aaron.ballman added inline comments.
================ Comment at: clang-tools-extra/clang-tidy/modernize/IntegralLiteralExpressionMatcher.cpp:99 + + if (!Current->isLiteral() || isStringLiteral(Current->getKind()) || + !isIntegralConstant(*Current)) { ---------------- LegalizeAdulthood wrote: > aaron.ballman wrote: > > LegalizeAdulthood wrote: > > > aaron.ballman wrote: > > > > LegalizeAdulthood wrote: > > > > > aaron.ballman wrote: > > > > > > LegalizeAdulthood wrote: > > > > > > > aaron.ballman wrote: > > > > > > > > I know this is code moved from elsewhere, but I suppose we > > > > > > > > never considered the odd edge case where a user does something > > > > > > > > like `"foo"[0]` as a really awful integer constant. :-D > > > > > > > It's always possible to write crazy contorted code and have a > > > > > > > check not recognize it. I don't think it's worthwhile to spend > > > > > > > time trying to handle torture cases unless we can find data that > > > > > > > shows it's prevalent in real world code. > > > > > > > > > > > > > > If I was doing a code review and saw this: > > > > > > > ``` > > > > > > > enum { > > > > > > > FOO = "foo"[0] > > > > > > > }; > > > > > > > ``` > > > > > > > I'd flag that in a code review as bogus, whereas if I saw: > > > > > > > ``` > > > > > > > enum { > > > > > > > FOO = 'f' > > > > > > > }; > > > > > > > ``` > > > > > > > That would be acceptable, which is why character literals are > > > > > > > accepted as an integral literal initializer for an enum in this > > > > > > > check. > > > > > > > I don't think it's worthwhile to spend time trying to handle > > > > > > > torture cases unless we can find data that shows it's prevalent > > > > > > > in real world code. > > > > > > > > > > > > I think I'm okay agreeing to that in this particular case, but this > > > > > > is more to point out that writing your own parser is a maintenance > > > > > > burden. Users will hit cases we've both forgotten about here, > > > > > > they'll file a bug, then someone has to deal with it. It's very > > > > > > hard to justify to users "we think you write silly code" because > > > > > > they often have creative ways in which their code is not actually > > > > > > so silly, especially when we support "most" valid expressions. > > > > > Writing your own parser is unavoidable here because we can't just > > > > > assume that any old thing will be a valid initializer just by looking > > > > > at the set of tokens present in the macro body. (There is a separate > > > > > discussion going on about improving the preprocessor support and > > > > > parsing things more deeply, but that isn't even to the point of a > > > > > prototype yet.) The worst thing we can do is create "fixits" that > > > > > produce invalid code. > > > > > > > > > > The worst that happens if your expression isn't recognized is that > > > > > your macro isn't matched as a candidate for an enum. You can always > > > > > make it an enum manually and join it with adjacent macros that were > > > > > recognized and converted. > > > > > > > > > > As it stands, the check only recognizes a single literal with an > > > > > optional unary operator. > > > > > > > > > > This change expands the check to recognize a broad range of > > > > > expressions, allowing those macros to be converted to enums. I > > > > > opened the issue because running modernize-macro-to-enum on the [[ > > > > > https://github.com/InsightSoftwareConsortium/ITK | ITK codebase ]] > > > > > showed some simple expressions involving literals that weren't > > > > > recognized and converted. > > > > > > > > > > If an expression isn't recognized and an issue is opened, it will be > > > > > an enhancement request to support a broader range of expression, not > > > > > a bug that this check created invalid code. > > > > > > > > > > IMO, the more useful thing that's missing from the grammar is > > > > > recognizing `sizeof` expressions rather than indexing string literals > > > > > with an integral literal subscript. > > > > > > > > > > I had planned on doing another increment to recognize `sizeof` > > > > > expressions. > > > > > Writing your own parser is unavoidable here because we can't just > > > > > assume that any old thing will be a valid initializer just by looking > > > > > at the set of tokens present in the macro body. > > > > > > > > If you ran the token sequence through clang's parser and got an AST > > > > node out, you'd have significantly *more* information as to whether > > > > something is a valid enum constant initializer because you can check > > > > that it's an actual constant expression *and* that it's within a valid > > > > range of values. This not only fixes edge case bugs with your approach > > > > (like the fact that you can generate a series of literal expressions > > > > that result in a value too large to store within an enumerator > > > > constant), but it enables new functionality your approach currently > > > > disallows (like using constexpr variables instead of just numeric > > > > literals). > > > > > > > > So I don't agree that it's unavoidable to write