Reimplement isPotentiallyReachable to make nocapture deduction much stronger.

Adds unit tests for it too.

Split BasicBlockUtils into an analysis-half and a transforms-half, and put the
analysis bits into a new Analysis/CFG.{h,cpp}. Promote isPotentiallyReachable
into llvm::isPotentiallyReachable and move it into Analysis/CFG.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187283 91177308-0d34-0410-b5e6-96231b3b80d8

13 files changed, 666 insertions, 142 deletions

diff --git a/include/llvm/Analysis/CFG.h b/include/llvm/Analysis/CFG.h
new file mode 100644
index 0000000000..08fdc2689d
--- /dev/null
+++ b/include/llvm/Analysis/CFG.h
@@ -0,0 +1,69 @@
+//===-- Analysis/CFG.h - BasicBlock Analyses --------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This family of functions performs analyses on basic blocks, and instructions
+// contained within basic blocks.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ANALYSIS_CFG_H
+#define LLVM_ANALYSIS_CFG_H
+
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/Support/CFG.h"
+
+namespace llvm {
+
+class BasicBlock;
+class DominatorTree;
+class Function;
+class Instruction;
+class LoopInfo;
+class TerminatorInst;
+
+/// Analyze the specified function to find all of the loop backedges in the
+/// function and return them.  This is a relatively cheap (compared to
+/// computing dominators and loop info) analysis.
+///
+/// The output is added to Result, as pairs of <from,to> edge info.
+void FindFunctionBackedges(
+    const Function &F,
+    SmallVectorImpl<std::pair<const BasicBlock *, const BasicBlock *> > &
+        Result);
+
+/// Search for the specified successor of basic block BB and return its position
+/// in the terminator instruction's list of successors.  It is an error to call
+/// this with a block that is not a successor.
+unsigned GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ);
+
+/// Return true if the specified edge is a critical edge. Critical edges are
+/// edges from a block with multiple successors to a block with multiple
+/// predecessors.
+///
+bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
+                    bool AllowIdenticalEdges = false);
+
+/// Determine whether there is a path from From to To within a single function.
+/// Returns false only if we can prove that once 'From' has been executed then
+/// 'To' can not be executed. Conservatively returns true.
+///
+/// This function is linear with respect to the number of blocks in the CFG,
+/// walking down successors from From to reach To, with a fixed threshold.
+/// Using DT or LI allows us to answer more quickly. LI reduces the cost of
+/// an entire loop of any number of blocsk to be the same as the cost of a
+/// single block. DT reduces the cost by allowing the search to terminate when
+/// we find a block that dominates the block containing 'To'. DT is most useful
+/// on branchy code but not loops, and LI is most useful on code with loops but
+/// does not help on branchy code outside loops.
+bool isPotentiallyReachable(const Instruction *From, const Instruction *To,
+                            DominatorTree *DT = 0, LoopInfo *LI = 0);
+
+} // End llvm namespace
+
+#endif
diff --git a/include/llvm/Transforms/Utils/BasicBlockUtils.h b/include/llvm/Transforms/Utils/BasicBlockUtils.h
index 8f1a6e2b75..b5478cb669 100644
--- a/include/llvm/Transforms/Utils/BasicBlockUtils.h
+++ b/include/llvm/Transforms/Utils/BasicBlockUtils.h
@@ -70,28 +70,6 @@ void ReplaceInstWithInst(BasicBlock::InstListType &BIL,
 //
 void ReplaceInstWithInst(Instruction *From, Instruction *To);
 
-/// FindFunctionBackedges - Analyze the specified function to find all of the
-/// loop backedges in the function and return them.  This is a relatively cheap
-/// (compared to computing dominators and loop info) analysis.
-///
-/// The output is added to Result, as pairs of <from,to> edge info.
-void FindFunctionBackedges(const Function &F,
-      SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result);
-
-
-/// GetSuccessorNumber - Search for the specified successor of basic block BB
-/// and return its position in the terminator instruction's list of
-/// successors.  It is an error to call this with a block that is not a
-/// successor.
