[PATCH] D76977: [lld-macho] Implement basic export trie

Tue Apr 21 23:57:46 PDT 2020

int3 added inline comments.

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Comment at: lld/MachO/ExportTrie.cpp:192-195
+  std::vector<TrieNode *> orderedNodes;
+  orderedNodes.reserve(allNodes.size());
+  for (const Symbol *sym : exported)
+    rootNode->orderNodes(*sym, orderedNodes);
----------------
int3 wrote:
> smeenai wrote:
> > int3 wrote:
> > > This is basically a topological sort. It's unclear to me though why this approach (of finding every element starting from the root) was chosen vs doing a single preorder traversal of the trie...
> > Not having looked at this in detail yet, would they give comparable results?
> I think the only difference would be the order (and therefore the offsets) of the serialized nodes. Might make things more or less compact, it's unclear to me...
Aha, I see that `ld64` has an `-exported_symbols_order` flag, so the export trie "can be optimized to make lookups of popular symbols faster". The implementation also secondarily sorts the trie by address. I guess that means dyld loads this trie lazily. I'll add a TODO to support this...

I think we can still implement it more efficiently than looking up every element from the root: While building the trie, we should keep track of the lowest user-ordered leaf that it contains. Then we can sort the nodes based on that.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D76977/new/

https://reviews.llvm.org/D76977