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

[Felix Berger via Phabricator via cfe-commits]

flx marked an inline comment as done.
flx 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);
----------------
ymandel wrote:
> 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.
I was not able to pass the (possibly more narrow) DeclContext to the match function as scope since match does not support DeclContexts.

I implemented  isDeclaredInFunction() which iterates through the decl contexts as you suggested. I'm not sure though whether we should start with VarDecl::getDeclContext() or VarDecl::getLexicalDeclContext()?

While looking at VarDecl::getLexicalDeclContext() I discovered is VarDecl has the following method:

```
  /// Returns true for local variable declarations other than parameters.                                                    
  /// Note that this includes static variables inside of functions. It also                                                  
  /// includes variables inside blocks.                                                                                      
  ///                                                                                                                        
  ///   void foo() { int x; static int y; extern int z; }                                                                    
  bool isLocalVarDecl() const;
```

I think this is exactly what we'd want here. What do you think?



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

https://reviews.llvm.org/D103021