Fix PR/33305. caused by trying to simplify expressions in phi of ops that should have no leaders.

Summary:
After a discussion with Rekka, i believe this (or a small variant)
should fix the remaining phi-of-ops problems.

Rekka's algorithm for completeness relies on looking up expressions
that should have no leader, and expecting it to fail (IE looking up
expressions that can't exist in a predecessor, and expecting it to
find nothing).

Unfortunately, sometimes these expressions can be simplified to
constants, but we need the lookup to fail anyway.  Additionally, our
simplifier outsmarts this by taking these "not quite right"
expressions, and simplifying them into other expressions or walking
through phis, etc.  In the past, we've sometimes been able to find
leaders for these expressions, incorrectly.

This change causes us to not to try to phi of ops such expressions.
We determine safety by seeing if they depend on a phi node in our
block.

This is not perfect, we can do a bit better, but this should be a
"correctness start" that we can then improve.  It also requires a
bunch of caching that i'll eventually like to eliminate.

The right solution, longer term, to the simplifier issues, is to make
the query interface for the instruction simplifier/constant folder
have the flags we need, so that we can keep most things going, but
turn off the possibly-invalid parts (threading through phis, etc).
This is an issue in another wrong code bug as well.

Reviewers: davide, mcrosier

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D37175

llvm-svn: 312401

1 files changed, 144 insertions, 50 deletions

diff --git a/llvm/lib/Transforms/Scalar/NewGVN.cpp b/llvm/lib/Transforms/Scalar/NewGVN.cpp
index b946350d264..45f49d4d12e 100644
--- a/llvm/lib/Transforms/Scalar/NewGVN.cpp
+++ b/llvm/lib/Transforms/Scalar/NewGVN.cpp
@@ -128,7 +128,7 @@ static cl::opt<bool> EnableStoreRefinement("enable-store-refinement",
                                            cl::init(false), cl::Hidden);
 
 /// Currently, the generation "phi of ops" can result in correctness issues.
-static cl::opt<bool> EnablePhiOfOps("enable-phi-of-ops", cl::init(false),
+static cl::opt<bool> EnablePhiOfOps("enable-phi-of-ops", cl::init(true),
                                     cl::Hidden);
 
 //===----------------------------------------------------------------------===//
@@ -475,6 +475,7 @@ class NewGVN {
   // These mappings just store various data that would normally be part of the
   // IR.
   DenseSet<const Instruction *> PHINodeUses;
+  DenseMap<const Value *, bool> OpSafeForPHIOfOps;
   // Map a temporary instruction we created to a parent block.
   DenseMap<const Value *, BasicBlock *> TempToBlock;
   // Map between the already in-program instructions and the temporary phis we
@@ -643,6 +644,14 @@ private:
   void initializeCongruenceClasses(Function &F);
   const Expression *makePossiblePhiOfOps(Instruction *,
                                          SmallPtrSetImpl<Value *> &);
+  Value *findLeaderForInst(Instruction *ValueOp,
+                           SmallPtrSetImpl<Value *> &Visited,
+                           MemoryAccess *MemAccess, Instruction *OrigInst,
+                           BasicBlock *PredBB);
+
+  bool OpIsSafeForPHIOfOps(Value *Op, Instruction *OrigInst,
+                           const BasicBlock *PHIBlock,
+                           SmallPtrSetImpl<const Value *> &);
   void addPhiOfOps(PHINode *Op, BasicBlock *BB, Instruction *ExistingValue);
   void removePhiOfOps(Instruction *I, PHINode *PHITemp);
 
@@ -669,6 +678,7 @@ private:
   // Congruence finding.
   bool someEquivalentDominates(const Instruction *, const Instruction *) const;
   Value *lookupOperandLeader(Value *) const;
+  CongruenceClass *getClassForExpression(const Expression *E) const;
   void performCongruenceFinding(Instruction *, const Expression *);
   void moveValueToNewCongruenceClass(Instruction *, const Expression *,
                                      CongruenceClass *, CongruenceClass *);
@@ -703,8 +713,7 @@ private:
   void replaceInstruction(Instruction *, Value *);
   void markInstructionForDeletion(Instruction *);
   void deleteInstructionsInBlock(BasicBlock *);
-  Value *findPhiOfOpsLeader(const Expression *E, const BasicBlock *BB) const;
-
+  Value *findPHIOfOpsLeader(const Expression *E, const BasicBlock *BB) const;
   // New instruction creation.
   void handleNewInstruction(Instruction *){};
 
