1 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
17 //===----------------------------------------------------------------------===//
19 #include "llvm/Transforms/Utils/Local.h"
20 #include "llvm/Function.h"
21 #include "llvm/Instructions.h"
22 #include "llvm/Type.h"
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.
32 AllocaInst* llvm::DemoteRegToStack(LLVMContext &Context,
33 Instruction &I, bool VolatileLoads,
34 Instruction *AllocaPoint) {
40 // Create a stack slot to hold the value.
43 Slot = new AllocaInst(Context, I.getType(), 0,
44 I.getName()+".reg2mem", AllocaPoint);
46 Function *F = I.getParent()->getParent();
47 Slot = new AllocaInst(Context, I.getType(), 0, I.getName()+".reg2mem",
48 F->getEntryBlock().begin());
51 // Change all of the users of the instruction to read from the stack slot
53 while (!I.use_empty()) {
54 Instruction *U = cast<Instruction>(I.use_back());
55 if (PHINode *PN = dyn_cast<PHINode>(U)) {
56 // If this is a PHI node, we can't insert a load of the value before the
57 // use. Instead, insert the load in the predecessor block corresponding
58 // to the incoming value.
60 // Note that if there are multiple edges from a basic block to this PHI
61 // node that we cannot multiple loads. The problem is that the resultant
62 // PHI node will have multiple values (from each load) coming in from the
63 // same block, which is illegal SSA form. For this reason, we keep track
64 // and reuse loads we insert.
65 std::map<BasicBlock*, Value*> Loads;
66 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
67 if (PN->getIncomingValue(i) == &I) {
68 Value *&V = Loads[PN->getIncomingBlock(i)];
70 // Insert the load into the predecessor block
71 V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
72 PN->getIncomingBlock(i)->getTerminator());
74 PN->setIncomingValue(i, V);
78 // If this is a normal instruction, just insert a load.
79 Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
80 U->replaceUsesOfWith(&I, V);
85 // Insert stores of the computed value into the stack slot. We have to be
86 // careful is I is an invoke instruction though, because we can't insert the
87 // store AFTER the terminator instruction.
88 BasicBlock::iterator InsertPt;
89 if (!isa<TerminatorInst>(I)) {
93 // We cannot demote invoke instructions to the stack if their normal edge
95 InvokeInst &II = cast<InvokeInst>(I);
96 assert(II.getNormalDest()->getSinglePredecessor() &&
97 "Cannot demote invoke with a critical successor!");
98 InsertPt = II.getNormalDest()->begin();
101 for (; isa<PHINode>(InsertPt); ++InsertPt)
102 /* empty */; // Don't insert before any PHI nodes.
103 new StoreInst(&I, Slot, InsertPt);
109 /// DemotePHIToStack - This function takes a virtual register computed by a phi
110 /// node and replaces it with a slot in the stack frame, allocated via alloca.
111 /// The phi node is deleted and it returns the pointer to the alloca inserted.
112 AllocaInst* llvm::DemotePHIToStack(LLVMContext &Context, PHINode *P,
113 Instruction *AllocaPoint) {
114 if (P->use_empty()) {
115 P->eraseFromParent();
119 // Create a stack slot to hold the value.
122 Slot = new AllocaInst(Context, P->getType(), 0,
123 P->getName()+".reg2mem", AllocaPoint);
125 Function *F = P->getParent()->getParent();
126 Slot = new AllocaInst(Context, P->getType(), 0, P->getName()+".reg2mem",
127 F->getEntryBlock().begin());
130 // Iterate over each operand, insert store in each predecessor.
131 for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
132 if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
133 assert(II->getParent() != P->getIncomingBlock(i) &&
134 "Invoke edge not supported yet"); II=II;
136 new StoreInst(P->getIncomingValue(i), Slot,
137 P->getIncomingBlock(i)->getTerminator());
140 // Insert load in place of the phi and replace all uses.
141 Value *V = new LoadInst(Slot, P->getName()+".reload", P);
142 P->replaceAllUsesWith(V);
145 P->eraseFromParent();