recordDecl() AST matcher

[Manuel Klimek via cfe-commits]

On Sat, Sep 12, 2015, 9:25 PM Aaron Ballman <aaron at aaronballman.com> wrote:

> On Sat, Sep 12, 2015 at 8:22 AM, Manuel Klimek <klimek at google.com> wrote:
> >
> >
> > On Fri, Sep 11, 2015 at 10:39 PM Aaron Ballman <aaron at aaronballman.com>
> > wrote:
> >>
> >> On Fri, Sep 11, 2015 at 4:30 PM, Richard Smith <richard at metafoo.co.uk>
> >> wrote:
> >> > I don't think CXXRecordDecl is an anachronism, so much as an
> >> > implementation
> >> > detail; it makes sense to use a smaller class when in C mode, as we
> >> > don't
> >> > need most of the features and complexity that CXXRecordDecl brings
> with
> >> > it.
> >> > But... as a user of clang matchers, I don't think I'd want to care
> about
> >> > the
> >> > difference, and it'd be more convenient if I could nest (say) a
> >> > hasMethod
> >> > matcher within a recordDecl matcher, since it's completely obvious
> what
> >> > that
> >> > should mean. If I have a matcher that says:
> >> >
> >> >   recordDecl(or(hasMethod(...), hasField(...)))
> >> >
> >> > I would expect that to work in both C and C++ (and the only way it
> could
> >> > match in C would be on a record with the specified field, since the
> >> > hasMethod matcher would always fail).
> >>
> >> Okay, so then it sounds like we want recordDecl to *mean* RecordDecl,
> >> but we want the traversal and narrowing matchers that currently take a
> >> CXXRecordDecl to instead take a RecordDecl and handle the CXX part
> >> transparently? This means we would not need to add a cxxRecordDecl()
> >> matcher, but could still access CXX-only functionality (like access
> >> control, base classes, etc) through recordDecl()?
> >
> >
> > I'm against that proposal. I think we have tried to make the matchers
> more
> > "user friendly" in the past, and all those attempts have failed
> miserably;
> > in the end, users will do ast-dump to see what they want to match, and
> then
> > be confused when the matchers do follow the AST 99% of the time, but try
> to
> > be smart 1% of the time.
> > I think given that we want to keep CXXRecordDecl, the right solution is
> to
> > have a cxxRecordDecl() matcher.
>
> Personally, I think this makes the most sense, at least to me. The
> recommendation I've always heard (and given) is to use -ast-dump and
> write matchers from there. (Consequently, the more I work with type
> traversal matchers, the more I wish we had -ast-dump-types to give
> even *more* information for writing matchers.)
>
> But the question still remains, what do we do with recordDecl? Right
> now, it means CXXRecordDecl instead of RecordDecl. If we change it to
> mean RecordDecl instead, there's a chance we'll break existing,
> reasonable code. Are we okay with that risk? If we're at least
> conceptually okay with it, I could make the change locally and see
> just how much of our own code breaks, and report back. But if that
> turns out to be problematic, do we want to deprecate recordDecl and
> replace it with structDecl as our fallback position? Or is there a
> better solution?
>
> Basically, I see a few ways to solve this (and there may be other ways
> I'm not thinking about yet):
>
> 1) Undocument/deprecate recordDecl, add structDecl, unionDecl, and
> cxxRecordDecl. This does not match the AST because we have no
> StructDecl or UnionDecl types. The more I think about this option, the
> less I like it. It's easy to implement, but seems hard to relate to
> the AST.
> 2) Make recordDecl match RecordDecl, don't touch other matchers. Add
> way to distinguish unions from structs (e.g., isUnion(), isStruct()),
> add cxxRecordDecl. This matches the AST most closely, but may break
> code. I think that I prefer this approach, but it depends heavily on
> what "may break code" looks like in practice.