@@ -963,7 +972,9 @@ const Expression *NewGVN::checkSimplificationResults(Expression *E,
   CongruenceClass *CC = ValueToClass.lookup(V);
   if (CC) {
     if (CC->getLeader() && CC->getLeader() != I) {
-      addAdditionalUsers(V, I);
+      // Don't add temporary instructions to the user lists.
+      if (!AllTempInstructions.count(I))
+        addAdditionalUsers(V, I);
       return createVariableOrConstant(CC->getLeader());
     }
 
@@ -988,6 +999,9 @@ const Expression *NewGVN::checkSimplificationResults(Expression *E,
   return nullptr;
 }
 
+// Create a value expression from the instruction I, replacing operands with
+// their leaders.
+
 const Expression *NewGVN::createExpression(Instruction *I) const {
   auto *E = new (ExpressionAllocator) BasicExpression(I->getNumOperands());
 
@@ -1002,7 +1016,6 @@ const Expression *NewGVN::createExpression(Instruction *I) const {
     if (shouldSwapOperands(E->getOperand(0), E->getOperand(1)))
       E->swapOperands(0, 1);
   }
-
   // Perform simplification.
   if (auto *CI = dyn_cast<CmpInst>(I)) {
     // Sort the operand value numbers so x<y and y>x get the same value
@@ -1389,8 +1402,8 @@ const Expression *NewGVN::performSymbolicLoadEvaluation(Instruction *I) const {
     }
   }
 
-  const auto *LE = createLoadExpression(LI->getType(), LoadAddressLeader,
-                                             LI, DefiningAccess);
+  const auto *LE = createLoadExpression(LI->getType(), LoadAddressLeader, LI,
+                                        DefiningAccess);
   // If our MemoryLeader is not our defining access, add a use to the
   // MemoryLeader, so that we get reprocessed when it changes.
   if (LE->getMemoryLeader() != DefiningAccess)
@@ -2464,6 +2477,92 @@ static bool okayForPHIOfOps(const Instruction *I) {
          isa<LoadInst>(I);
 }
 
+// Return true if this operand will be safe to use for phi of ops.
+//
+// The reason some operands are unsafe is that we are not trying to recursively
+// translate everything back through phi nodes.  We actually expect some lookups
+// of expressions to fail.  In particular, a lookup where the expression cannot
+// exist in the predecessor.  This is true even if the expression, as shown, can
+// be determined to be constant.
+bool NewGVN::OpIsSafeForPHIOfOps(Value *V, Instruction *OrigInst,
+                                 const BasicBlock *PHIBlock,
+                                 SmallPtrSetImpl<const Value *> &Visited) {
+  if (!isa<Instruction>(V))
+    return true;
+  auto OISIt = OpSafeForPHIOfOps.find(V);
+  if (OISIt != OpSafeForPHIOfOps.end())
+    return OISIt->second;
+  // Keep walking until we either dominate the phi block, or hit a phi, or run
+  // out of things to check.
+  if (DT->properlyDominates(getBlockForValue(V), PHIBlock)) {
+    OpSafeForPHIOfOps.insert({V, true});
+    return true;
+  }
+  // PHI in the same block.
+  if (isa<PHINode>(V) && getBlockForValue(V) == PHIBlock) {
+    OpSafeForPHIOfOps.insert({V, false});
+    return false;
+  }
+  for (auto Op : cast<Instruction>(V)->operand_values()) {
+    if (!isa<Instruction>(Op))
+      continue;
+    // See if we already know the answer for this node.
+    auto OISIt = OpSafeForPHIOfOps.find(Op);
+    if (OISIt != OpSafeForPHIOfOps.end()) {
+      if (!OISIt->second) {
+        OpSafeForPHIOfOps.insert({V, false});
+        return false;
+      }
+    }
+    if (!Visited.insert(Op).second)
+      continue;
+    if (!OpIsSafeForPHIOfOps(Op, OrigInst, PHIBlock, Visited)) {
+      OpSafeForPHIOfOps.insert({V, false});
+      return false;
+    }
+  }
+  OpSafeForPHIOfOps.insert({V, true});
+  return true;
+}
+
+// Try to find a leader for instruction TransInst, which is a phi translated
+// version of something in our original program.  Visited is used to ensure we
+// don't infinite loop during translations of cycles.  OrigInst is the
+// instruction in the original program, and PredBB is the predecessor we
+// translated it through.
+Value *NewGVN::findLeaderForInst(Instruction *TransInst,
+                                 SmallPtrSetImpl<Value *> &Visited,
+                                 MemoryAccess *MemAccess, Instruction *OrigInst,
+                                 BasicBlock *PredBB) {
+  unsigned IDFSNum = InstrToDFSNum(OrigInst);
+  // Make sure it's marked as a temporary instruction.
+  AllTempInstructions.insert(TransInst);
+  // and make sure anything that tries to add it's DFS number is
+  // redirected to the instruction we are making a phi of ops
+  // for.
+  TempToBlock.insert({TransInst, PredBB});
+  InstrDFS.insert({TransInst, IDFSNum});
+
+  const Expression *E = performSymbolicEvaluation(TransInst, Visited);
+  InstrDFS.erase(TransInst);
+  AllTempInstructions.erase(TransInst);
+  TempToBlock.erase(TransInst);
+  if (MemAccess)
+    TempToMemory.erase(TransInst);
+  if (!E)
+    return nullptr;
+  auto *FoundVal = findPHIOfOpsLeader(E, PredBB);
+  if (!FoundVal || FoundVal == OrigInst) {
+    ExpressionToPhiOfOps[E].insert(OrigInst);
+    DEBUG(dbgs() << "Cannot find phi of ops operand for " << *TransInst
+                 << " in block " << getBlockName(PredBB) << "\n");
+    return nullptr;
+  }
+  if (auto *SI = dyn_cast<StoreInst>(FoundVal))
+    FoundVal = SI->getValueOperand();
+  return FoundVal;
+}
+
 // When we see an instruction that is an op of phis, generate the equivalent phi
 // of ops form.
 const Expression *
@@ -2481,7 +2580,6 @@ NewGVN::makePossiblePhiOfOps(Instruction *I,
   if (!isCycleFree(I))
     return nullptr;
 
