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      1 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 
     10 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
     11 #include "llvm/ADT/DenseMap.h"
     12 #include "llvm/Analysis/CFG.h"
     13 #include "llvm/IR/Function.h"
     14 #include "llvm/IR/Instructions.h"
     15 #include "llvm/IR/Type.h"
     16 #include "llvm/Transforms/Utils/Local.h"
     17 using namespace llvm;
     18 
     19 /// DemoteRegToStack - This function takes a virtual register computed by an
     20 /// Instruction and replaces it with a slot in the stack frame, allocated via
     21 /// alloca.  This allows the CFG to be changed around without fear of
     22 /// invalidating the SSA information for the value.  It returns the pointer to
     23 /// the alloca inserted to create a stack slot for I.
     24 AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
     25                                    Instruction *AllocaPoint) {
     26   if (I.use_empty()) {
     27     I.eraseFromParent();
     28     return nullptr;
     29   }
     30 
     31   // Create a stack slot to hold the value.
     32   AllocaInst *Slot;
     33   if (AllocaPoint) {
     34     Slot = new AllocaInst(I.getType(), nullptr,
     35                           I.getName()+".reg2mem", AllocaPoint);
     36   } else {
     37     Function *F = I.getParent()->getParent();
     38     Slot = new AllocaInst(I.getType(), nullptr, I.getName()+".reg2mem",
     39                           F->getEntryBlock().begin());
     40   }
     41 
     42   // We cannot demote invoke instructions to the stack if their normal edge
     43   // is critical. Therefore, split the critical edge and create a basic block
     44   // into which the store can be inserted.
     45   if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
     46     if (!II->getNormalDest()->getSinglePredecessor()) {
     47       unsigned SuccNum = GetSuccessorNumber(II->getParent(), II->getNormalDest());
     48       assert(isCriticalEdge(II, SuccNum) && "Expected a critical edge!");
     49       BasicBlock *BB = SplitCriticalEdge(II, SuccNum);
     50       assert(BB && "Unable to split critical edge.");
     51       (void)BB;
     52     }
     53   }
     54 
     55   // Change all of the users of the instruction to read from the stack slot.
     56   while (!I.use_empty()) {
     57     Instruction *U = cast<Instruction>(I.user_back());
     58     if (PHINode *PN = dyn_cast<PHINode>(U)) {
     59       // If this is a PHI node, we can't insert a load of the value before the
     60       // use.  Instead insert the load in the predecessor block corresponding
     61       // to the incoming value.
     62       //
     63       // Note that if there are multiple edges from a basic block to this PHI
     64       // node that we cannot have multiple loads. The problem is that the
     65       // resulting PHI node will have multiple values (from each load) coming in
     66       // from the same block, which is illegal SSA form. For this reason, we
     67       // keep track of and reuse loads we insert.
     68       DenseMap<BasicBlock*, Value*> Loads;
     69       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
     70         if (PN->getIncomingValue(i) == &I) {
     71           Value *&V = Loads[PN->getIncomingBlock(i)];
     72           if (!V) {
     73             // Insert the load into the predecessor block
     74             V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
     75                              PN->getIncomingBlock(i)->getTerminator());
     76           }
     77           PN->setIncomingValue(i, V);
     78         }
     79 
     80     } else {
     81       // If this is a normal instruction, just insert a load.
     82       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
     83       U->replaceUsesOfWith(&I, V);
     84     }
     85   }
     86 
     87   // Insert stores of the computed value into the stack slot. We have to be
     88   // careful if I is an invoke instruction, because we can't insert the store
     89   // AFTER the terminator instruction.
     90   BasicBlock::iterator InsertPt;
     91   if (!isa<TerminatorInst>(I)) {
     92     InsertPt = &I;
     93     ++InsertPt;
     94     for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
     95       /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
     96   } else {
     97     InvokeInst &II = cast<InvokeInst>(I);
     98     InsertPt = II.getNormalDest()->getFirstInsertionPt();
     99   }
    100 
    101   new StoreInst(&I, Slot, InsertPt);
    102   return Slot;
    103 }
    104 
    105 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
    106 /// node and replaces it with a slot in the stack frame allocated via alloca.
    107 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
    108 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
    109   if (P->use_empty()) {
    110     P->eraseFromParent();
    111     return nullptr;
    112   }
    113 
    114   // Create a stack slot to hold the value.
    115   AllocaInst *Slot;
    116   if (AllocaPoint) {
    117     Slot = new AllocaInst(P->getType(), nullptr,
    118                           P->getName()+".reg2mem", AllocaPoint);
    119   } else {
    120     Function *F = P->getParent()->getParent();
    121     Slot = new AllocaInst(P->getType(), nullptr, P->getName()+".reg2mem",
    122                           F->getEntryBlock().begin());
    123   }
    124 
    125   // Iterate over each operand inserting a store in each predecessor.
    126   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
    127     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
    128       assert(II->getParent() != P->getIncomingBlock(i) &&
    129              "Invoke edge not supported yet"); (void)II;
    130     }
    131     new StoreInst(P->getIncomingValue(i), Slot,
    132                   P->getIncomingBlock(i)->getTerminator());
    133   }
    134 
    135   // Insert a load in place of the PHI and replace all uses.
    136   BasicBlock::iterator InsertPt = P;
    137 
    138   for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
    139     /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
    140 
    141   Value *V = new LoadInst(Slot, P->getName()+".reload", InsertPt);
    142   P->replaceAllUsesWith(V);
    143 
    144   // Delete PHI.
    145   P->eraseFromParent();
    146   return Slot;
    147 }
    148