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 //===- SSAUpdaterImpl.h - SSA Updater Implementation ------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file provides a template that implements the core algorithm for the
 // SSAUpdater and MachineSSAUpdater.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
 #define LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 
 #define DEBUG_TYPE "ssaupdater"
 
 namespace llvm {
 
 template<typename T> class SSAUpdaterTraits;
 
 template<typename UpdaterT>
 class SSAUpdaterImpl {
 private:
   UpdaterT *Updater;
 
   using Traits = SSAUpdaterTraits<UpdaterT>;
   using BlkT = typename Traits::BlkT;
   using ValT = typename Traits::ValT;
   using PhiT = typename Traits::PhiT;
 
   /// BBInfo - Per-basic block information used internally by SSAUpdaterImpl.
   /// The predecessors of each block are cached here since pred_iterator is
   /// slow and we need to iterate over the blocks at least a few times.
   class BBInfo {
   public:
     // Back-pointer to the corresponding block.
     BlkT *BB;
 
     // Value to use in this block.
     ValT AvailableVal;
 
     // Block that defines the available value.
     BBInfo *DefBB;
 
     // Postorder number.
     int BlkNum = 0;
 
     // Immediate dominator.
     BBInfo *IDom = nullptr;
 
     // Number of predecessor blocks.
     unsigned NumPreds = 0;
 
     // Array[NumPreds] of predecessor blocks.
     BBInfo **Preds = nullptr;
 
     // Marker for existing PHIs that match.
     PhiT *PHITag = nullptr;
 
     BBInfo(BlkT *ThisBB, ValT V)
       : BB(ThisBB), AvailableVal(V), DefBB(V ? this : nullptr) {}
   };
 
   using AvailableValsTy = DenseMap<BlkT *, ValT>;
 
   AvailableValsTy *AvailableVals;
 
   SmallVectorImpl<PhiT *> *InsertedPHIs;
 
   using BlockListTy = SmallVectorImpl<BBInfo *>;
   using BBMapTy = DenseMap<BlkT *, BBInfo *>;
 
   BBMapTy BBMap;
   BumpPtrAllocator Allocator;
 
 public:
   explicit SSAUpdaterImpl(UpdaterT *U, AvailableValsTy *A,
                           SmallVectorImpl<PhiT *> *Ins) :
     Updater(U), AvailableVals(A), InsertedPHIs(Ins) {}
 
   /// GetValue - Check to see if AvailableVals has an entry for the specified
   /// BB and if so, return it.  If not, construct SSA form by first
   /// calculating the required placement of PHIs and then inserting new PHIs
   /// where needed.
   ValT GetValue(BlkT *BB) {
     SmallVector<BBInfo *, 100> BlockList;
     BBInfo *PseudoEntry = BuildBlockList(BB, &BlockList);
 
     // Special case: bail out if BB is unreachable.
     if (BlockList.size() == 0) {
       ValT V = Traits::GetPoisonVal(BB, Updater);
       (*AvailableVals)[BB] = V;
       return V;
     }
 
     FindDominators(&BlockList, PseudoEntry);
     FindPHIPlacement(&BlockList);
     FindAvailableVals(&BlockList);
 
     return BBMap[BB]->DefBB->AvailableVal;
   }
 
   /// BuildBlockList - Starting from the specified basic block, traverse back
   /// through its predecessors until reaching blocks with known values.
   /// Create BBInfo structures for the blocks and append them to the block
   /// list.
   BBInfo *BuildBlockList(BlkT *BB, BlockListTy *BlockList) {
     SmallVector<BBInfo *, 10> RootList;
     SmallVector<BBInfo *, 64> WorkList;
 
     BBInfo *Info = new (Allocator) BBInfo(BB, 0);
     BBMap[BB] = Info;
     WorkList.push_back(Info);
 
     // Search backward from BB, creating BBInfos along the way and stopping
     // when reaching blocks that define the value.  Record those defining
     // blocks on the RootList.
     SmallVector<BlkT *, 10> Preds;
     while (!WorkList.empty()) {
       Info = WorkList.pop_back_val();
       Preds.clear();
       Traits::FindPredecessorBlocks(Info->BB, &Preds);
       Info->NumPreds = Preds.size();
       if (Info->NumPreds == 0)
         Info->Preds = nullptr;
       else
         Info->Preds = static_cast<BBInfo **>(Allocator.Allocate(
             Info->NumPreds * sizeof(BBInfo *), alignof(BBInfo *)));
 
       for (unsigned p = 0; p != Info->NumPreds; ++p) {
         BlkT *Pred = Preds[p];
         // Check if BBMap already has a BBInfo for the predecessor block.
         BBInfo *&BBMapBucket = BBMap[Pred];
         if (BBMapBucket) {
           Info->Preds[p] = BBMapBucket;
           continue;
         }
 
