ymandel marked 2 inline comments as done. ymandel added inline comments.
================ Comment at: clang-tidy/readability/ConstValueReturnCheck.cpp:107 + diag(Def->getInnerLocStart(), "return type %0 is 'const'-qualified " + "hindering compiler optimizations") + << Def->getReturnType(); ---------------- aaron.ballman wrote: > ymandel wrote: > > aaron.ballman wrote: > > > ymandel wrote: > > > > aaron.ballman wrote: > > > > > It seems strange to me that this is in the readability module but the > > > > > diagnostic here is about compiler optimizations. Should this be in > > > > > the performance module instead? Or should this diagnostic be reworded > > > > > about the readability concerns? > > > > Good point. The readability angle is that the const clutters the code > > > > to no benefit. That is, even if it was performance neutral, I'd still > > > > want to discourage this practice. But, it does seem like the primary > > > > impact is performance. > > > > > > > > I'm fine either way -- moving it to performance or emphasizing the > > > > clutter of the unhelpful "const". I'm inclined to moving it, but don't > > > > have good sense of how strict these hierarchies are. What do you think? > > > I'm not sold that `const`-qualified return types always pessimize > > > optimizations. However, I'm also not sold that it's *always* a bad idea > > > to have a top-level cv-qualifier on a return type either (for instance, > > > this is one way to prevent callers from modifying a temp returned by a > > > function). How about leaving this in readability and changing the > > > diagnostic into: "top-level 'const' qualifier on a return type may reduce > > > code readability with limited benefit" or something equally weasely? > > I talked this over with other google folks and seems like the consensus is: > > > > 1. The readability benefits may be controversial. Some folks might not > > view `const` as clutter and there are some corner cases where the qualifier > > may prevent something harmful. > > 2. The performance implication is real, if not guaranteed to be a problem. > > > > Based on this, seems best to move it to performance, but water down the > > performance claims. That keeps the focus to an issue we can assume nearly > > everyone agrees on. > I'm not convinced the performance implications are real compared to how the > check is currently implemented. I know there are performance concerns when > you return a const value of class type because it pessimizes the ability to > use move constructors, but that's a very specific performance concern that I > don't believe is present in general. For instance, I don't know of any > performance concerns regarding `const int f();` as a declaration. I'd be fine > moving this to the performance module, but I think the check would need to be > tightened to only focus on actual performance concerns. > > FWIW, I do agree that the readability benefits may be controversial, but I > kind of thought that was the point to the check and as such, it's a very > reasonable check. Almost everything in readability is subjective to some > degree and our metric has always been "if you agree with a style check, don't > enable it". > > It's up to you how you want to proceed, but I see two ways forward: 1) keep > this as a readability check that broadly finds top-level const qualifiers and > removes them, 2) move this to the performance module and rework the check to > focus on only the scenarios that have concrete performance impact. > It's up to you how you want to proceed, but I see two ways forward: 1) keep > this as a readability check that broadly finds top-level const qualifiers and > removes them, 2) move this to the performance module and rework the check to > focus on only the scenarios that have concrete performance impact. Aaron, thank you for the detailed response. I'm inclined to agree with you that this belongs in readabilty and I spoke with sbenza who feels the same. The high-level point is that the `const` is noise in most cases. You suggested above a warning along the lines of: "top-level 'const' qualifier on a return type may reduce code readability with limited benefit" I like this, but think it should be a little stronger. Perhaps: "top-level 'const' qualifier on a return type may reduce code readability while rarely having an effect" I also propose updating the explanation in the documentation file from: Such use of ``const`` is superfluous, and prevents valuable compiler optimizations. to the weaker Such use of ``const`` is usually superfluous, and can prevent valuable compiler optimizations. WDYT? Repository: rCTE Clang Tools Extra https://reviews.llvm.org/D53025 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
