[llvm-dev] [RFC] Refactor Clang: move frontend/driver/diagnostics code to LLVM

Tue Jun 2 05:08:06 PDT 2020

*TL;DR*

We propose some non-trivial refactoring in Clang and LLVM to enable 
further work on Flang driver.

*SUMMARY*
We would like to start extracting the driver/frontend code from Clang 
(alongside the code that the driver/frontend depends on, e.g. 
Diagnostics) and move the components that could be re-used by 
non-C-based languages to LLVM. From our initial investigation we see 
that these changes will impact many projects (upstream and downstream) 
and will require big mechanical patches (our first attempt is 
implemented in [8]). This is not ideal, but seems unavoidable in the 
long-term. We would like to do this refactoring _before_ we start 
implementing the Flang driver upstream (OPTION 1 below). This way we avoid:

* contaminating Clang with Fortran specific code (and vice versa)
* introducing dependency on Clang in Flang

The downside is that the refactoring is likely to be disruptive for many 
projects that use Clang. We will try our best to minimise this.

Does this approach make sense? Are there any preferred alternatives? At 
this stage we'd like to discuss the overall direction. If folks are in 
favour, we'll send a separate RFC with a finer breakdown and more 
technical details for the refactoring.

Below you will find more context for our use-case (the Flang driver) and 
possible alternatives. We hope that this will help the discussion. We 
would really appreciate your feedback!

*BACKGROUND*
Flang (formerly known as F18) has recently been merged into LLVM [1]. 
Our ambition, as a community, is to make it as flexible, robust and nice 
to work with as Clang. One of the major items to address is the 
implementation of a driver that would provide the flexibility and user 
experience similar to that available in Clang. The F18/Flang driver was 
already discussed on cfe-dev last year [2], but back then F18 (now llvm 
project/flang) was a separate project. In the original proposal it was 
assumed that initially Flang would depend (and extend where necessary) 
Clang's driver/frontend code. Since F18/Flang was an independent 
project, the refactoring of Clang/LLVM wasn't really considered. That 
design has been challenged since ([3], [10]), and also not much progress 
has been made. We would like to revisit that RFC from a slightly 
different angle. Since Flang is now part of LLVM's monorepo, we feel 
that refactoring Clang/LLVM _before_ we upstream the driver makes a lot 
of sense and is the natural first step.

*ASSUMPTIONS & DESIGN GOALS*
1. We will re-use as much of the Clang's driver/frontend code as 
possible (this was previously proposed in [2]).

2. We want to avoid dependencies from Flang to Clang, both long-term 
(strong requirement) and short-term (might be difficult to achieve). 
This has recently come up in a discussion on one of our early patches 
[3] (tl;dr Steve Scalpone, the code owner of Flang, would prefer us to 
avoid this dependency), and was also suggested before by Eric 
Christopher [10].

3. We will move the code that can be shared between Flang and Clang (and 
other projects) to LLVM. This idea has already come up on llvm-dev 
before [7] (in a slightly different context, and to a slightly different 
extent). The methods that are not language specific would be shared in 
an LLVM library.

4. The classes/types/methods that need specific changes for Fortran will 
be "copied" to Flang and adapted as needed. We should minimize (or even 
eliminate) any Fortran specific code from Clang and make sure that that 
lives in llvm-project/flang.

*FLANG'S DEPENDENCIES ON CLANG*
These are the dependencies on Clang that we have identified so far while 
prototyping the Flang driver.

1. All the machinery related to Diagnostics & SourceLocation.

This is currently part of libclangBasic [4] and is used in _many_ places 
in Clang. The official documentation [5] suggests that this could be 
re-used for non-C-based languages. In particular, we feel that It would 
make a lot of sense for Flang to use it. Also, separating Clang's 
driver/frontend code and the diagnostics would require a lot of 
refactoring for no real benefit (and we feel that Flang should re-use 
Clang's driver/frontend code, see below). This dependency is used in 
many places, so moving it to LLVM will require a lot of (mostly) 
mechanical changes. We can't see an obvious way to split it into smaller 
chunks (see also below where we discuss the impact).

2. libclangFrontend & libclangDriver

The Flang driver will use many methods from libClangDriver, 
libClangFrontend and libClangFrontendTool. Driver.h and Compilation.h 
from libClangDriver are responsible to call, pass the correct arguments 
and execute the driver. TextDiagnosticPrinter.h takes care of printing 
the driver diagnostics in case of errors.

