LLVM 17.0.0git
DemoteRegToStack.cpp
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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"
15using 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.
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.
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 // Don't insert before PHI nodes or landingpad instrs.
96 for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
97 if (isa<CatchSwitchInst>(InsertPt))
98 break;
99 if (isa<CatchSwitchInst>(InsertPt)) {
100 for (BasicBlock *Handler : successors(&*InsertPt))
101 new StoreInst(&I, Slot, &*Handler->getFirstInsertionPt());
102 return Slot;
103 }
104 } else {
105 InvokeInst &II = cast<InvokeInst>(I);
106 InsertPt = II.getNormalDest()->getFirstInsertionPt();
107 }
108
109 new StoreInst(&I, Slot, &*InsertPt);
110 return Slot;
111}
112
113/// DemotePHIToStack - This function takes a virtual register computed by a PHI
114/// node and replaces it with a slot in the stack frame allocated via alloca.
115/// The PHI node is deleted. It returns the pointer to the alloca inserted.
117 if (P->use_empty()) {
118 P->eraseFromParent();
119 return nullptr;
120 }
121
122 const DataLayout &DL = P->getModule()->getDataLayout();
123
124 // Create a stack slot to hold the value.
125 AllocaInst *Slot;
126 if (AllocaPoint) {
127 Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
128 P->getName()+".reg2mem", AllocaPoint);
129 } else {
130 Function *F = P->getParent()->getParent();
131 Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
132 P->getName() + ".reg2mem",
133 &F->getEntryBlock().front());
134 }
135
136 // Iterate over each operand inserting a store in each predecessor.
137 for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
138 if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
139 assert(II->getParent() != P->getIncomingBlock(i) &&
140 "Invoke edge not supported yet"); (void)II;
141 }
142 new StoreInst(P->getIncomingValue(i), Slot,
143 P->getIncomingBlock(i)->getTerminator());
144 }
145
146 // Insert a load in place of the PHI and replace all uses.
147 BasicBlock::iterator InsertPt = P->getIterator();
148 // Don't insert before PHI nodes or landingpad instrs.
149 for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
150 if (isa<CatchSwitchInst>(InsertPt))
151 break;
152 if (isa<CatchSwitchInst>(InsertPt)) {
153 // We need a separate load before each actual use of the PHI
155 for (User *U : P->users()) {
156 Instruction *User = cast<Instruction>(U);
157 Users.push_back(User);
158 }
159 for (Instruction *User : Users) {
160 Value *V =
161 new LoadInst(P->getType(), Slot, P->getName() + ".reload", User);
163 }
164 } else {
165 Value *V =
166 new LoadInst(P->getType(), Slot, P->getName() + ".reload", &*InsertPt);
167 P->replaceAllUsesWith(V);
168 }
169 // Delete PHI.
170 P->eraseFromParent();
171 return Slot;
172}
MachineBasicBlock MachineBasicBlock::iterator DebugLoc DL
This file defines the DenseMap class.
iv Induction Variable Users
Definition: IVUsers.cpp:48
#define F(x, y, z)
Definition: MD5.cpp:55
#define I(x, y, z)
Definition: MD5.cpp:58
#define P(N)
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
an instruction to allocate memory on the stack
Definition: Instructions.h:58
LLVM Basic Block Representation.
Definition: BasicBlock.h:56
const_iterator getFirstInsertionPt() const
Returns an iterator to the first instruction in this block that is suitable for inserting a non-PHI i...
Definition: BasicBlock.cpp:245
InstListType::iterator iterator
Instruction iterators...
Definition: BasicBlock.h:87
A parsed version of the target data layout string in and methods for querying it.
Definition: DataLayout.h:114
Invoke instruction.
BasicBlock * getNormalDest() const
An instruction for reading from memory.
Definition: Instructions.h:177
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1200
An instruction for storing to memory.
Definition: Instructions.h:301
bool replaceUsesOfWith(Value *From, Value *To)
Replace uses of one Value with another.
Definition: User.cpp:21
LLVM Value Representation.
Definition: Value.h:74
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ)
Search for the specified successor of basic block BB and return its position in the terminator instru...
Definition: CFG.cpp:79
auto successors(const MachineBasicBlock *BB)
BasicBlock * SplitCriticalEdge(Instruction *TI, unsigned SuccNum, const CriticalEdgeSplittingOptions &Options=CriticalEdgeSplittingOptions(), const Twine &BBName="")
If this edge is a critical edge, insert a new node to split the critical edge.
bool isCriticalEdge(const Instruction *TI, unsigned SuccNum, bool AllowIdenticalEdges=false)
Return true if the specified edge is a critical edge.
Definition: CFG.cpp:95
AllocaInst * DemoteRegToStack(Instruction &X, bool VolatileLoads=false, Instruction *AllocaPoint=nullptr)
This function takes a virtual register computed by an Instruction and replaces it with a slot in the ...
AllocaInst * DemotePHIToStack(PHINode *P, Instruction *AllocaPoint=nullptr)
This function takes a virtual register computed by a phi node and replaces it with a slot in the stac...