[clang][dataflow] Add flowConditionIsTautology function

Provide a way for users to check if a flow condition is
unconditionally true.

Differential Revision: https://reviews.llvm.org/D124943

Author: tJener

clang/include/clang/Analysis/FlowSensitive/DataflowAnalysisContext.h

@@ -173,6 +173,10 @@ class DataflowAnalysisContext {
   /// identified by `Token` imply that `Val` is true.
   bool flowConditionImplies(AtomicBoolValue &Token, BoolValue &Val);
 
+  /// Returns true if and only if the constraints of the flow condition
+  /// identified by `Token` are always true.
+  bool flowConditionIsTautology(AtomicBoolValue &Token);
+
 private:
   /// Adds all constraints of the flow condition identified by `Token` and all
   /// of its transitive dependencies to `Constraints`. `VisitedTokens` is used

clang/lib/Analysis/FlowSensitive/DataflowAnalysisContext.cpp

@@ -117,6 +117,19 @@ bool DataflowAnalysisContext::flowConditionImplies(AtomicBoolValue &Token,
   return S->solve(std::move(Constraints)) == Solver::Result::Unsatisfiable;
 }
 
+bool DataflowAnalysisContext::flowConditionIsTautology(AtomicBoolValue &Token) {
+  // Returns true if and only if we cannot prove that the flow condition can
+  // ever be false.
+  llvm::DenseSet<BoolValue *> Constraints = {
+      &getBoolLiteralValue(true),
+      &getOrCreateNegationValue(getBoolLiteralValue(false)),
+      &getOrCreateNegationValue(Token),
+  };
+  llvm::DenseSet<AtomicBoolValue *> VisitedTokens;
+  addTransitiveFlowConditionConstraints(Token, Constraints, VisitedTokens);
+  return S->solve(std::move(Constraints)) == Solver::Result::Unsatisfiable;
+}
+
 void DataflowAnalysisContext::addTransitiveFlowConditionConstraints(
     AtomicBoolValue &Token, llvm::DenseSet<BoolValue *> &Constraints,
     llvm::DenseSet<AtomicBoolValue *> &VisitedTokens) const {

clang/unittests/Analysis/FlowSensitive/DataflowAnalysisContextTest.cpp

@@ -140,4 +140,33 @@ TEST_F(DataflowAnalysisContextTest, JoinFlowConditions) {
   EXPECT_TRUE(Context.flowConditionImplies(FC3, C3));
 }
 
+TEST_F(DataflowAnalysisContextTest, FlowConditionTautologies) {
+  // Fresh flow condition with empty/no constraints is always true.
+  auto &FC1 = Context.makeFlowConditionToken();
+  EXPECT_TRUE(Context.flowConditionIsTautology(FC1));
+
+  // Literal `true` is always true.
+  auto &FC2 = Context.makeFlowConditionToken();
+  Context.addFlowConditionConstraint(FC2, Context.getBoolLiteralValue(true));
+  EXPECT_TRUE(Context.flowConditionIsTautology(FC2));
+
+  // Literal `false` is never true.
+  auto &FC3 = Context.makeFlowConditionToken();
+  Context.addFlowConditionConstraint(FC3, Context.getBoolLiteralValue(false));
+  EXPECT_FALSE(Context.flowConditionIsTautology(FC3));
+
+  // We can't prove that an arbitrary bool A is always true...
+  auto &C1 = Context.createAtomicBoolValue();
+  auto &FC4 = Context.makeFlowConditionToken();
+  Context.addFlowConditionConstraint(FC4, C1);
+  EXPECT_FALSE(Context.flowConditionIsTautology(FC4));
+
+  // ... but we can prove A || !A is true.
+  auto &FC5 = Context.makeFlowConditionToken();
+  Context.addFlowConditionConstraint(
+      FC5, Context.getOrCreateDisjunctionValue(
+               C1, Context.getOrCreateNegationValue(C1)));
+  EXPECT_TRUE(Context.flowConditionIsTautology(FC5));
+}
+
 } // namespace