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

[Reid Kleckner via llvm-commits]

rnk wrote:

> 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'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. 

I share this concern about aligning the needs . At the end of the day, the compiler only emits one kind of debug info, and it's used for many purposes. We need to make reasonable tradeoffs. I view this copt as a form of short term tech debt to unblock current PGO use cases. It doesn't feel reasonable to me to insist that at this moment, to uphold the desired PGO invariant that all call sites have determinstic source locations, that we stop now and come up with a design for that up front.

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 (I won't even say short term, because everything lives longer than you expect) tech debt tradeoff.

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