-unsigned GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ);
-
-/// isCriticalEdge - Return true if the specified edge is a critical edge.
-/// Critical edges are edges from a block with multiple successors to a block
-/// with multiple predecessors.
-///
-bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
-                    bool AllowIdenticalEdges = false);
-
 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
 /// split the critical edge.  This will update DominatorTree and
 /// DominatorFrontier information if it is available, thus calling this pass
diff --git a/lib/Analysis/AliasAnalysis.cpp b/lib/Analysis/AliasAnalysis.cpp
index 5be5613b12..b8b6d37a79 100644
--- a/lib/Analysis/AliasAnalysis.cpp
+++ b/lib/Analysis/AliasAnalysis.cpp
@@ -26,6 +26,7 @@
 
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/BasicBlock.h"
@@ -361,26 +362,6 @@ AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
 }
 
 namespace {
-  /// Determine whether there is a path from From to To within a single
-  /// function. Returns false only if we can prove that once 'From' has been
-  /// executed then 'To' can not be executed. Conservatively returns true.
-  static bool isPotentiallyReachable(const BasicBlock *From,
-                                     const BasicBlock *To) {
-    const unsigned MaxCheck = 5;
-    const BasicBlock *Current = From;
-    for (unsigned I = 0; I < MaxCheck; I++) {
-      unsigned NumSuccs = Current->getTerminator()->getNumSuccessors();
-      if (NumSuccs > 1)
-        return true;
-      if (NumSuccs == 0)
-        return false;
-      Current = Current->getTerminator()->getSuccessor(0);
-      if (Current == To)
-        return true;
-    }
-    return true;
-  }
-
   /// Only find pointer captures which happen before the given instruction. Uses
   /// the dominator tree to determine whether one instruction is before another.
   /// Only support the case where the Value is defined in the same basic block
@@ -402,7 +383,7 @@ namespace {
       // there is no need to explore the use if BeforeHere dominates use.
       // Check whether there is a path from I to BeforeHere.
       if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
-          !isPotentiallyReachable(BB, BeforeHere->getParent()))
+          !isPotentiallyReachable(I, BeforeHere, DT))
         return false;
       return true;
     }
@@ -414,7 +395,7 @@ namespace {
       if (BeforeHere != I && !DT->isReachableFromEntry(BB))
         return false;
       if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
-          !isPotentiallyReachable(BB, BeforeHere->getParent()))
+          !isPotentiallyReachable(I, BeforeHere, DT))
         return false;
       Captured = true;
       return true;
diff --git a/lib/Analysis/CFG.cpp b/lib/Analysis/CFG.cpp
new file mode 100644
index 0000000000..a5ed21afbf
--- /dev/null
+++ b/lib/Analysis/CFG.cpp
@@ -0,0 +1,227 @@
+//===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This family of functions performs analyses on basic blocks, and instructions
+// contained within basic blocks.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/CFG.h"
+
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/LoopInfo.h"
+
+using namespace llvm;
+
+/// FindFunctionBackedges - Analyze the specified function to find all of the
+/// loop backedges in the function and return them.  This is a relatively cheap
+/// (compared to computing dominators and loop info) analysis.
+///
+/// The output is added to Result, as pairs of <from,to> edge info.
+void llvm::FindFunctionBackedges(const Function &F,
+     SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
+  const BasicBlock *BB = &F.getEntryBlock();
+  if (succ_begin(BB) == succ_end(BB))
+    return;
+
+  SmallPtrSet<const BasicBlock*, 8> Visited;
+  SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
+  SmallPtrSet<const BasicBlock*, 8> InStack;
+
+  Visited.insert(BB);
+  VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
+  InStack.insert(BB);
+  do {
+    std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
+    const BasicBlock *ParentBB = Top.first;
+    succ_const_iterator &I = Top.second;
+
+    bool FoundNew = false;
+    while (I != succ_end(ParentBB)) {
+      BB = *I++;
+      if (Visited.insert(BB)) {
+        FoundNew = true;
+        break;
+      }
+      // Successor is in VisitStack, it's a back edge.
