DemoteRegToStack.cpp revision 249423
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/Transforms/Utils/Local.h" 12#include "llvm/ADT/DenseMap.h" 13#include "llvm/IR/Function.h" 14#include "llvm/IR/Instructions.h" 15#include "llvm/IR/Type.h" 16using namespace llvm; 17 18/// DemoteRegToStack - This function takes a virtual register computed by an 19/// Instruction and replaces it with a slot in the stack frame, allocated via 20/// alloca. This allows the CFG to be changed around without fear of 21/// invalidating the SSA information for the value. It returns the pointer to 22/// the alloca inserted to create a stack slot for I. 23AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads, 24 Instruction *AllocaPoint) { 25 if (I.use_empty()) { 26 I.eraseFromParent(); 27 return 0; 28 } 29 30 // Create a stack slot to hold the value. 31 AllocaInst *Slot; 32 if (AllocaPoint) { 33 Slot = new AllocaInst(I.getType(), 0, 34 I.getName()+".reg2mem", AllocaPoint); 35 } else { 36 Function *F = I.getParent()->getParent(); 37 Slot = new AllocaInst(I.getType(), 0, I.getName()+".reg2mem", 38 F->getEntryBlock().begin()); 39 } 40 41 // Change all of the users of the instruction to read from the stack slot. 42 while (!I.use_empty()) { 43 Instruction *U = cast<Instruction>(I.use_back()); 44 if (PHINode *PN = dyn_cast<PHINode>(U)) { 45 // If this is a PHI node, we can't insert a load of the value before the 46 // use. Instead insert the load in the predecessor block corresponding 47 // to the incoming value. 48 // 49 // Note that if there are multiple edges from a basic block to this PHI 50 // node that we cannot have multiple loads. The problem is that the 51 // resulting PHI node will have multiple values (from each load) coming in 52 // from the same block, which is illegal SSA form. For this reason, we 53 // keep track of and reuse loads we insert. 54 DenseMap<BasicBlock*, Value*> Loads; 55 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 56 if (PN->getIncomingValue(i) == &I) { 57 Value *&V = Loads[PN->getIncomingBlock(i)]; 58 if (V == 0) { 59 // Insert the load into the predecessor block 60 V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, 61 PN->getIncomingBlock(i)->getTerminator()); 62 } 63 PN->setIncomingValue(i, V); 64 } 65 66 } else { 67 // If this is a normal instruction, just insert a load. 68 Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U); 69 U->replaceUsesOfWith(&I, V); 70 } 71 } 72 73 74 // Insert stores of the computed value into the stack slot. We have to be 75 // careful if I is an invoke instruction, because we can't insert the store 76 // AFTER the terminator instruction. 77 BasicBlock::iterator InsertPt; 78 if (!isa<TerminatorInst>(I)) { 79 InsertPt = &I; 80 ++InsertPt; 81 } else { 82 InvokeInst &II = cast<InvokeInst>(I); 83 if (II.getNormalDest()->getSinglePredecessor()) 84 InsertPt = II.getNormalDest()->getFirstInsertionPt(); 85 else { 86 // We cannot demote invoke instructions to the stack if their normal edge 87 // is critical. Therefore, split the critical edge and insert the store 88 // in the newly created basic block. 89 unsigned SuccNum = GetSuccessorNumber(I.getParent(), II.getNormalDest()); 90 TerminatorInst *TI = &cast<TerminatorInst>(I); 91 assert (isCriticalEdge(TI, SuccNum) && 92 "Expected a critical edge!"); 93 BasicBlock *BB = SplitCriticalEdge(TI, SuccNum); 94 assert (BB && "Unable to split critical edge."); 95 InsertPt = BB->getFirstInsertionPt(); 96 } 97 } 98 99 for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt) 100 /* empty */; // Don't insert before PHI nodes or landingpad instrs. 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. 109AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) { 110 if (P->use_empty()) { 111 P->eraseFromParent(); 112 return 0; 113 } 114 115 // Create a stack slot to hold the value. 116 AllocaInst *Slot; 117 if (AllocaPoint) { 118 Slot = new AllocaInst(P->getType(), 0, 119 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 inserting a 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"); (void)II; 131 } 132 new StoreInst(P->getIncomingValue(i), Slot, 133 P->getIncomingBlock(i)->getTerminator()); 134 } 135 136 // Insert a load in place of the PHI and replace all uses. 137 BasicBlock::iterator InsertPt = P; 138 139 for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt) 140 /* empty */; // Don't insert before PHI nodes or landingpad instrs. 141 142 Value *V = new LoadInst(Slot, P->getName()+".reload", InsertPt); 143 P->replaceAllUsesWith(V); 144 145 // Delete PHI. 146 P->eraseFromParent(); 147 return Slot; 148} 149