Fix PR17952.

  The symptom is that an assertion is triggered. The assertion was added by
me to detect the situation when value is propagated from dead blocks.
(We can certainly get rid of assertion; it is safe to do so, because propagating
 value from dead block to alive join node is certainly ok.)

  The root cause of this bug is : edge-splitting is conducted on the fly,
the edge being split could be a dead edge, therefore the block that 
split the critial edge needs to be flagged "dead" as well.

  There are 3 ways to fix this bug:
  1) Get rid of the assertion as I mentioned eariler 
  2) When an dead edge is split, flag the inserted block "dead".
  3) proactively split the critical edges connecting dead and live blocks when
     new dead blocks are revealed.

  This fix go for 3) with additional 2 LOC.

  Testing case was added by Rafael the other day.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194424 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 175 insertions, 6 deletions

diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index 957c123034..d353765b42 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -21,6 +21,7 @@
 #include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/CFG.h"
@@ -507,7 +508,9 @@ namespace {
     enum ValType {
       SimpleVal,  // A simple offsetted value that is accessed.
       LoadVal,    // A value produced by a load.
-      MemIntrin   // A memory intrinsic which is loaded from.
+      MemIntrin,  // A memory intrinsic which is loaded from.
+      UndefVal    // A UndefValue representing a value from dead block (which
+                  // is not yet physically removed from the CFG). 
     };
   
     /// V - The value that is live out of the block.
@@ -545,10 +548,20 @@ namespace {
       Res.Offset = Offset;
       return Res;
     }
-  
+
+    static AvailableValueInBlock getUndef(BasicBlock *BB) {
+      AvailableValueInBlock Res;
+      Res.BB = BB;
+      Res.Val.setPointer(0);
+      Res.Val.setInt(UndefVal);
+      Res.Offset = 0;
+      return Res;
+    }
+
     bool isSimpleValue() const { return Val.getInt() == SimpleVal; }
     bool isCoercedLoadValue() const { return Val.getInt() == LoadVal; }
     bool isMemIntrinValue() const { return Val.getInt() == MemIntrin; }
+    bool isUndefValue() const { return Val.getInt() == UndefVal; }
   
     Value *getSimpleValue() const {
       assert(isSimpleValue() && "Wrong accessor");
@@ -576,6 +589,7 @@ namespace {
     DominatorTree *DT;
     const DataLayout *TD;
     const TargetLibraryInfo *TLI;
+    SetVector<BasicBlock *> DeadBlocks;
 
     ValueTable VN;
 
@@ -698,6 +712,9 @@ namespace {
     unsigned replaceAllDominatedUsesWith(Value *From, Value *To,
                                          const BasicBlockEdge &Root);
     bool propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root);
+    bool processFoldableCondBr(BranchInst *BI);
+    void addDeadBlock(BasicBlock *BB);
+    void assignValNumForDeadCode();
   };
 
   char GVN::ID = 0;
@@ -1255,8 +1272,10 @@ static Value *ConstructSSAForLoadSet(LoadInst *LI,
   // just use the dominating value directly.
   if (ValuesPerBlock.size() == 1 &&
       gvn.getDominatorTree().properlyDominates(ValuesPerBlock[0].BB,
-                                               LI->getParent()))
+                                               LI->getParent())) {
+    assert(!ValuesPerBlock[0].isUndefValue() && "Dead BB dominate this block");
     return ValuesPerBlock[0].MaterializeAdjustedValue(LI->getType(), gvn);
+  }
 
