[CFG][Analyzer] Add LoopExit element to the CFG in more cases

[szepet]

Summary

This patch adds a LoopExit element to the CFG whenever a loop is exited by a ReturnStmt, GotoStmt or IndirectGotoStmt.
The LoopExit element is consumed by the Static Analyzer in order to simulate the loops more precisely. This patch aims to ensure that the simulation will be always consistent. So, whenever a loop is entered a loop exit element will be encountered after leaving it.

The idea is the following: In cases where a 'jump' is made we check (by walking up on the AST) the containing loops for both (From and To) locations. Then, generate the LoopExit element for the exited loops (which is the difference of the two "containing loop" set.

Note: In case of IndirectGotoStmt, it could happen that we generate LoopExit elements even for loops which is not exited by that jump. However, it does not seem to be a problem. This could result that we can not apply some more precise modeling feature (like unrolling and widening) but not any more - as I can see. (Also, this is a rare case.)

[szepet]

Just removed some accidentally left changes from the patch.

[NoQ]

These CFGs make perfect sense to me so far, and i guess this is a must-have. Thank you!

Note: In case of IndirectGotoStmt, it could happen that we generate LoopExit elements even for loops which is not exited by that jump. However, it does not seem to be a problem. This could result that we can not apply some more precise modeling feature (like unrolling and widening) but not any more - as I can see. (Also, this is a rare case.)

This seems a bit scary because if there's no obvious one-to-once correspondence between entrances and exits along every path, how would we know when to pop loop contexts from the stack of location contexts? Like, if we attach variable lifetime checks to the loop exit, would we need to write additional code in the analyzer to see if the variable indeed goes out of scope on the current path? Could you demonstrate how it works in a test case over D41151?

include/clang/Analysis/CFG.h

178–189

I'd be glad to see more documentation on where does this CFGElement appear, when, and why:

• At the end of the block within the loop?
• At the beginning of the block after the loop?
• In its own block constructed with the sole purpose of sheltering it?

Same goes for the CFGLoopEntrace, i guess.

test/Analysis/loopexit-cfg-output.cpp

912–914

P.S. This is not a regression introduced by your patch, but i'm really curious what does this block even do.

[szepet]

Added comments and removed indirect goto support from this patch.

This seems a bit scary because if there's no obvious one-to-once correspondence between entrances and exits along every path, how would we know when to pop loop contexts from the stack of location contexts? Like, if we attach variable lifetime checks to the loop exit, would we need to write additional code in the analyzer to see if the variable indeed goes out of scope on the current path.

Yepp, we should check if the current loop (one quick method on the LocationContext) belongs to the LoopExit. This was already necessary in the previous patch since cases like:

if(Cond)
  for(;;){...}
stmt1
stmt2
...

In the above example even if Cond is false the analyzer will encounter the LoopExit since it is the first element of the breakBlock of the loop. (So it is within the same block as stmt1 and stmt2)

I have removed IndirectGoto support since it was not precise enough and rather handle this in the analyzer. (isPrecisableModelableLoop function in the D41151)

test/Analysis/loopexit-cfg-output.cpp

912–914

I believe this is the corresponding TransitionBlock to the WhileStmt (there is always a block which marks the looping back to the head of the loop event) but the optimization phase was good enough to detect that we will never proceed on that (since the first statement of the while loop is a goto jump).

[szepet]

ping

[george.karpenkov]

lib/Analysis/CFG.cpp

1348

shouldn't we have a visited list as well, to avoid exponential queue explosion if nodes are repeated?

include/clang/Analysis/CFG.h

private:
 
   CFGNewAllocator() = default;
 
   static bool isKind(const CFGElement &elem) {
     return elem.getKind() == NewAllocator;
   }
 };
 
