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//: Start allowing us to not specify precise addresses for the start of each
//: segment.
//: This gives up a measure of control in placing code and data.
//: segment address computation requires setting Mem_offset in test mode to what it'll be in run mode
:(scenario segment_name)
% Mem_offset = CODE_START;
== code
05/add 0x0d0c0b0a/imm32 # add 0x0d0c0b0a to EAX
# code starts at 0x08048000 + p_offset, which is 0x54 for a single-segment binary
+load: 0x08048054 -> 05
+load: 0x08048055 -> 0a
+load: 0x08048056 -> 0b
+load: 0x08048057 -> 0c
+load: 0x08048058 -> 0d
+run: add imm32 0x0d0c0b0a to reg EAX
+run: storing 0x0d0c0b0a
:(before "End Level-2 Transforms")
Transform.push_back(compute_segment_starts);
:(code)
void compute_segment_starts(program& p) {
trace(99, "transform") << "-- compute segment addresses" << end();
uint32_t p_offset = /*size of ehdr*/0x34 + SIZE(p.segments)*0x20/*size of each phdr*/;
for (size_t i = 0; i < p.segments.size(); ++i) {
segment& curr = p.segments.at(i);
if (curr.start == 0) {
curr.start = CODE_START + i*SEGMENT_SIZE + p_offset;
trace(99, "transform") << "segment " << i << " begins at address 0x" << HEXWORD << curr.start << end();
}
p_offset += size_of(curr);
assert(p_offset < SEGMENT_SIZE); // for now we get less and less available space in each successive segment
}
}
uint32_t size_of(const segment& s) {
uint32_t sum = 0;
for (int i = 0; i < SIZE(s.lines); ++i)
sum += num_bytes(s.lines.at(i));
return sum;
}
// 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_operand_metadata(curr, "disp32") || has_operand_metadata(curr, "imm32")) // only multi-byte operands
sum += 4;
// End num_bytes(curr) Special-cases
else
sum++;
}
return sum;
}
//: Dependencies:
//: - We'd like to compute segment addresses before setting up global variables,
//: because computing addresses for global variables requires knowing where
//: the data segment starts.
//: - We'd like to finish expanding labels before computing segment addresses,
//: because it would make computing the sizes of segments more self-contained
//: (num_bytes).
//:
//: Decision: compute segment addresses before expanding labels, by being
//: aware in this layer of certain operand types that will eventually occupy
//: multiple bytes.
//:
//: The layer to expand labels later hooks into num_bytes() to teach this
//: layer that labels occupy zero space in the binary.
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