+      if (InStack.count(BB))
+        Result.push_back(std::make_pair(ParentBB, BB));
+    }
+
+    if (FoundNew) {
+      // Go down one level if there is a unvisited successor.
+      InStack.insert(BB);
+      VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
+    } else {
+      // Go up one level.
+      InStack.erase(VisitStack.pop_back_val().first);
+    }
+  } while (!VisitStack.empty());
+}
+
+/// GetSuccessorNumber - Search for the specified successor of basic block BB
+/// and return its position in the terminator instruction's list of
+/// successors.  It is an error to call this with a block that is not a
+/// successor.
+unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
+  TerminatorInst *Term = BB->getTerminator();
+#ifndef NDEBUG
+  unsigned e = Term->getNumSuccessors();
+#endif
+  for (unsigned i = 0; ; ++i) {
+    assert(i != e && "Didn't find edge?");
+    if (Term->getSuccessor(i) == Succ)
+      return i;
+  }
+}
+
+/// isCriticalEdge - Return true if the specified edge is a critical edge.
+/// Critical edges are edges from a block with multiple successors to a block
+/// with multiple predecessors.
+bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
+                          bool AllowIdenticalEdges) {
+  assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
+  if (TI->getNumSuccessors() == 1) return false;
+
+  const BasicBlock *Dest = TI->getSuccessor(SuccNum);
+  const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
+
+  // If there is more than one predecessor, this is a critical edge...
+  assert(I != E && "No preds, but we have an edge to the block?");
+  const BasicBlock *FirstPred = *I;
+  ++I;        // Skip one edge due to the incoming arc from TI.
+  if (!AllowIdenticalEdges)
+    return I != E;
+
+  // If AllowIdenticalEdges is true, then we allow this edge to be considered
+  // non-critical iff all preds come from TI's block.
+  while (I != E) {
+    const BasicBlock *P = *I;
+    if (P != FirstPred)
+      return true;
+    // Note: leave this as is until no one ever compiles with either gcc 4.0.1
+    // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
+    E = pred_end(P);
+    ++I;
+  }
+  return false;
+}
+
+// LoopInfo contains a mapping from basic block to the innermost loop. Find
+// the outermost loop in the loop nest that contains BB.
+static const Loop *getOutermostLoop(LoopInfo *LI, const BasicBlock *BB) {
+  const Loop *L = LI->getLoopFor(BB);
+  if (L) {
+    while (const Loop *Parent = L->getParentLoop())
+      L = Parent;
+  }
+  return L;
+}
+
+// True if there is a loop which contains both BB1 and BB2.
+static bool loopContainsBoth(LoopInfo *LI,
+                             const BasicBlock *BB1, const BasicBlock *BB2) {
+  const Loop *L1 = getOutermostLoop(LI, BB1);
+  const Loop *L2 = getOutermostLoop(LI, BB2);
+  return L1 != NULL && L1 == L2;
+}
+
+static bool isPotentiallyReachableSameBlock(const Instruction *A,
+                                            const Instruction *B,
+                                            LoopInfo *LI) {
+  // The same block case is special because it's the only time we're looking
+  // within a single block to see which comes first. Once we start looking at
+  // multiple blocks, the first instruction of the block is reachable, so we
+  // only need to determine reachability between whole blocks.
+
+  const BasicBlock *BB = A->getParent();
+  // If the block is in a loop then we can reach any instruction in the block
+  // from any other instruction in the block by going around the backedge.
+  // Check whether we're in a loop (or aren't sure).
+
+  // Can't be in a loop if it's the entry block -- the entry block may not
+  // have predecessors.
+  bool HasLoop = BB != &BB->getParent()->getEntryBlock();
+
+  // Can't be in a loop if LoopInfo doesn't know about it.
+  if (LI && HasLoop) {
+    HasLoop = LI->getLoopFor(BB) != 0;
+  }
+  if (HasLoop)
+    return true;
+
+  // Linear scan, start at 'A', see whether we hit 'B' or the end first.
+  for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
+    if (&*I == B)
+      return true;
+  }
+  return false;
+}
+
+bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
+                                  DominatorTree *DT, LoopInfo *LI) {
+  assert(A->getParent()->getParent() == B->getParent()->getParent() &&
+         "This analysis is function-local!");
+
+  const BasicBlock *StopBB = B->getParent();
+
+  if (A->getParent() == B->getParent())
+    return isPotentiallyReachableSameBlock(A, B, LI);
+
+  if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
+    return true;
+  if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
+    return false;
+
+  // When the stop block is unreachable, it's dominated from everywhere,
+  // regardless of whether there's a path between the two blocks.
+  if (DT && !DT->isReachableFromEntry(StopBB))
+    DT = 0;
+
+  // Limit the number of blocks we visit. The goal is to avoid run-away compile
+  // times on large CFGs without hampering sensible code. Arbitrarily chosen.
+  unsigned Limit = 32;
+
+  SmallSet<const BasicBlock*, 64> Visited;
+  SmallVector<BasicBlock*, 32> Worklist;
+  Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
+
+  do {
+    BasicBlock *BB = Worklist.pop_back_val();
+    if (!Visited.insert(BB))
+      continue;
+    if (BB == StopBB)
+      return true;
+    if (DT && DT->dominates(BB, StopBB))
+      return true;
+    if (LI && loopContainsBoth(LI, BB, StopBB))
+      return true;
+
+    if (!--Limit) {
+      // We haven't been able to prove it one way or the other. Conservatively
+      // answer true -- that there is potentially a path.
+      return true;
+    }
+
+    if (const Loop *Outer = LI ? getOutermostLoop(LI, BB) : 0) {
+      // All blocks in a single loop are reachable from all other blocks. From
+      // any of these blocks, we can skip directly to the exits of the loop,
+      // ignoring any other blocks inside the loop body.
+      Outer->getExitBlocks(Worklist);
+    } else {
+      for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
+        Worklist.push_back(*I);
+    }
+  } while (!Worklist.empty());
+
+  // We have exhaustived all possible paths and are certain that 'To' can not
+  // be reached from 'From'.
+  return false;
+}
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index 98879649c8..01da51c5f2 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -19,6 +19,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/BranchProbabilityInfo.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/CodeGen/FastISel.h"
 #include "llvm/CodeGen/FunctionLoweringInfo.h"
diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index dad3147aa4..bc418af142 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -23,6 +23,7 @@
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/InstructionSimplify.h"
diff --git a/lib/Transforms/Scalar/JumpThreading.cpp b/lib/Transforms/Scalar/JumpThreading.cpp
index b61c5ba56e..2d961ee630 100644
--- a/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/lib/Transforms/Scalar/JumpThreading.cpp
@@ -19,6 +19,7 @@
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/LazyValueInfo.h"
@@ -1614,5 +1615,3 @@ bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
   ++NumDupes;
   return true;
 }
-
-
diff --git a/lib/Transforms/Utils/BasicBlockUtils.cpp b/lib/Transforms/Utils/BasicBlockUtils.cpp
index 9167795654..28c08d5861 100644
--- a/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -14,6 +14,7 @@
 
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
@@ -235,22 +236,6 @@ void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
   ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
 }
 
-/// GetSuccessorNumber - Search for the specified successor of basic block BB
-/// and return its position in the terminator instruction's list of
-/// successors.  It is an error to call this with a block that is not a
-/// successor.
-unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) {
-  TerminatorInst *Term = BB->getTerminator();
-#ifndef NDEBUG
-  unsigned e = Term->getNumSuccessors();
-#endif
-  for (unsigned i = 0; ; ++i) {
-    assert(i != e && "Didn't find edge?");
-    if (Term->getSuccessor(i) == Succ)
-      return i;
-  }
-}
-
 /// SplitEdge -  Split the edge connecting specified block. Pass P must
 /// not be NULL.
 BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) {
@@ -598,52 +583,6 @@ void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
   }
 }
 
-/// FindFunctionBackedges - Analyze the specified function to find all of the
-/// loop backedges in the function and return them.  This is a relatively cheap
-/// (compared to computing dominators and loop info) analysis.
-///
-/// The output is added to Result, as pairs of <from,to> edge info.
-void llvm::FindFunctionBackedges(const Function &F,
-     SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
-  const BasicBlock *BB = &F.getEntryBlock();
-  if (succ_begin(BB) == succ_end(BB))
-    return;
-
-  SmallPtrSet<const BasicBlock*, 8> Visited;
-  SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
-  SmallPtrSet<const BasicBlock*, 8> InStack;
-
-  Visited.insert(BB);
-  VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
-  InStack.insert(BB);
-  do {
-    std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
-    const BasicBlock *ParentBB = Top.first;
-    succ_const_iterator &I = Top.second;
-
-    bool FoundNew = false;
-    while (I != succ_end(ParentBB)) {
-      BB = *I++;
-      if (Visited.insert(BB)) {
-        FoundNew = true;
-        break;
-      }
-      // Successor is in VisitStack, it's a back edge.
-      if (InStack.count(BB))
-        Result.push_back(std::make_pair(ParentBB, BB));
-    }
-
-    if (FoundNew) {
-      // Go down one level if there is a unvisited successor.
-      InStack.insert(BB);
-      VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
-    } else {
-      // Go up one level.
-      InStack.erase(VisitStack.pop_back_val().first);
-    }
-  } while (!VisitStack.empty());
-}
-
 /// FoldReturnIntoUncondBranch - This method duplicates the specified return
 /// instruction into a predecessor which ends in an unconditional branch. If
 /// the return instruction returns a value defined by a PHI, propagate the
diff --git a/lib/Transforms/Utils/BreakCriticalEdges.cpp b/lib/Transforms/Utils/BreakCriticalEdges.cpp
index 8513772da2..8f3ff96d7e 100644
--- a/lib/Transforms/Utils/BreakCriticalEdges.cpp
+++ b/lib/Transforms/Utils/BreakCriticalEdges.cpp
@@ -19,6 +19,7 @@
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/ProfileInfo.h"
@@ -84,39 +85,6 @@ bool BreakCriticalEdges::runOnFunction(Function &F) {
 //    Implementation of the external critical edge manipulation functions
 //===----------------------------------------------------------------------===//
 
-// isCriticalEdge - Return true if the specified edge is a critical edge.
-// Critical edges are edges from a block with multiple successors to a block
-// with multiple predecessors.
-//
-bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
-                          bool AllowIdenticalEdges) {
-  assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
-  if (TI->getNumSuccessors() == 1) return false;
-
-  const BasicBlock *Dest = TI->getSuccessor(SuccNum);
-  const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);
-
-  // If there is more than one predecessor, this is a critical edge...
-  assert(I != E && "No preds, but we have an edge to the block?");
-  const BasicBlock *FirstPred = *I;
-  ++I;        // Skip one edge due to the incoming arc from TI.
-  if (!AllowIdenticalEdges)
-    return I != E;
-
-  // If AllowIdenticalEdges is true, then we allow this edge to be considered
-  // non-critical iff all preds come from TI's block.
-  while (I != E) {
-    const BasicBlock *P = *I;
-    if (P != FirstPred)
-      return true;
-    // Note: leave this as is until no one ever compiles with either gcc 4.0.1
-    // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
-    E = pred_end(P);
-    ++I;
-  }
-  return false;
-}
-
 /// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
 /// may require new PHIs in the new exit block. This function inserts the
 /// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB
diff --git a/lib/Transforms/Utils/DemoteRegToStack.cpp b/lib/Transforms/Utils/DemoteRegToStack.cpp
index db525cdc24..0723b3534c 100644
--- a/lib/Transforms/Utils/DemoteRegToStack.cpp
+++ b/lib/Transforms/Utils/DemoteRegToStack.cpp
@@ -10,6 +10,7 @@
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Type.h"
diff --git a/unittests/Analysis/CFGTest.cpp b/unittests/Analysis/CFGTest.cpp
new file mode 100644
index 0000000000..2358803778
--- /dev/null
+++ b/unittests/Analysis/CFGTest.cpp
@@ -0,0 +1,359 @@
+//===- CFGTest.cpp - CFG tests --------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/CFG.h"
+#include "llvm/ADT/OwningPtr.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Assembly/Parser.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/InstIterator.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/Pass.h"
+#include "llvm/PassManager.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+// This fixture assists in running the isPotentiallyReachable utility four ways
+// and ensuring it produces the correct answer each time.
+class IsPotentiallyReachableTest : public testing::Test {
+protected:
+  void ParseAssembly(const char *Assembly) {
+    M.reset(new Module("Module", getGlobalContext()));
+
+    SMDiagnostic Error;
+    bool Parsed = ParseAssemblyString(Assembly, M.get(),
+                                      Error, M->getContext()) == M.get();
+
+    std::string errMsg;
+    raw_string_ostream os(errMsg);
+    Error.print("", os);
+
+    if (!Parsed) {
+      // A failure here means that the test itself is buggy.
+      report_fatal_error(os.str().c_str());
+    }
+
+    Function *F = M->getFunction("test");
+    if (F == NULL)
+      report_fatal_error("Test must have a function named @test");
+
+    A = B = NULL;
+    for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) {
+      if (I->hasName()) {
+        if (I->getName() == "A")
+          A = &*I;
+        else if (I->getName() == "B")
+          B = &*I;
+      }
+    }
+    if (A == NULL)
+      report_fatal_error("@test must have an instruction %A");
+    if (B == NULL)
+      report_fatal_error("@test must have an instruction %B");
+  }
+
+  void ExpectPath(bool ExpectedResult) {
+    static char ID;
+    class IsPotentiallyReachableTestPass : public FunctionPass {
+     public:
+      IsPotentiallyReachableTestPass(bool ExpectedResult,
+                                     Instruction *A, Instruction *B)
+          : FunctionPass(ID), ExpectedResult(ExpectedResult), A(A), B(B) {}
+
+      static int initialize() {
+        PassInfo *PI = new PassInfo("isPotentiallyReachable testing pass",
+                                    "", &ID, 0, true, true);
+        PassRegistry::getPassRegistry()->registerPass(*PI, false);
+        initializeLoopInfoPass(*PassRegistry::getPassRegistry());
+        initializeDominatorTreePass(*PassRegistry::getPassRegistry());
+        return 0;
+      }
+
+      void getAnalysisUsage(AnalysisUsage &AU) const {
+        AU.setPreservesAll();
+        AU.addRequired<LoopInfo>();
+        AU.addRequired<DominatorTree>();
+      }
+
+      bool runOnFunction(Function &F) {
+        if (!F.hasName() || F.getName() != "test")
+          return false;
+
+        LoopInfo *LI = &getAnalysis<LoopInfo>();
+        DominatorTree *DT = &getAnalysis<DominatorTree>();