another custom parser. > > > You keep bringing up the idea that the values have to be known, but so > > > far they don't. > > > > > > Remember, we are replacing macro identifiers with anonymous enum > > > identifiers. We aren't specifying a restricting type to the enum, so as > > > long as it's a valid integral literal expression, we're not changing any > > > semantics. Unscoped enums also allow arbitrary conversions to/from an > > > underlying integral type chosen by the compiler. > > > > > > C++20 9.7.1 paragraph 7 says: > > > > > > > For an enumeration whose underlying type is not fixed, the underlying > > > > type is an integral type that can > > > > represent all the enumerator values defined in the enumeration. If no > > > > integral type can represent all the > > > > enumerator values, the enumeration is ill-formed. It is > > > > implementation-defined which integral type is used > > > > as the underlying type except that the underlying type shall not be > > > > larger than int unless the value of an > > > > enumerator cannot fit in an int or unsigned int . If the > > > > enumerator-list is empty, the underlying type is as > > > > if the enumeration had a single enumerator with value 0. > > > > > > So the compiler is free to pick an underlying type that's large enough to > > > handle all the explicitly listed initial values. Do we actually need to > > > know the values for this check? I don't think so, because we aren't > > > changing anything about the type of the named values. When the compiler > > > evaluates an integral literal, it goes through a similar algorithm > > > assigning the appropriate type to those integral values: > > > > > > C++20 5.9 paragraph 2: > > > > > > > A preprocessing number does not have a type or a value; it acquires > > > > both after a successful conversion to an > > > > integer-literal token or a floating-point-literal token. > > > > > > C++20 5.13.2 paragraph 3: > > > > > > > The type of an integer-literal is the first type in the list in Table 8 > > > > corresponding to its optional integer-suffix > > > > in which its value can be represented. > > > > > > The table says the type is int, unsigned int, long int, unsigned long > > > int, long long int, or unsigned long long int based on the suffix and the > > > value and that the type is chosen to be big enough to hold the value if > > > the suffix is unspecified. > > > > > > > but [using `clangParse`] enables new functionality your approach > > > > currently disallows (like using constexpr variables instead of just > > > > numeric literals). > > > > > > I agree that if we used the full parser, we'd bring in `constexpr` > > > expressions as valid initializers for the enums. However, before > > > engaging in all that work, I'd like to see how likely this is in existing > > > codebases by feedback from users requesting the support. Maybe engaging > > > the parser isn't a big amount of work, I don't actually know. I've never > > > looked deeply at the actual parsing code in clang. Maybe it's easy > > > enough to throw a bag of tokens at it and get back an AST node, maybe > > > not. (I suspect not based on my experience with the code base so far.) > > > > > > My suspicion is that code bases that are heavy with macros for constants > > > //aren't// using modern C++ in the body of those macros to define the > > > values of those constants. Certainly this is 100% true for C code that > > > uses macros to define constants, by definition. This check applies > > > equally well to C code as C has had enums forever but even recent C code > > > still tends to use macros for constants. > > > > > > Still, my suspicions aren't data. I'd like to get this check deployed in > > > a basic fashion and let user feedback provide data on what is important. > > > > > > > So I don't agree that it's unavoidable to write another custom parser. > > > > > > That's a fair point. //Some// kind of parser is needed to recognize > > > valid initializer expressions or we run the risk of transforming valid > > > code into invalid code. Whether it is a custom recognizer as I've done > > > or `clangParse` is what we're debating here. > > > You keep bringing up the idea that the values have to be known, but so > > > far they don't. > > > > See comments at the top level. > > > > > So the compiler is free to pick an underlying type that's large enough to > > > handle all the explicitly listed initial values. Do we actually need to > > > know the values for this check? > > > > Yes, C requires the enumeration constants to be representable with `int`. > > But also, because this is in the `modernize` module, it's very likely we'll > > be getting a request to convert to using a scoped enumeration or an > > enumeration with the appropriate fixed underlying type in C++ as well. > Oh, I see now, thanks for explaining it. I didn't realize that C restricts > the values to `int`. You're welcome, sorry for not pointing it out sooner! CHANGES SINCE LAST ACTION https://reviews.llvm.org/D124500/new/ https://reviews.llvm.org/D124500 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits