1 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file was developed by the LLVM research group and is distributed under 6 // the University of Illinois Open Source 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) { 33 if (I.use_empty()) return 0; // nothing to do! 34 35 // Create a stack slot to hold the value. 36 Function *F = I.getParent()->getParent(); 37 AllocaInst *Slot = new AllocaInst(I.getType(), 0, I.getName(), 38 F->getEntryBlock().begin()); 39 40 // Change all of the users of the instruction to read from the stack slot 41 // instead. 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 multiple loads. The problem is that the resultant 51 // PHI node will have multiple values (from each load) coming in from the 52 // same block, which is illegal SSA form. For this reason, we keep track 53 // and reuse loads we insert. 54 std::map<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", 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", 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 is I is an invoke instruction though, because we can't insert the 76 // store AFTER the terminator instruction. 77 if (!isa<TerminatorInst>(I)) { 78 BasicBlock::iterator InsertPt = &I; 79 for (++InsertPt; isa<PHINode>(InsertPt); ++InsertPt) 80 /* empty */; // Don't insert before any PHI nodes. 81 new StoreInst(&I, Slot, InsertPt); 82 } else { 83 // FIXME: We cannot yet demote invoke instructions to the stack, because 84 // doing so would require breaking critical edges. This should be fixed 85 // eventually. 86 assert(0 && 87 "Cannot demote the value computed by an invoke instruction yet!"); 88 } 89 90 return Slot; 91 } 92