On Mon, Sep 14, 2015 at 2:29 PM Aaron Ballman <aa...@aaronballman.com> wrote:
> On Mon, Sep 14, 2015 at 4:38 PM, Manuel Klimek <kli...@google.com> wrote: > > > > > > On Mon, Sep 14, 2015 at 12:26 PM Aaron Ballman <aa...@aaronballman.com> > > wrote: > >> > >> On Mon, Sep 14, 2015 at 2:45 PM, Daniel Jasper <djas...@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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