Limited source mapping information in compiler debug output

[JoshuaBatty]

I'm currently trying to implement source <-> bytecode mapping for compiler-explorer integration and noticed that the generated debug info (both DWARF and json) is quite limited. This is calling the debug build with forc build using OptLevel::Opt0.

For example:

script;

fn get_num() -> u64 {
    5
}

fn main() {
    let x = 5;
    let y = get_num();
    let res = x + y;
}

The generated symbols.json only contains mappings for the main function entry point:

{
  "paths": [...],
  "map": {
    "58": {"path": 1, "range": {"start": {"line": 7, "col": 1}, "end": {"line": 11,"col": 2}}},
    "59": {"path": 1, "range": {"start": {"line": 7, "col": 1}, "end": {"line": 11,"col": 2}}}
  }
}

DWARF output shows the same limited info:

Address            Line   Column File   ISA Discriminator Flags
------------------ ------ ------ ------ --- ------------- -------------
0x000000000000003a      7      1      2   0             0  is_stmt
0x000000000000003c      7      1      2   0             0  is_stmt end_sequence

Looking at the bytecode, we have operations like ADD, MOVE etc that should map to specific source locations (e.g. res = x + y on line 10), but these mappings are missing. This seems to be because many AllocatedOps aren't getting their owning_span populated which is required for them to be inserted into the sourcemap.

if let Some(span) = &span {
    source_map.insert(source_engine, half_word_ix, span);
}

[vaivaswatha]

See #5511 (comment). Even at -O0, we remove some dead code, and that's why res here (and consequently x and y) get eliminated, thus not seen in the output. If you return res from main, you should see the instructions as well as its debug info.

[IGI-111]

I'll close this since it seems to be working as intended, feel free to reopen if there's more to do here @JoshuaBatty