[cfe-dev] [F18/AST] Using clang tooling with f18

Petr Penzin via cfe-dev cfe-dev at lists.llvm.org
Thu Oct 18 19:32:11 PDT 2018

Hi David,

There was a different Flang project before Nvidia's involvement, which 
successfully used and extended Clang ASTs. The code is still on Github 
<https://github.com/llvm-flang/flang>, along with updated fork 
<https://github.com/llvm-fortran/fort/> (which builds with trunk). It 
implements a fair share of the Fortran features that you have mentioned 
using (what is essentially) Clang's infrastructure.

IMO, it is possible to use Clang ASTs, but also there is a lot of other 
code and patterns in Clang that would be useful for a Fortran frontend 
-  error handling, driver, testing, etc. It would make sense to take 
those things from Clang rather than reinvent them in F18.


On 10/14/2018 09:21, David Greene via cfe-dev wrote:
> F18 currently lowers to LLVM IR, but that is too low-level for the kind of tooling we
> want to have.  Clang's tooling works on the AST because it is high-level enough to
> express source language concepts and reason about them.
> For example, one can't easily write a source-level documentation tool that works
> with  LLVM IR.  The same goes for tools like code formatters, syntax highlighters
> and anything else that wants to analyze and/or modify source.
>                                   -David
> ________________________________________
> From: Marcus Johnson<bumblebritches57 at gmail.com>
> Sent: Friday, October 12, 2018 7:20:51 PM
> To: David Greene
> Cc: Flang development; Clang development
> Subject: Re: [cfe-dev] [F18/AST] Using clang tooling with f18
> I know absolutely nothing about the AST, but why not just use LLVM's IR?
>> On Oct 12, 2018, at 1:58 PM, David Greene via cfe-dev<cfe-dev at lists.llvm.org>  wrote:
>> [Reposted with the correct clang mailing list -- sorry about the
>> duplicate...]
>> Hi all,
>> We've been having a bit of discussion over on flang-dev and wanted to
>> bring in clang people to comment/brainstorm.  Here is the original post
>> that kicked this off on flang-dev:
>>   I was hoping f18 would lower to something akin to clang's AST.
>>   Obviously clang's AST doesn't directly apply to Fortran but perhaps
>>   some kind of common interface could exist so that clang tools could
>>   work with Fortran codes. It would be great to have things like the
>>   clang static analyzer and clang-doc for Fortran.
>>   Some tools will be language-specific of course but it seems like
>>   Fortran and C-family languages share enough common concepts that some
>>   tooling could work with both, given a common interface.
>>   Language-specific tools would work with a more language-specific
>>   interface.
>>   My impression from the presentation is that there's a lot more that
>>   could be shared with clang. The messaging system and command-line
>>   options infrastructure should be shareable, for example.  Maybe
>>   there's already work being done in these areas to make f18 a
>>   first-class LLVM project.
>> Folks raised some concerns/areas to explore:
>> - Can we represent various Fortran constructs with additions to the
>>   clang AST?  For example:
>>   * Implied DO loops
>>   * Array syntax
>>   * I/O statements (FORMAT, READ, NAMELIST, etc.)
>>   * Array declarations (DIMENSION, etc.)
>>   * Array syntax
>>   * ...
>> - Can clang's infrastructure handle various Fortran oddities like
>>   non-reserved keyworks and the ability to redefine constants?
>>   These may primarily be "dusty deck" issues and perhaps for tooling
>>   purposes a 98% solution is ok.  F18 still needs to fully handle them,
>>   of course.
>> - Can we modify clang's AST and surrounding infrastructure to re-use
>>   bits of clang tooling for f18 or should we create some kind of common
>>   tooling interface for clang/f18 tooling that can also support
>>   language-specific bits?
>> - Can this be forward-looking for tooling for other languages (Rust,
>>   Chapel, Go, etc.)?
>> We pretty quickly came to a point where we needed input from clang
>> folks, so here we are.  :)
>>                                  -David
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