[llvm-dev] RFC for f18+runtimes in LLVM

[Chris Lattner via llvm-dev]

On Mar 1, 2019, at 5:12 AM, Petr Penzin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> At this point, we have documented and implemented a healthy subset of the compiler for symbol tables and scoping, name resolution, USE statements and module files, constant representation, constant folding and much of declaration, label and expression semantics. The parser handles all of Fortran 2018 and OpenMP 4.5 and implements a Fortran-aware preprocessor. The Fortran control flow graph (CFG) is in review now.  We continue to update other documentation, such as the style guide and runtime descriptor design.
>> 
> Currently it looks like only the parser is partially implemented in f18, there is no code generator (via LLVM or otherwise) and, obviously, no object output. For that reason and due to the condition of its test suite it is impossible to reliably assess the state of Fortran 18 support (thought it does look like a fair amount of effort went into it). State of OpenMP support actually got me a bit puzzled, more on that below.
> 
> As I understand the announcement, f18 is intended to be used or merged with Flang sources at some point, but that still does not explain how it would integrate with LLVM, since Flang does not seem invoke LLVM directly either (it used to produce LLVM IR as text files). Because of this, it is likely that its code generator component would have to be written from scratch. It is also unclear if and how it would provide the library API which has been announced.
> A bit about the test suite -- I looked at the Fortran (regression) part of it (as opposed to unit tests, which hopefully are a relatively simple affair). Maybe nitpicking, but despite "handles all of OpenMP 4.5" statement in the announcement there seem to be only two 
> 

I haven’t looked at f18 at all, but if this is true, it wouldn’t be concerning to me.  There is a lot of work at the parser level that can be developed and tested without code generation at all - all semantic analysis, error message generation, sub-dialect support, etc all require a significant amount of work in the parser.  This is all generally useful for tooling, even without codegen.

That said, codegen would obviously be great to see, and my understanding is that the f18 community is committed to building that.

-Chris

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