[clang][nullability] Don't discard expression state before end of full-expression. (#82611)

In #72985, I made a change to
discard
expression state (`ExprToLoc` and `ExprToVal`) at the beginning of each
basic
block. I did so with the claim that "we never need to access entries
from these
maps outside of the current basic block", noting that there are
exceptions to
this claim when control flow happens inside a full-expression (the
operands of
`&&`, `||`, and the conditional operator live in different basic blocks
than the
operator itself) but that we already have a mechanism for retrieving the
values
of these operands from the environment for the block they are computed
in.

It turns out, however, that the operands of these operators aren't the
only
expressions whose values can be accessed from a different basic block;
when
control flow happens within a full-expression, that control flow can be
"interposed" between an expression and its parent. Here is an example:

```cxx
void f(int*, int);
bool cond();

void target() {
  int i = 0;
  f(&i, cond() ? 1 : 0);
}
```

([godbolt](https://godbolt.org/z/hrbj1Mj3o))

In the CFG[^1] , note how the expression for `&i` is computed in block
B4,
but the parent of this expression (the `CallExpr`) is located in block
B1.
The the argument expression `&i` and the `CallExpr` are essentially
"torn apart"
into different basic blocks by the conditional operator in the second
argument.
In other words, the edge between the `CallExpr` and its argument `&i`
straddles
the boundary between two blocks.

I used to think that this scenario -- where an edge between an
expression and
one of its children straddles a block boundary -- could only happen
between the
expression that triggers the control flow (`&&`, `||`, or the
conditional
operator) and its children, but the example above shows that other
expressions
can be affected as well; the control flow is still triggered by `&&`,
`||` or
the conditional operator, but the expressions affected lie outside these
operators.

Discarding expression state too soon is harmful. For example, an
analysis that
checks the arguments of the `CallExpr` above would not be able to
retrieve a
value for the `&i` argument.

This patch therefore ensures that we don't discard expression state
before the
end of a full-expression. In other cases -- when the evaluation of a
full-expression is complete -- we still want to discard expression state
for the
reasons explained in #72985
(avoid
performing joins on boolean values that are no longer needed, which
unnecessarily extends the flow condition; improve debuggability by
removing
clutter from the expression state).

The impact on performance from this change is about a 1% slowdown in the
Crubit nullability check benchmarks:

```
name                              old cpu/op   new cpu/op   delta
BM_PointerAnalysisCopyPointer     71.9µs ± 1%  71.9µs ± 2%    ~     (p=0.987 n=15+20)
BM_PointerAnalysisIntLoop          190µs ± 1%   192µs ± 2%  +1.06%  (p=0.000 n=14+16)
BM_PointerAnalysisPointerLoop      325µs ± 5%   324µs ± 4%    ~     (p=0.496 n=18+20)
BM_PointerAnalysisBranch           193µs ± 0%   192µs ± 4%    ~     (p=0.488 n=14+18)
BM_PointerAnalysisLoopAndBranch    521µs ± 1%   525µs ± 3%  +0.94%  (p=0.017 n=18+19)
BM_PointerAnalysisTwoLoops         337µs ± 1%   341µs ± 3%  +1.19%  (p=0.004 n=17+19)
BM_PointerAnalysisJoinFilePath    1.62ms ± 2%  1.64ms ± 3%  +0.92%  (p=0.021 n=20+20)
BM_PointerAnalysisCallInLoop      1.14ms ± 1%  1.15ms ± 4%    ~     (p=0.135 n=16+18)
```

[^1]:
```
 [B5 (ENTRY)]
   Succs (1): B4

 [B1]
   1: [B4.9] ? [B2.1] : [B3.1]
   2: [B4.4]([B4.6], [B1.1])
   Preds (2): B2 B3
   Succs (1): B0

 [B2]
   1: 1
   Preds (1): B4
   Succs (1): B1

 [B3]
   1: 0
   Preds (1): B4
   Succs (1): B1

 [B4]
   1: 0
   2: int i = 0;
   3: f
   4: [B4.3] (ImplicitCastExpr, FunctionToPointerDecay, void (*)(int *, int))
   5: i
   6: &[B4.5]
   7: cond
   8: [B4.7] (ImplicitCastExpr, FunctionToPointerDecay, _Bool (*)(void))
   9: [B4.8]()
   T: [B4.9] ? ... : ...
   Preds (1): B5
   Succs (2): B2 B3

 [B0 (EXIT)]
   Preds (1): B1
```

