//: Labels are defined by ending names with a ':'. This layer will compute
//: displacements for labels, and compute the offset for instructions using them.
//:
//: We won't check this, but our convention will be that jump targets will
//: start with a '$', while functions will not. Function names will never be
//: jumped to, and jump targets will never be called.
//: We're introducing non-number names for the first time, so it's worth
//: laying down some ground rules all transforms will follow, so things don't
//: get too confusing:
//: - if it starts with a digit, it's treated as a number. If it can't be
//: parsed as hex it will raise an error.
//: - if it starts with '-' it's treated as a number.
//: - if it starts with '0x' it's treated as a number.
//: - if it's two characters long, it can't be a name. Either it's a hex
//: byte, or it raises an error.
//: That's it. Names can start with any non-digit that isn't a dash. They can
//: be a single character long. 'a' is not a hex number, it's a variable.
//: Later layers may add more conventions partitioning the space of names. But
//: the above rules will remain inviolate.
void check_valid_name(const string& s) {
if (s.empty()) {
raise << "empty name!\n" << end();
return;
}
if (s.at(0) == '-')
raise << "'" << s << "' starts with '-', which can be confused with a negative number; use a different name\n" << end();
if (s.substr(0, 2) == "0x") {
raise << "'" << s << "' looks like a hex number; use a different name\n" << end();
return;
}
if (isdigit(s.at(0)))
raise << "'" << s << "' starts with a digit, and so can be confused with a negative number; use a different name.\n" << end();
if (SIZE(s) == 2)
raise << "'" << s << "' is two characters long which can look like raw hex bytes at a glance; use a different name\n" << end();
}
:(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> byte_index; // values are unsigned, but we're going to do subtractions on them so they need to fit in 31 bits
compute_byte_indices_for_labels(code, byte_index);
if (trace_contains_errors()) return;
drop_labels(code);
if (trace_contains_errors()) return;
replace_labels_with_displacements(code, byte_index);
}
void compute_byte_indices_for_labels(const segment& code, map<string, int32_t>& byte_index) {
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_operand_metadata(curr, "disp32") || has_operand_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);
// ensure labels look sufficiently different from raw hex
check_valid_name(label);
if (trace_contains_errors()) 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(byte_index, 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_displacements(segment& code, const map<string, int32_t>& byte_index) {
int32_t byte_index_next_instruction_starts_at = 0;
for (int i = 0; i < SIZE(code.lines); ++i) {
line& inst = code.lines.at(i);
byte_index_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(byte_index, curr.data)) {
int32_t displacement = static_cast<int32_t>(get(byte_index, curr.data)) - byte_index_next_instruction_starts_at;
if (has_operand_metadata(curr, "disp8") || has_operand_metadata(curr, "imm8")) {
if (displacement > 0xff || displacement < -0x7f)
raise << "'" << to_string(inst) << "': label too far away for displacement " << std::hex << displacement << " to fit in 8 bits\n" << end();
else
emit_hex_bytes(new_inst, displacement, 1);
}
else if (has_operand_metadata(curr, "disp16")) {
if (displacement > 0xffff || displacement < -0x7fff)
raise << "'" << to_string(inst) << "': label too far away for displacement " << std::hex << displacement << " to fit in 16 bits\n" << end();
else
emit_hex_bytes(new_inst, displacement, 2);
}
else if (has_operand_metadata(curr, "disp32") || has_operand_metadata(curr, "imm32")) {
emit_hex_bytes(new_inst, displacement, 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();
}
}
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 0xa
$loop3:
+transform: label 'loop' is at address 1
+transform: label '$loop2' is at address 1
+transform: label '$loop3' is at address a
# 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: 'xz' is two characters long which can look like raw hex bytes at a glance; use a different name
:(scenario label_hex)
% 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
0xab:
05 0x0d0c0b0a/imm32 # add to EAX
+error: '0xab' looks like a hex number; use a different name
:(scenario label_negative_hex)
% 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
-a: # indent to avoid looking like a trace_should_not_contain command for this scenario
05 0x0d0c0b0a/imm32 # add to EAX
+error: '-a' starts with '-', which can be confused with a negative number; use a different name
//: now that we have labels, we need to adjust segment size computation to
//: ignore them.
:(scenario segment_size_ignores_labels)
% Mem_offset = CODE_START;
== code # 0x08048074
05/add 0x0d0c0b0a/imm32 # 5 bytes
foo: # 0 bytes
== data # 0x08049079
bar:
00
+transform: segment 1 begins at address 0x08049079
:(before "End num_bytes(curr) Special-cases")
else if (is_label(curr))
; // don't count it