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