 /// Represents the point where a loop ends.
 /// This element is is only produced when building the CFG for the static
-/// analyzer and hidden behind the 'cfg-loopexit' analyzer config flag.
+/// analyzer and hidden behind the 'cfg-loopexit' analyzer config flag. It is
+/// placed at the beginning of the block after the loop, however, in special
+/// cases it can appear different positions:
+/// - Right before a ReturnStmt inside the loop
+/// - In the end of a goto terminated block (inside the loop as well)
 ///
-/// Note: a loop exit element can be reached even when the loop body was never
-/// entered.
+/// Note: whenever a loop is entered a loop exit element will be encountered
+/// after leaving it. However, a loop exit element can be reached even when the
+/// loop body was never entered.
 class CFGLoopExit : public CFGElement {
 public:
   explicit CFGLoopExit(const Stmt *stmt) : CFGElement(LoopExit, stmt) {}
 
   const Stmt *getLoopStmt() const {
     return static_cast<Stmt *>(Data1.getPointer());
   }
 
 private:

lib/Analysis/CFG.cpp

 #include "llvm/Support/DOTGraphTraits.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/GraphWriter.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <memory>
+#include <queue>
 #include <string>
 #include <tuple>
 #include <utility>
 #include <vector>
 
 using namespace clang;
 
 static SourceLocation GetEndLoc(Decl *D) {
private:
   CFGBlock *createNoReturnBlock();
 
   CFGBlock *addStmt(Stmt *S) {
     return Visit(S, AddStmtChoice::AlwaysAdd);
   }
 
   CFGBlock *addInitializer(CXXCtorInitializer *I);
   void addLoopExit(const Stmt *LoopStmt);
+  void addLoopExit(const Stmt *FromStmt, const Stmt *ToStmt);
+
   void addAutomaticObjDtors(LocalScope::const_iterator B,
                             LocalScope::const_iterator E, Stmt *S);
   void addLifetimeEnds(LocalScope::const_iterator B,
                        LocalScope::const_iterator E, Stmt *S);
   void addAutomaticObjHandling(LocalScope::const_iterator B,
                                LocalScope::const_iterator E, Stmt *S);
   void addImplicitDtorsForDestructor(const CXXDestructorDecl *DD);
 
while (true) {
     
     break;
   }
 
   return Init->getType();
 }
 
 // TODO: Support adding LoopExit element to the CFG in case where the loop is
-// ended by ReturnStmt, GotoStmt or ThrowExpr.
+// ended by an IndirectGotoStmt or ThrowExpr. Since this element is consumed
+// only by the Static Analyzer which does not support modeling of ThrowExpr
+// yet, it does not causes any problem.
 void CFGBuilder::addLoopExit(const Stmt *LoopStmt){
   if(!BuildOpts.AddLoopExit)
     return;
   autoCreateBlock();
   appendLoopExit(Block, LoopStmt);
 }
 
+llvm::SmallSetVector<const Stmt *, 4>
+collectContainingLoops(const Stmt *S, ASTContext &ASTCtx) {
+  llvm::SmallSetVector<const Stmt *, 4> LoopStmts;
+
+  if (!S)
+    return LoopStmts;
+
+  std::queue<ast_type_traits::DynTypedNode> NodesToVisit;
+  NodesToVisit.push(ast_type_traits::DynTypedNode::create(*S));
+
+  while (!NodesToVisit.empty()) {
+    ast_type_traits::DynTypedNode Node = NodesToVisit.front();
+    NodesToVisit.pop();
+
+    for (auto &Parent : ASTCtx.getParents(Node)) {
+      NodesToVisit.push(Parent);
+    }
+
+    const Stmt *LoopStmt = Node.get<Stmt>();
+    if (LoopStmt && (isa<ForStmt>(LoopStmt) || isa<WhileStmt>(LoopStmt) ||
+                     isa<DoStmt>(LoopStmt)))
+      LoopStmts.insert(LoopStmt);
+  }
+  return LoopStmts;
+}
+
+void CFGBuilder::addLoopExit(const Stmt *FromStmt, const Stmt *ToStmt) {
+  if (!BuildOpts.AddLoopExit)
+    return;
+
+  llvm::SmallSetVector<const Stmt *, 4> FromLoopStmts =
+      collectContainingLoops(FromStmt, *Context);
+
+  llvm::SmallSetVector<const Stmt *, 4> ToLoopStmts =
+      collectContainingLoops(ToStmt, *Context);
+
+  FromLoopStmts.set_subtract(ToLoopStmts);
+  for (llvm::SmallSetVector<const Stmt *, 4>::reverse_iterator
+           I = FromLoopStmts.rbegin(),
+           E = FromLoopStmts.rend();
+       I != E; ++I)
+    appendLoopExit(Block, *I);
+}
+
 void CFGBuilder::addAutomaticObjHandling(LocalScope::const_iterator B,
                                          LocalScope::const_iterator E,
                                          Stmt *S) {
   if (BuildOpts.AddImplicitDtors)
     addAutomaticObjDtors(B, E, S);
   if (BuildOpts.AddLifetime)
     addLifetimeEnds(B, E, S);
 }
CFGBlock *CFGBuilder::VisitReturnStmt(ReturnStmt *R) {
 