Author: martinboehme

clang/include/clang/Analysis/FlowSensitive/ControlFlowContext.h

@@ -58,19 +58,36 @@ class ControlFlowContext {
     return BlockReachable[B.getBlockID()];
   }
 
+  /// Returns whether `B` contains an expression that is consumed in a
+  /// different block than `B` (i.e. the parent of the expression is in a
+  /// different block).
+  /// This happens if there is control flow within a full-expression (triggered
+  /// by `&&`, `||`, or the conditional operator). Note that the operands of
+  /// these operators are not the only expressions that can be consumed in a
+  /// different block. For example, in the function call
+  /// `f(&i, cond() ? 1 : 0)`, `&i` is in a different block than the `CallExpr`.
+  bool containsExprConsumedInDifferentBlock(const CFGBlock &B) const {
+    return ContainsExprConsumedInDifferentBlock.contains(&B);
+  }
+
 private:
-  ControlFlowContext(const Decl &D, std::unique_ptr<CFG> Cfg,
-                     llvm::DenseMap<const Stmt *, const CFGBlock *> StmtToBlock,
-                     llvm::BitVector BlockReachable)
+  ControlFlowContext(
+      const Decl &D, std::unique_ptr<CFG> Cfg,
+      llvm::DenseMap<const Stmt *, const CFGBlock *> StmtToBlock,
+      llvm::BitVector BlockReachable,
+      llvm::DenseSet<const CFGBlock *> ContainsExprConsumedInDifferentBlock)
       : ContainingDecl(D), Cfg(std::move(Cfg)),
         StmtToBlock(std::move(StmtToBlock)),
-        BlockReachable(std::move(BlockReachable)) {}
+        BlockReachable(std::move(BlockReachable)),
+        ContainsExprConsumedInDifferentBlock(
+            std::move(ContainsExprConsumedInDifferentBlock)) {}
 
   /// The `Decl` containing the statement used to construct the CFG.
   const Decl &ContainingDecl;
   std::unique_ptr<CFG> Cfg;
   llvm::DenseMap<const Stmt *, const CFGBlock *> StmtToBlock;
   llvm::BitVector BlockReachable;
+  llvm::DenseSet<const CFGBlock *> ContainsExprConsumedInDifferentBlock;
 };
 
 } // namespace dataflow

clang/include/clang/Analysis/FlowSensitive/DataflowEnvironment.h

@@ -210,6 +210,14 @@ class Environment {
   bool equivalentTo(const Environment &Other,
                     Environment::ValueModel &Model) const;
 
+  /// How to treat expression state (`ExprToLoc` and `ExprToVal`) in a join.
+  /// If the join happens within a full expression, expression state should be
+  /// kept; otherwise, we can discard it.
+  enum ExprJoinBehavior {
+    DiscardExprState,
+    KeepExprState,
+  };
+
   /// Joins two environments by taking the intersection of storage locations and
   /// values that are stored in them. Distinct values that are assigned to the
   /// same storage locations in `EnvA` and `EnvB` are merged using `Model`.
@@ -218,7 +226,8 @@ class Environment {
   ///
   ///  `EnvA` and `EnvB` must use the same `DataflowAnalysisContext`.
   static Environment join(const Environment &EnvA, const Environment &EnvB,
-                          Environment::ValueModel &Model);
+                          Environment::ValueModel &Model,
+                          ExprJoinBehavior ExprBehavior);
 
   /// Widens the environment point-wise, using `PrevEnv` as needed to inform the
   /// approximation.

clang/lib/Analysis/FlowSensitive/ControlFlowContext.cpp

@@ -94,6 +94,38 @@ static llvm::BitVector findReachableBlocks(const CFG &Cfg) {
   return BlockReachable;
 }
 
