With this, we can update DWARF debug line info properly as we write a new binary. To do that we track binary locations as we write. Each instruction is mapped to the location it is written to. We must also adjust them as we move code around because of LEB optimization (we emit a function or a section with a 5-byte LEB placeholder, the maximal size; later we shrink it which is almost always possible). writeDWARFSections() now takes a second param, the new locations of instructions. It then maps debug line info from the original offsets in the binary to the new offsets in the binary being written. The core logic for updating the debug line section is in wasm-debug.cpp. It basically tracks state machine logic both to read the existing debug lines and to emit the new ones. I couldn't find a way to reuse LLVM code for this, but reading LLVM's code was very useful here. A final tricky thing we need to do is to update the DWARF section's internal size annotation. The LLVM YAML writing code doesn't do that for us. Luckily it's pretty easy, in fixEmittedSection we just update the first 4 bytes in place to have the section size, after we've emitted it and know the size. This ignores debug lines with a 0 in the line, col, or addr, see WebAssembly/debugging#9 (comment) This ignores debug line offsets into the middle of instructions, which LLVM sometimes emits for some reason, see WebAssembly/debugging#9 (comment) Handling that would likely at least double our memory usage, which is unfortunate - we are run in an LTO manner, where the entire app's DWARF is present, and it may be massive. I think we should see if such odd offsets are a bug in LLVM, and if we can fix or prevent that. This does not emit "special" opcodes for debug lines. Those are purely an optimization, which I wanted to leave for later. (Even without them we decrease the size quite a lot, btw, as many lines have 0s in them...) This adds some testing that shows we can load and save fib2.c and fannkuch.cpp properly. The latter includes more than one function and has nontrivial code. To actually emit correct offsets a few minor fixes are done here: * Fix the code section location tracking during reading - the correct offset we care about is the body of the code section, not including the section declaration and size. * Fix wasm-stack debug line emitting. We need to update in BinaryInstWriter::visit(), that is, right before writing bytes for the instruction. That differs from * BinaryenIRWriter::visit which is a recursive function that also calls the children - so the offset there would be of the first child. For some reason that is correct with source maps, I don't understand why, but it's wrong for DWARF... * Print code section offsets in hex, to match other tools. Remove DWARFUpdate pass, which was useful for testing temporarily, but doesn't make sense now (it just updates without writing a binary). cc @yurydelendik