[Lldb-commits] [lldb] [llvm] Annotate disassembly with register‐resident variable locations (PR #147460)

[Abdullah Mohammad Amin via lldb-commits]

UltimateForce21 wrote:

> As high-level comments:
> 
> * I would filter out the entry values, since this feature is meant to help a user understand the disassembly, and the entry values don't really exist in the function.
> * It would help readability if all annotations started at the same column
> * in a follow-up patch it would be great to outline the valid range of each expression
> * is it possible to hide the DW_OP... part and just show the human-readable register name?
> * `DW_OP_consts +0, DW_OP_stack_value` -> I would filter out anything that isn't a pure register. Then we can later recognize certain kinds of expression, for example, this is a constant, so it should just print `= 0`

Thank you for the thoughtful feedback! Here's a breakdown of what has been addressed so far and what I’m currently working on:

1. DW_OP_ suppression and human-readable register names
The most recent commit https://github.com/llvm/llvm-project/pull/147460/commits/912ba6d5a7a00c9111aae12a89ef84ccf0de00b4 introduces a new `PrintRegisterOnly `flag in `DIDumpOptions`, along with a `DumpLocationWithOptions()` API. This allows us to display clean, architecture-specific register names (e.g. `RDI`) without the `DW_OP_` prefix or LLVM-internal formatting.

- It also suppresses `<decoding error>` messages when `PrintRegisterOnly `is enabled, helping keep output clean for disassembly annotations.


2. Column alignment for annotations
I attempted to align annotations to a fixed column, and this works well in non-colored output. However, with color enabled, the byte-length of character sequences skews alignment. I'm investigating this, and suspect that the issue stems from the color codes affecting character counting without accounting for their visual width.

3. Variable invalidation (end of live range)
I'm currently working on modifying `Disassembler::PrintInstructions()` to allow tracking of variable live ranges across instructions. The goal is to emit annotations like `var = <invalid>` at the exact PC where a variable’s location range ends—while ensuring this is printed only once per variable.

4. `DW_OP_consts `filtering
I’ll follow up with a commit to recognize and handle `DW_OP_consts `and similar constant expressions more gracefully (e.g. print `var = 0`), while continuing to filter out more complex or mixed expressions for now, as suggested.

Thanks again—will continue to iterate on this based on your guidance and on all the other comments you have made!

https://github.com/llvm/llvm-project/pull/147460