+static llvm::DenseSet<const CFGBlock *>
+buildContainsExprConsumedInDifferentBlock(
+    const CFG &Cfg,
+    const llvm::DenseMap<const Stmt *, const CFGBlock *> &StmtToBlock) {
+  llvm::DenseSet<const CFGBlock *> Result;
+
+  auto CheckChildExprs = [&Result, &StmtToBlock](const Stmt *S,
+                                                 const CFGBlock *Block) {
+    for (const Stmt *Child : S->children()) {
+      if (!isa<Expr>(Child))
+        continue;
+      const CFGBlock *ChildBlock = StmtToBlock.lookup(Child);
+      if (ChildBlock != Block)
+        Result.insert(ChildBlock);
+    }
+  };
+
+  for (const CFGBlock *Block : Cfg) {
+    if (Block == nullptr)
+      continue;
+
+    for (const CFGElement &Element : *Block)
+      if (auto S = Element.getAs<CFGStmt>())
+        CheckChildExprs(S->getStmt(), Block);
+
+    if (const Stmt *TerminatorCond = Block->getTerminatorCondition())
+      CheckChildExprs(TerminatorCond, Block);
+  }
+
+  return Result;
+}
+
 llvm::Expected<ControlFlowContext>
 ControlFlowContext::build(const FunctionDecl &Func) {
   if (!Func.doesThisDeclarationHaveABody())
@@ -140,8 +172,12 @@ ControlFlowContext::build(const Decl &D, Stmt &S, ASTContext &C) {
 
   llvm::BitVector BlockReachable = findReachableBlocks(*Cfg);
 
+  llvm::DenseSet<const CFGBlock *> ContainsExprConsumedInDifferentBlock =
+      buildContainsExprConsumedInDifferentBlock(*Cfg, StmtToBlock);
+
   return ControlFlowContext(D, std::move(Cfg), std::move(StmtToBlock),
-                            std::move(BlockReachable));
+                            std::move(BlockReachable),
+                            std::move(ContainsExprConsumedInDifferentBlock));
 }
 
 } // namespace dataflow

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

@@ -48,6 +48,24 @@ static llvm::DenseMap<const ValueDecl *, StorageLocation *> intersectDeclToLoc(
   return Result;
 }
 
+// Performs a join on either `ExprToLoc` or `ExprToVal`.
+// The maps must be consistent in the sense that any entries for the same
+// expression must map to the same location / value. This is the case if we are
+// performing a join for control flow within a full-expression (which is the
+// only case when this function should be used).
+template <typename MapT> MapT joinExprMaps(const MapT &Map1, const MapT &Map2) {
+  MapT Result = Map1;
+
+  for (const auto &Entry : Map2) {
+    [[maybe_unused]] auto [It, Inserted] = Result.insert(Entry);
+    // If there was an existing entry, its value should be the same as for the
+    // entry we were trying to insert.
+    assert(It->second == Entry.second);
+  }
+
+  return Result;
+}
+
 // Whether to consider equivalent two values with an unknown relation.
 //
 // FIXME: this function is a hack enabling unsoundness to support
@@ -627,7 +645,8 @@ LatticeJoinEffect Environment::widen(const Environment &PrevEnv,
 }
 
 Environment Environment::join(const Environment &EnvA, const Environment &EnvB,
-                              Environment::ValueModel &Model) {
+                              Environment::ValueModel &Model,
+                              ExprJoinBehavior ExprBehavior) {
   assert(EnvA.DACtx == EnvB.DACtx);
   assert(EnvA.ThisPointeeLoc == EnvB.ThisPointeeLoc);
   assert(EnvA.CallStack == EnvB.CallStack);
@@ -675,9 +694,10 @@ Environment Environment::join(const Environment &EnvA, const Environment &EnvB,
   JoinedEnv.LocToVal =
       joinLocToVal(EnvA.LocToVal, EnvB.LocToVal, EnvA, EnvB, JoinedEnv, Model);
 
