1 //===-- PPCBranchSelector.cpp - Emit long conditional branches ------------===//
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 // This file contains a pass that scans a machine function to determine which
11 // conditional branches need more than 16 bits of displacement to reach their
12 // target basic block. It does this in two passes; a calculation of basic block
13 // positions pass, and a branch pseudo op to machine branch opcode pass. This
14 // pass should be run last, just before the assembly printer.
15 //
16 //===----------------------------------------------------------------------===//
17
18 #include "PPC.h"
19 #include "MCTargetDesc/PPCPredicates.h"
20 #include "PPCInstrBuilder.h"
21 #include "PPCInstrInfo.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/CodeGen/MachineFunctionPass.h"
24 #include "llvm/Support/MathExtras.h"
25 #include "llvm/Target/TargetMachine.h"
26 #include "llvm/Target/TargetSubtargetInfo.h"
27 using namespace llvm;
28
29 #define DEBUG_TYPE "ppc-branch-select"
30
31 STATISTIC(NumExpanded, "Number of branches expanded to long format");
32
33 namespace llvm {
34 void initializePPCBSelPass(PassRegistry&);
35 }
36
37 namespace {
38 struct PPCBSel : public MachineFunctionPass {
39 static char ID;
PPCBSel__anon264cb36a0111::PPCBSel40 PPCBSel() : MachineFunctionPass(ID) {
41 initializePPCBSelPass(*PassRegistry::getPassRegistry());
42 }
43
44 /// BlockSizes - The sizes of the basic blocks in the function.
45 std::vector<unsigned> BlockSizes;
46
47 bool runOnMachineFunction(MachineFunction &Fn) override;
48
getPassName__anon264cb36a0111::PPCBSel49 const char *getPassName() const override {
50 return "PowerPC Branch Selector";
51 }
52 };
53 char PPCBSel::ID = 0;
54 }
55
56 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
57 false, false)
58
59 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
60 /// Pass
61 ///
createPPCBranchSelectionPass()62 FunctionPass *llvm::createPPCBranchSelectionPass() {
63 return new PPCBSel();
64 }
65
runOnMachineFunction(MachineFunction & Fn)66 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
67 const PPCInstrInfo *TII =
68 static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
69 // Give the blocks of the function a dense, in-order, numbering.
70 Fn.RenumberBlocks();
71 BlockSizes.resize(Fn.getNumBlockIDs());
72
73 auto GetAlignmentAdjustment =
74 [TII](MachineBasicBlock &MBB, unsigned Offset) -> unsigned {
75 unsigned Align = MBB.getAlignment();
76 if (!Align)
77 return 0;
78
79 unsigned AlignAmt = 1 << Align;
80 unsigned ParentAlign = MBB.getParent()->getAlignment();
81
82 if (Align <= ParentAlign)
83 return OffsetToAlignment(Offset, AlignAmt);
84
85 // The alignment of this MBB is larger than the function's alignment, so we
86 // can't tell whether or not it will insert nops. Assume that it will.
87 return AlignAmt + OffsetToAlignment(Offset, AlignAmt);
88 };
89
90 // Measure each MBB and compute a size for the entire function.
91 unsigned FuncSize = 0;
92 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
93 ++MFI) {
94 MachineBasicBlock *MBB = &*MFI;
95
96 // The end of the previous block may have extra nops if this block has an
97 // alignment requirement.
98 if (MBB->getNumber() > 0) {
99 unsigned AlignExtra = GetAlignmentAdjustment(*MBB, FuncSize);
100 BlockSizes[MBB->getNumber()-1] += AlignExtra;
101 FuncSize += AlignExtra;
102 }
103
104 unsigned BlockSize = 0;
105 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
106 MBBI != EE; ++MBBI)
107 BlockSize += TII->GetInstSizeInBytes(MBBI);
108
109 BlockSizes[MBB->getNumber()] = BlockSize;
110 FuncSize += BlockSize;
111 }
112
113 // If the entire function is smaller than the displacement of a branch field,
114 // we know we don't need to shrink any branches in this function. This is a
115 // common case.