   // If the one of the destructors does not return, we already have the Exit
   // block as a successor.
   if (!Block->hasNoReturnElement())
     addSuccessor(Block, &cfg->getExit());
 
   // Add the return statement to the block.  This may create new blocks if R
   // contains control-flow (short-circuit operations).
-  return VisitStmt(R, AddStmtChoice::AlwaysAdd);
+  CFGBlock* ReturnBlock = VisitStmt(R, AddStmtChoice::AlwaysAdd);
+  addLoopExit(R, nullptr);
+  return ReturnBlock;
 }
 
 CFGBlock *CFGBuilder::VisitSEHExceptStmt(SEHExceptStmt *ES) {
   // SEHExceptStmt are treated like labels, so they are the first statement in a
   // block.
 
   // Save local scope position because in case of exception variable ScopePos
   // won't be restored when traversing AST.
CFGBlock *CFGBuilder::VisitGotoStmt(GotoStmt *G) {
   if (I == LabelMap.end())
     // We will need to backpatch this block later.
     BackpatchBlocks.push_back(JumpSource(Block, ScopePos));
   else {
     JumpTarget JT = I->second;
     addAutomaticObjHandling(ScopePos, JT.scopePosition, G);
     addSuccessor(Block, JT.block);
   }
+  addLoopExit(G, G->getLabel()->getStmt());
 
   return Block;
 }
 
 CFGBlock *CFGBuilder::VisitForStmt(ForStmt *F) {
   CFGBlock *LoopSuccessor = nullptr;
 
   // Save local scope position because in case of condition variable ScopePos

test/Analysis/loopexit-cfg-output.cpp

 // CHECK:       [B11]
 // CHECK-NEXT:   1: 2
 // CHECK-NEXT:   2: int i = 2;
 // CHECK-NEXT:   Preds (1): B12
 // CHECK-NEXT:   Succs (1): B10
 
 // CHECK:       [B0 (EXIT)]
 // CHECK-NEXT:   Preds (1): B1
-void check_break()
-{
-  for(int i = 2; i < 6; i++) {
-    if(i == 4)
+void check_break() {
+  for (int i = 2; i < 6; i++) {
+    if (i == 4)
       break;
   }
 
