[llvm] [CodeLayout] Faster basic block reordering, ext-tsp (PR #68617)

[Fangrui Song via llvm-commits]

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@@ -725,6 +734,7 @@ class ExtTSPImpl {
       return std::make_tuple(A1->Id, B1->Id) < std::make_tuple(A2->Id, B2->Id);
     };
 
+    double PrevScore = 1e9;
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MaskRay wrote:

This should use a sentinel value instead of a magic 1e9.

You can use `std::optional<double>` for an absent value.

https://github.com/llvm/llvm-project/pull/68617