[clang] [Clang] Dump minimization hints for namespaces (PR #151534)

[Ilya Biryukov via cfe-commits]

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@@ -188,10 +205,49 @@ class DeserializedDeclsSourceRangePrinter : public ASTConsumer,
 
 private:
   std::vector<const Decl *> PendingDecls;
+  llvm::DenseSet<const NamespaceDecl *> ProcessedNamespaces;
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ilya-biryukov wrote:

There is a misunderstanding. The documentation says:
> DenseSet is a simple **quadratically probed** hash table.

So it would only be quadratic if the hash function is bad (which is not the case for pointers), otherwise it's amortized `O(1)` like one would expect.

The documentation does seem to suggest that `SmallPtrSet` is a better alternative, so I have switched to it. I am using the size `0`, though, because I anticipate that the number of namespaces we put is going to be quite large on average and the small-set optimization is not going to be useful.

`std::set` would be my last choice as it definitely underperforms compared to both alternatives, the hash tables are just so much faster in those cases in practice.

https://github.com/llvm/llvm-project/pull/151534