1 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "llvm/ADT/DenseMap.h"
10 #include "llvm/Analysis/CFG.h"
11 #include "llvm/IR/Function.h"
12 #include "llvm/IR/Instructions.h"
13 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
14 #include "llvm/Transforms/Utils/Local.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.
22 AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
23                                    Instruction *AllocaPoint) {
24   if (I.use_empty()) {
25     I.eraseFromParent();
26     return nullptr;
27   }
28 
29   Function *F = I.getParent()->getParent();
30   const DataLayout &DL = F->getParent()->getDataLayout();
31 
32   // Create a stack slot to hold the value.
33   AllocaInst *Slot;
34   if (AllocaPoint) {
35     Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
36                           I.getName()+".reg2mem", AllocaPoint);
37   } else {
38     Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
39                           I.getName() + ".reg2mem", &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(I.getType(), Slot, I.getName() + ".reload",
75                              VolatileLoads,
76                              PN->getIncomingBlock(i)->getTerminator());
77             Loads[PN->getIncomingBlock(i)] = V;
78           }
79           PN->setIncomingValue(i, V);
80         }
81 
82     } else {
83       // If this is a normal instruction, just insert a load.
84       Value *V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
85                               VolatileLoads, U);
86       U->replaceUsesOfWith(&I, V);
87     }
88   }
89 
90   // Insert stores of the computed value into the stack slot. We have to be
91   // careful if I is an invoke instruction, because we can't insert the store
92   // AFTER the terminator instruction.
93   BasicBlock::iterator InsertPt;
94   if (!I.isTerminator()) {
95     InsertPt = ++I.getIterator();
96     // Don't insert before PHI nodes or landingpad instrs.
97     for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
98       if (isa<CatchSwitchInst>(InsertPt))
99         break;
100     if (isa<CatchSwitchInst>(InsertPt)) {
101       for (BasicBlock *Handler : successors(&*InsertPt))
102         new StoreInst(&I, Slot, &*Handler->getFirstInsertionPt());
103       return Slot;
104     }
105   } else {
106     InvokeInst &II = cast<InvokeInst>(I);
107     InsertPt = II.getNormalDest()->getFirstInsertionPt();
108   }
109 
110   new StoreInst(&I, Slot, &*InsertPt);
111   return Slot;
112 }
113 
114 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
115 /// node and replaces it with a slot in the stack frame allocated via alloca.
116 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
117 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
118   if (P->use_empty()) {
119     P->eraseFromParent();
120     return nullptr;
121   }
122 
123   const DataLayout &DL = P->getModule()->getDataLayout();
124 
125   // Create a stack slot to hold the value.
126   AllocaInst *Slot;
127   if (AllocaPoint) {
128     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
129                           P->getName()+".reg2mem", AllocaPoint);
130   } else {
131     Function *F = P->getParent()->getParent();
132     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
133                           P->getName() + ".reg2mem",
134                           &F->getEntryBlock().front());
135   }
136 
137   // Iterate over each operand inserting a store in each predecessor.
138   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
139     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
140       assert(II->getParent() != P->getIncomingBlock(i) &&
141              "Invoke edge not supported yet"); (void)II;
142     }
143     new StoreInst(P->getIncomingValue(i), Slot,
144                   P->getIncomingBlock(i)->getTerminator());
145   }
146 
147   // Insert a load in place of the PHI and replace all uses.
148   BasicBlock::iterator InsertPt = P->getIterator();
149   // Don't insert before PHI nodes or landingpad instrs.
150   for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
151     if (isa<CatchSwitchInst>(InsertPt))
152       break;
153   if (isa<CatchSwitchInst>(InsertPt)) {
154     // We need a separate load before each actual use of the PHI
155     SmallVector<Instruction *, 4> Users;
156     for (User *U : P->users()) {
157       Instruction *User = cast<Instruction>(U);
158       Users.push_back(User);
159     }
160     for (Instruction *User : Users) {
161       Value *V =
162           new LoadInst(P->getType(), Slot, P->getName() + ".reload", User);
163       User->replaceUsesOfWith(P, V);
164     }
165   } else {
166     Value *V =
167         new LoadInst(P->getType(), Slot, P->getName() + ".reload", &*InsertPt);
168     P->replaceAllUsesWith(V);
169   }
170   // Delete PHI.
171   P->eraseFromParent();
172   return Slot;
173 }
174