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.
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 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.
DemotePHIToStack(PHINode * P,Instruction * AllocaPoint)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