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 // This file provide the function DemoteRegToStack().  This function takes a
  11 // virtual register computed by an Instruction and replaces it with a slot in
  12 // the stack frame, allocated via alloca. It returns the pointer to the
  13 // AllocaInst inserted.  After this function is called on an instruction, we are
  14 // guaranteed that the only user of the instruction is a store that is
  15 // immediately after it.
  16 //
  17 //===----------------------------------------------------------------------===//
  18
  19 #include "llvm/Transforms/Utils/Local.h"
  20 #include "llvm/Function.h"
  21 #include "llvm/Instructions.h"
  22 #include "llvm/Type.h"
  23 #include <map>
  24 using namespace llvm;
  25
  26 /// DemoteRegToStack - This function takes a virtual register computed by an
  27 /// Instruction and replaces it with a slot in the stack frame, allocated via
  28 /// alloca.  This allows the CFG to be changed around without fear of
  29 /// invalidating the SSA information for the value.  It returns the pointer to
  30 /// the alloca inserted to create a stack slot for I.
  31 ///
  32 AllocaInst* llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
  33                                    Instruction *AllocaPoint) {
  34   if (I.use_empty()) {
  35     I.eraseFromParent();
  36     return 0;
  37   }
  38
  39   // Create a stack slot to hold the value.
  40   AllocaInst *Slot;
  41   if (AllocaPoint) {
  42     Slot = new AllocaInst(I.getType(), 0, I.getName()+".reg2mem", AllocaPoint);
  43   } else {
  44     Function *F = I.getParent()->getParent();
  45     Slot = new AllocaInst(I.getType(), 0, I.getName()+".reg2mem",
  46                           F->getEntryBlock().begin());
  47   }
  48
  49   // Change all of the users of the instruction to read from the stack slot
  50   // instead.
  51   while (!I.use_empty()) {
  52     Instruction *U = cast<Instruction>(I.use_back());
  53     if (PHINode *PN = dyn_cast<PHINode>(U)) {
  54       // If this is a PHI node, we can't insert a load of the value before the
  55       // use.  Instead, insert the load in the predecessor block corresponding
  56       // to the incoming value.
  57       //
  58       // Note that if there are multiple edges from a basic block to this PHI
  59       // node that we cannot multiple loads.  The problem is that the resultant
  60       // PHI node will have multiple values (from each load) coming in from the
  61       // same block, which is illegal SSA form.  For this reason, we keep track
  62       // and reuse loads we insert.
  63       std::map<BasicBlock*, Value*> Loads;
  64       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
  65         if (PN->getIncomingValue(i) == &I) {
  66           Value *&V = Loads[PN->getIncomingBlock(i)];
  67           if (V == 0) {
  68             // Insert the load into the predecessor block
  69             V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
  70                              PN->getIncomingBlock(i)->getTerminator());
  71           }
  72           PN->setIncomingValue(i, V);
  73         }
  74
  75     } else {
  76       // If this is a normal instruction, just insert a load.
  77       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
  78       U->replaceUsesOfWith(&I, V);
  79     }
  80   }
  81
  82
  83   // Insert stores of the computed value into the stack slot.  We have to be
  84   // careful is I is an invoke instruction though, because we can't insert the
  85   // store AFTER the terminator instruction.
  86   BasicBlock::iterator InsertPt;
  87   if (!isa<TerminatorInst>(I)) {
  88     InsertPt = &I;
  89     ++InsertPt;
  90   } else {
  91     // We cannot demote invoke instructions to the stack if their normal edge
  92     // is critical.
  93     InvokeInst &II = cast<InvokeInst>(I);
  94     assert(II.getNormalDest()->getSinglePredecessor() &&
  95            "Cannot demote invoke with a critical successor!");
  96     InsertPt = II.getNormalDest()->begin();
  97   }
  98
  99   for (; isa<PHINode>(InsertPt); ++InsertPt)
 100   /* empty */;   // Don't insert before any PHI nodes.
 101   new StoreInst(&I, Slot, InsertPt);
 102
 103   return Slot;
 104 }
 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 and 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 0;
 114   }
 115
 116   // Create a stack slot to hold the value.
 117   AllocaInst *Slot;
 118   if (AllocaPoint) {
 119     Slot = new AllocaInst(P->getType(), 0, P->getName()+".reg2mem", AllocaPoint);
 120   } else {
 121     Function *F = P->getParent()->getParent();
 122     Slot = new AllocaInst(P->getType(), 0, P->getName()+".reg2mem",
 123                           F->getEntryBlock().begin());
 124   }
 125
 126   // Iterate over each operand, insert store in each predecessor.
 127   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
 128     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
 129       assert(II->getParent() != P->getIncomingBlock(i) &&
 130              "Invoke edge not supported yet"); II=II;
 131     }
 132     new StoreInst(P->getIncomingValue(i), Slot,
 133                   P->getIncomingBlock(i)->getTerminator());
 134   }
 135
 136   // Insert load in place of the phi and replace all uses.
 137   Value *V = new LoadInst(Slot, P->getName()+".reload", P);
 138   P->replaceAllUsesWith(V);
 139
 140   // Delete phi.
 141   P->eraseFromParent();
 142
 143   return Slot;
 144 }