   // Otherwise, we have to construct SSA form.
   SmallVector<PHINode*, 8> NewPHIs;
@@ -1326,7 +1345,7 @@ Value *AvailableValueInBlock::MaterializeAdjustedValue(Type *LoadTy, GVN &gvn) c
                    << *getCoercedLoadValue() << '\n'
                    << *Res << '\n' << "\n\n\n");
     }
-  } else {
+  } else if (isMemIntrinValue()) {
     const DataLayout *TD = gvn.getDataLayout();
     assert(TD && "Need target data to handle type mismatch case");
     Res = GetMemInstValueForLoad(getMemIntrinValue(), Offset,
@@ -1334,6 +1353,10 @@ Value *AvailableValueInBlock::MaterializeAdjustedValue(Type *LoadTy, GVN &gvn) c
     DEBUG(dbgs() << "GVN COERCED NONLOCAL MEM INTRIN:\nOffset: " << Offset
                  << "  " << *getMemIntrinValue() << '\n'
                  << *Res << '\n' << "\n\n\n");
+  } else {
+    assert(isUndefValue() && "Should be UndefVal");
+    DEBUG(dbgs() << "GVN COERCED NONLOCAL Undef:\n";);
+    return UndefValue::get(LoadTy);
   }
   return Res;
 }
@@ -1357,6 +1380,13 @@ void GVN::AnalyzeLoadAvailability(LoadInst *LI, LoadDepVect &Deps,
     BasicBlock *DepBB = Deps[i].getBB();
     MemDepResult DepInfo = Deps[i].getResult();
 
+    if (DeadBlocks.count(DepBB)) {
+      // Dead dependent mem-op disguise as a load evaluating the same value
+      // as the load in question.
+      ValuesPerBlock.push_back(AvailableValueInBlock::getUndef(DepBB));
+      continue;
+    }
+
     if (!DepInfo.isDef() && !DepInfo.isClobber()) {
       UnavailableBlocks.push_back(DepBB);
       continue;
@@ -2193,11 +2223,13 @@ bool GVN::processInstruction(Instruction *I) {
   // For conditional branches, we can perform simple conditional propagation on
   // the condition value itself.
   if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
-    if (!BI->isConditional() || isa<Constant>(BI->getCondition()))
+    if (!BI->isConditional())
       return false;
 
-    Value *BranchCond = BI->getCondition();
+    if (isa<Constant>(BI->getCondition()))
+      return processFoldableCondBr(BI);
 
+    Value *BranchCond = BI->getCondition();
     BasicBlock *TrueSucc = BI->getSuccessor(0);
     BasicBlock *FalseSucc = BI->getSuccessor(1);
     // Avoid multiple edges early.
@@ -2314,6 +2346,9 @@ bool GVN::runOnFunction(Function& F) {
   }
 
   if (EnablePRE) {
+    // Fabricate val-num for dead-code in order to suppress assertion in
+    // performPRE().
+    assignValNumForDeadCode();
     bool PREChanged = true;
     while (PREChanged) {
       PREChanged = performPRE(F);
@@ -2327,6 +2362,9 @@ bool GVN::runOnFunction(Function& F) {
   // Actually, when this happens, we should just fully integrate PRE into GVN.
 
   cleanupGlobalSets();
+  // Do not cleanup DeadBlocks in cleanupGlobalSets() as it's called for each
+  // iteration. 
+  DeadBlocks.clear();
 
   return Changed;
 }
@@ -2337,6 +2375,9 @@ bool GVN::processBlock(BasicBlock *BB) {
   // (and incrementing BI before processing an instruction).
   assert(InstrsToErase.empty() &&
          "We expect InstrsToErase to be empty across iterations");
+  if (DeadBlocks.count(BB))
+    return false;
+
   bool ChangedFunction = false;
 