-  // We intentionally leave `JoinedEnv.ExprToLoc` and `JoinedEnv.ExprToVal`
-  // empty, as we never need to access entries in these maps outside of the
-  // basic block that sets them.
+  if (ExprBehavior == KeepExprState) {
+    JoinedEnv.ExprToVal = joinExprMaps(EnvA.ExprToVal, EnvB.ExprToVal);
+    JoinedEnv.ExprToLoc = joinExprMaps(EnvA.ExprToLoc, EnvB.ExprToLoc);
+  }
 
   return JoinedEnv;
 }

clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp

@@ -221,18 +221,21 @@ class PrettyStackTraceCFGElement : public llvm::PrettyStackTraceEntry {
 // Avoids unneccesary copies of the environment.
 class JoinedStateBuilder {
   AnalysisContext ∾
+  Environment::ExprJoinBehavior JoinBehavior;
   std::vector<const TypeErasedDataflowAnalysisState *> All;
   std::deque<TypeErasedDataflowAnalysisState> Owned;
 
   TypeErasedDataflowAnalysisState
   join(const TypeErasedDataflowAnalysisState &L,
        const TypeErasedDataflowAnalysisState &R) {
     return {AC.Analysis.joinTypeErased(L.Lattice, R.Lattice),
-            Environment::join(L.Env, R.Env, AC.Analysis)};
+            Environment::join(L.Env, R.Env, AC.Analysis, JoinBehavior)};
   }
 
 public:
-  JoinedStateBuilder(AnalysisContext &AC) : AC(AC) {}
+  JoinedStateBuilder(AnalysisContext &AC,
+                     Environment::ExprJoinBehavior JoinBehavior)
+      : AC(AC), JoinBehavior(JoinBehavior) {}
 
   void addOwned(TypeErasedDataflowAnalysisState State) {
     Owned.push_back(std::move(State));
@@ -248,12 +251,12 @@ class JoinedStateBuilder {
       // initialize the state of each basic block differently.
       return {AC.Analysis.typeErasedInitialElement(), AC.InitEnv.fork()};
     if (All.size() == 1)
-      // Join the environment with itself so that we discard the entries from
-      // `ExprToLoc` and `ExprToVal`.
+      // Join the environment with itself so that we discard expression state if
+      // desired.
       // FIXME: We could consider writing special-case code for this that only
       // does the discarding, but it's not clear if this is worth it.
-      return {All[0]->Lattice,
-              Environment::join(All[0]->Env, All[0]->Env, AC.Analysis)};
+      return {All[0]->Lattice, Environment::join(All[0]->Env, All[0]->Env,
+                                                 AC.Analysis, JoinBehavior)};
 
     auto Result = join(*All[0], *All[1]);
     for (unsigned I = 2; I < All.size(); ++I)
@@ -307,7 +310,22 @@ computeBlockInputState(const CFGBlock &Block, AnalysisContext &AC) {
     }
   }
 
-  JoinedStateBuilder Builder(AC);
+  // If any of the predecessor blocks contains an expression consumed in a
+  // different block, we need to keep expression state.
+  // Note that in this case, we keep expression state for all predecessors,
+  // rather than only those predecessors that actually contain an expression
+  // consumed in a different block. While this is potentially suboptimal, it's
+  // actually likely, if we have control flow within a full expression, that
+  // all predecessors have expression state consumed in a different block.
+  Environment::ExprJoinBehavior JoinBehavior = Environment::DiscardExprState;
+  for (const CFGBlock *Pred : Preds) {
+    if (Pred && AC.CFCtx.containsExprConsumedInDifferentBlock(*Pred)) {
+      JoinBehavior = Environment::KeepExprState;
+      break;
+    }
+  }
+
+  JoinedStateBuilder Builder(AC, JoinBehavior);
   for (const CFGBlock *Pred : Preds) {
     // Skip if the `Block` is unreachable or control flow cannot get past it.
     if (!Pred || Pred->hasNoReturnElement())

clang/unittests/Analysis/FlowSensitive/DataflowEnvironmentTest.cpp

@@ -190,7 +190,8 @@ TEST_F(EnvironmentTest, JoinRecords) {
     Env2.setValue(Loc, Val2);
 