   int i = 1;
-  while(i<5){
+  while (i < 5) {
     i++;
-    if(i%2)
+    if (i % 2)
       break;
   }
-  
   return;
 }
+
+// CHECK:       [B11 (ENTRY)]
+// CHECK-NEXT:   Succs (1): B10
+
+// CHECK:       [B1]
+// CHECK-NEXT:   1: ForStmt (LoopExit)
+// CHECK-NEXT:   2: return;
+// CHECK-NEXT:   Preds (1): B9
+// CHECK-NEXT:   Succs (1): B0
+
+// CHECK:       [B2]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B2.1]++
+// CHECK-NEXT:   Preds (1): B3
+// CHECK-NEXT:   Succs (1): B9
+
+// CHECK:       [B3]
+// CHECK-NEXT:   1: WhileStmt (LoopExit)
+// CHECK-NEXT:   Preds (1): B7
+// CHECK-NEXT:   Succs (1): B2
+
+// CHECK:       [B4]
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B7
+
+// CHECK:       [B5]
+// CHECK-NEXT:   1: WhileStmt (LoopExit)
+// CHECK-NEXT:   2: ForStmt (LoopExit)
+// CHECK-NEXT:   T: goto lab;
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B10
+
+// CHECK:       [B6]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B6.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 2
+// CHECK-NEXT:   4: [B6.2] % [B6.3]
+// CHECK-NEXT:   5: [B6.4] (ImplicitCastExpr, IntegralToBoolean, _Bool)
+// CHECK-NEXT:   T: if [B6.5]
+// CHECK-NEXT:   Preds (1): B7
+// CHECK-NEXT:   Succs (2): B5 B4
+
+// CHECK:       [B7]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B7.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 12
+// CHECK-NEXT:   4: [B7.2] < [B7.3]
+// CHECK-NEXT:   T: while [B7.4]
+// CHECK-NEXT:   Preds (2): B4 B8
+// CHECK-NEXT:   Succs (2): B6 B3
+
+// CHECK:       [B8]
+// CHECK-NEXT:   1: 1
+// CHECK-NEXT:   2: int j = 1;
+// CHECK-NEXT:   Preds (1): B9
+// CHECK-NEXT:   Succs (1): B7
+
+// CHECK:       [B9]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B9.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 10
+// CHECK-NEXT:   4: [B9.2] < [B9.3]
+// CHECK-NEXT:   T: for (...; [B9.4]; ...)
+// CHECK-NEXT:   Preds (2): B2 B10
+// CHECK-NEXT:   Succs (2): B8 B1
+
+// CHECK:       [B10]
+// CHECK-NEXT:   lab:
+// CHECK-NEXT:   1: 0
+// CHECK-NEXT:   2: int i = 0;
+// CHECK-NEXT:   Preds (2): B5 B11
+// CHECK-NEXT:   Succs (1): B9
+
+// CHECK:       [B0 (EXIT)]
+// CHECK-NEXT:   Preds (1): B1
+void check_goto() {
+lab:
+  for (int i = 0; i < 10; i++) {
+    int j = 1;
+    while (j < 12) {
+      if (j % 2)
+        goto lab;
+    }
+  }
+  return;
+}
+
+// CHECK:       [B11 (ENTRY)]
+// CHECK-NEXT:   Succs (1): B10
+
+// CHECK:       [B1]
+// CHECK-NEXT:   1: ForStmt (LoopExit)
+// CHECK-NEXT:   2: return;
+// CHECK-NEXT:   Preds (1): B9
+// CHECK-NEXT:   Succs (1): B0
+
+// CHECK:       [B2]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B2.1]++
+// CHECK-NEXT:   Preds (1): B3
+// CHECK-NEXT:   Succs (1): B9
+
+// CHECK:       [B3]
+// CHECK-NEXT:   1: WhileStmt (LoopExit)
+// CHECK-NEXT:   Preds (1): B7
+// CHECK-NEXT:   Succs (1): B2
+
+// CHECK:       [B4]
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B7
+
+// CHECK:       [B5]
+// CHECK-NEXT:   1: WhileStmt (LoopExit)
+// CHECK-NEXT:   T: goto lab;
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B8
+
+// CHECK:       [B6]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B6.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 2
+// CHECK-NEXT:   4: [B6.2] % [B6.3]
+// CHECK-NEXT:   5: [B6.4] (ImplicitCastExpr, IntegralToBoolean, _Bool)
+// CHECK-NEXT:   T: if [B6.5]
+// CHECK-NEXT:   Preds (1): B7
+// CHECK-NEXT:   Succs (2): B5 B4
+
+// CHECK:       [B7]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B7.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 12