The Flang frontend will use CompilerInstance, CompilerInvocation, 
FrontendOptions, FrontendActions and Utils from libClangFrontend and 
libClangFrontendTool. These methods are responsible for translating the 
command line arguments to frontend Options and later to Actions to be 
executed by ExecuteCompilerInvocation. The translation from arguments to 
Actions happens with FrontendOption and FrontendActions. But it is the 
CompilerInvocation that has the pointers for the sequence of Actions 
that are required in a Compiler Instance. These methods are needed to 
implement Flang driver/frontend and contain actions/method/functions 
that seem to be language agnostic.

*ALTERNATIVES*
This is a summary of the alternative ways of implementing the Flang 
driver. We propose OPTION 1. If there are no major objections, we will 
draft a separate RFC with more technical details (we will also break it 
down into smaller pieces). Otherwise, what would be your preferred 
alternative and why?

OPTION 1
We avoid dependency on Clang from Day 1.

This is the ideal scenario that would guarantee that Clang and Flang are 
completely separate and that the common bits stay in LLVM instead. It 
would mean slower progress for us initially, but then other projects 
could benefit from the refactoring sooner rather than later.

OPTION 2
We avoid dependency on clangBasic from day 1, but initially allow 
dependency on libClangFrontend & libClangDriver (or other libs specific 
to the driver/frontend).

The dependency on libclang{Driver|Frontend} would gradually be 
removed/refactored out as the driver for Flang gains momentum. As 
mentioned earlier, there is plenty of code in libClangFrontend and 
libClangDriver that we'd like to re-use, but the separation between code 
that's specific to C-based languages and generic driver/frontend code is 
not always obvious. We think that refactoring the common bits in 
libClangFrontend and libClangDriver might simply be easier once:

  * we have a Flang driver that leverages these libraries, and, as a result,
  * we understand better what we could re-use and what's not that 
relevant to non-C-based languages.

OPTION 3
We initially keep the dependency on Clang and re-visit this RFC later.

This would be the least disruptive approach (at least for the time 
being) and would allow us to make us the most rapid progress (i.e. we 
would be focusing on implementing the features rather than refactoring). 
It would also inform the future refactoring better. But it was already 
pointed out that we should avoid dependencies on clang [3] and this 
would be a step in the opposite direction. Also, the build requirements 
for Flang would increase, and we feel that we should strive to reduce 
them instead [6].

If we missed any alternatives, please bring them up.

*IMPACT ON OTHER PROJECTS*
The refactoring will have non-trivial impact on other projects:

* OPTION 1 and OPTION 2 - huge impact initially.
* OPTION 3 - no impact initially, but most likely similar impact as 
OPTION 1 and OPTION 2 in the long term.

 From our initial investigation, extracting Diagnostics/SourceLocation 
from clangBasic and moving it to LLVM will be the most impactful change. 
Within llvm-project it is used in clang, clang-tools-extra, lldb and 
polly. Most of the changes will be mechanical, but will require touching 
many files. In order to get to a state where we could build libclang 
using the newly defined LLVM library, we had to touch ~850 files and 
make ~30k insertions/deletions. The result of this exercise is available 
in our development fork of llvm-project [8].

Please note: our patches on GitHub [8] are just experiments to 
illustrate the idea. It's work-in-progress that requires a lot of 
polishing. When/if up-streaming this, we would need to do some 
low-impact refactoring first. For example, currently ASTReader & 
ASTWriter are `friends` with DiagnosticsEngine [9]. That won't be 
possible when DiagnosticsEngine is moved to LLVM.

On behalf of the Arm Fortran Team,
Andrzej Warzynski

REFERENCES

[1] 
https://github.com/llvm/llvm-project/commit/b98ad941a40c96c841bceb171725c925500fce6c
[2] http://lists.llvm.org/pipermail/cfe-dev/2019-June/062669.html
[3] https://reviews.llvm.org/D79092
[4] 
https://github.com/llvm/llvm-project/blob/ad5d319ee85d31ee2b1ca5c29b3a10b340513fec/clang/lib/Basic/CMakeLists.txt#L45-L47
[5] https://clang.llvm.org/docs/InternalsManual.html#the-clang-basic-library
[6] http://lists.llvm.org/pipermail/flang-dev/2019-November/000061.html
[7] http://lists.llvm.org/pipermail/llvm-dev/2019-November/136743.html
[8] 
https://github.com/banach-space/llvm-project/commits/andrzej/refactor_clangBasic
[9] 
https://github.com/llvm/llvm-project/blob/b11ecd196540d87cb7db190d405056984740d2ce/clang/include/clang/Basic/Diagnostic.h#L985-L986
[10] https://reviews.llvm.org/D63607