1 //===------------- PPCEarlyReturn.cpp - Form Early Returns ----------------===//
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 // A pass that form early (predicated) returns. If-conversion handles some of
10 // this, but this pass picks up some remaining cases.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "MCTargetDesc/PPCPredicates.h"
15 #include "PPC.h"
16 #include "PPCInstrBuilder.h"
17 #include "PPCInstrInfo.h"
18 #include "PPCMachineFunctionInfo.h"
19 #include "PPCTargetMachine.h"
20 #include "llvm/ADT/STLExtras.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/CodeGen/MachineFrameInfo.h"
23 #include "llvm/CodeGen/MachineFunctionPass.h"
24 #include "llvm/CodeGen/MachineInstrBuilder.h"
25 #include "llvm/CodeGen/MachineMemOperand.h"
26 #include "llvm/CodeGen/MachineRegisterInfo.h"
27 #include "llvm/MC/MCAsmInfo.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/ErrorHandling.h"
30 #include "llvm/Support/TargetRegistry.h"
31 #include "llvm/Support/raw_ostream.h"
32 
33 using namespace llvm;
34 
35 #define DEBUG_TYPE "ppc-early-ret"
36 STATISTIC(NumBCLR, "Number of early conditional returns");
37 STATISTIC(NumBLR,  "Number of early returns");
38 
39 namespace {
40   // PPCEarlyReturn pass - For simple functions without epilogue code, move
41   // returns up, and create conditional returns, to avoid unnecessary
42   // branch-to-blr sequences.
43   struct PPCEarlyReturn : public MachineFunctionPass {
44     static char ID;
45     PPCEarlyReturn() : MachineFunctionPass(ID) {
46       initializePPCEarlyReturnPass(*PassRegistry::getPassRegistry());
47     }
48 
49     const TargetInstrInfo *TII;
50 
51 protected:
52     bool processBlock(MachineBasicBlock &ReturnMBB) {
53       bool Changed = false;
54 
55       MachineBasicBlock::iterator I = ReturnMBB.begin();
56       I = ReturnMBB.SkipPHIsLabelsAndDebug(I);
57 
58       // The block must be essentially empty except for the blr.
59       if (I == ReturnMBB.end() ||
60           (I->getOpcode() != PPC::BLR && I->getOpcode() != PPC::BLR8) ||
61           I != ReturnMBB.getLastNonDebugInstr())
62         return Changed;
63 
64       SmallVector<MachineBasicBlock*, 8> PredToRemove;
65       for (MachineBasicBlock::pred_iterator PI = ReturnMBB.pred_begin(),
66            PIE = ReturnMBB.pred_end(); PI != PIE; ++PI) {
67         bool OtherReference = false, BlockChanged = false;
68 
69         if ((*PI)->empty())
70           continue;
71 
72         for (MachineBasicBlock::iterator J = (*PI)->getLastNonDebugInstr();;) {
73           if (J == (*PI)->end())
74             break;
75 
76           if (J->getOpcode() == PPC::B) {
77             if (J->getOperand(0).getMBB() == &ReturnMBB) {
78               // This is an unconditional branch to the return. Replace the
79               // branch with a blr.
80               MachineInstr *MI = ReturnMBB.getParent()->CloneMachineInstr(&*I);
81               (*PI)->insert(J, MI);
82 
83               MachineBasicBlock::iterator K = J--;
84               K->eraseFromParent();
85               BlockChanged = true;
86               ++NumBLR;
87               continue;
88             }
89           } else if (J->getOpcode() == PPC::BCC) {
90             if (J->getOperand(2).getMBB() == &ReturnMBB) {
91               // This is a conditional branch to the return. Replace the branch
92               // with a bclr.
93               MachineInstr *MI = ReturnMBB.getParent()->CloneMachineInstr(&*I);
94               MI->setDesc(TII->get(PPC::BCCLR));
95               MachineInstrBuilder(*ReturnMBB.getParent(), MI)
96                   .add(J->getOperand(0))
97                   .add(J->getOperand(1));
98               (*PI)->insert(J, MI);
99 
100               MachineBasicBlock::iterator K = J--;
101               K->eraseFromParent();
102               BlockChanged = true;
103               ++NumBCLR;
104               continue;
105             }
106           } else if (J->getOpcode() == PPC::BC || J->getOpcode() == PPC::BCn) {
107             if (J->getOperand(1).getMBB() == &ReturnMBB) {
108               // This is a conditional branch to the return. Replace the branch
109               // with a bclr.
