Rework CFG edges to encode potentially unreachable edges, instead of just making them NULL.

This is to support some analyses, like -Wunreachable-code, that
will need to recover the original unprunned CFG edges in order
to suppress issues that aren't really bugs in practice.

There are two important changes here:

- AdjacentBlock replaces CFGBlock* for CFG successors/predecessors.
  This has the size of 2 pointers, instead of 1.  This is unlikely
  to have a significant memory impact on Sema since a single
  CFG usually exists at one time, but could impact the memory
  usage of the static analyzer.  This could possibly be optimized
  down to a single pointer with some cleverness.

- Predecessors can now contain null predecessors, which means
  some analyses doing a reverse traversal will need to take into
  account.  This already exists for successors, which contain
  successor slots for specific branch kinds (e.g., 'if') that
  expect a fixed number of successors, even if a branch is
  not reachable.

llvm-svn: 202325

clang/include/clang/Analysis/CFG.h

@@ -412,9 +412,64 @@ class CFGBlock {
   ///   of the CFG.
   unsigned BlockID;
 
+public:
+  /// This class represents a potential adjacent block in the CFG.  It encodes
+  /// whether or not the block is actually reachable, or can be proved to be
+  /// trivially unreachable.  For some cases it allows one to encode scenarios
+  /// where a block was substituted because the original (now alternate) block
+  /// is unreachable.
+  class AdjacentBlock {
+    enum Kind {
+      AB_Normal,
+      AB_Unreachable,
+      AB_Alternate
+    };
+
+    CFGBlock *ReachableBlock;
+    llvm::PointerIntPair<CFGBlock*, 2> UnreachableBlock;
+
+  public:
+    /// Construct an AdjacentBlock with a possibly unreachable block.
+    AdjacentBlock(CFGBlock *B, bool IsReachable);
+    
+    /// Construct an AdjacentBlock with a reachable block and an alternate
+    /// unreachable block.
+    AdjacentBlock(CFGBlock *B, CFGBlock *AlternateBlock);
+
+    /// Get the reachable block, if one exists.
+    CFGBlock *getReachableBlock() const {
+      return ReachableBlock;
+    }
+
+    /// Get the potentially unreachable block.
+    CFGBlock *getPossiblyUnreachableBlock() const {
+      return UnreachableBlock.getPointer();
+    }
+
+    /// Provide an implicit conversion to CFGBlock* so that
+    /// AdjacentBlock can be substituted for CFGBlock*.
+    operator CFGBlock*() const {
+      return getReachableBlock();
+    }
+
+    CFGBlock& operator *() const {
+      return *getReachableBlock();
+    }
+
+    CFGBlock* operator ->() const {
+      return getReachableBlock();
+    }
+
+    bool isReachable() const {
+      Kind K = (Kind) UnreachableBlock.getInt();
+      return K == AB_Normal || K == AB_Alternate;
+    }
+  };
+
+private:
   /// Predecessors/Successors - Keep track of the predecessor / successor
   /// CFG blocks.
-  typedef BumpVector<CFGBlock*> AdjacentBlocks;
+  typedef BumpVector<AdjacentBlock> AdjacentBlocks;
   AdjacentBlocks Preds;
   AdjacentBlocks Succs;
 
@@ -504,9 +559,11 @@ public:
   class FilterOptions {
   public:
     FilterOptions() {
+      IgnoreNullPredecessors = 1;
       IgnoreDefaultsWithCoveredEnums = 0;
     }
 
+    unsigned IgnoreNullPredecessors : 1;
     unsigned IgnoreDefaultsWithCoveredEnums : 1;
   };
 
@@ -588,11 +645,8 @@ public:
     OS << "BB#" << getBlockID();
   }
 
-  void addSuccessor(CFGBlock *Block, BumpVectorContext &C) {
-    if (Block)
-      Block->Preds.push_back(this, C);
-    Succs.push_back(Block, C);
-  }
+  /// Adds a (potentially unreachable) successor block to the current block.
+  void addSuccessor(AdjacentBlock Succ, BumpVectorContext &C);
 
   void appendStmt(Stmt *statement, BumpVectorContext &C) {
     Elements.push_back(CFGStmt(statement), C);

clang/lib/Analysis/CFG.cpp

@@ -483,8 +483,16 @@ private:
   void prependAutomaticObjDtorsWithTerminator(CFGBlock *Blk,
       LocalScope::const_iterator B, LocalScope::const_iterator E);
 