> 3) Make recordDecl match RecordDecl, fix other matchers that currently
> take CXXRecordDecl to instead take RecordDecl and handle sensibly when
> possible. Add a way to distinguish unions from structs, add
> cxxRecordDecl. This doesn't match the AST because we will have
> matchers taking a RecordDecl when the AST would require a
> CXXRecordDecl, but is likely to break less code.
>

That sums it up. My preferences are 2, 3, 1 in that order :)


> ~Aaron
>
>
> > Richard: if CXXRecordDecl was really an implementation detail, it would
> be
> > hidden behind a RecordDecl class, as an implementation detail. The
> reasons
> > why we don't want it to be an implementation detail in the code
> > (performance, data structure size) don't matter - in the end, it's in the
> > AST API.
> >
> >>
> >>
> >> ~Aaron
> >>
> >> >
> >> > On Fri, Sep 11, 2015 at 6:30 AM, Manuel Klimek <klimek at google.com>
> >> > wrote:
> >> >>
> >> >> Richard! We need an informed opinion :D
> >> >>
> >> >> On Fri, Sep 11, 2015 at 3:07 PM Aaron Ballman <
> aaron at aaronballman.com>
> >> >> wrote:
> >> >>>
> >> >>> Ping?
> >> >>>
> >> >>> On Tue, Sep 8, 2015 at 9:26 AM, Manuel Klimek <klimek at google.com>
> >> >>> wrote:
> >> >>> > On Tue, Sep 8, 2015 at 3:23 PM Aaron Ballman
> >> >>> > <aaron at aaronballman.com>
> >> >>> > wrote:
> >> >>> >>
> >> >>> >> On Tue, Sep 8, 2015 at 9:18 AM, Manuel Klimek <klimek at google.com
> >
> >> >>> >> wrote:
> >> >>> >> > On Tue, Sep 8, 2015 at 2:23 PM Aaron Ballman
> >> >>> >> > <aaron at aaronballman.com>
> >> >>> >> > wrote:
> >> >>> >> >>
> >> >>> >> >> On Tue, Sep 8, 2015 at 5:40 AM, Manuel Klimek
> >> >>> >> >> <klimek at google.com>
> >> >>> >> >> wrote:
> >> >>> >> >> > Yea, we had that discussion a few times, and I can never
> >> >>> >> >> > remember
> >> >>> >> >> > why
> >> >>> >> >> > we
> >> >>> >> >> > ended up in the state we're in.
> >> >>> >> >> > We definitely had a time where we switched to just using the
> >> >>> >> >> > exact
> >> >>> >> >> > same
> >> >>> >> >> > name
> >> >>> >> >> > as the node's class name for the matchers.
> >> >>> >> >> > I *think* we didn't do it for cxxRecordDecl, because Richard
> >> >>> >> >> > said
> >> >>> >> >> > that's
> >> >>> >> >> > a
> >> >>> >> >> > relic we should get rid of anyway, but I'm not sure.
> >> >>> >> >>
> >> >>> >> >> FWIW, I think the state we're in is the worst of all worlds.
> >> >>> >> >> It's
> >> >>> >> >> not
> >> >>> >> >> intuitive that recordDecl() doesn't match a struct in C mode,
> >> >>> >> >> and
> >> >>> >> >> as
> >> >>> >> >> it stands, there is no way to match a struct or union
> >> >>> >> >> declaration
> >> >>> >> >> in C
> >> >>> >> >> at all.
> >> >>> >> >
> >> >>> >> >
> >> >>> >> > Agreed. Best intentions. Worst possible outcome. That's
> software
> >> >>> >> > development
> >> >>> >> > :)
> >> >>> >> >
> >> >>> >> >> >
> >> >>> >> >> > On Fri, Sep 4, 2015 at 8:32 PM Aaron Ballman
> >> >>> >> >> > <aaron at aaronballman.com>
> >> >>> >> >> > wrote:
> >> >>> >> >> >>
> >> >>> >> >> >> It turns out that the recordDecl() AST matcher doesn't
> match
> >> >>> >> >> >> RecordDecl objects; instead, it matches CXXRecordDecl
> >> >>> >> >> >> objects.
