[llvm] [DomTree] Reduce number of hash table lookups (NFC) (PR #73097)

[Nikita Popov via llvm-commits]

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@@ -304,8 +306,7 @@ struct SemiNCAInfo {
     // Note that the parents were stored in IDoms and later got invalidated
     // during path compression in Eval.
     for (unsigned i = 2; i < NextDFSNum; ++i) {
-      const NodePtr W = NumToNode[i];
-      auto &WInfo = NodeToInfo[W];
+      auto &WInfo = *NumToInfo[i];
       const unsigned SDomNum = NodeToInfo[NumToNode[WInfo.Semi]].DFSNum;
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nikic wrote:

If you're wondering why I didn't change this to `NumToNode[WInfo.Semi]` as well, the reason is that `WInfo.Semi` can be `0` for PDTs here. Because the PDT virtual root `nullptr` happens to be the same as the `NumToNode` sentinel, this ends up looking up the info for the virtual root.

I could easily work around this here, but I didn't do this because I think this indicates a (probably harmless) bug somewhere else, and Semi is supposed to be 1 (the virtual root) rather than 0 here.

https://github.com/llvm/llvm-project/pull/73097