[llvm] [SampleFDO] Improve stale profile matching by diff algorithm (PR #87375)

[via llvm-commits]

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@@ -19,6 +19,59 @@
 
 namespace llvm {
 
+// Callsite location based matching anchor.
+struct Anchor {
+  LineLocation Loc;
+  FunctionId FuncId;
+
+  Anchor(const LineLocation &Loc, const FunctionId &FuncId)
+      : Loc(Loc), FuncId(FuncId) {}
+  Anchor(const LineLocation &Loc, StringRef &FName) : Loc(Loc), FuncId(FName) {}
+  bool operator==(const Anchor &Other) const {
+    return this->FuncId == Other.FuncId;
+  }
+};
+
+// This class implements the Myers diff algorithm used for stale profile
+// matching. The algorithm provides a simple and efficient way to find the
+// Longest Common Subsequence(LCS) or the Shortest Edit Script(SES) of two
+// sequences. For more details, refer to the paper 'An O(ND) Difference
+// Algorithm and Its Variations' by Eugene W. Myers.
+// In the scenario of profile fuzzy matching, the two sequences are the IR
+// callsite anchors and profile callsite anchors. The subsequence equivalent
+// parts from the resulting SES are used to remap the IR locations to the
+// profile locations. As the number of function callsite is usually not big, we
+// currently just implements the basic greedy version(page 6 of the paper).
+class MyersDiff {
+public:
+  struct DiffResult {
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WenleiHe wrote:

I hope we can simplify this. The abstractions here seem unnecessary -- 1) `MyersDiff` is entirely stateless, so no need for class / instances.  2) `DiffResult` is really just a `LocToLocMap` if we remove all the test/debug related stuff. 

https://github.com/llvm/llvm-project/pull/87375