         // Create a new BBInfo for the predecessor.
         ValT PredVal = AvailableVals->lookup(Pred);
         BBInfo *PredInfo = new (Allocator) BBInfo(Pred, PredVal);
         BBMapBucket = PredInfo;
         Info->Preds[p] = PredInfo;
 
         if (PredInfo->AvailableVal) {
           RootList.push_back(PredInfo);
           continue;
         }
         WorkList.push_back(PredInfo);
       }
     }
 
     // Now that we know what blocks are backwards-reachable from the starting
     // block, do a forward depth-first traversal to assign postorder numbers
     // to those blocks.
     BBInfo *PseudoEntry = new (Allocator) BBInfo(nullptr, 0);
     unsigned BlkNum = 1;
 
     // Initialize the worklist with the roots from the backward traversal.
     while (!RootList.empty()) {
       Info = RootList.pop_back_val();
       Info->IDom = PseudoEntry;
       Info->BlkNum = -1;
       WorkList.push_back(Info);
     }
 
     while (!WorkList.empty()) {
       Info = WorkList.back();
 
       if (Info->BlkNum == -2) {
         // All the successors have been handled; assign the postorder number.
         Info->BlkNum = BlkNum++;
         // If not a root, put it on the BlockList.
         if (!Info->AvailableVal)
           BlockList->push_back(Info);
         WorkList.pop_back();
         continue;
       }
 
       // Leave this entry on the worklist, but set its BlkNum to mark that its
       // successors have been put on the worklist.  When it returns to the top
       // the list, after handling its successors, it will be assigned a
       // number.
       Info->BlkNum = -2;
 
       // Add unvisited successors to the work list.
       for (typename Traits::BlkSucc_iterator SI =
              Traits::BlkSucc_begin(Info->BB),
              E = Traits::BlkSucc_end(Info->BB); SI != E; ++SI) {
         BBInfo *SuccInfo = BBMap[*SI];
         if (!SuccInfo || SuccInfo->BlkNum)
           continue;
         SuccInfo->BlkNum = -1;
         WorkList.push_back(SuccInfo);
       }
     }
     PseudoEntry->BlkNum = BlkNum;
     return PseudoEntry;
   }
 
   /// IntersectDominators - This is the dataflow lattice "meet" operation for
   /// finding dominators.  Given two basic blocks, it walks up the dominator
   /// tree until it finds a common dominator of both.  It uses the postorder
   /// number of the blocks to determine how to do that.
   BBInfo *IntersectDominators(BBInfo *Blk1, BBInfo *Blk2) {
     while (Blk1 != Blk2) {
       while (Blk1->BlkNum < Blk2->BlkNum) {
         Blk1 = Blk1->IDom;
         if (!Blk1)
           return Blk2;
       }
       while (Blk2->BlkNum < Blk1->BlkNum) {
         Blk2 = Blk2->IDom;
         if (!Blk2)
           return Blk1;
       }
     }
     return Blk1;
   }
 
   /// FindDominators - Calculate the dominator tree for the subset of the CFG
   /// corresponding to the basic blocks on the BlockList.  This uses the
   /// algorithm from: "A Simple, Fast Dominance Algorithm" by Cooper, Harvey
   /// and Kennedy, published in Software--Practice and Experience, 2001,
   /// 4:1-10.  Because the CFG subset does not include any edges leading into
   /// blocks that define the value, the results are not the usual dominator
   /// tree.  The CFG subset has a single pseudo-entry node with edges to a set
   /// of root nodes for blocks that define the value.  The dominators for this
   /// subset CFG are not the standard dominators but they are adequate for
   /// placing PHIs within the subset CFG.
   void FindDominators(BlockListTy *BlockList, BBInfo *PseudoEntry) {
     bool Changed;
     do {
       Changed = false;
       // Iterate over the list in reverse order, i.e., forward on CFG edges.
       for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
              E = BlockList->rend(); I != E; ++I) {
         BBInfo *Info = *I;
         BBInfo *NewIDom = nullptr;
 
         // Iterate through the block's predecessors.
         for (unsigned p = 0; p != Info->NumPreds; ++p) {
           BBInfo *Pred = Info->Preds[p];
 