+// CHECK-NEXT:   4: [B7.2] < [B7.3]
+// CHECK-NEXT:   T: while [B7.4]
+// CHECK-NEXT:   Preds (2): B4 B8
+// CHECK-NEXT:   Succs (2): B6 B3
+
+// CHECK:       [B8]
+// CHECK-NEXT:   lab:
+// CHECK-NEXT:   1: 1
+// CHECK-NEXT:   2: int j = 1;
+// CHECK-NEXT:   Preds (2): B9 B5
+// CHECK-NEXT:   Succs (1): B7
+
+// CHECK:       [B9]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B9.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 10
+// CHECK-NEXT:   4: [B9.2] < [B9.3]
+// CHECK-NEXT:   T: for (...; [B9.4]; ...)
+// CHECK-NEXT:   Preds (2): B2 B10
+// CHECK-NEXT:   Succs (2): B8 B1
+
+// CHECK:       [B10]
+// CHECK-NEXT:   1: 0
+// CHECK-NEXT:   2: int i = 0;
+// CHECK-NEXT:   Preds (1): B11
+// CHECK-NEXT:   Succs (1): B9
+
+// CHECK:       [B0 (EXIT)]
+// CHECK-NEXT:   Preds (1): B1
+
+void check_goto2() {
+  for (int i = 0; i < 10; i++) {
+  lab:
+    int j = 1;
+    while (j < 12) {
+      if (j % 2)
+        goto lab;
+    }
+  }
+  return;
+}
+
+// CHECK:       [B10 (ENTRY)]
+// CHECK-NEXT:   Succs (1): B9
+
+// CHECK:       [B1]
+// CHECK-NEXT:   1: ForStmt (LoopExit)
+// CHECK-NEXT:   2: return;
+// CHECK-NEXT:   Preds (1): B8
+// CHECK-NEXT:   Succs (1): B0
+
+// CHECK:       [B2]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B2.1]++
+// CHECK-NEXT:   Preds (1): B3
+// CHECK-NEXT:   Succs (1): B8
+
+// CHECK:       [B3]
+// CHECK-NEXT:   1: WhileStmt (LoopExit)
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B2
+
+// CHECK:       [B4]
+// CHECK-NEXT:   Preds (1): B5
+// CHECK-NEXT:   Succs (1): B6
+
+// CHECK:       [B5]
+// CHECK-NEXT:   lab:
+// CHECK-NEXT:   1: 2
+// CHECK-NEXT:   2: j
+// CHECK-NEXT:   3: [B5.2] = [B5.1]
+// CHECK-NEXT:   Preds (2): B6 B7
+// CHECK-NEXT:   Succs (1): B4
+
+// CHECK:       [B6]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B6.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 12
+// CHECK-NEXT:   4: [B6.2] < [B6.3]
+// CHECK-NEXT:   T: while [B6.4]
+// CHECK-NEXT:   Preds (1): B4
+// CHECK-NEXT:   Succs (2): B5 B3
+
+// CHECK:       [B7]
+// CHECK-NEXT:   1: 1
+// CHECK-NEXT:   2: int j = 1;
+// CHECK-NEXT:   T: goto lab;
+// CHECK-NEXT:   Preds (1): B8
+// CHECK-NEXT:   Succs (1): B5
+
+// CHECK:       [B8]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B8.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 10
+// CHECK-NEXT:   4: [B8.2] < [B8.3]
+// CHECK-NEXT:   T: for (...; [B8.4]; ...)
+// CHECK-NEXT:   Preds (2): B2 B9
+// CHECK-NEXT:   Succs (2): B7 B1
+
+// CHECK:       [B9]
+// CHECK-NEXT:   1: 0
+// CHECK-NEXT:   2: int i = 0;
+// CHECK-NEXT:   Preds (1): B10
+// CHECK-NEXT:   Succs (1): B8
+
+// CHECK:       [B0 (EXIT)]
+// CHECK-NEXT:   Preds (1): B1
+void check_goto3() {
+  for (int i = 0; i < 10; i++) {
+    int j = 1;
+    goto lab;
+    while (j < 12) {
+    lab:
+      j = 2;
+    }
+  }
+  return;
+}
+
+// CHECK:       [B11 (ENTRY)]
+// CHECK-NEXT:   Succs (1): B10
+
+// CHECK:       [B1]
+// CHECK-NEXT:   1: ForStmt (LoopExit)
+// CHECK-NEXT:   2: 1
+// CHECK-NEXT:   3: return [B1.2];
+// CHECK-NEXT:   Preds (1): B9
+// CHECK-NEXT:   Succs (1): B0
+
+// CHECK:       [B2]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B2.1]++
+// CHECK-NEXT:   Preds (1): B3
+// CHECK-NEXT:   Succs (1): B9
+
+// CHECK:       [B3]
+// CHECK-NEXT:   1: WhileStmt (LoopExit)
+// CHECK-NEXT:   Preds (1): B7
+// CHECK-NEXT:   Succs (1): B2
+
+// CHECK:       [B4]
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B7
+
+// CHECK:       [B5]
