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/ADT/DenseMap.h"
12 #include "llvm/Analysis/CFG.h"
13 #include "llvm/IR/Function.h"
14 #include "llvm/IR/Instructions.h"
15 #include "llvm/IR/Type.h"
16 #include "llvm/Transforms/Utils/Local.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 nullptr;
29 }
30
31 // Create a stack slot to hold the value.
32 AllocaInst *Slot;
33 if (AllocaPoint) {
34 Slot = new AllocaInst(I.getType(), nullptr,
35 I.getName()+".reg2mem", AllocaPoint);
36 } else {
37 Function *F = I.getParent()->getParent();
38 Slot = new AllocaInst(I.getType(), nullptr, I.getName() + ".reg2mem",
39 &F->getEntryBlock().front());
40 }
41
42 // We cannot demote invoke instructions to the stack if their normal edge
43 // is critical. Therefore, split the critical edge and create a basic block
44 // into which the store can be inserted.
45 if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
46 if (!II->getNormalDest()->getSinglePredecessor()) {
47 unsigned SuccNum = GetSuccessorNumber(II->getParent(), II->getNormalDest());
48 assert(isCriticalEdge(II, SuccNum) && "Expected a critical edge!");
49 BasicBlock *BB = SplitCriticalEdge(II, SuccNum);
50 assert(BB && "Unable to split critical edge.");
51 (void)BB;
52 }
53 }
54
55 // Change all of the users of the instruction to read from the stack slot.
56 while (!I.use_empty()) {
57 Instruction *U = cast<Instruction>(I.user_back());
58 if (PHINode *PN = dyn_cast<PHINode>(U)) {
59 // If this is a PHI node, we can't insert a load of the value before the
60 // use. Instead insert the load in the predecessor block corresponding
61 // to the incoming value.
62 //
63 // Note that if there are multiple edges from a basic block to this PHI
64 // node that we cannot have multiple loads. The problem is that the
65 // resulting PHI node will have multiple values (from each load) coming in
66 // from the same block, which is illegal SSA form. For this reason, we
67 // keep track of and reuse loads we insert.
68 DenseMap<BasicBlock*, Value*> Loads;
69 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
70 if (PN->getIncomingValue(i) == &I) {
71 Value *&V = Loads[PN->getIncomingBlock(i)];
72 if (!V) {
73 // Insert the load into the predecessor block
74 V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
75 PN->getIncomingBlock(i)->getTerminator());
76 }
77 PN->setIncomingValue(i, V);
78 }
79
80 } else {
81 // If this is a normal instruction, just insert a load.
82 Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
83 U->replaceUsesOfWith(&I, V);
84 }
85 }
86
87 // Insert stores of the computed value into the stack slot. We have to be
88 // careful if I is an invoke instruction, because we can't insert the store
89 // AFTER the terminator instruction.
90 BasicBlock::iterator InsertPt;
91 if (!isa<TerminatorInst>(I)) {
92 InsertPt = ++I.getIterator();
93 for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
94 /* empty */; // Don't insert before PHI nodes or landingpad instrs.
95 } else {
96 InvokeInst &II = cast<InvokeInst>(I);
97 InsertPt = II.getNormalDest()->getFirstInsertionPt();
98 }
99
100 new StoreInst(&I, Slot, &*InsertPt);
101 return Slot;
102 }
103
104 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
105 /// node and replaces it with a slot in the stack frame allocated via alloca.
106 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
DemotePHIToStack(PHINode * P,Instruction * AllocaPoint)107 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
108 if (P->use_empty()) {
109 P->eraseFromParent();
110 return nullptr;
111 }
112
113 // Create a stack slot to hold the value.
114 AllocaInst *Slot;
115 if (AllocaPoint) {
116 Slot = new AllocaInst(P->getType(), nullptr,
117 P->getName()+".reg2mem", AllocaPoint);
118 } else {
119 Function *F = P->getParent()->getParent();
120 Slot = new AllocaInst(P->getType(), nullptr, P->getName() + ".reg2mem",
121 &F->getEntryBlock().front());
122 }
123
124 // Iterate over each operand inserting a store in each predecessor.
125 for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
126 if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
127 assert(II->getParent() != P->getIncomingBlock(i) &&
128 "Invoke edge not supported yet"); (void)II;
129 }
130 new StoreInst(P->getIncomingValue(i), Slot,
131 P->getIncomingBlock(i)->getTerminator());
132 }
133
134 // Insert a load in place of the PHI and replace all uses.
135 BasicBlock::iterator InsertPt = P->getIterator();
136
137 for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
138 /* empty */; // Don't insert before PHI nodes or landingpad instrs.
139
140 Value *V = new LoadInst(Slot, P->getName() + ".reload", &*InsertPt);
141 P->replaceAllUsesWith(V);
142
143 // Delete PHI.
144 P->eraseFromParent();
145 return Slot;
146 }
147