+        EXPECT_EQ(isPotentiallyReachable(A, B, 0, 0), ExpectedResult);
+        EXPECT_EQ(isPotentiallyReachable(A, B, DT, 0), ExpectedResult);
+        EXPECT_EQ(isPotentiallyReachable(A, B, 0, LI), ExpectedResult);
+        EXPECT_EQ(isPotentiallyReachable(A, B, DT, LI), ExpectedResult);
+        return false;
+      }
+      bool ExpectedResult;
+      Instruction *A, *B;
+    };
+
+    static int initialize = IsPotentiallyReachableTestPass::initialize();
+    (void)initialize;
+
+    IsPotentiallyReachableTestPass *P =
+        new IsPotentiallyReachableTestPass(ExpectedResult, A, B);
+    PassManager PM;
+    PM.add(P);
+    PM.run(*M);
+  }
+private:
+  OwningPtr<Module> M;
+  Instruction *A, *B;
+};
+
+}
+
+TEST_F(IsPotentiallyReachableTest, SameBlockNoPath) {
+  ParseAssembly(
+      "define void @test() {\n"
+      "entry:\n"
+      "  bitcast i8 undef to i8\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  bitcast i8 undef to i8\n"
+      "  bitcast i8 undef to i8\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  ret void\n"
+      "}\n");
+  ExpectPath(false);
+}
+
+TEST_F(IsPotentiallyReachableTest, SameBlockPath) {
+  ParseAssembly(
+      "define void @test() {\n"
+      "entry:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  bitcast i8 undef to i8\n"
+      "  bitcast i8 undef to i8\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  ret void\n"
+      "}\n");
+  ExpectPath(true);
+}
+
+TEST_F(IsPotentiallyReachableTest, StraightNoPath) {
+  ParseAssembly(
+      "define void @test() {\n"
+      "entry:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  br label %exit\n"
+      "exit:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  ret void\n"
+      "}");
+  ExpectPath(false);
+}
+
+TEST_F(IsPotentiallyReachableTest, StraightPath) {
+  ParseAssembly(
+      "define void @test() {\n"
+      "entry:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  br label %exit\n"
+      "exit:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  ret void\n"
+      "}");
+  ExpectPath(true);
+}
+
+TEST_F(IsPotentiallyReachableTest, DestUnreachable) {
+  ParseAssembly(
+      "define void @test() {\n"
+      "entry:\n"
+      "  br label %midblock\n"
+      "midblock:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  ret void\n"
+      "unreachable:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  br label %midblock\n"
+      "}");
+  ExpectPath(false);
+}
+
+TEST_F(IsPotentiallyReachableTest, BranchToReturn) {
+  ParseAssembly(
+      "define void @test(i1 %x) {\n"
+      "entry:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  br i1 %x, label %block1, label %block2\n"
+      "block1:\n"
+      "  ret void\n"
+      "block2:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  ret void\n"
+      "}");
+  ExpectPath(true);
+}
+
+TEST_F(IsPotentiallyReachableTest, SimpleLoop1) {
+  ParseAssembly(
+      "declare i1 @switch()\n"
+      "\n"
+      "define void @test() {\n"
+      "entry:\n"
+      "  br label %loop\n"
+      "loop:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  %x = call i1 @switch()\n"
+      "  br i1 %x, label %loop, label %exit\n"
+      "exit:\n"
+      "  ret void\n"
+      "}");
+  ExpectPath(true);
+}
+
+TEST_F(IsPotentiallyReachableTest, SimpleLoop2) {
+  ParseAssembly(
+      "declare i1 @switch()\n"
+      "\n"
+      "define void @test() {\n"
+      "entry:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  br label %loop\n"
+      "loop:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  %x = call i1 @switch()\n"
+      "  br i1 %x, label %loop, label %exit\n"
+      "exit:\n"
+      "  ret void\n"
+      "}");
+  ExpectPath(false);
+}
+
+TEST_F(IsPotentiallyReachableTest, SimpleLoop3) {
+  ParseAssembly(
+      "declare i1 @switch()\n"
+      "\n"
+      "define void @test() {\n"
