[flang-dev] RFC: new Flang driver options

[Andrzej Warzynski via flang-dev]

Hi All,

We would like to start adding the remaining features in the new Flang 
driver and would appreciate your feedback on the list of proposed 
compiler options.

Flang - llvm-project/flang
"flang" - current Flang compiler and frontend driver
"flang-new" - new Flang compiler driver
"flang-new -fc1" - new Flang frontend driver

The list of proposed options is available in Google Docs:

https://docs.google.com/spreadsheets/d/1JRe39lP_KhtkYxFEIvwrCFlE5v1Ofa_krOHI-XXXWPY/edit?usp=sharing

If you are unable to read Google Docs, please suggest a different format 
and I will port it for you. The "unsupported options" sheet lists 
options from "flang" that won't be supported in "flang-new".

Below you will find our rationale.

# *SPELLINGS*
The current driver implements both  "gfortran " and "Classic Flang" 
spellings for options. For the sake of simplicity (it's easier to focus 
on 1 set) and consistency with libclangDriver (which the new driver is 
implemented in terms of), we propose that for now we focus on "gfortran" 
style spellings.

# *COMPILER VS FRONTED DRIVER OPTIONS*
Options available in the frontend driver ("flang-new -fc1") won't be 
automatically available in the compile driver ("flang-new"). We propose 
the following rule of thumb:
   * if an option is available in "gfortran" then it should also be 
available in "flang-new"
For all other options one either has to use the frontend driver or the 
`-Xflang` compiler driver flag (equivalent to `-Xclang`).

# *EXTERNAL TOOL*
"flang" calls a tool defined with the "F18_FC" environment variable to 
perform the actual compilation. In the new driver we want to focus on 
typical compiler driver tasks, the interface and the user experience. 
Also, with the ongoing efforts to bring the code-generation capabilities 
to llvm-project/flang, we assume that this functionality won't be needed 
in "flang-new". In other words, we suggest that this is not supported.

# *FUTURE CHANGES*
The proposed set of options is _not_ set in stone and can be refined in 
future. In particular, "Classic Flang" spellings can be added in a 
similar manner as MSVC spelling were added to Clang (e.g. "flang-new-cf" 
could represent the new driver in "Classic Flang" compatibility mode).

# *FINER DETAILS*
* In some cases we suggest a completely new spelling (e.g. 
"-fdebug-unparse" instead of "-unparse") - please see the "Notes" column
* "flang-new" already supports a few options that are not supported by 
"flang" (e.g. "-###") - these are also listed
* "-fget-definition" seems like a rather complex and obscure 
proof-of-concept option and we suggest that it's skipped for now

# *QUESTIONS*
* Are the descriptions accurate? We might have missed or misinterpreted 
something.
* What are `-ed` and `-fdebug-module-writer` for? We couldn't find any 
documentation.

Is this sufficient for you to adopt the new Flang driver once these 
options are available? Is there anything that we missed? Does this sound 
sensible to you?

Thank you for reading,
-Andrzej