           // Treat an unreachable predecessor as a definition with 'poison'.
           if (Pred->BlkNum == 0) {
             Pred->AvailableVal = Traits::GetPoisonVal(Pred->BB, Updater);
             (*AvailableVals)[Pred->BB] = Pred->AvailableVal;
             Pred->DefBB = Pred;
             Pred->BlkNum = PseudoEntry->BlkNum;
             PseudoEntry->BlkNum++;
           }
 
           if (!NewIDom)
             NewIDom = Pred;
           else
             NewIDom = IntersectDominators(NewIDom, Pred);
         }
 
         // Check if the IDom value has changed.
         if (NewIDom && NewIDom != Info->IDom) {
           Info->IDom = NewIDom;
           Changed = true;
         }
       }
     } while (Changed);
   }
 
   /// IsDefInDomFrontier - Search up the dominator tree from Pred to IDom for
   /// any blocks containing definitions of the value.  If one is found, then
   /// the successor of Pred is in the dominance frontier for the definition,
   /// and this function returns true.
   bool IsDefInDomFrontier(const BBInfo *Pred, const BBInfo *IDom) {
     for (; Pred != IDom; Pred = Pred->IDom) {
       if (Pred->DefBB == Pred)
         return true;
     }
     return false;
   }
 
   /// FindPHIPlacement - PHIs are needed in the iterated dominance frontiers
   /// of the known definitions.  Iteratively add PHIs in the dom frontiers
   /// until nothing changes.  Along the way, keep track of the nearest
   /// dominating definitions for non-PHI blocks.
   void FindPHIPlacement(BlockListTy *BlockList) {
     bool Changed;
     do {
       Changed = false;
       // Iterate over the list in reverse order, i.e., forward on CFG edges.
       for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
              E = BlockList->rend(); I != E; ++I) {
         BBInfo *Info = *I;
 
         // If this block already needs a PHI, there is nothing to do here.
         if (Info->DefBB == Info)
           continue;
 
         // Default to use the same def as the immediate dominator.
         BBInfo *NewDefBB = Info->IDom->DefBB;
         for (unsigned p = 0; p != Info->NumPreds; ++p) {
           if (IsDefInDomFrontier(Info->Preds[p], Info->IDom)) {
             // Need a PHI here.
             NewDefBB = Info;
             break;
           }
         }
 
         // Check if anything changed.
         if (NewDefBB != Info->DefBB) {
           Info->DefBB = NewDefBB;
           Changed = true;
         }
       }
     } while (Changed);
   }
 
   /// Check all predecessors and if all of them have the same AvailableVal use
   /// it as value for block represented by Info. Return true if singluar value
   /// is found.
   bool FindSingularVal(BBInfo *Info) {
     if (!Info->NumPreds)
       return false;
     ValT Singular = Info->Preds[0]->DefBB->AvailableVal;
     if (!Singular)
       return false;
     for (unsigned Idx = 1; Idx < Info->NumPreds; ++Idx) {
       ValT PredVal = Info->Preds[Idx]->DefBB->AvailableVal;
       if (!PredVal || Singular != PredVal)
         return false;
     }
     // Record Singular value.
     (*AvailableVals)[Info->BB] = Singular;
     assert(BBMap[Info->BB] == Info && "Info missed in BBMap?");
     Info->AvailableVal = Singular;
     Info->DefBB = Info->Preds[0]->DefBB;
     return true;
   }
 
   /// FindAvailableVal - If this block requires a PHI, first check if an
   /// existing PHI matches the PHI placement and reaching definitions computed
   /// earlier, and if not, create a new PHI.  Visit all the block's
   /// predecessors to calculate the available value for each one and fill in
   /// the incoming values for a new PHI.
   void FindAvailableVals(BlockListTy *BlockList) {
     // Go through the worklist in forward order (i.e., backward through the CFG)
     // and check if existing PHIs can be used.  If not, create empty PHIs where
     // they are needed.
     for (typename BlockListTy::iterator I = BlockList->begin(),
            E = BlockList->end(); I != E; ++I) {
       BBInfo *Info = *I;
       // Check if there needs to be a PHI in BB.
       if (Info->DefBB != Info)
         continue;
 
       // Look for singular value.
       if (FindSingularVal(Info))
         continue;
 
       // Look for an existing PHI.
       FindExistingPHI(Info->BB, BlockList);
       if (Info->AvailableVal)
         continue;
 
       ValT PHI = Traits::CreateEmptyPHI(Info->BB, Info->NumPreds, Updater);
       Info->AvailableVal = PHI;
       (*AvailableVals)[Info->BB] = PHI;
     }
 
     // Now go back through the worklist in reverse order to fill in the
     // arguments for any new PHIs added in the forward traversal.
     for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
            E = BlockList->rend(); I != E; ++I) {
       BBInfo *Info = *I;
 
       if (Info->DefBB != Info) {
         // Record the available value to speed up subsequent uses of this
         // SSAUpdater for the same value.
         (*AvailableVals)[Info->BB] = Info->DefBB->AvailableVal;
         continue;
       }
 
