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

[David Blaikie via llvm-commits]

dwblaikie wrote:

> > > I think we will find additional opportunities and once we have a better idea, I can update the documentation and guidance at https://llvm.org/docs/HowToUpdateDebugInfo.html.
> > > Does that sound reasonable to you?
> > 
> > 
> > I'd be inclined to figure out the principles here a bit earlier/somewhat up front, before we make changes. I'm especially concerned about a divergence between what's good for profilers and what's good for humans.
> 
> I think this is probably the most important thing: documenting the principles of what we do, why, what AFDO needs, what profilers need, etc, what modes we have, and what we expect them to do.

Documenting it seems good - perhaps more than has been posted so far (the AFDO documentation doesn't discuss the issue of "samples of a source location inside a block could lead to misconstrued assumptions about that block being reachable"). But even then, I'm still really worried that this makes the needs of different consumers divergent to a problematic degree - we managed to make the direction align between a variety of consumers (humans, profilers, profile driven optimization, etc) so far and diverging from that seems like a real problem. (even just at Google - humans use profiles from this same debug info, and if we tell them that some block is hot when it isn't, that seems potentially really unfortunate)

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