-  void addSuccessor(CFGBlock *B, CFGBlock *S) {
-    B->addSuccessor(S, cfg->getBumpVectorContext());
+  void addSuccessor(CFGBlock *B, CFGBlock *S, bool IsReachable = true) {
+    B->addSuccessor(CFGBlock::AdjacentBlock(S, IsReachable),
+                    cfg->getBumpVectorContext());
+  }
+
+  /// Add a reachable successor to a block, with the alternate variant that is
+  /// unreachable.
+  void addSuccessor(CFGBlock *B, CFGBlock *ReachableBlock, CFGBlock *AltBlock) {
+    B->addSuccessor(CFGBlock::AdjacentBlock(ReachableBlock, AltBlock),
+                    cfg->getBumpVectorContext());
   }
 
   /// Try and evaluate an expression to an integer constant.
@@ -3495,13 +3503,37 @@ bool CFGImplicitDtor::isNoReturn(ASTContext &astContext) const {
 }
 
 //===----------------------------------------------------------------------===//
-// Filtered walking of the CFG.
+// CFGBlock operations.
 //===----------------------------------------------------------------------===//
 
+CFGBlock::AdjacentBlock::AdjacentBlock(CFGBlock *B, bool IsReachable)
+  : ReachableBlock(IsReachable ? B : 0),
+    UnreachableBlock(!IsReachable ? B : 0,
+                     B && IsReachable ? AB_Normal : AB_Unreachable) {}
+
+CFGBlock::AdjacentBlock::AdjacentBlock(CFGBlock *B, CFGBlock *AlternateBlock)
+  : ReachableBlock(B),
+    UnreachableBlock(B == AlternateBlock ? 0 : AlternateBlock,
+                     B == AlternateBlock ? AB_Alternate : AB_Normal) {}
+
+void CFGBlock::addSuccessor(AdjacentBlock Succ,
+                            BumpVectorContext &C) {
+  if (CFGBlock *B = Succ.getReachableBlock())
+    B->Preds.push_back(CFGBlock::AdjacentBlock(this, Succ.isReachable()), C);
+
+  if (CFGBlock *UnreachableB = Succ.getPossiblyUnreachableBlock())
+    UnreachableB->Preds.push_back(CFGBlock::AdjacentBlock(this, false), C);
+
+  Succs.push_back(Succ, C);
+}
+
 bool CFGBlock::FilterEdge(const CFGBlock::FilterOptions &F,
         const CFGBlock *From, const CFGBlock *To) {
 
-  if (To && F.IgnoreDefaultsWithCoveredEnums) {
+  if (F.IgnoreNullPredecessors && !From)
+    return true;
+
+  if (To && From && F.IgnoreDefaultsWithCoveredEnums) {
     // If the 'To' has no label or is labeled but the label isn't a
     // CaseStmt then filter this edge.
     if (const SwitchStmt *S =
@@ -3963,7 +3995,16 @@ static void print_block(raw_ostream &OS, const CFG* cfg,
         if (i % 10 == 8)
           OS << "\n     ";
 
-        OS << " B" << (*I)->getBlockID();
+        CFGBlock *B = *I;
+        bool Reachable = true;
+        if (!B) {
+          Reachable = false;
+          B = I->getPossiblyUnreachableBlock();
+        }
+
+        OS << " B" << B->getBlockID();
+        if (!Reachable)
+          OS << "(Unreachable)";
       }
       
       if (ShowColors)

clang/lib/Analysis/CFGReachabilityAnalysis.cpp

@@ -69,7 +69,8 @@ void CFGReverseBlockReachabilityAnalysis::mapReachability(const CFGBlock *Dst) {
     // Add the predecessors to the worklist.
     for (CFGBlock::const_pred_iterator i = block->pred_begin(), 
          e = block->pred_end(); i != e; ++i) {
-      worklist.push_back(*i);
+      if (*i)
+        worklist.push_back(*i);
     }
   }
 }

clang/lib/Analysis/UninitializedValues.cpp

@@ -535,6 +535,9 @@ public:
       for (CFGBlock::const_pred_iterator I = B->pred_begin(), E = B->pred_end();
            I != E; ++I) {
         const CFGBlock *Pred = *I;
+        if (!Pred)
+          continue;
+        
         Value AtPredExit = vals.getValue(Pred, B, vd);
         if (AtPredExit == Initialized)
           // This block initializes the variable.
@@ -751,6 +754,8 @@ static bool runOnBlock(const CFGBlock *block, const CFG &cfg,
   for (CFGBlock::const_pred_iterator I = block->pred_begin(),
        E = block->pred_end(); I != E; ++I) {
     const CFGBlock *pred = *I;
+    if (!pred)
+      continue;
     if (wasAnalyzed[pred->getBlockID()]) {
       vals.mergeIntoScratch(vals.getValueVector(pred), isFirst);
       isFirst = false;