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           }
78           PN->setIncomingValue(i, V);
79         }
80 
81     } else {
82       // If this is a normal instruction, just insert a load.
83       Value *V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
84                               VolatileLoads, U);
85       U->replaceUsesOfWith(&I, V);
86     }
87   }
88 
89   // Insert stores of the computed value into the stack slot. We have to be
90   // careful if I is an invoke instruction, because we can't insert the store
91   // AFTER the terminator instruction.
92   BasicBlock::iterator InsertPt;
93   if (!I.isTerminator()) {
94     InsertPt = ++I.getIterator();
95     for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
96       /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
97   } else {
98     InvokeInst &II = cast<InvokeInst>(I);
99     InsertPt = II.getNormalDest()->getFirstInsertionPt();
100   }
101 
102   new StoreInst(&I, Slot, &*InsertPt);
103   return Slot;
104 }
105 
106 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
107 /// node and replaces it with a slot in the stack frame allocated via alloca.
108 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
109 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
110   if (P->use_empty()) {
111     P->eraseFromParent();
112     return nullptr;
113   }
114 
115   const DataLayout &DL = P->getModule()->getDataLayout();
116 
117   // Create a stack slot to hold the value.
118   AllocaInst *Slot;
119   if (AllocaPoint) {
120     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
121                           P->getName()+".reg2mem", AllocaPoint);
122   } else {
123     Function *F = P->getParent()->getParent();
124     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
125                           P->getName() + ".reg2mem",
126                           &F->getEntryBlock().front());
127   }
128 
129   // Iterate over each operand inserting a store in each predecessor.
130   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
131     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
132       assert(II->getParent() != P->getIncomingBlock(i) &&
133              "Invoke edge not supported yet"); (void)II;
134     }
135     new StoreInst(P->getIncomingValue(i), Slot,
136                   P->getIncomingBlock(i)->getTerminator());
137   }
138 
139   // Insert a load in place of the PHI and replace all uses.
140   BasicBlock::iterator InsertPt = P->getIterator();
141 
142   for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
143     /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
144 
145   Value *V =
146       new LoadInst(P->getType(), Slot, P->getName() + ".reload", &*InsertPt);
147   P->replaceAllUsesWith(V);
148 
149   // Delete PHI.
150   P->eraseFromParent();
151   return Slot;
152 }
153