[llvm] [DebugInfo] Preserve line and column number when merging debug info. (PR #129960)

[Snehasish Kumar via llvm-commits]

snehasish wrote:

> Thanks for the docs!
> 
> This makes me wonder if we should look into some higher-level integration testing that tracks the number of call sites that lack source locations, since call site source locations are critical for so many applications.

+1 to this. We've had similar discussions about [profile information propagation](https://discourse.llvm.org/t/rfc-profile-information-propagation-unittesting/73595) before.

>  I think it would be helpful to document that maintaining deterministic and distinct source locations are what's important for sample PGO. 

Yes, the need for distinction also motivated the work on DWARF path [discriminators](https://reviews.llvm.org/D2691) for duplicated basic blocks. More recent work on [flow sensitive AFDO](https://discourse.llvm.org/t/rfc-control-flow-sensitive-autofdo-fs-afdo/56965) has extended the use of discriminators to enhance profile accuracy particularly in the backend.

> As long as we have agreement that we need such a design and that this is the long term goal, I'm comfortable living with this 

Let me know if you want me to file an issue or if you want any further clarification on SamplePGO needs in the debug info documentation. Thanks!

https://github.com/llvm/llvm-project/pull/129960