4468

1 files changed, 161 insertions, 0 deletions

diff --git a/subx/034labels.cc b/subx/034labels.cc
new file mode 100644
index 00000000..78190c6d
--- /dev/null
+++ b/subx/034labels.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 One-time Setup")
+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'