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//: 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 Level-2 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 {
string label = drop_last(curr.data);
if (SIZE(label) <= 2) {
raise << "label '" << label << "' is too short; must be more than two characters long\n" << end();
return;
}
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();
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();
}
string drop_last(const string& s) {
return string(s.begin(), --s.end());
}
//: 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'
:(scenario label_too_short)
% Hide_errors = true;
== 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
xz:
05 0x0d0c0b0a/imm32 # add to EAX
+error: label 'xz' is too short; must be more than two characters long
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