lib/Transforms/Utils/DemoteRegToStack.cpp

   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/ADT/DenseMap.h"
  11 #include "llvm/Analysis/CFG.h"
  12 #include "llvm/IR/Function.h"
  13 #include "llvm/IR/Instructions.h"
  14 #include "llvm/IR/Type.h"
  15 #include "llvm/Transforms/Utils/BasicBlockUtils.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   Function *F = I.getParent()->getParent();
  32   const DataLayout &DL = F->getParent()->getDataLayout();
  33
  34   // Create a stack slot to hold the value.
  35   AllocaInst *Slot;
  36   if (AllocaPoint) {
  37     Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
  38                           I.getName()+".reg2mem", AllocaPoint);
  39   } else {
  40     Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
  41                           I.getName() + ".reg2mem", &F->getEntryBlock().front());
  42   }
  43
  44   // We cannot demote invoke instructions to the stack if their normal edge
  45   // is critical. Therefore, split the critical edge and create a basic block
  46   // into which the store can be inserted.
  47   if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
  48     if (!II->getNormalDest()->getSinglePredecessor()) {
  49       unsigned SuccNum = GetSuccessorNumber(II->getParent(), II->getNormalDest());
  50       assert(isCriticalEdge(II, SuccNum) && "Expected a critical edge!");
  51       BasicBlock *BB = SplitCriticalEdge(II, SuccNum);
  52       assert(BB && "Unable to split critical edge.");
  53       (void)BB;
  54     }
  55   }
  56
  57   // Change all of the users of the instruction to read from the stack slot.
  58   while (!I.use_empty()) {
  59     Instruction *U = cast<Instruction>(I.user_back());
  60     if (PHINode *PN = dyn_cast<PHINode>(U)) {
  61       // If this is a PHI node, we can't insert a load of the value before the
  62       // use.  Instead insert the load in the predecessor block corresponding
  63       // to the incoming value.
  64       //
  65       // Note that if there are multiple edges from a basic block to this PHI
  66       // node that we cannot have multiple loads. The problem is that the
  67       // resulting PHI node will have multiple values (from each load) coming in
  68       // from the same block, which is illegal SSA form. For this reason, we
  69       // keep track of and reuse loads we insert.
  70       DenseMap<BasicBlock*, Value*> Loads;
  71       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
  72         if (PN->getIncomingValue(i) == &I) {
  73           Value *&V = Loads[PN->getIncomingBlock(i)];
  74           if (!V) {
  75             // Insert the load into the predecessor block
  76             V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
  77                              PN->getIncomingBlock(i)->getTerminator());
  78           }
  79           PN->setIncomingValue(i, V);
  80         }
  81
  82     } else {
  83       // If this is a normal instruction, just insert a load.
  84       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
  85       U->replaceUsesOfWith(&I, V);
  86     }
  87   }
  88
  89   // Insert stores of the computed value into the stack slot. We have to be
  90   // careful if I is an invoke instruction, because we can't insert the store
  91   // AFTER the terminator instruction.
  92   BasicBlock::iterator InsertPt;
  93   if (!isa<TerminatorInst>(I)) {
  94     InsertPt = ++I.getIterator();
  95     for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
  96       /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
  97   } else {
  98     InvokeInst &II = cast<InvokeInst>(I);
  99     InsertPt = II.getNormalDest()->getFirstInsertionPt();
 100   }
 101
 102   new StoreInst(&I, Slot, &*InsertPt);
 103   return Slot;
 104 }
 105
 106 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
 107 /// node and replaces it with a slot in the stack frame allocated via alloca.
 108 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
 109 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
 110   if (P->use_empty()) {
 111     P->eraseFromParent();
 112     return nullptr;
 113   }
 114
 115   const DataLayout &DL = P->getModule()->getDataLayout();
 116
 117   // Create a stack slot to hold the value.
 118   AllocaInst *Slot;
 119   if (AllocaPoint) {
 120     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
 121                           P->getName()+".reg2mem", AllocaPoint);
 122   } else {
 123     Function *F = P->getParent()->getParent();
 124     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
 125                           P->getName() + ".reg2mem",
 126                           &F->getEntryBlock().front());
 127   }
 128
 129   // Iterate over each operand inserting a store in each predecessor.
 130   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
 131     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
 132       assert(II->getParent() != P->getIncomingBlock(i) &&
 133              "Invoke edge not supported yet"); (void)II;
 134     }
 135     new StoreInst(P->getIncomingValue(i), Slot,
 136                   P->getIncomingBlock(i)->getTerminator());
 137   }
 138
 139   // Insert a load in place of the PHI and replace all uses.
 140   BasicBlock::iterator InsertPt = P->getIterator();
 141
 142   for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
 143     /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
 144
 145   Value *V = new LoadInst(Slot, P->getName() + ".reload", &*InsertPt);
 146   P->replaceAllUsesWith(V);
 147
 148   // Delete PHI.
 149   P->eraseFromParent();
 150   return Slot;
 151 }