Poor debug info in optimized code

[jsgf]

At the moment debugging optimized code is pretty tricky. Even with full debug info enabled, most/all of the locals and parameters are <optimized out>. (Linux + gdb)

I believe that DWARF can include info to regenerate the logical value of a variable that has been removed by optimization. Is this something llvm needs to support, or rustc, or both?

In general how can this be improved?

cc @tromey @Mark-Simulacrum

[cuviper]

I expect that's totally up to LLVM.

GCC has had a few DW_OP_GNU_* extensions that are now in DWARF5, like DW_OP_entry_value and DW_OP_implicit_pointer, which are useful for recovering optimized values. I don't see anything creating those in LLVM -- just basic declarations in their Dwarf.def.

[tromey]

The way to check a specific case is to following the instructions here: https://rust-lang-nursery.github.io/rustc-guide/compiler-debugging.html#debugging-llvm. In particular what you want to do is compile a small test program, have it emit the LLVM IR (see the discussion of -C llvm-args=-print-after-all), and then search through the dumps to see when the debug information is lost.

[tromey]

Bug #46698 is a specific case to still be investigated, FWIW.

[nical]

I expect that's totally up to LLVM.

For what it's worth, Gecko's code when compiled with LLVM contains both a lot of C++ and a lot of Rust at this point, and the debug information we are getting from Rust code in optimized builds are a lot worse than the C++ ones.
If there is any kind of massaging that can be done by rustc to make LLVM happier in that regard, it could potentially make a big difference for the productivity of Gecko developers. At the moment crash reports in Rust code are quite a bit harder to investigate.

[steveklabnik]

Triage: while this bug does contain one specific case, it's also very general. We have a bunch of bugs open related to debuginfo not being sufficient. I'm going to make a judgement call and give this one a close. Please file follow-up issues for specific things that can be improved, if issues aren't open already. Thanks!