recordDecl() AST matcher

[Manuel Klimek via cfe-commits]

On Mon, Sep 14, 2015 at 2:29 PM Aaron Ballman <aaron at aaronballman.com>
wrote:

> On Mon, Sep 14, 2015 at 4:38 PM, Manuel Klimek <klimek at google.com> wrote:
> >
> >
> > On Mon, Sep 14, 2015 at 12:26 PM Aaron Ballman <aaron at aaronballman.com>
> > wrote:
> >>
> >> On Mon, Sep 14, 2015 at 2:45 PM, Daniel Jasper <djasper at google.com>
> wrote:
> >> > By this point, I see that change might be profitable overall. However,
> >> > lets
> >> > completely map this out. Changing just cxxRecordDecl() can actually
> >> > increase
> >> > confusion in other areas. Right now, not a single AST matcher has the
> >> > cxx
> >> > prefix (although a total of 28 stand for the corresponding CXX.. AST
> >> > node).
> >> > This is consistent and people knowing this will never try to write
> >> > cxxConstructExpr(). As soon as people have used cxxRecordDecl(), the
> >> > chance
> >> > of them trying cxxConstructExpr() increases. You have spent a long
> time
> >> > figuring out that recordDecl means cxxRecordDecl(), which is one
> >> > datapoint,
> >> > but I am not aware of anyone else having this specific issue. And we
> >> > could
> >> > make this less bad with better documentation, I think.
> >> >
> >> > So, for me, the questions are:
> >> > 1) Do we want/need this change?
> >>
> >> We definitely need *a* change because there currently is no way to
> >> match a C struct or union when compiling in C mode. I discovered this
> >> because I was trying to write a new checker for clang-tidy that
> >> focuses on C code and it would fail to match when compiling in C mode.
> >> Whether we decide to go with cxxRecordDecl vs recordDecl vs structDecl
> >> (etc) is less important to me than the ability to write clang-tidy
> >> checks for C code.
> >>
> >> > 2) Do we want to be consistent and change the other 27 matchers as
> well?
> >>
> >> I'm on the fence about this for all the reasons you point out.
> >>
> >> > A fundamental question is whether we want AST matchers to match AST
> >> > nodes
> >> > 1:1 or whether they should be an abstraction from some implementation
> >> > details of the AST.
> >>
> >> I absolutely agree that this is a fundamental question. I think a
> >> higher priority fundamental question that goes along with it is: are
> >> we okay with breaking a lot of user code (are these meant to be stable
> >> APIs like the LLVM C APIs)? If we want these APIs to be stable, that
> >> changes the answer of what kind of mapping we can have.
> >
> >
> > I think the AST matchers are so closely coupled to the AST that it
> trying to
> > be more stable than the AST doesn't help. Basically all uses of AST
> matchers
> > do something with the AST nodes afterwards, which will break anyway.
>
> I can get behind that logic. So we're okay with breaking their code
> because there's no way around it -- it's tied to the AST, so users
> cannot rely on the AST APIs remaining the same from release to release
> anyway.
>

We might even *want* the code to break, as the use of the AST node might
now be incorrect on a semantic level.


