitions won't be in the final binary + } + } + } +} + +void drop_labels(segment& code) { + for (int i = 0; i < SIZE(code.lines); ++i) { + line& inst = code.lines.at(i); + vector<word>::iterator new_end = remove_if(inst.words.begin(), inst.words.end(), is_label); + inst.words.erase(new_end, inst.words.end()); + } +} + +bool is_label(const word& w) { + return *w.data.rbegin() == ':'; +} + +void replace_labels_with_addresses(segment& code, const map<string, int32_t>& address) { + int32_t byte_next_instruction_starts_at = 0; + for (int i = 0; i < SIZE(code.lines); ++i) { + line& inst = code.lines.at(i); + byte_next_instruction_starts_at += num_bytes(inst); + line new_inst; + for (int j = 0; j < SIZE(inst.words); ++j) { + const word& curr = inst.words.at(j); + if (contains_key(address, curr.data)) { + int32_t offset = static_cast<int32_t>(get(address, curr.data)) - byte_next_instruction_starts_at; + if (has_metadata(curr, "disp8") || has_metadata(curr, "imm8")) { + if (offset > 0xff || offset < -0x7f) + raise << "'" << to_string(inst) << "': label too far away for distance " << std::hex << offset << " to fit in 8 bits\n" << end(); + else + emit_hex_bytes(new_inst, offset, 1); + } + else if (has_metadata(curr, "disp16")) { + if (offset > 0xffff || offset < -0x7fff) + raise << "'" << to_string(inst) << "': label too far away for distance " << std::hex << offset << " to fit in 16 bits\n" << end(); + else + emit_hex_bytes(new_inst, offset, 2); + } + else if (has_metadata(curr, "disp32") || has_metadata(curr, "imm32")) { + emit_hex_bytes(new_inst, offset, 4); + } + } + else { + new_inst.words.push_back(curr); + } + } + inst.words.swap(new_inst.words); + trace(99, "transform") << "instruction after transform: '" << data_to_string(inst) << "'" << end(); + } +} + +// Assumes all bitfields are packed. +uint32_t num_bytes(const line& inst) { + uint32_t sum = 0; + for (int i = 0; i < SIZE(inst.words); ++i) { + const word& curr = inst.words.at(i); + if (has_metadata(curr, "disp32") || has_metadata(curr, "imm32")) // only multi-byte operands + sum += 4; + else + sum++; + } + return sum; +} + +string data_to_string(const line& inst) { + ostringstream out; + for (int i = 0; i < SIZE(inst.words); ++i) { + if (i > 0) out << ' '; + out << inst.words.at(i).data; + } + return out.str(); +} + +//: Label definitions must be the first word on a line. No jumping inside +//: instructions. +//: They should also be the only word on a line. +//: However, you can absolutely have multiple labels map to the same address, +//: as long as they're on separate lines. + +:(scenario multiple_labels_at) +== 0x1 + # instruction effective address operand displacement immediate + # op subop mod rm32 base index scale r32 + # 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes +# address 1 +loop: +loop2: +# address 1 (labels take up no space) + 05 0x0d0c0b0a/imm32 # add to EAX +# address 6 + eb loop2/disp8 +# address 8 + eb loop3/disp8 +# address 10 +loop3: ++transform: label 'loop' is at address 1 ++transform: label 'loop2' is at address 1 ++transform: label 'loop3' is at address 10 +# first jump is to -7 ++transform: instruction after transform: 'eb f9' +# second jump is to 0 (fall through) ++transform: instruction after transform: 'eb 00' |