[cfe-dev] RFC: Easier AST Matching by Default

Stephen Kelly via cfe-dev cfe-dev at lists.llvm.org
Wed May 27 02:34:31 PDT 2020

On Wed 27 May 2020, 09:25 Sam McCall, <sammccall at google.com> wrote:

> On Mon, May 25, 2020 at 11:56 PM Stephen Kelly via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>> On 20/12/2019 21:01, Stephen Kelly via cfe-dev wrote:
>> >
>> > Hi,
>> >
>> > (Apologies if you receive this twice. GMail classified the first one as
>> > spam)
>> >
>> > Aaron Ballman and I met by chance in Belfast and we discussed a way
>> > forward towards making AST Matchers easier to use, particularly for C++
>> > developers who are not familiar with the details of the Clang AST.
>> >
>> > For those unaware, I expanded on this in the EuroLLVM conference this
>> > year, and then expanded on it at ACCU:
>> >
>> >   https://steveire.wordpress.com/2019/04/30/the-future-of-ast-matching
>> >
>> > One step in the process of getting there is changing the default
>> > behavior of AST Matchers to ignore invisible nodes while matching using
>> > the C++ API, and while matching and dumping AST nodes in clang-query.
>> >
>> > I think this is the most important change in the entire proposal as it
>> > sets out the intention of making the AST Matchers easier to use for C++
>> > developers who are not already familiar with Clang APIs.
>> >
>> > To that end, I've written an AST to motivate the change:
>> >
>> >
>> >
>> https://docs.google.com/document/d/17Z6gAwwc3HoRXvsy0OdwU0X5MFQEuiGeSu3i6ICOB90
>> >
>> >
>> > We're looking for feedback before pressing forward with the change. I
>> > already have some patches written to port clang-tidy and unit tests to
>> > account for the change of default.
>> This change is now in master.
> Thanks for simplifying the matchers! Unfortunately I missed the discussion
> on this.

That's a pity. This thread was bumped a few times since December.

> Unfortunately this is causing widespread problems in downstream tools that
> are difficult to track:

Sorry this is causing problems in downstream tools.

 - this is a prominent API that is used in many places
>  - the semantics have changed subtly but pervasively, so all usage sites
> need to be audited, but because there was no API syntax change (e.g. build
> break), it's not trivial to find all the affected locations to audit

Is it trivial to set the TraversalKind presumptively on the
ParentMapContext early in the runtime of the downstream tool? The
ParentMapContext is available via your ASTContext.

I ask because reading your email it doesn't look like you considered this
option? If you did consider it and rejected it, please say more about why
it is not an option.

That way you don't have to try to audit any matchers yet. You would just
set the TraversalKind to AsIs in the entire tool.

 - the differences between old and new only occur when running against
> certain code patterns that often aren't covered by tests, therefore each
> audit is a lot of work

If that audit can be deferred, does the situation change?

 - because the default was flipped globally, all usage sites must be dealt
> with at once

Does this become manageable if you flip the setting locally in your tool in
your ParentMapContext?

 - the "revert-to-old-behaviour" is invasive at all usage sites (hurts
> readability a lot) and apparently has bugs (D80606)

If changing the TraversalKind in the  ParentMapContext in your tool is a
potential solution, then I can look into the above issue this morning.

Please let me know.

> In summary: it requires changes in consuming tools that are numerous, hard
> to find, hard to analyze, must all be done at once, and the changes aren't
> mechanically reliable.

Hmm, the location to call setTraversalKind on the ParentMapContext in each
external tool should be easy to find? Should be easy to analyze too. There
is presumably fewer places in your tool (or just one place in the entire
tool)  where ASTContexts are created, so "all" is either "1" or close to it?

I expect this to bite people building tools against release versions to hit
> the same problems in a few months.

Perhaps the solution of changing the setting in the ParentMapContext needs
to be spelled out better in the release notes. It would also be good to
know whether doing so makes the impact on your tools manageable. Can you
look into that?

We should consider reverting this and finding a less-silent way to roll out
> the change (like adding it with different syntax, or having no default for
> a transition period covering a stable release).

Before doing that I'd like to find out more about whether the above
solution works.


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