>
> >
> >>
> >> > And this is not an easy question to answer. There are
> >> > many places where we don't follow a strict 1:1 mapping. Mostly node
> >> > matchers, but also in traversal matchers, e.g. isDerivedFrom().
> >> >
> >> > Personally, I'd really hate to have the cxx Prefix everywhere, but
> >> > that's
> >> > just my personal opinion. I would even prefer matchers like record()
> and
> >> > method(), but I think somebody convinced me that that would be a very
> >> > bad
> >> > idea ;-).
> >>
> >> My personal opinion is that (1) breaking code is fine, but we should
> >> avoid doing it without very clear benefit, and (2) the mapping between
> >> AST node identifiers and AST matcher identifiers needs to be
> >> incredibly obvious, but perhaps not slavishly 1:1. If we instead
> >> decide we want a 1:1 mapping, then I think we need to start seriously
> >> considering auto-generating the AST node (and type) matchers from
> >> tablegen so that the AST nodes *cannot* get out of sync with the AST
> >> matchers, otherwise we'll be right back here again in a few years when
> >> we modify the name of an AST node.
> >
> >
> > I do think we want to auto-generate the matchers, but I don't think
> tablegen
> > is the right approach (I think an ast-matcher based tool is ;)
> > That said, auto-generating all the matchers is a) a lot of effort and b)
> the
> > code-size and compile time of matchers already matters, so it's unclear
> > which ones we would want to generate, especially for traversal matchers
> :(
>
> Oh, that's an excellent point (I'm talking about (b), I already knew
> (a) was a lot of work). Thank you for pointing that out!
>
> >
> >>
> >> My definition of "incredibly obvious" is: if the AST has a prefixed
> >> and unprefixed version, or two different prefixes, we should mimic
> >> that directly with the matchers. Otherwise, existing AST matchers
> >> without prefix shenanigans can remain as they are, and new AST
> >> matchers should prefix as-required. If we decide we're okay breaking
> >> code, then I don't see a problem with changing ctorInitializer() into
> >> cxxCtorInitializer() when C adds constructors. ;-)
> >
> >
> > I think the main things is cost for developers who try to write matchers
> and
> > work from the -ast-dump. Figuring out that there *is* a matcher with an
> > unprefixed node can take a while.
>
> Hmm, yes, but "take a while" should be relatively short, I would
> think. In that use-case, the user does an -ast-dump, sees
> "CXXFrobbleGnasher", they go to the AST matcher reference and they
> search for "CXXFrobberGnasher." The first hit won't be
> cxxFrobbleGnasher, but the entry for frobbleGnasher (which is still
> the first hit when searching from the top of the document due to the
> way we position node matchers) will have a parameter of
> CXXFrobbleGnasher, so they will find still get to the right matcher on
> the first hit. If someone doesn't read the documentation at all,
> they're going to try cxxFrobbleGnasher() and get a compile error/no
> known matcher. Then they'll go look at ASTMatchers.h and figure out
> it's called frobbleGnasher by searching there instead of the
> documentation.
>

The problem is that I've learned that sometimes people try to make things
work in ways that I couldn't even imagine, and they lose more time than I
could ever imagine them using :) Also, I agree the time is probably on
average not that large, but we pay it over a long time in the future, and
it tends to add up.


> That's compared to having the matcher name always be the same as the
> AST node, where the user writes cxxFrobbleGnasher and it just works,
> which is definitely a mark in favor of making everything consistent. I
> just don't think the current approach is too onerous in the case where
> the matcher is at least *provided* for the user with a relatively sane
> name.
>
> >> I should be clear, I'm not opposed to just having a 1:1 mapping. I'm
> >> just not certain the benefits of being strict about that outweigh the
> >> costs to broken code. cxxCtorInitializer will break someone's code,
> >> but I don't think it adds any clarity to their code, so I don't see
> >> the benefit of forcing the change.
> >
> > Well, I think there's the cost of broken code *once* now, vs. the
> (smaller)
> > cost for users in all future.
> > I'm still strongly in favor of breaking now, and having a simpler model
> > going forward.
>
> I'm definitely in favor of breaking now in the case of RecordDecl vs
> CXXRecordDecl. I think having recordDecl match CXXRecordDecl is a bug
> given that there's no way to match a RecordDecl.
>
> I would also be totally in favor of being consistent if we were
> starting from scratch. I'm very, very weakly opposed to breaking more
> user's code than we have to in order to get usable matchers because it
> seems gratuitous. Breaking code to get something that works seems
> reasonable. Breaking code that already works just to change the name
> for consistency elsewhere, I'm a bit less keen on. But the fact that
> we already can break user's code at-will because of the reliance on
> the AST nodes makes me think it may be the right approach for the best
> API, since that's what I would want if we were starting from scratch.
>
> Okay, I'm convinced. I think we should rename the type and node
> matchers (not traversal and narrowing matchers) to match the AST node
> names in all cases. We can document the breakage in the release notes,
> but (hopefully) only have to do this dance one time instead of
> spreading the pain out as it happens to eventually get to the same
> place anyway.
>

Yea, people who want more stability do use releases anyway.


> Daniel, is this something you would be okay with? (I'm assuming
> Richard finds it acceptable based on previous comments from Manuel,
> but Richard, feel free to chime in.)
>

Offline conversation with Richard says that he is convinced.


>
> ~Aaron
>
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