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 } 147