   for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
@@ -2630,3 +2671,131 @@ void GVN::verifyRemoved(const Instruction *Inst) const {
     }
   }
 }
+
+// BB is declared dead, which implied other blocks become dead as well. This
+// function is to add all these blocks to "DeadBlocks". For the dead blocks'
+// live successors, update their phi nodes by replacing the operands
+// corresponding to dead blocks with UndefVal.
+//
+void GVN::addDeadBlock(BasicBlock *BB) {
+  SmallVector<BasicBlock *, 4> NewDead;
+  SmallSetVector<BasicBlock *, 4> DF;
+
+  NewDead.push_back(BB);
+  while (!NewDead.empty()) {
+    BasicBlock *D = NewDead.pop_back_val();
+    if (DeadBlocks.count(D))
+      continue;
+
+    // All blocks dominated by D are dead.
+    SmallVector<BasicBlock *, 8> Dom;
+    DT->getDescendants(D, Dom);
+    DeadBlocks.insert(Dom.begin(), Dom.end());
+    
+    // Figure out the dominance-frontier(D).
+    for (SmallVectorImpl<BasicBlock *>::iterator I = Dom.begin(),
+           E = Dom.end(); I != E; I++) {
+      BasicBlock *B = *I;
+      for (succ_iterator SI = succ_begin(B), SE = succ_end(B); SI != SE; SI++) {
+        BasicBlock *S = *SI;
+        if (DeadBlocks.count(S))
+          continue;
+
+        bool AllPredDead = true;
+        for (pred_iterator PI = pred_begin(S), PE = pred_end(S); PI != PE; PI++)
+          if (!DeadBlocks.count(*PI)) {
+            AllPredDead = false;
+            break;
+          }
+
+        if (!AllPredDead) {
+          // S could be proved dead later on. That is why we don't update phi
+          // operands at this moment.
+          DF.insert(S);
+        } else {
+          // While S is not dominated by D, it is dead by now. This could take
+          // place if S already have a dead predecessor before D is declared
+          // dead.
+          NewDead.push_back(S);
+        }
+      }
+    }
+  }
+
+  // For the dead blocks' live successors, update their phi nodes by replacing
+  // the operands corresponding to dead blocks with UndefVal.
+  for(SmallSetVector<BasicBlock *, 4>::iterator I = DF.begin(), E = DF.end();
+        I != E; I++) {
+    BasicBlock *B = *I;
+    if (DeadBlocks.count(B))
+      continue;
+
+    for (pred_iterator PI = pred_begin(B), PE = pred_end(B); PI != PE; PI++) {
+      BasicBlock *P = *PI;
+
+      if (!DeadBlocks.count(P))
+        continue;
+
+      if (isCriticalEdge(P->getTerminator(), GetSuccessorNumber(P, B))) {
+        if (BasicBlock *S = splitCriticalEdges(P, B))
+          DeadBlocks.insert(P = S);
+      }
+
+      for (BasicBlock::iterator II = B->begin(); isa<PHINode>(II); ++II) {
+        PHINode &Phi = cast<PHINode>(*II);
+        Phi.setIncomingValue(Phi.getBasicBlockIndex(P),
+                             UndefValue::get(Phi.getType()));
+      }
+    }
+  }
+}
+
+// If the given branch is recognized as a foldable branch (i.e. conditional
+// branch with constant condition), it will perform following analyses and
+// transformation.
+//  1) If the dead out-coming edge is a critical-edge, split it. Let 
+//     R be the target of the dead out-coming edge.
+//  1) Identify the set of dead blocks implied by the branch's dead outcoming
+//     edge. The result of this step will be {X| X is dominated by R}
+//  2) Identify those blocks which haves at least one dead prodecessor. The
+//     result of this step will be dominance-frontier(R).
+//  3) Update the PHIs in DF(R) by replacing the operands corresponding to 
+//     dead blocks with "UndefVal" in an hope these PHIs will optimized away.
+//
+// Return true iff *NEW* dead code are found.
+bool GVN::processFoldableCondBr(BranchInst *BI) {
+  if (!BI || BI->isUnconditional())
+    return false;
+
+  ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
+  if (!Cond)
+    return false;
+
+  BasicBlock *DeadRoot = Cond->getZExtValue() ? 
+                         BI->getSuccessor(1) : BI->getSuccessor(0);
+  if (DeadBlocks.count(DeadRoot))
+    return false;
+
+  if (!DeadRoot->getSinglePredecessor())
+    DeadRoot = splitCriticalEdges(BI->getParent(), DeadRoot);
+
+  addDeadBlock(DeadRoot);
+  return true;
+}
+
+// performPRE() will trigger assert if it come across an instruciton without
+// associated val-num. As it normally has far more live instructions than dead
+// instructions, it makes more sense just to "fabricate" a val-number for the
+// dead code than checking if instruction involved is dead or not.
+void GVN::assignValNumForDeadCode() {
+  for (SetVector<BasicBlock *>::iterator I = DeadBlocks.begin(),
+        E = DeadBlocks.end(); I != E; I++) {
+    BasicBlock *BB = *I;
+    for (BasicBlock::iterator II = BB->begin(), EE = BB->end();
+          II != EE; II++) {
+      Instruction *Inst = &*II;
+      unsigned ValNum = VN.lookup_or_add(Inst);
+      addToLeaderTable(ValNum, Inst, BB);
+    }
+  }
+}