[flang][debug] Number function arguments in FIR

[vzakhari]

AddDebugInfo pass uses fragile logic for figuring out the positional number of a dummy argument in the list of function arguments. This causes issues like #131672

It seems that we need to number the dummy arguments in a more reliable way, e.g. attribute [hl]fir.declare operations with the proper number early in the pipeline (maybe even during lowering).

There are some questions that I do not know answers for. @abidh please feel free to comment, if you know the answers.

1. How is the numbering affected by AbstractResultOpt pass that may introduce a new function argument for the result of function? E.g. let's say we set number 1 for the first Fortran dummy argument of a function. Then AbstractResultOpt adds a new dummy argument before the first one. Should all the numbers shift at this time?
2. How is the numbering affected by MLIR inlining? E.g. when a callee with dummy arguments is inlined into the caller, should we still attribute those used to be dummy arguments with arg = ...?
3. How is the numbering affected by ABI handling. E.g. if a single Fortran dummy argument is received by a function via two low-level function arguments, does it mean the positional numbers should be shifted at this point?

[llvmbot]

@llvm/issue-subscribers-flang-ir

Author: Slava Zakharin (vzakhari)

AddDebugInfo pass uses fragile logic for figuring out the positional number of a dummy argument in the list of function arguments. This causes issues like https://github.com//pull/131672

It seems that we need to number the dummy arguments in a more reliable way, e.g. attribute [hl]fir.declare operations with the proper number early in the pipeline (maybe even during lowering).

There are some questions that I do not know answers for. @abidh please feel free to comment, if you know the answers.

1. How is the numbering affected by AbstractResultOpt pass that may introduce a new function argument for the result of function? E.g. let's say we set number 1 for the first Fortran dummy argument of a function. Then AbstractResultOpt adds a new dummy argument before the first one. Should all the numbers shift at this time?
2. How is the numbering affected by MLIR inlining? E.g. when a callee with dummy arguments is inlined into the caller, should we still attribute those used to be dummy arguments with arg = ...?
3. How is the numbering affected by ABI handling. E.g. if a single Fortran dummy argument is received by a function via two low-level function arguments, does it mean the positional numbers should be shifted at this point?