[llvm] [Dwarf][Transforms] Add dwarf support when func signature changed (PR #127855)

[Paul T Robinson via llvm-commits]

pogo59 wrote:

> > How about an extension to the standard attribute? It's a pity the DW_CC_* doesn't have a user defined extension space, but could get a bit weird and use some high-ish value to avoid colliding with likely standard features added to the enum.
> > Perhaps you could file a DWARF issue to discuss the long-term/standardized solution to this?
> 
> I could do this. But one thing I want to emphasize is for tag LLVM_func_args_changed. Currently we have not figured out what sub-tags or attributes for LLVM_func_args_changed. It won't be easy to express changed arguments, relation to original arguments, etc. But using a tag allows future extension.

It might help with the review to have some idea of how that information would be conveyed. It sounds like DeadArgumentElimination simply drops an unused argument (in effect, removes it from the argument list), so that could be described with a flag on the original DW_TAG_formal_parameter (DW_AT_omitted_in_practice, or something), which avoids the need for a DW_TAG_func_args_changed. I don't know what ArgumentPromotion does so I can't suggest how to describe what it does. If it changes the argument type, we could have a DW_AT_type_in_practice that points to the type that is actually used.

If we have an idea what the final DWARF would look like (acknowledging that the DWARF committee might make different decisions), that would help figure out whether DW_TAG_LLVM_func_args_changed is actually needed.

https://github.com/llvm/llvm-project/pull/127855