[PATCH] D31757: [clang-tidy] Add a clang-tidy check for possible inefficient vector operations

[Haojian Wu via Phabricator via cfe-commits]

hokein marked an inline comment as done.
hokein added a comment.

friendly ping @alexfh.



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Comment at: clang-tidy/performance/InefficientVectorOperationCheck.cpp:53-54
+                        PushBackCall)),
+          hasParent(compoundStmt(unless(has(ReserveCall)),
+                                 has(VectorVarDefStmt))))
+          .bind("for_loop"),
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aaron.ballman wrote:
> hokein wrote:
> > aaron.ballman wrote:
> > > hokein wrote:
> > > > aaron.ballman wrote:
> > > > > hokein wrote:
> > > > > > aaron.ballman wrote:
> > > > > > > I'm really not keen on this. It will catch trivial cases, so there is some utility, but this will quickly fall apart with anything past the trivial case.
> > > > > > The motivation of this check is to find code patterns like `for (int i = 0; i < n; ++i) { v.push_back(i); }` and clean them in our codebase (we have lots of similar cases). [These](https://docs.google.com/document/d/1Bbc-6DlNs6zQujWD5-XOHWbfPJVMG7Z_T27Kv0WcFb4/edit?usp=sharing) are all cases we want to support. Using `hasParent` is a simple and sufficient way to do it IMO.
> > > > > I'm not convinced of the utility without implementing this in a more sensitive way. Have you run this across any large code bases and found that it catches issues?
> > > > Yeah, the check catches ~2800 cases (regexp shows ~17,000 total usages) in our internal codebase. And all caught cases are what we are interested in. It would catch more if we support for-range loops and iterator-based for-loops. 
> > > I wasn't worried about it not triggering often enough, I was worried about it triggering too often because of the lack of sensitivity. If you randomly sample some of those 2800 cases, do they reserve the space in a way that your check isn't catching?
> > Ok, I see your concern now, thanks for pointing it out.
> > 
> > I have read through these caught cases. The results look reasonable. Most cases (> 95%) are what we expected, the code pattern is like `vector<T> v; for (...) { v.push_back(...); }` where the vector definition statement and for-loop statement are consecutive. Another option is to make the check more strict (only detects the consecutive code pattern).
> Okay, that sounds like it has utility then. Thank you for clarifying!
I improved the way of catching the preallocations before the loop. Could you take a look again?


https://reviews.llvm.org/D31757