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 // 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, bool VolatileLoads,
33                                    Instruction *AllocaPoint) {
34   if (I.use_empty()) {
35     I.eraseFromParent();
36     return 0;
37   }
38 
39   // Create a stack slot to hold the value.
40   AllocaInst *Slot;
41   if (AllocaPoint) {
42     Slot = new AllocaInst(I.getType(), 0, I.getName()+".reg2mem", AllocaPoint);
43   } else {
44     Function *F = I.getParent()->getParent();
45     Slot = new AllocaInst(I.getType(), 0, I.getName()+".reg2mem",
46                           F->getEntryBlock().begin());
47   }
48 
49   // Change all of the users of the instruction to read from the stack slot
50   // instead.
51   while (!I.use_empty()) {
52     Instruction *U = cast<Instruction>(I.use_back());
53     if (PHINode *PN = dyn_cast<PHINode>(U)) {
54       // If this is a PHI node, we can't insert a load of the value before the
55       // use.  Instead, insert the load in the predecessor block corresponding
56       // to the incoming value.
57       //
58       // Note that if there are multiple edges from a basic block to this PHI
59       // node that we cannot multiple loads.  The problem is that the resultant
60       // PHI node will have multiple values (from each load) coming in from the
61       // same block, which is illegal SSA form.  For this reason, we keep track
62       // and reuse loads we insert.
63       std::map<BasicBlock*, Value*> Loads;
64       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
65         if (PN->getIncomingValue(i) == &I) {
66           Value *&V = Loads[PN->getIncomingBlock(i)];
67           if (V == 0) {
68             // Insert the load into the predecessor block
69             V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
70                              PN->getIncomingBlock(i)->getTerminator());
71           }
72           PN->setIncomingValue(i, V);
73         }
74 
75     } else {
76       // If this is a normal instruction, just insert a load.
77       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
78       U->replaceUsesOfWith(&I, V);
79     }
80   }
81 
82 
83   // Insert stores of the computed value into the stack slot.  We have to be
84   // careful is I is an invoke instruction though, because we can't insert the
85   // store AFTER the terminator instruction.
86   BasicBlock::iterator InsertPt;
87   if (!isa<TerminatorInst>(I)) {
88     InsertPt = &I;
89     ++InsertPt;
90   } else {
91     // We cannot demote invoke instructions to the stack if their normal edge
92     // is critical.
93     InvokeInst &II = cast<InvokeInst>(I);
94     assert(II.getNormalDest()->getSinglePredecessor() &&
95            "Cannot demote invoke with a critical successor!");
96     InsertPt = II.getNormalDest()->begin();
97   }
98 
99   for (; isa<PHINode>(InsertPt); ++InsertPt)
100   /* empty */;   // Don't insert before any PHI nodes.
101   new StoreInst(&I, Slot, InsertPt);
102 
103   return Slot;
104 }
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 and it returns the pointer to the alloca inserted.
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, P->getName()+".reg2mem", AllocaPoint);
120   } else {
121     Function *F = P->getParent()->getParent();
122     Slot = new AllocaInst(P->getType(), 0, P->getName()+".reg2mem",
123                           F->getEntryBlock().begin());
124   }
125 
126   // Iterate over each operand, insert store in each predecessor.
127   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
128     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
129       assert(II->getParent() != P->getIncomingBlock(i) &&
130              "Invoke edge not supported yet"); II=II;
131     }
132     new StoreInst(P->getIncomingValue(i), Slot,
133                   P->getIncomingBlock(i)->getTerminator());
134   }
135 
136   // Insert load in place of the phi and replace all uses.
137   Value *V = new LoadInst(Slot, P->getName()+".reload", P);
138   P->replaceAllUsesWith(V);
139 
140   // Delete phi.
141   P->eraseFromParent();
142 
143   return Slot;
144 }
145