     Environment::ValueModel Model;
-    Environment EnvJoined = Environment::join(Env1, Env2, Model);
+    Environment EnvJoined =
+        Environment::join(Env1, Env2, Model, Environment::DiscardExprState);
     auto *JoinedVal = cast<RecordValue>(EnvJoined.getValue(Loc));
     EXPECT_NE(JoinedVal, &Val1);
     EXPECT_NE(JoinedVal, &Val2);

clang/unittests/Analysis/FlowSensitive/TypeErasedDataflowAnalysisTest.cpp

@@ -244,62 +244,98 @@ TEST_F(DiscardExprStateTest, WhileStatement) {
   EXPECT_NE(NotEqOpState.Env.getValue(NotEqOp), nullptr);
 
   // In the block that calls `foo(p)`, the value for `p != nullptr` is discarded
-  // because it is not consumed by this block.
+  // because it is not consumed outside the block it is in.
   const auto &CallFooState = blockStateForStmt(BlockStates, CallFoo);
   EXPECT_EQ(CallFooState.Env.getValue(NotEqOp), nullptr);
 }
 
 TEST_F(DiscardExprStateTest, BooleanOperator) {
   std::string Code = R"(
-    bool target(bool b1, bool b2) {
-      return b1 && b2;
+    void f();
+    void target(bool b1, bool b2) {
+      if (b1 && b2)
+        f();
     }
   )";
   auto BlockStates = llvm::cantFail(runAnalysis<NoopAnalysis>(
       Code, [](ASTContext &C) { return NoopAnalysis(C); }));
 
   const auto &AndOp =
       matchNode<BinaryOperator>(binaryOperator(hasOperatorName("&&")));
-  const auto &Return = matchNode<ReturnStmt>(returnStmt());
+  const auto &CallF =
+      matchNode<CallExpr>(callExpr(callee(functionDecl(hasName("f")))));
 
   // In the block that evaluates the LHS of the `&&` operator, the LHS is
   // associated with a value, while the right-hand side is not (unsurprisingly,
   // as it hasn't been evaluated yet).
   const auto &LHSState = blockStateForStmt(BlockStates, *AndOp.getLHS());
   auto *LHSValue = cast<BoolValue>(LHSState.Env.getValue(*AndOp.getLHS()));
-  ASSERT_NE(LHSValue, nullptr);
+  EXPECT_NE(LHSValue, nullptr);
   EXPECT_EQ(LHSState.Env.getValue(*AndOp.getRHS()), nullptr);
 