-  unsigned IDFSNum = InstrToDFSNum(I);
   SmallPtrSet<const Value *, 8> ProcessedPHIs;
   // TODO: We don't do phi translation on memory accesses because it's
   // complicated. For a load, we'd need to be able to simulate a new memoryuse,
@@ -2494,18 +2592,9 @@ NewGVN::makePossiblePhiOfOps(Instruction *I,
       MemAccess->getDefiningAccess()->getBlock() == I->getParent())
     return nullptr;
 
+  SmallPtrSet<const Value *, 10> VisitedOps;
   // Convert op of phis to phi of ops
   for (auto &Op : I->operands()) {
-    // TODO: We can't handle expressions that must be recursively translated
-    // IE
-    // a = phi (b, c)
-    // f = use a
-    // g = f + phi of something
-    // To properly make a phi of ops for g, we'd have to properly translate and
-    // use the instruction for f.  We should add this by splitting out the
-    // instruction creation we do below.
-    if (isa<Instruction>(Op) && PHINodeUses.count(cast<Instruction>(Op)))
-      return nullptr;
     if (!isa<PHINode>(Op))
       continue;
     auto *OpPHI = cast<PHINode>(Op);
@@ -2526,36 +2615,30 @@ NewGVN::makePossiblePhiOfOps(Instruction *I,
         Instruction *ValueOp = I->clone();
         if (MemAccess)
           TempToMemory.insert({ValueOp, MemAccess});
-
+        bool SafeForPHIOfOps = true;
+        VisitedOps.clear();
         for (auto &Op : ValueOp->operands()) {
+          auto *OrigOp = &*Op;
           Op = Op->DoPHITranslation(PHIBlock, PredBB);
           // When this operand changes, it could change whether there is a
           // leader for us or not.
           addAdditionalUsers(Op, I);
+          // If we phi-translated the op, it must be safe.
+          SafeForPHIOfOps = SafeForPHIOfOps &&
+                            (Op != OrigOp ||
+                             OpIsSafeForPHIOfOps(Op, I, PHIBlock, VisitedOps));
         }
-        // Make sure it's marked as a temporary instruction.
-        AllTempInstructions.insert(ValueOp);
-        // and make sure anything that tries to add it's DFS number is
-        // redirected to the instruction we are making a phi of ops
-        // for.
-        TempToBlock.insert({ValueOp, PredBB});
-        InstrDFS.insert({ValueOp, IDFSNum});
-        const Expression *E = performSymbolicEvaluation(ValueOp, Visited);
-        InstrDFS.erase(ValueOp);
-        AllTempInstructions.erase(ValueOp);
+        // FIXME: For those things that are not safe We could generate
+        // expressions all the way down, and see if this comes out to a
+        // constant.  For anything where that is true, and unsafe, we should
+        // have made a phi-of-ops (or value numbered it equivalent to something)
+        // for the pieces already.
+        FoundVal = !SafeForPHIOfOps ? nullptr
+                                    : findLeaderForInst(ValueOp, Visited,
+                                                        MemAccess, I, PredBB);
         ValueOp->deleteValue();
-        TempToBlock.erase(ValueOp);
-        if (MemAccess)
-          TempToMemory.erase(ValueOp);