+// CHECK-NEXT:   1: WhileStmt (LoopExit)
+// CHECK-NEXT:   2: ForStmt (LoopExit)
+// CHECK-NEXT:   3: 0
+// CHECK-NEXT:   4: return [B5.3];
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B0
+
+// CHECK:       [B6]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B6.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 2
+// CHECK-NEXT:   4: [B6.2] % [B6.3]
+// CHECK-NEXT:   5: [B6.4] (ImplicitCastExpr, IntegralToBoolean, _Bool)
+// CHECK-NEXT:   T: if [B6.5]
+// CHECK-NEXT:   Preds (1): B7
+// CHECK-NEXT:   Succs (2): B5 B4
+
+// CHECK:       [B7]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B7.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 12
+// CHECK-NEXT:   4: [B7.2] < [B7.3]
+// CHECK-NEXT:   T: while [B7.4]
+// CHECK-NEXT:   Preds (2): B4 B8
+// CHECK-NEXT:   Succs (2): B6 B3
+
+// CHECK:       [B8]
+// CHECK-NEXT:   1: 1
+// CHECK-NEXT:   2: int j = 1;
+// CHECK-NEXT:   Preds (1): B9
+// CHECK-NEXT:   Succs (1): B7
+
+// CHECK:       [B9]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B9.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 10
+// CHECK-NEXT:   4: [B9.2] < [B9.3]
+// CHECK-NEXT:   T: for (...; [B9.4]; ...)
+// CHECK-NEXT:   Preds (2): B2 B10
+// CHECK-NEXT:   Succs (2): B8 B1
+
+// CHECK:       [B10]
+// CHECK-NEXT:   1: 0
+// CHECK-NEXT:   2: int i = 0;
+// CHECK-NEXT:   Preds (1): B11
+// CHECK-NEXT:   Succs (1): B9
+
+// CHECK:       [B0 (EXIT)]
+// CHECK-NEXT:   Preds (2): B1 B5
+int check_return() {
+  for (int i = 0; i < 10; i++) {
+    int j = 1;
+    while (j < 12) {
+      if (j % 2)
+        return 0;
+    }
+  }
+  return 1;
+}
+
+// CHECK:       [B10 (ENTRY)]
+// CHECK-NEXT:   Succs (1): B9
+
+// CHECK:       [B1]
+// CHECK-NEXT:   1: ForStmt (LoopExit)
+// CHECK-NEXT:   2: 1
+// CHECK-NEXT:   3: return [B1.2];
+// CHECK-NEXT:   Preds (1): B8
+// CHECK-NEXT:   Succs (1): B0
+
+// CHECK:       [B2]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B2.1]++
+// CHECK-NEXT:   Succs (1): B8
+
+// CHECK:       [B3]
+// CHECK-NEXT:   1: WhileStmt (LoopExit)
+// CHECK-NEXT:   2: ForStmt (LoopExit)
+// CHECK-NEXT:   3: 0
+// CHECK-NEXT:   4: return [B3.3];
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B0
+
+// CHECK:       [B4]
+// CHECK-NEXT:   Preds (1): B5
+// CHECK-NEXT:   Succs (1): B6
+
+// CHECK:       [B5]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B5.1]++
+// CHECK-NEXT:   Preds (1): B6
+// CHECK-NEXT:   Succs (1): B4
+
+// CHECK:       [B6]
+// CHECK-NEXT:   1: j
+// CHECK-NEXT:   2: [B6.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 12
+// CHECK-NEXT:   4: [B6.2] < [B6.3]
+// CHECK-NEXT:   T: while [B6.4]
+// CHECK-NEXT:   Preds (2): B4 B7
+// CHECK-NEXT:   Succs (2): B5 B3
+
+// CHECK:       [B7]
+// CHECK-NEXT:   1: 1
+// CHECK-NEXT:   2: int j = 1;
+// CHECK-NEXT:   Preds (1): B8
+// CHECK-NEXT:   Succs (1): B6
+
+// CHECK:       [B8]
+// CHECK-NEXT:   1: i
+// CHECK-NEXT:   2: [B8.1] (ImplicitCastExpr, LValueToRValue, int)
+// CHECK-NEXT:   3: 10
+// CHECK-NEXT:   4: [B8.2] < [B8.3]
+// CHECK-NEXT:   T: for (...; [B8.4]; ...)
+// CHECK-NEXT:   Preds (2): B2 B9
+// CHECK-NEXT:   Succs (2): B7 B1
+
+// CHECK:       [B9]
+// CHECK-NEXT:   1: 0
+// CHECK-NEXT:   2: int i = 0;
+// CHECK-NEXT:   Preds (1): B10
+// CHECK-NEXT:   Succs (1): B8
+
+// CHECK:       [B0 (EXIT)]
+// CHECK-NEXT:   Preds (2): B1 B3
+int check_return2() {
+  for (int i = 0; i < 10; i++) {
+    int j = 1;
+    while (j < 12)
+      j++;
+    return 0;
+  }
+  return 1;
+}
 No newline at end of file