-  // In the block that evaluates the RHS, the RHS is associated with a
-  // value. The value for the LHS has been discarded as it is not consumed by
-  // this block.
+  // In the block that evaluates the RHS, both the LHS and RHS are associated
+  // with values, as they are both subexpressions of the `&&` operator, which
+  // is evaluated in a later block.
   const auto &RHSState = blockStateForStmt(BlockStates, *AndOp.getRHS());
-  EXPECT_EQ(RHSState.Env.getValue(*AndOp.getLHS()), nullptr);
-  auto *RHSValue = cast<BoolValue>(RHSState.Env.getValue(*AndOp.getRHS()));
-  ASSERT_NE(RHSValue, nullptr);
-
-  // In the block that evaluates the return statement, the expression `b1 && b2`
-  // is associated with a value (and check that it's the right one).
-  // The expressions `b1` and `b2` are _not_ associated with a value in this
-  // block, even though they are consumed by the block, because:
-  // * This block has two prececessor blocks (the one that evaluates `b1` and
-  //   the one that evaluates `b2`).
-  // * `b1` is only associated with a value in the block that evaluates `b1` but
-  //   not the block that evalutes `b2`, so the join operation discards the
-  //   value for `b1`.
-  // * `b2` is only associated with a value in the block that evaluates `b2` but
-  //   not the block that evaluates `b1`, the the join operation discards the
-  //   value for `b2`.
-  // Nevertheless, the analysis generates the correct formula for `b1 && b2`
-  // because the transfer function for the `&&` operator retrieves the values
-  // for its operands from the environments for the blocks that compute the
-  // operands, rather than from the environment for the block that contains the
-  // `&&`.
-  const auto &ReturnState = blockStateForStmt(BlockStates, Return);
-  EXPECT_EQ(ReturnState.Env.getValue(*AndOp.getLHS()), nullptr);
-  EXPECT_EQ(ReturnState.Env.getValue(*AndOp.getRHS()), nullptr);
-  EXPECT_EQ(ReturnState.Env.getValue(AndOp),
-            &ReturnState.Env.makeAnd(*LHSValue, *RHSValue));
+  EXPECT_EQ(RHSState.Env.getValue(*AndOp.getLHS()), LHSValue);
+  auto *RHSValue = RHSState.Env.get<BoolValue>(*AndOp.getRHS());
+  EXPECT_NE(RHSValue, nullptr);
+
+  // In the block that evaluates `b1 && b2`, the `&&` as well as its operands
+  // are associated with values.
+  const auto &AndOpState = blockStateForStmt(BlockStates, AndOp);
+  EXPECT_EQ(AndOpState.Env.getValue(*AndOp.getLHS()), LHSValue);
+  EXPECT_EQ(AndOpState.Env.getValue(*AndOp.getRHS()), RHSValue);
+  EXPECT_EQ(AndOpState.Env.getValue(AndOp),
+            &AndOpState.Env.makeAnd(*LHSValue, *RHSValue));
+
+  // In the block that calls `f()`, none of `b1`, `b2`, or `b1 && b2` should be
+  // associated with values.
+  const auto &CallFState = blockStateForStmt(BlockStates, CallF);
+  EXPECT_EQ(CallFState.Env.getValue(*AndOp.getLHS()), nullptr);
+  EXPECT_EQ(CallFState.Env.getValue(*AndOp.getRHS()), nullptr);
+  EXPECT_EQ(CallFState.Env.getValue(AndOp), nullptr);
+}
+
+TEST_F(DiscardExprStateTest, ConditionalOperator) {
+  std::string Code = R"(
+    void f(int*, int);
+    void g();
+    bool cond();
+
+    void target() {
+      int i = 0;
+      if (cond())
+        f(&i, cond() ? 1 : 0);
+      g();
+    }
+  )";
+  auto BlockStates = llvm::cantFail(runAnalysis<NoopAnalysis>(
+      Code, [](ASTContext &C) { return NoopAnalysis(C); }));
+
+  const auto &AddrOfI =
+      matchNode<UnaryOperator>(unaryOperator(hasOperatorName("&")));
+  const auto &CallF =
+      matchNode<CallExpr>(callExpr(callee(functionDecl(hasName("f")))));
+  const auto &CallG =
+      matchNode<CallExpr>(callExpr(callee(functionDecl(hasName("g")))));
+
+  // In the block that evaluates `&i`, it should obviously have a value.
+  const auto &AddrOfIState = blockStateForStmt(BlockStates, AddrOfI);
+  auto *AddrOfIVal = AddrOfIState.Env.get<PointerValue>(AddrOfI);
+  EXPECT_NE(AddrOfIVal, nullptr);
+
+  // Because of the conditional operator, the `f(...)` call is evaluated in a
+  // different block than `&i`, but `&i` still needs to have a value here
+  // because it's a subexpression of the call.
+  const auto &CallFState = blockStateForStmt(BlockStates, CallF);
+  EXPECT_NE(&CallFState, &AddrOfIState);
+  EXPECT_EQ(CallFState.Env.get<PointerValue>(AddrOfI), AddrOfIVal);
+
+  // In the block that calls `g()`, `&i` should no longer be associated with a
+  // value.
+  const auto &CallGState = blockStateForStmt(BlockStates, CallG);
+  EXPECT_EQ(CallGState.Env.get<PointerValue>(AddrOfI), nullptr);
 }
 
 struct NonConvergingLattice {