[PATCH] D53025: [clang-tidy] implement new check for const return types.

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman 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();
----------------
ymandel wrote:
> 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?
> 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'm not opposed to it, but I worry about people getting hung up on "rarely having an effect". For instance, a const return type will definitely have an impact on code using template metaprogramming to inspect the return type of a Callable. How about:

"top-level 'const' qualifier on a return type may reduce code readability without improving const correctness"

> Such use of `const` is usually superfluous, and can
> prevent valuable compiler optimizations.

Sounds good to me!


Repository:
  rCTE Clang Tools Extra

https://reviews.llvm.org/D53025