+      "entry:\n"
+      "  br label %loop\n"
+      "loop:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  %x = call i1 @switch()\n"
+      "  br i1 %x, label %loop, label %exit\n"
+      "exit:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  ret void\n"
+      "}");
+  ExpectPath(false);
+}
+
+
+TEST_F(IsPotentiallyReachableTest, OneLoopAfterTheOther1) {
+  ParseAssembly(
+      "declare i1 @switch()\n"
+      "\n"
+      "define void @test() {\n"
+      "entry:\n"
+      "  br label %loop1\n"
+      "loop1:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  %x = call i1 @switch()\n"
+      "  br i1 %x, label %loop1, label %loop1exit\n"
+      "loop1exit:\n"
+      "  br label %loop2\n"
+      "loop2:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  %y = call i1 @switch()\n"
+      "  br i1 %x, label %loop2, label %loop2exit\n"
+      "loop2exit:"
+      "  ret void\n"
+      "}");
+  ExpectPath(true);
+}
+
+TEST_F(IsPotentiallyReachableTest, OneLoopAfterTheOther2) {
+  ParseAssembly(
+      "declare i1 @switch()\n"
+      "\n"
+      "define void @test() {\n"
+      "entry:\n"
+      "  br label %loop1\n"
+      "loop1:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  %x = call i1 @switch()\n"
+      "  br i1 %x, label %loop1, label %loop1exit\n"
+      "loop1exit:\n"
+      "  br label %loop2\n"
+      "loop2:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  %y = call i1 @switch()\n"
+      "  br i1 %x, label %loop2, label %loop2exit\n"
+      "loop2exit:"
+      "  ret void\n"
+      "}");
+  ExpectPath(false);
+}
+
+TEST_F(IsPotentiallyReachableTest, OneLoopAfterTheOtherInsideAThirdLoop) {
+  ParseAssembly(
+      "declare i1 @switch()\n"
+      "\n"
+      "define void @test() {\n"
+      "entry:\n"
+      "  br label %outerloop3\n"
+      "outerloop3:\n"
+      "  br label %innerloop1\n"
+      "innerloop1:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  %x = call i1 @switch()\n"
+      "  br i1 %x, label %innerloop1, label %innerloop1exit\n"
+      "innerloop1exit:\n"
+      "  br label %innerloop2\n"
+      "innerloop2:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  %y = call i1 @switch()\n"
+      "  br i1 %x, label %innerloop2, label %innerloop2exit\n"
+      "innerloop2exit:"
+      "  ;; In outer loop3 now.\n"
+      "  %z = call i1 @switch()\n"
+      "  br i1 %z, label %outerloop3, label %exit\n"
+      "exit:\n"
+      "  ret void\n"
+      "}");
+  ExpectPath(true);
+}
+
+TEST_F(IsPotentiallyReachableTest, BranchInsideLoop) {
+  ParseAssembly(
+      "declare i1 @switch()\n"
+      "\n"
+      "define void @test() {\n"
+      "entry:\n"
+      "  br label %loop\n"
+      "loop:\n"
+      "  %x = call i1 @switch()\n"
+      "  br i1 %x, label %nextloopblock, label %exit\n"
+      "nextloopblock:\n"
+      "  %y = call i1 @switch()\n"
+      "  br i1 %y, label %left, label %right\n"
+      "left:\n"
+      "  %A = bitcast i8 undef to i8\n"
+      "  br label %loop\n"
+      "right:\n"
+      "  %B = bitcast i8 undef to i8\n"
+      "  br label %loop\n"
+      "exit:\n"
+      "  ret void\n"
+      "}");
+  ExpectPath(true);
+}
diff --git a/unittests/Analysis/CMakeLists.txt b/unittests/Analysis/CMakeLists.txt
index 7991a4101c..5cae68a467 100644
--- a/unittests/Analysis/CMakeLists.txt
+++ b/unittests/Analysis/CMakeLists.txt
@@ -1,5 +1,6 @@
 set(LLVM_LINK_COMPONENTS
   Analysis
+  AsmParser
   )
 
 add_llvm_unittest(AnalysisTests
diff --git a/unittests/Analysis/Makefile b/unittests/Analysis/Makefile
index b548d25d1e..527f4525e8 100644
--- a/unittests/Analysis/Makefile
+++ b/unittests/Analysis/Makefile
@@ -9,7 +9,7 @@
 
 LEVEL = ../..
 TESTNAME = Analysis
-LINK_COMPONENTS := analysis
+LINK_COMPONENTS := analysis asmparser
 
 include $(LEVEL)/Makefile.config
 include $(LLVM_SRC_ROOT)/unittests/Makefile.unittest