> >> >>> >> >> >> This
> >> >>> >> >> >> is... unfortunate... as it makes writing AST matchers more
> >> >>> >> >> >> complicated
> >> >>> >> >> >> because of having to translate between
> >> >>> >> >> >> recordDecl()/CXXRecordDecl.
> >> >>> >> >> >> It
> >> >>> >> >> >> also makes it impossible to match a struct or union
> >> >>> >> >> >> declaration
> >> >>> >> >> >> in C
> >> >>> >> >> >> or ObjC. However, given how prevalent recordDecl()'s use is
> >> >>> >> >> >> in
> >> >>> >> >> >> the
> >> >>> >> >> >> wild (I'm guessing), changing it at this point would be a
> Bad
> >> >>> >> >> >> Thing.
> >> >>> >> >> >>
> >> >>> >> >> >> For people trying to write AST matchers for languages like
> C
> >> >>> >> >> >> or
> >> >>> >> >> >> ObjC,
> >> >>> >> >> >> I would like to propose adding:
> >> >>> >> >> >>
> >> >>> >> >> >> structDecl()
> >> >>> >> >> >> unionDecl()
> >> >>> >> >> >> tagDecl()
> >> >>> >> >> >>
> >> >>> >> >> >> These will match nicely with the existing enumDecl() AST
> >> >>> >> >> >> matcher.
> >> >>> >> >> >>
> >> >>> >> >> >> Additionally, I would like to add cxxRecordDecl() to match
> >> >>> >> >> >> CXXRecordDecl objects. While it duplicates the
> functionality
> >> >>> >> >> >> exposed
> >> >>> >> >> >> by recordDecl(), it more clearly matches the intention of
> >> >>> >> >> >> which
> >> >>> >> >> >> AST
> >> >>> >> >> >> node it corresponds to.
> >> >>> >> >> >>
> >> >>> >> >> >> Finally, I would like to undocument recordDecl() and change
> >> >>> >> >> >> our
> >> >>> >> >> >> existing documentation and AST matcher uses to use
> >> >>> >> >> >> cxxRecordDecl/structDecl() instead. Maybe someday we can
> >> >>> >> >> >> deprecate
> >> >>> >> >> >> recordDecl() more officially.
> >> >>> >> >> >>
> >> >>> >> >> >> I'm open to other ideas if there are better ways to move
> >> >>> >> >> >> forward. If
> >> >>> >> >> >> you think changing the meaning of recordDecl() is
> acceptable,
> >> >>> >> >> >> I
> >> >>> >> >> >> can
> >> >>> >> >> >> also go that route (though I would still propose adding
> >> >>> >> >> >> unionDecl()
> >> >>> >> >> >> and cxxRecordDecl() in that case).
> >> >>> >> >> >
> >> >>> >> >> >
> >> >>> >> >> > I think changing recordDecl is acceptable. I believe very
> few
> >> >>> >> >> > tools
> >> >>> >> >> > will
> >> >>> >> >> > actually start doing wrong things because of it. I'd like
> more
> >> >>> >> >> > opinions
> >> >>> >> >> > first, though :)
> >> >>> >> >>
> >> >>> >> >> I was giving this more thought over the long weekend, and I
> >> >>> >> >> think
> >> >>> >> >> you
> >> >>> >> >> may be right. I think changing recordDecl() to mean RecordDecl
> >> >>> >> >> will
> >> >>> >> >> fix more code than it breaks, so long as we take a holistic
> >> >>> >> >> approach
> >> >>> >> >> to the change and see which narrowing and traversal matchers
> we
> >> >>> >> >> need
> >> >>> >> >> to fix up at the same time. When I tried to think of AST
> >> >>> >> >> matchers
> >> >>> >> >> that
> >> >>> >> >> mean CXXRecordDecl but *not* RecordDecl, they were horribly
> >> >>> >> >> contrived
> >> >>> >> >> because you usually are matching on additional selection
> >> >>> >> >> criteria
> >> >>> >> >> that
> >> >>> >> >> is specific to C++ (such as hasMethod() or isDerivedFrom())
> >> >>> >> >> which
> >> >>> >> >> would cause the match to continue to fail, as expected. Code
> >> >>> >> >> that
> >> >>> >> >> uses
> >> >>> >> >> recordDecl() to mean RecordDecl will suddenly start to match
> in
> >> >>> >> >> more
> >> >>> >> >> cases, but that's likely to be a bug fix more than a breaking
> >> >>> >> >> change.
