Allow PRE to insert no-cost phi nodes

llvm-svn: 228024

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -711,6 +711,8 @@ namespace {
     bool iterateOnFunction(Function &F);
     bool performPRE(Function &F);
     bool performScalarPRE(Instruction *I);
+    bool performScalarPREInsertion(Instruction *Instr, BasicBlock *Pred,
+                                   unsigned int ValNo);
     Value *findLeader(const BasicBlock *BB, uint32_t num);
     void cleanupGlobalSets();
     void verifyRemoved(const Instruction *I) const;
@@ -2454,6 +2456,43 @@ bool GVN::processBlock(BasicBlock *BB) {
   return ChangedFunction;
 }
 
+// Instantiate an expression in a predecessor that lacked it.
+bool GVN::performScalarPREInsertion(Instruction *Instr, BasicBlock *Pred,
+                                    unsigned int ValNo) {
+  // Because we are going top-down through the block, all value numbers
+  // will be available in the predecessor by the time we need them.  Any
+  // that weren't originally present will have been instantiated earlier
+  // in this loop.
+  bool success = true;
+  for (unsigned i = 0, e = Instr->getNumOperands(); i != e; ++i) {
+    Value *Op = Instr->getOperand(i);
+    if (isa<Argument>(Op) || isa<Constant>(Op) || isa<GlobalValue>(Op))
+      continue;
+
+    if (Value *V = findLeader(Pred, VN.lookup(Op))) {
+      Instr->setOperand(i, V);
+    } else {
+      success = false;
+      break;
+    }
+  }
+
+  // Fail out if we encounter an operand that is not available in
+  // the PRE predecessor.  This is typically because of loads which
+  // are not value numbered precisely.
+  if (!success)
+    return false;
+
+  Instr->insertBefore(Pred->getTerminator());
+  Instr->setName(Instr->getName() + ".pre");
+  Instr->setDebugLoc(Instr->getDebugLoc());
+  VN.add(Instr, ValNo);
+
+  // Update the availability map to include the new instruction.
+  addToLeaderTable(ValNo, Instr, Pred);
+  return true;
+}
+
 bool GVN::performScalarPRE(Instruction *CurInst) {
   SmallVector<std::pair<Value*, BasicBlock*>, 8> predMap;
 
@@ -2520,60 +2559,43 @@ bool GVN::performScalarPRE(Instruction *CurInst) {
 
   // Don't do PRE when it might increase code size, i.e. when
   // we would need to insert instructions in more than one pred.
-  if (NumWithout != 1 || NumWith == 0)
+  if (NumWithout > 1 || NumWith == 0)
     return false;
 
-  // Don't do PRE across indirect branch.
-  if (isa<IndirectBrInst>(PREPred->getTerminator()))
-    return false;
+  // We may have a case where all predecessors have the instruction,
+  // and we just need to insert a phi node. Otherwise, perform
+  // insertion.
+  Instruction *PREInstr = nullptr;
 
-  // We can't do PRE safely on a critical edge, so instead we schedule
-  // the edge to be split and perform the PRE the next time we iterate
-  // on the function.
-  unsigned SuccNum = GetSuccessorNumber(PREPred, CurrentBlock);
-  if (isCriticalEdge(PREPred->getTerminator(), SuccNum)) {
-    toSplit.push_back(std::make_pair(PREPred->getTerminator(), SuccNum));
-    return false;
-  }
+  if (NumWithout != 0) {
+    // Don't do PRE across indirect branch.
+    if (isa<IndirectBrInst>(PREPred->getTerminator()))
+      return false;
 
-  // Instantiate the expression in the predecessor that lacked it.
-  // Because we are going top-down through the block, all value numbers
-  // will be available in the predecessor by the time we need them.  Any
-  // that weren't originally present will have been instantiated earlier
-  // in this loop.
-  Instruction *PREInstr = CurInst->clone();
-  bool success = true;
-  for (unsigned i = 0, e = CurInst->getNumOperands(); i != e; ++i) {
-    Value *Op = PREInstr->getOperand(i);
-    if (isa<Argument>(Op) || isa<Constant>(Op) || isa<GlobalValue>(Op))
-      continue;
-
-    if (Value *V = findLeader(PREPred, VN.lookup(Op))) {
-      PREInstr->setOperand(i, V);
-    } else {
-      success = false;
-      break;
+    // We can't do PRE safely on a critical edge, so instead we schedule
+    // the edge to be split and perform the PRE the next time we iterate
+    // on the function.
+    unsigned SuccNum = GetSuccessorNumber(PREPred, CurrentBlock);
+    if (isCriticalEdge(PREPred->getTerminator(), SuccNum)) {
+      toSplit.push_back(std::make_pair(PREPred->getTerminator(), SuccNum));
+      return false;
+    }
+    // We need to insert somewhere, so let's give it a shot
+    PREInstr = CurInst->clone();
+    if (!performScalarPREInsertion(PREInstr, PREPred, ValNo)) {
+      // If we failed insertion, make sure we remove the instruction.
+      DEBUG(verifyRemoved(PREInstr));
+      delete PREInstr;
+      return false;
     }
   }
 
-  // Fail out if we encounter an operand that is not available in
-  // the PRE predecessor.  This is typically because of loads which
-  // are not value numbered precisely.
-  if (!success) {
-    DEBUG(verifyRemoved(PREInstr));
-    delete PREInstr;
-    return false;
-  }
+  // Either we should have filled in the PRE instruction, or we should
+  // not have needed insertions.
+  assert (PREInstr != nullptr || NumWithout == 0);
 
-  PREInstr->insertBefore(PREPred->getTerminator());
-  PREInstr->setName(CurInst->getName() + ".pre");
-  PREInstr->setDebugLoc(CurInst->getDebugLoc());
-  VN.add(PREInstr, ValNo);
   ++NumGVNPRE;
 
-  // Update the availability map to include the new instruction.
-  addToLeaderTable(ValNo, PREInstr, PREPred);
-
   // Create a PHI to make the value available in this block.
   PHINode *Phi =
       PHINode::Create(CurInst->getType(), predMap.size(),
@@ -2609,6 +2631,8 @@ bool GVN::performScalarPRE(Instruction *CurInst) {
     MD->removeInstruction(CurInst);
   DEBUG(verifyRemoved(CurInst));
   CurInst->eraseFromParent();
+  ++NumGVNInstr;
+  
   return true;
 }
 

llvm/test/Transforms/GVN/pre-no-cost-phi.ll

@@ -0,0 +1,31 @@
+; RUN: opt < %s -gvn -S | FileCheck %s
+; This testcase tests insertion of no-cost phis.  That is,
+; when the value is already available in every predecessor,
+; and we just need to insert a phi node to merge the available values.
+
+@c = global i32 0, align 4
+@d = global i32 0, align 4
+
+
+define i32 @mai(i32 %foo, i32 %a, i32 %b) {
+  %1 = icmp ne i32 %foo, 0
+  br i1 %1, label %bb1, label %bb2
+
+bb1:
+  %2 = add nsw i32 %a, %b
+  store i32 %2, i32* @c, align 4
+  br label %mergeblock
+
+bb2:
+  %3 = add nsw i32 %a, %b
+  store i32 %3, i32* @d, align 4
+  br label %mergeblock
+
+mergeblock:
+; CHECK: pre-phi = phi i32 [ %3, %bb2 ], [ %2, %bb1 ]
+; CHECK-NEXT: ret i32 %.pre-phi
+  %4 = add nsw i32 %a, %b
+  ret i32 %4
+}
+
+