[PATCH] D103021: [clang-tidy] performance-unnecessary-copy-initialization: Search whole function body for variable initializations.

[Yitzhak Mandelbaum via Phabricator via cfe-commits]

ymandel added inline comments.


================
Comment at: clang-tools-extra/clang-tidy/performance/UnnecessaryCopyInitialization.cpp:98-101
   auto Matches =
       match(findAll(declStmt(has(varDecl(equalsNode(&InitializingVar))))
                         .bind("declStmt")),
+            Body, Context);
----------------
flx wrote:
> ymandel wrote:
> > Consider inspecting the `DeclContext`s instead, which should be much more efficient than searching the entire block.  Pass the `FunctionDecl` as an argument instead of `Body`, since it is a `DeclContext`.  e.g. `const DeclContext &Fun`
> > 
> > Then, either
> > 1. Call `Fun.containsDecl(InitializingVar)`, or
> > 2. Search through the contexts yourself; something like:
> > ```
> > DeclContext* DC = InitializingVar->getDeclContext(); 
> > while (DC != nullptr && DC != &Fun)
> >   DC = DC->getLexicalParent();
> > if (DC == nullptr)
> >   // The reference or pointer is not initialized anywhere witin the function. We
> >   // assume its pointee is not modified then.
> >   return true;
> > ```
> Are #1 and #2 equivalent? From the implementation and comment I cannot tell whether #1 would cover cases where the variable is not declared directly in the function, but in child block:
> 
> ```
> void Fun() {
>  {
>    var i;
>    {
>      i.usedHere();
>    }  
>  } 
> }
> ```
> 
> I'm also reading this as an optimization to more quickly determine whether we can stop here. We still need to find the matches for the next steps, but I  think I could then limit matching to the DeclContext I found here. Is this correct? For this I would actually need the DeclContext result from #2.
A. I think you're right that #2 is more suited to what you need. I wasn't sure, so included both. Agreed that the comments are ambiguous.
B. yes, this is just an optimization. it may be premature for that matter; just that match can be expensive and this seemed a more direct expression of the algorithm.


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  https://reviews.llvm.org/D103021/new/

https://reviews.llvm.org/D103021