[LifetimeSafety] Add loan expiry analysis

[usx95]

This PR adds the ExpiredLoansAnalysis class to track which loans have expired. The analysis uses a dataflow lattice (ExpiredLattice) to maintain the set of expired loans at each program point.

This is a very light weight dataflow analysis and is expected to reach fixed point in ~2 iterations.
In principle, this does not need a dataflow analysis but is used for convenience in favour of lean code.

[usx95]

• [LifetimeSafety] Add lattice tracking per program point #149069
• [LifetimeSafety] Implement LiveOrigins analysis #148976
• [LifetimeSafety] Add loan expiry analysis #148712 👈 (View in Graphite)
• [LifetimeSafety] Support bidirectional dataflow analysis #148967
• [LifetimeSafety] Make the dataflow analysis generic #148222
• [LifetimeSafety] Implement dataflow analysis for loan propagation #148065 : 1 other dependent PR (#147315 )
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[llvmbot]

@llvm/pr-subscribers-clang-analysis

@llvm/pr-subscribers-clang

Author: Utkarsh Saxena (usx95)

Changes

This PR adds the ExpiredLoansAnalysis class to track which loans have expired. The analysis uses a dataflow lattice (ExpiredLattice) to maintain the set of expired loans at each program point.

This is a very light weight dataflow analysis and is expected to reach fixed point in ~2 iterations.

---

Full diff: https://github.com/llvm/llvm-project/pull/148712.diff

1 Files Affected:

• (modified) clang/lib/Analysis/LifetimeSafety.cpp (+62)

diff --git a/clang/lib/Analysis/LifetimeSafety.cpp b/clang/lib/Analysis/LifetimeSafety.cpp
index 829da3ceb4c83..217b02b416c1a 100644
--- a/clang/lib/Analysis/LifetimeSafety.cpp
+++ b/clang/lib/Analysis/LifetimeSafety.cpp
@@ -777,6 +777,65 @@ class LoanPropagationAnalysis
   }
 };
 
+// ========================================================================= //
+//                         Expired Loans Analysis
+// ========================================================================= //
+
+/// The dataflow lattice for tracking the set of expired loans.
+struct ExpiredLattice {
+  LoanSet Expired;
+
+  ExpiredLattice() : Expired(nullptr) {};
+  explicit ExpiredLattice(LoanSet S) : Expired(S) {}
+
+  bool operator==(const ExpiredLattice &Other) const {
+    return Expired == Other.Expired;
+  }
+  bool operator!=(const ExpiredLattice &Other) const {
+    return !(*this == Other);
+  }
+
+  void dump(llvm::raw_ostream &OS) const {
+    OS << "ExpiredLattice State:\n";
+    if (Expired.isEmpty())
+      OS << "  <empty>\n";
+    for (const LoanID &LID : Expired)
+      OS << "  Loan " << LID << " is expired\n";
+  }
+};
+
+/// The analysis that tracks which loans have expired.
+class ExpiredLoansAnalysis
+    : public DataflowAnalysis<ExpiredLoansAnalysis, ExpiredLattice,
+                              Direction::Forward> {
+
+  LoanSet::Factory &Factory;
+
+public:
+  ExpiredLoansAnalysis(const CFG &C, AnalysisDeclContext &AC, FactManager &F,
+                       LifetimeFactory &SF)
+      : DataflowAnalysis(C, AC, F), Factory(SF.LoanSetFactory) {}
+
+  using Base::transfer;
+
+  StringRef getAnalysisName() const { return "ExpiredLoans"; }
+
+  Lattice getInitialState() { return Lattice(Factory.getEmptySet()); }
+
+  /// Merges two lattices by taking the union of the expired loan sets.
+  Lattice join(Lattice L1, Lattice L2) const {
+    return Lattice(utils::join(L1.Expired, L2.Expired, Factory));
+  }
+
+  Lattice transfer(Lattice In, const ExpireFact &F) {
+    return Lattice(Factory.add(In.Expired, F.getLoanID()));
+  }
+
+  Lattice transfer(Lattice In, const IssueFact &F) {
+    return Lattice(Factory.remove(In.Expired, F.getLoanID()));
+  }
+};
+
 // ========================================================================= //
 //  TODO:
 // - Modifying loan propagation to answer `LoanSet getLoans(Origin O, Point P)`
@@ -809,5 +868,8 @@ void runLifetimeSafetyAnalysis(const DeclContext &DC, const CFG &Cfg,
   LoanPropagationAnalysis LoanPropagation(Cfg, AC, FactMgr, Factory);
   LoanPropagation.run();
   DEBUG_WITH_TYPE("LifetimeLoanPropagation", LoanPropagation.dump());
+
+  ExpiredLoansAnalysis ExpiredLoans(Cfg, AC, FactMgr, Factory);
+  ExpiredLoans.run();
 }
 } // namespace clang

[ymand]

Looks good, but what's the plan for tests?

[usx95]

Looks good, but what's the plan for tests?

Good question :) I think adding tests for each analyses to the current llvm-lit tests is quite a pain for the eyes at this point. Also I want to test things at specific program points (is loan expired here, does origin contain loan here, etc). I am exploring a couple of options, primarily https://github.com/llvm/llvm-project/blob/main/clang/include/clang/Testing/TestAST.h, https://github.com/llvm/llvm-project/blob/main/clang/unittests/Analysis/FlowSensitive/TestingSupport.h and something like https://github.com/llvm/llvm-project/blob/main/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp#L874-L900

This will likely take some more time. Happy to hold this off until I add something like this for unit tests!

I would still rely on llvm-lit tests for final diagnostic reporting (or maybe that also can be done through unit testing)

[Xazax-hun]

Once the testing story is figured out this looks good to me.

[Xazax-hun]

So we have the expectation that we have a tight bound on this analysis. I wonder if there is a way to somehow add an assert to verify that the reality matches our expectations. Not super important but if it is not too complicated it could be nice.

We can also defer this to a later PR since we want to be able to add strict bounds to the number of iterations in the future and that might be required for us to easily assert on this.

[usx95]

Once the testing story is figured out this looks good to me.

Alright. I will add some unit test infra separately before this PR then!