110               MachineInstr *MI = ReturnMBB.getParent()->CloneMachineInstr(&*I);
111               MI->setDesc(
112                   TII->get(J->getOpcode() == PPC::BC ? PPC::BCLR : PPC::BCLRn));
113               MachineInstrBuilder(*ReturnMBB.getParent(), MI)
114                   .add(J->getOperand(0));
115               (*PI)->insert(J, MI);
116 
117               MachineBasicBlock::iterator K = J--;
118               K->eraseFromParent();
119               BlockChanged = true;
120               ++NumBCLR;
121               continue;
122             }
123           } else if (J->isBranch()) {
124             if (J->isIndirectBranch()) {
125               if (ReturnMBB.hasAddressTaken())
126                 OtherReference = true;
127             } else
128               for (unsigned i = 0; i < J->getNumOperands(); ++i)
129                 if (J->getOperand(i).isMBB() &&
130                     J->getOperand(i).getMBB() == &ReturnMBB)
131                   OtherReference = true;
132           } else if (!J->isTerminator() && !J->isDebugInstr())
133             break;
134 
135           if (J == (*PI)->begin())
136             break;
137 
138           --J;
139         }
140 
141         if ((*PI)->canFallThrough() && (*PI)->isLayoutSuccessor(&ReturnMBB))
142           OtherReference = true;
143 
144         // Predecessors are stored in a vector and can't be removed here.
145         if (!OtherReference && BlockChanged) {
146           PredToRemove.push_back(*PI);
147         }
148 
149         if (BlockChanged)
150           Changed = true;
151       }
152 
153       for (unsigned i = 0, ie = PredToRemove.size(); i != ie; ++i)
154         PredToRemove[i]->removeSuccessor(&ReturnMBB, true);
155 
156       if (Changed && !ReturnMBB.hasAddressTaken()) {
157         // We now might be able to merge this blr-only block into its
158         // by-layout predecessor.
159         if (ReturnMBB.pred_size() == 1) {
160           MachineBasicBlock &PrevMBB = **ReturnMBB.pred_begin();
161           if (PrevMBB.isLayoutSuccessor(&ReturnMBB) && PrevMBB.canFallThrough()) {
162             // Move the blr into the preceding block.
163             PrevMBB.splice(PrevMBB.end(), &ReturnMBB, I);
164             PrevMBB.removeSuccessor(&ReturnMBB, true);
165           }
166         }
167 
168         if (ReturnMBB.pred_empty())
169           ReturnMBB.eraseFromParent();
170       }
171 
172       return Changed;
173     }
174 
175 public:
176     bool runOnMachineFunction(MachineFunction &MF) override {
177       if (skipFunction(MF.getFunction()))
178         return false;
179 
180       TII = MF.getSubtarget().getInstrInfo();
181 
182       bool Changed = false;
183 
184       // If the function does not have at least two blocks, then there is
185       // nothing to do.
186       if (MF.size() < 2)
187         return Changed;
188 
189       // We can't use a range-based for loop due to clobbering the iterator.
190       for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E;) {
191         MachineBasicBlock &B = *I++;
192         Changed |= processBlock(B);
193       }
194 
195       return Changed;
196     }
197 
198     MachineFunctionProperties getRequiredProperties() const override {
199       return MachineFunctionProperties().set(
200           MachineFunctionProperties::Property::NoVRegs);
201     }
202 
203     void getAnalysisUsage(AnalysisUsage &AU) const override {
204       MachineFunctionPass::getAnalysisUsage(AU);
205     }
206   };
207 }
208 
209 INITIALIZE_PASS(PPCEarlyReturn, DEBUG_TYPE,
210                 "PowerPC Early-Return Creation", false, false)
211 
212 char PPCEarlyReturn::ID = 0;
213 FunctionPass*
214 llvm::createPPCEarlyReturnPass() { return new PPCEarlyReturn(); }
215