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/Analysis/CFG.h"
14 #include "llvm/IR/Function.h"
15 #include "llvm/IR/Instructions.h"
16 #include "llvm/IR/Type.h"
17 using namespace llvm;
18
19 /// DemoteRegToStack - This function takes a virtual register computed by an
20 /// Instruction and replaces it with a slot in the stack frame, allocated via
21 /// alloca. This allows the CFG to be changed around without fear of
22 /// invalidating the SSA information for the value. It returns the pointer to
23 /// the alloca inserted to create a stack slot for I.
DemoteRegToStack(Instruction & I,bool VolatileLoads,Instruction * AllocaPoint)24 AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
25 Instruction *AllocaPoint) {
26 if (I.use_empty()) {
27 I.eraseFromParent();
28 return 0;
29 }
30
31 // Create a stack slot to hold the value.
32 AllocaInst *Slot;
33 if (AllocaPoint) {
34 Slot = new AllocaInst(I.getType(), 0,
35 I.getName()+".reg2mem", AllocaPoint);
36 } else {
37 Function *F = I.getParent()->getParent();
38 Slot = new AllocaInst(I.getType(), 0, I.getName()+".reg2mem",
39 F->getEntryBlock().begin());
40 }
41
42 // Change all of the users of the instruction to read from the stack slot.
43 while (!I.use_empty()) {
44 Instruction *U = cast<Instruction>(I.use_back());
45 if (PHINode *PN = dyn_cast<PHINode>(U)) {
46 // If this is a PHI node, we can't insert a load of the value before the
47 // use. Instead insert the load in the predecessor block corresponding
48 // to the incoming value.
49 //
50 // Note that if there are multiple edges from a basic block to this PHI
51 // node that we cannot have multiple loads. The problem is that the
52 // resulting PHI node will have multiple values (from each load) coming in
53 // from the same block, which is illegal SSA form. For this reason, we
54 // keep track of and reuse loads we insert.
55 DenseMap<BasicBlock*, Value*> Loads;
56 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
57 if (PN->getIncomingValue(i) == &I) {
58 Value *&V = Loads[PN->getIncomingBlock(i)];
59 if (V == 0) {
60 // Insert the load into the predecessor block
61 V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
62 PN->getIncomingBlock(i)->getTerminator());
63 }
64 PN->setIncomingValue(i, V);
65 }
66
67 } else {
68 // If this is a normal instruction, just insert a load.
69 Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
70 U->replaceUsesOfWith(&I, V);
71 }
72 }
73
74
75 // Insert stores of the computed value into the stack slot. We have to be
76 // careful if I is an invoke instruction, because we can't insert the store
77 // AFTER the terminator instruction.
78 BasicBlock::iterator InsertPt;
79 if (!isa<TerminatorInst>(I)) {
80 InsertPt = &I;
81 ++InsertPt;
82 } else {
83 InvokeInst &II = cast<InvokeInst>(I);
84 if (II.getNormalDest()->getSinglePredecessor())
85 InsertPt = II.getNormalDest()->getFirstInsertionPt();
86 else {
87 // We cannot demote invoke instructions to the stack if their normal edge
88 // is critical. Therefore, split the critical edge and insert the store
89 // in the newly created basic block.
90 unsigned SuccNum = GetSuccessorNumber(I.getParent(), II.getNormalDest());
91 TerminatorInst *TI = &cast<TerminatorInst>(I);
92 assert (isCriticalEdge(TI, SuccNum) &&
93 "Expected a critical edge!");
94 BasicBlock *BB = SplitCriticalEdge(TI, SuccNum);
95 assert (BB && "Unable to split critical edge.");
96 InsertPt = BB->getFirstInsertionPt();
97 }
98 }
99
100 for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
101 /* empty */; // Don't insert before PHI nodes or landingpad instrs.
102
103 new StoreInst(&I, Slot, InsertPt);
104 return Slot;
105 }
106
107 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
108 /// node and replaces it with a slot in the stack frame allocated via alloca.
109 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
DemotePHIToStack(PHINode * P,Instruction * AllocaPoint)110 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
111 if (P->use_empty()) {
112 P->eraseFromParent();
113 return 0;
114 }
115
116 // Create a stack slot to hold the value.
117 AllocaInst *Slot;
118 if (AllocaPoint) {
119 Slot = new AllocaInst(P->getType(), 0,
120 P->getName()+".reg2mem", AllocaPoint);
121 } else {
122 Function *F = P->getParent()->getParent();
123 Slot = new AllocaInst(P->getType(), 0, P->getName()+".reg2mem",
124 F->getEntryBlock().begin());
125 }
126
127 // Iterate over each operand inserting a store in each predecessor.
128 for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
129 if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
130 assert(II->getParent() != P->getIncomingBlock(i) &&
131 "Invoke edge not supported yet"); (void)II;
132 }
133 new StoreInst(P->getIncomingValue(i), Slot,
134 P->getIncomingBlock(i)->getTerminator());
135 }
136
137 // Insert a load in place of the PHI and replace all uses.
138 BasicBlock::iterator InsertPt = P;
139
140 for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
141 /* empty */; // Don't insert before PHI nodes or landingpad instrs.
142
143 Value *V = new LoadInst(Slot, P->getName()+".reload", InsertPt);
144 P->replaceAllUsesWith(V);
145
146 // Delete PHI.
147 P->eraseFromParent();
148 return Slot;
149 }
150