1 //===------------- PPCEarlyReturn.cpp - Form Early Returns ----------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // A pass that form early (predicated) returns. If-conversion handles some of
11 // this, but this pass picks up some remaining cases.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "PPCInstrInfo.h"
16 #include "MCTargetDesc/PPCPredicates.h"
17 #include "PPC.h"
18 #include "PPCInstrBuilder.h"
19 #include "PPCMachineFunctionInfo.h"
20 #include "PPCTargetMachine.h"
21 #include "llvm/ADT/STLExtras.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/CodeGen/MachineFrameInfo.h"
24 #include "llvm/CodeGen/MachineFunctionPass.h"
25 #include "llvm/CodeGen/MachineInstrBuilder.h"
26 #include "llvm/CodeGen/MachineMemOperand.h"
27 #include "llvm/CodeGen/MachineRegisterInfo.h"
28 #include "llvm/MC/MCAsmInfo.h"
29 #include "llvm/Support/CommandLine.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/ErrorHandling.h"
32 #include "llvm/Support/TargetRegistry.h"
33 #include "llvm/Support/raw_ostream.h"
34 
35 using namespace llvm;
36 
37 #define DEBUG_TYPE "ppc-early-ret"
38 STATISTIC(NumBCLR, "Number of early conditional returns");
39 STATISTIC(NumBLR,  "Number of early returns");
40 
41 namespace llvm {
42   void initializePPCEarlyReturnPass(PassRegistry&);
43 }
44 
45 namespace {
46   // PPCEarlyReturn pass - For simple functions without epilogue code, move
47   // returns up, and create conditional returns, to avoid unnecessary
48   // branch-to-blr sequences.
49   struct PPCEarlyReturn : public MachineFunctionPass {
50     static char ID;
PPCEarlyReturn__anon8419d4370111::PPCEarlyReturn51     PPCEarlyReturn() : MachineFunctionPass(ID) {
52       initializePPCEarlyReturnPass(*PassRegistry::getPassRegistry());
53     }
54 
55     const TargetInstrInfo *TII;
56 
57 protected:
processBlock__anon8419d4370111::PPCEarlyReturn58     bool processBlock(MachineBasicBlock &ReturnMBB) {
59       bool Changed = false;
60 
61       MachineBasicBlock::iterator I = ReturnMBB.begin();
62       I = ReturnMBB.SkipPHIsAndLabels(I);
63 
64       // The block must be essentially empty except for the blr.
65       if (I == ReturnMBB.end() ||
66           (I->getOpcode() != PPC::BLR && I->getOpcode() != PPC::BLR8) ||
67           I != ReturnMBB.getLastNonDebugInstr())
68         return Changed;
69 
70       SmallVector<MachineBasicBlock*, 8> PredToRemove;
71       for (MachineBasicBlock::pred_iterator PI = ReturnMBB.pred_begin(),
72            PIE = ReturnMBB.pred_end(); PI != PIE; ++PI) {
73         bool OtherReference = false, BlockChanged = false;
74 
75         if ((*PI)->empty())
76           continue;
77 
78         for (MachineBasicBlock::iterator J = (*PI)->getLastNonDebugInstr();;) {
79           if (J == (*PI)->end())
80             break;
81 
82           if (J->getOpcode() == PPC::B) {
83             if (J->getOperand(0).getMBB() == &ReturnMBB) {
84               // This is an unconditional branch to the return. Replace the
85               // branch with a blr.
86               BuildMI(**PI, J, J->getDebugLoc(), TII->get(I->getOpcode()))
87                   .copyImplicitOps(I);
88               MachineBasicBlock::iterator K = J--;
89               K->eraseFromParent();
90               BlockChanged = true;
91               ++NumBLR;
92               continue;
93             }
94           } else if (J->getOpcode() == PPC::BCC) {
95             if (J->getOperand(2).getMBB() == &ReturnMBB) {
96               // This is a conditional branch to the return. Replace the branch
97               // with a bclr.
98               BuildMI(**PI, J, J->getDebugLoc(), TII->get(PPC::BCCLR))
99                   .addImm(J->getOperand(0).getImm())
100                   .addReg(J->getOperand(1).getReg())
101                   .copyImplicitOps(I);
102               MachineBasicBlock::iterator K = J--;
103               K->eraseFromParent();
104               BlockChanged = true;
105               ++NumBCLR;
106               continue;
107             }
108           } else if (J->getOpcode() == PPC::BC || J->getOpcode() == PPC::BCn) {
109             if (J->getOperand(1).getMBB() == &ReturnMBB) {
110               // This is a conditional branch to the return. Replace the branch
111               // with a bclr.
112               BuildMI(
113                   **PI, J, J->getDebugLoc(),
114                   TII->get(J->getOpcode() == PPC::BC ? PPC::BCLR : PPC::BCLRn))
115                   .addReg(J->getOperand(0).getReg())
116                   .copyImplicitOps(I);
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->isDebugValue())
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:
runOnMachineFunction__anon8419d4370111::PPCEarlyReturn176     bool runOnMachineFunction(MachineFunction &MF) override {
177       TII = MF.getSubtarget().getInstrInfo();
178 
179       bool Changed = false;
180 
181       // If the function does not have at least two blocks, then there is
182       // nothing to do.
183       if (MF.size() < 2)
184         return Changed;
185 
186       for (MachineFunction::iterator I = MF.begin(); I != MF.end();) {
187         MachineBasicBlock &B = *I++;
188         if (processBlock(B))
189           Changed = true;
190       }
191 
192       return Changed;
193     }
194 
getAnalysisUsage__anon8419d4370111::PPCEarlyReturn195     void getAnalysisUsage(AnalysisUsage &AU) const override {
196       MachineFunctionPass::getAnalysisUsage(AU);
197     }
198   };
199 }
200 
201 INITIALIZE_PASS(PPCEarlyReturn, DEBUG_TYPE,
202                 "PowerPC Early-Return Creation", false, false)
203 
204 char PPCEarlyReturn::ID = 0;
205 FunctionPass*
createPPCEarlyReturnPass()206 llvm::createPPCEarlyReturnPass() { return new PPCEarlyReturn(); }
207 
208