       // Check if this block contains a newly added PHI.
       PhiT *PHI = Traits::ValueIsNewPHI(Info->AvailableVal, Updater);
       if (!PHI)
         continue;
 
       // Iterate through the block's predecessors.
       for (unsigned p = 0; p != Info->NumPreds; ++p) {
         BBInfo *PredInfo = Info->Preds[p];
         BlkT *Pred = PredInfo->BB;
         // Skip to the nearest preceding definition.
         if (PredInfo->DefBB != PredInfo)
           PredInfo = PredInfo->DefBB;
         Traits::AddPHIOperand(PHI, PredInfo->AvailableVal, Pred);
       }
 
       LLVM_DEBUG(dbgs() << "  Inserted PHI: " << *PHI << "\n");
 
       // If the client wants to know about all new instructions, tell it.
       if (InsertedPHIs) InsertedPHIs->push_back(PHI);
     }
   }
 
   /// FindExistingPHI - Look through the PHI nodes in a block to see if any of
   /// them match what is needed.
   void FindExistingPHI(BlkT *BB, BlockListTy *BlockList) {
     SmallVector<BBInfo *, 20> TaggedBlocks;
     for (auto &SomePHI : BB->phis()) {
       if (CheckIfPHIMatches(&SomePHI, TaggedBlocks)) {
         RecordMatchingPHIs(BlockList);
         break;
       }
     }
   }
 
   /// CheckIfPHIMatches - Check if a PHI node matches the placement and values
   /// in the BBMap.
   bool CheckIfPHIMatches(PhiT *PHI, SmallVectorImpl<BBInfo *> &TaggedBlocks) {
     // Match failed: clear all the PHITag values. Only need to clear visited
     // blocks.
     auto Cleanup = make_scope_exit([&]() {
       for (BBInfo *TaggedBlock : TaggedBlocks)
         TaggedBlock->PHITag = nullptr;
       TaggedBlocks.clear();
     });
 
     SmallVector<PhiT *, 20> WorkList;
     WorkList.push_back(PHI);
 
     // Mark that the block containing this PHI has been visited.
     BBInfo *PHIBlock = BBMap[PHI->getParent()];
     PHIBlock->PHITag = PHI;
     TaggedBlocks.push_back(PHIBlock);
 
     while (!WorkList.empty()) {
       PHI = WorkList.pop_back_val();
 
       // Iterate through the PHI's incoming values.
       for (typename Traits::PHI_iterator I = Traits::PHI_begin(PHI),
              E = Traits::PHI_end(PHI); I != E; ++I) {
         ValT IncomingVal = I.getIncomingValue();
         BBInfo *PredInfo = BBMap[I.getIncomingBlock()];
         // Skip to the nearest preceding definition.
         if (PredInfo->DefBB != PredInfo)
           PredInfo = PredInfo->DefBB;
 
         // Check if it matches the expected value.
         if (PredInfo->AvailableVal) {
           if (IncomingVal == PredInfo->AvailableVal)
             continue;
           return false;
         }
 
         // Check if the value is a PHI in the correct block.
         PhiT *IncomingPHIVal = Traits::ValueIsPHI(IncomingVal, Updater);
         if (!IncomingPHIVal || IncomingPHIVal->getParent() != PredInfo->BB)
           return false;
 
         // If this block has already been visited, check if this PHI matches.
         if (PredInfo->PHITag) {
           if (IncomingPHIVal == PredInfo->PHITag)
             continue;
           return false;
         }
         PredInfo->PHITag = IncomingPHIVal;
         TaggedBlocks.push_back(PredInfo);
 
         WorkList.push_back(IncomingPHIVal);
       }
     }
     // Match found, keep PHITags.
     Cleanup.release();
     return true;
   }
 
   /// RecordMatchingPHIs - For each PHI node that matches, record it in both
   /// the BBMap and the AvailableVals mapping.
   void RecordMatchingPHIs(BlockListTy *BlockList) {
     for (typename BlockListTy::iterator I = BlockList->begin(),
            E = BlockList->end(); I != E; ++I)
       if (PhiT *PHI = (*I)->PHITag) {
         BlkT *BB = PHI->getParent();
         ValT PHIVal = Traits::GetPHIValue(PHI);
         (*AvailableVals)[BB] = PHIVal;
         BBMap[BB]->AvailableVal = PHIVal;
       }
   }
 };
 
 } // end namespace llvm
 
 #undef DEBUG_TYPE // "ssaupdater"
 
 #endif // LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H

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