-        if (!E)
-          return nullptr;
-        FoundVal = findPhiOfOpsLeader(E, PredBB);
-        if (!FoundVal) {
-          ExpressionToPhiOfOps[E].insert(I);
+        if (!FoundVal)
           return nullptr;
-        }
-        if (auto *SI = dyn_cast<StoreInst>(FoundVal))
-          FoundVal = SI->getValueOperand();
       } else {
         DEBUG(dbgs() << "Skipping phi of ops operand for incoming block "
                      << getBlockName(PredBB)
@@ -2570,7 +2653,8 @@ NewGVN::makePossiblePhiOfOps(Instruction *I,
     auto *ValuePHI = RealToTemp.lookup(I);
     bool NewPHI = false;
     if (!ValuePHI) {
-      ValuePHI = PHINode::Create(I->getType(), OpPHI->getNumOperands());
+      ValuePHI =
+          PHINode::Create(I->getType(), OpPHI->getNumOperands(), "phiofops");
       addPhiOfOps(ValuePHI, PHIBlock, I);
       NewPHI = true;
       NumGVNPHIOfOpsCreated++;
@@ -2695,6 +2779,8 @@ void NewGVN::cleanupTables() {
   TempToBlock.clear();
   TempToMemory.clear();
   PHIOfOpsPHIs.clear();
+  PHINodeUses.clear();
+  OpSafeForPHIOfOps.clear();
   ReachableBlocks.clear();
   ReachableEdges.clear();
 #ifndef NDEBUG
@@ -3524,17 +3610,29 @@ private:
 };
 }
 
+// Given an expression, get the congruence class for it.
+CongruenceClass *NewGVN::getClassForExpression(const Expression *E) const {
+  if (auto *VE = dyn_cast<VariableExpression>(E))
+    return ValueToClass.lookup(VE->getVariableValue());
+  else if (isa<DeadExpression>(E))
+    return TOPClass;
+  return ExpressionToClass.lookup(E);
+}
+
 // Given a value and a basic block we are trying to see if it is available in,
 // see if the value has a leader available in that block.
-Value *NewGVN::findPhiOfOpsLeader(const Expression *E,
+Value *NewGVN::findPHIOfOpsLeader(const Expression *E,
                                   const BasicBlock *BB) const {
   // It would already be constant if we could make it constant
   if (auto *CE = dyn_cast<ConstantExpression>(E))
     return CE->getConstantValue();
-  if (auto *VE = dyn_cast<VariableExpression>(E))
-    return VE->getVariableValue();
+  if (auto *VE = dyn_cast<VariableExpression>(E)) {
+    auto *V = VE->getVariableValue();
+    if (alwaysAvailable(V) || DT->dominates(getBlockForValue(V), BB))
+      return VE->getVariableValue();
+  }
 
-  auto *CC = ExpressionToClass.lookup(E);
+  auto *CC = getClassForExpression(E);
   if (!CC)
     return nullptr;
   if (alwaysAvailable(CC->getLeader()))
@@ -3545,12 +3643,8 @@ Value *NewGVN::findPhiOfOpsLeader(const Expression *E,
     // Anything that isn't an instruction is always available.
     if (!MemberInst)
       return Member;
-    // If we are looking for something in the same block as the member, it must
-    // be a leader because this function is looking for operands for a phi node.
-    if (MemberInst->getParent() == BB ||
-        DT->dominates(MemberInst->getParent(), BB)) {
+    if (DT->dominates(getBlockForValue(MemberInst), BB))
       return Member;
-    }
   }
   return nullptr;
 }