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