> >> >>> >> >> To the best of my understanding, the only breaking cases would
> >> >>> >> >> be
> >> >>> >> >> where you wrote recordDecl(), meant CXXRecordDecl, had no
> >> >>> >> >> further
> >> >>> >> >> narrowing or traversal matchers, and were compiling in C mode;
> >> >>> >> >> with
> >> >>> >> >> the result being additional unexpected matches.
> >> >>> >> >
> >> >>> >> >
> >> >>> >> > Ah, there's one thing that can break: the compile can break:
> >> >>> >> > recordDecl(hasMethod(...)) will *not* compile (it'll work in
> the
> >> >>> >> > dynamic
> >> >>> >> > matchers and fail as you suggest, but the in-C++ DSL does more
> >> >>> >> > static
> >> >>> >> > type
> >> >>> >> > checking).
> >> >>> >> > I don't think that's super bad though.
> >> >>> >> >
> >> >>> >> >>
> >> >>> >> >> So perhaps it would make sense to:
> >> >>> >> >>
> >> >>> >> >> 1) Make recordDecl() mean RecordDecl
> >> >>> >> >> 2) Do a comprehensive review of matchers that take a
> >> >>> >> >> CXXRecordDecl
> >> >>> >> >> and
> >> >>> >> >> see if they should instead take a RecordDecl
> >> >>> >> >> 3) Add unionDecl() as a node matcher (or should we add
> isUnion()
> >> >>> >> >> and
> >> >>> >> >> isStruct() as narrowing matchers?)
> >> >>> >> >> 4) Add tagDecl() as a node matcher, but not add
> cxxRecordDecl()
> >> >>> >> >
> >> >>> >> >
> >> >>> >> > Why not add cxxRecordDecl()? I think we need it if we want
> >> >>> >> > narrowing
> >> >>> >> > matchers on CXXRecordDecl?
> >> >>> >>
> >> >>> >> If Richard thinks CXXRecordDecl is an anachronism, I figured we
> >> >>> >> didn't
> >> >>> >> want to expose it. Instead, we could make hasMethod (et al)
> accept
> >> >>> >> a
> >> >>> >> RecordDecl and do the type checking for the caller. Then
> >> >>> >> recordDecl(hasMethod(...)) continues to compile and work, and
> when
> >> >>> >> hasMethod is given a RecordDecl instead of a CXXRecordDecl, it
> >> >>> >> simply
> >> >>> >> matches nothing. But you bring up a good point about the C++ DSL
> >> >>> >> being
> >> >>> >> a problem still, I hadn't considered that.
> >> >>> >
> >> >>> >
> >> >>> > First I want Richard to confirm that. I have a very bad memory,
> so I
> >> >>> > might
> >> >>> > as well misremember :)
> >> >>> >
> >> >>> >>
> >> >>> >>
> >> >>> >> ~Aaron
> >> >>> >>
> >> >>> >> >
> >> >>> >> >>
> >> >>> >> >>
> >> >>> >> >> ~Aaron
> >> >>> >> >>
> >> >>> >> >> >
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >> Thanks!
> >> >>> >> >> >>
> >> >>> >> >> >> ~Aaron
> >> >
> >> >
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20150913/89975fc4/attachment-0001.html>