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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   // Change all of the users of the instruction to read from the stack slot.
     43   while (!I.use_empty()) {
     44     Instruction *U = cast<Instruction>(I.user_back());
     45     if (PHINode *PN = dyn_cast<PHINode>(U)) {
     46       // If this is a PHI node, we can't insert a load of the value before the
     47       // use.  Instead insert the load in the predecessor block corresponding
     48       // to the incoming value.
     49       //
     50       // Note that if there are multiple edges from a basic block to this PHI
     51       // node that we cannot have multiple loads. The problem is that the
     52       // resulting PHI node will have multiple values (from each load) coming in
     53       // from the same block, which is illegal SSA form. For this reason, we
     54       // keep track of and reuse loads we insert.
     55       DenseMap<BasicBlock*, Value*> Loads;
     56       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
     57         if (PN->getIncomingValue(i) == &I) {
     58           Value *&V = Loads[PN->getIncomingBlock(i)];
     59           if (!V) {
     60             // Insert the load into the predecessor block
     61             V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
     62                              PN->getIncomingBlock(i)->getTerminator());
     63           }
     64           PN->setIncomingValue(i, V);
     65         }
     66 
     67     } else {
     68       // If this is a normal instruction, just insert a load.
     69       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
     70       U->replaceUsesOfWith(&I, V);
     71     }
     72   }
     73 
     74 
     75   // Insert stores of the computed value into the stack slot. We have to be
     76   // careful if I is an invoke instruction, because we can't insert the store
     77   // AFTER the terminator instruction.
     78   BasicBlock::iterator InsertPt;
     79   if (!isa<TerminatorInst>(I)) {
     80     InsertPt = &I;
     81     ++InsertPt;
     82   } else {
     83     InvokeInst &II = cast<InvokeInst>(I);
     84     if (II.getNormalDest()->getSinglePredecessor())
     85       InsertPt = II.getNormalDest()->getFirstInsertionPt();
     86     else {
     87       // We cannot demote invoke instructions to the stack if their normal edge
     88       // is critical.  Therefore, split the critical edge and insert the store
     89       // in the newly created basic block.
     90       unsigned SuccNum = GetSuccessorNumber(I.getParent(), II.getNormalDest());
     91       TerminatorInst *TI = &cast<TerminatorInst>(I);
     92       assert (isCriticalEdge(TI, SuccNum) &&
     93               "Expected a critical edge!");
     94       BasicBlock *BB = SplitCriticalEdge(TI, SuccNum);
     95       assert (BB && "Unable to split critical edge.");
     96       InsertPt = BB->getFirstInsertionPt();
     97     }
     98   }
     99 
    100   for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
    101     /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
    102 
    103   new StoreInst(&I, Slot, InsertPt);
    104   return Slot;
    105 }
    106 
    107 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
    108 /// node and replaces it with a slot in the stack frame allocated via alloca.
    109 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
    110 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
    111   if (P->use_empty()) {
    112     P->eraseFromParent();
    113     return nullptr;
    114   }
    115 
    116   // Create a stack slot to hold the value.
    117   AllocaInst *Slot;
    118   if (AllocaPoint) {
    119     Slot = new AllocaInst(P->getType(), nullptr,
    120                           P->getName()+".reg2mem", AllocaPoint);
    121   } else {
    122     Function *F = P->getParent()->getParent();
    123     Slot = new AllocaInst(P->getType(), nullptr, P->getName()+".reg2mem",
    124                           F->getEntryBlock().begin());
    125   }
    126 
    127   // Iterate over each operand inserting a store in each predecessor.
    128   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
    129     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
    130       assert(II->getParent() != P->getIncomingBlock(i) &&
    131              "Invoke edge not supported yet"); (void)II;
    132     }
    133     new StoreInst(P->getIncomingValue(i), Slot,
    134                   P->getIncomingBlock(i)->getTerminator());
    135   }
    136 
    137   // Insert a load in place of the PHI and replace all uses.
    138   BasicBlock::iterator InsertPt = P;
    139 
    140   for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
    141     /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
    142 
    143   Value *V = new LoadInst(Slot, P->getName()+".reload", InsertPt);
    144   P->replaceAllUsesWith(V);
    145 
    146   // Delete PHI.
    147   P->eraseFromParent();
    148   return Slot;
    149 }
    150