116 if (FuncSize < (1 << 15)) {
117 BlockSizes.clear();
118 return false;
119 }
120
121 // For each conditional branch, if the offset to its destination is larger
122 // than the offset field allows, transform it into a long branch sequence
123 // like this:
124 // short branch:
125 // bCC MBB
126 // long branch:
127 // b!CC $PC+8
128 // b MBB
129 //
130 bool MadeChange = true;
131 bool EverMadeChange = false;
132 while (MadeChange) {
133 // Iteratively expand branches until we reach a fixed point.
134 MadeChange = false;
135
136 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
137 ++MFI) {
138 MachineBasicBlock &MBB = *MFI;
139 unsigned MBBStartOffset = 0;
140 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
141 I != E; ++I) {
142 MachineBasicBlock *Dest = nullptr;
143 if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
144 Dest = I->getOperand(2).getMBB();
145 else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
146 !I->getOperand(1).isImm())
147 Dest = I->getOperand(1).getMBB();
148 else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
149 I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
150 !I->getOperand(0).isImm())
151 Dest = I->getOperand(0).getMBB();
152
153 if (!Dest) {
154 MBBStartOffset += TII->GetInstSizeInBytes(I);
155 continue;
156 }
157
158 // Determine the offset from the current branch to the destination
159 // block.
160 int BranchSize;
161 if (Dest->getNumber() <= MBB.getNumber()) {
162 // If this is a backwards branch, the delta is the offset from the
163 // start of this block to this branch, plus the sizes of all blocks
164 // from this block to the dest.
165 BranchSize = MBBStartOffset;
166
167 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
168 BranchSize += BlockSizes[i];
169 } else {
170 // Otherwise, add the size of the blocks between this block and the
171 // dest to the number of bytes left in this block.
172 BranchSize = -MBBStartOffset;
173
174 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
175 BranchSize += BlockSizes[i];
176 }
177
178 // If this branch is in range, ignore it.
179 if (isInt<16>(BranchSize)) {
180 MBBStartOffset += 4;
181 continue;
182 }
183
184 // Otherwise, we have to expand it to a long branch.
185 MachineInstr *OldBranch = I;
186 DebugLoc dl = OldBranch->getDebugLoc();
187
188 if (I->getOpcode() == PPC::BCC) {
189 // The BCC operands are:
190 // 0. PPC branch predicate
191 // 1. CR register
192 // 2. Target MBB
193 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
194 unsigned CRReg = I->getOperand(1).getReg();
195
196 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
197 BuildMI(MBB, I, dl, TII->get(PPC::BCC))
198 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
199 } else if (I->getOpcode() == PPC::BC) {
200 unsigned CRBit = I->getOperand(0).getReg();
201 BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
202 } else if (I->getOpcode() == PPC::BCn) {
203 unsigned CRBit = I->getOperand(0).getReg();
204 BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
205 } else if (I->getOpcode() == PPC::BDNZ) {
206 BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
207 } else if (I->getOpcode() == PPC::BDNZ8) {
208 BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
209 } else if (I->getOpcode() == PPC::BDZ) {
210 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
211 } else if (I->getOpcode() == PPC::BDZ8) {
212 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
213 } else {
214 llvm_unreachable("Unhandled branch type!");
215 }
216
217 // Uncond branch to the real destination.
218 I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
219
220 // Remove the old branch from the function.
221 OldBranch->eraseFromParent();
222
223 // Remember that this instruction is 8-bytes, increase the size of the
224 // block by 4, remember to iterate.
225 BlockSizes[MBB.getNumber()] += 4;
226 MBBStartOffset += 8;
227 ++NumExpanded;
228 MadeChange = true;
229 }
230 }
231 EverMadeChange |= MadeChange;
232 }
233
234 BlockSizes.clear();
235 return true;
236 }
237
238