[llvm-branch-commits] [clang] [LifetimeSafety] Introduce a liveness-based lifetime policy (PR #159991)

[Utkarsh Saxena via llvm-branch-commits]

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@@ -1085,11 +1080,17 @@ class LoanPropagationAnalysis
   /// Merges two lattices by taking the union of loans for each origin.
   // TODO(opt): Keep the state small by removing origins which become dead.
   Lattice join(Lattice A, Lattice B) {
-    OriginLoanMap JoinedOrigins =
-        utils::join(A.Origins, B.Origins, OriginLoanMapFactory,
-                    [&](LoanSet S1, LoanSet S2) {
-                      return utils::join(S1, S2, LoanSetFactory);
-                    });
+    OriginLoanMap JoinedOrigins = utils::join(
+        A.Origins, B.Origins, OriginLoanMapFactory,
+        [&](const LoanSet *S1, const LoanSet *S2) {
+          assert((S1 || S2) && "unexpectedly merging 2 empty sets");
+          if (!S1)
+            return *S2;
+          if (!S2)
+            return *S1;
+          return utils::join(*S1, *S2, LoanSetFactory);
+        },
+        /*JoinAllKeys=*/false);
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usx95 wrote:

Primarily for performance. We can join all keys but we do not need to in case of Loan propagation analysis: we want the set difference to make it into the merged set "as-is". Same is not true for `Liveness` because of "confidence". We also want to demote the set difference to "Maybe" confidence.
Let me add a comment about this.

https://github.com/llvm/llvm-project/pull/159991