1 //===-- PPCBranchSelector.cpp - Emit long conditional branches ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
16 //===----------------------------------------------------------------------===//
19 #include "MCTargetDesc/PPCPredicates.h"
20 #include "PPCInstrBuilder.h"
21 #include "PPCInstrInfo.h"
22 #include "PPCSubtarget.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/CodeGen/MachineFunctionPass.h"
25 #include "llvm/CodeGen/MachineRegisterInfo.h"
26 #include "llvm/Support/MathExtras.h"
27 #include "llvm/Target/TargetMachine.h"
28 #include "llvm/Target/TargetSubtargetInfo.h"
31 #define DEBUG_TYPE "ppc-branch-select"
33 STATISTIC(NumExpanded, "Number of branches expanded to long format");
36 void initializePPCBSelPass(PassRegistry&);
40 struct PPCBSel : public MachineFunctionPass {
42 PPCBSel() : MachineFunctionPass(ID) {
43 initializePPCBSelPass(*PassRegistry::getPassRegistry());
46 // The sizes of the basic blocks in the function (the first
47 // element of the pair); the second element of the pair is the amount of the
48 // size that is due to potential padding.
49 std::vector<std::pair<unsigned, unsigned>> BlockSizes;
51 bool runOnMachineFunction(MachineFunction &Fn) override;
53 MachineFunctionProperties getRequiredProperties() const override {
54 return MachineFunctionProperties().set(
55 MachineFunctionProperties::Property::NoVRegs);
58 StringRef getPassName() const override { return "PowerPC Branch Selector"; }
63 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
66 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
69 FunctionPass *llvm::createPPCBranchSelectionPass() {
73 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
74 const PPCInstrInfo *TII =
75 static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
76 // Give the blocks of the function a dense, in-order, numbering.
78 BlockSizes.resize(Fn.getNumBlockIDs());
80 auto GetAlignmentAdjustment =
81 [TII](MachineBasicBlock &MBB, unsigned Offset) -> unsigned {
82 unsigned Align = MBB.getAlignment();
86 unsigned AlignAmt = 1 << Align;
87 unsigned ParentAlign = MBB.getParent()->getAlignment();
89 if (Align <= ParentAlign)
90 return OffsetToAlignment(Offset, AlignAmt);
92 // The alignment of this MBB is larger than the function's alignment, so we
93 // can't tell whether or not it will insert nops. Assume that it will.
94 return AlignAmt + OffsetToAlignment(Offset, AlignAmt);
97 // We need to be careful about the offset of the first block in the function
98 // because it might not have the function's alignment. This happens because,
99 // under the ELFv2 ABI, for functions which require a TOC pointer, we add a
100 // two-instruction sequence to the start of the function.
101 // Note: This needs to be synchronized with the check in
102 // PPCLinuxAsmPrinter::EmitFunctionBodyStart.
103 unsigned InitialOffset = 0;
104 if (Fn.getSubtarget<PPCSubtarget>().isELFv2ABI() &&
105 !Fn.getRegInfo().use_empty(PPC::X2))
108 // Measure each MBB and compute a size for the entire function.
109 unsigned FuncSize = InitialOffset;
110 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
112 MachineBasicBlock *MBB = &*MFI;
114 // The end of the previous block may have extra nops if this block has an
115 // alignment requirement.
116 if (MBB->getNumber() > 0) {
117 unsigned AlignExtra = GetAlignmentAdjustment(*MBB, FuncSize);
119 auto &BS = BlockSizes[MBB->getNumber()-1];
120 BS.first += AlignExtra;
121 BS.second = AlignExtra;
123 FuncSize += AlignExtra;
126 unsigned BlockSize = 0;
127 for (MachineInstr &MI : *MBB)
128 BlockSize += TII->getInstSizeInBytes(MI);
130 BlockSizes[MBB->getNumber()].first = BlockSize;
131 FuncSize += BlockSize;
134 // If the entire function is smaller than the displacement of a branch field,
135 // we know we don't need to shrink any branches in this function. This is a
137 if (FuncSize < (1 << 15)) {
142 // For each conditional branch, if the offset to its destination is larger
143 // than the offset field allows, transform it into a long branch sequence
151 bool MadeChange = true;
152 bool EverMadeChange = false;
154 // Iteratively expand branches until we reach a fixed point.
157 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
159 MachineBasicBlock &MBB = *MFI;
160 unsigned MBBStartOffset = 0;
161 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
163 MachineBasicBlock *Dest = nullptr;
164 if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
165 Dest = I->getOperand(2).getMBB();
166 else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
167 !I->getOperand(1).isImm())
168 Dest = I->getOperand(1).getMBB();
169 else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
170 I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
171 !I->getOperand(0).isImm())
172 Dest = I->getOperand(0).getMBB();
175 MBBStartOffset += TII->getInstSizeInBytes(*I);
179 // Determine the offset from the current branch to the destination
182 if (Dest->getNumber() <= MBB.getNumber()) {
183 // If this is a backwards branch, the delta is the offset from the
184 // start of this block to this branch, plus the sizes of all blocks
185 // from this block to the dest.
186 BranchSize = MBBStartOffset;
188 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
189 BranchSize += BlockSizes[i].first;
191 // Otherwise, add the size of the blocks between this block and the
192 // dest to the number of bytes left in this block.
193 BranchSize = -MBBStartOffset;
195 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
196 BranchSize += BlockSizes[i].first;
199 // If this branch is in range, ignore it.
200 if (isInt<16>(BranchSize)) {
205 // Otherwise, we have to expand it to a long branch.
206 MachineInstr &OldBranch = *I;
207 DebugLoc dl = OldBranch.getDebugLoc();
209 if (I->getOpcode() == PPC::BCC) {
210 // The BCC operands are:
211 // 0. PPC branch predicate
214 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
215 unsigned CRReg = I->getOperand(1).getReg();
217 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
218 BuildMI(MBB, I, dl, TII->get(PPC::BCC))
219 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
220 } else if (I->getOpcode() == PPC::BC) {
221 unsigned CRBit = I->getOperand(0).getReg();
222 BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
223 } else if (I->getOpcode() == PPC::BCn) {
224 unsigned CRBit = I->getOperand(0).getReg();
225 BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
226 } else if (I->getOpcode() == PPC::BDNZ) {
227 BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
228 } else if (I->getOpcode() == PPC::BDNZ8) {
229 BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
230 } else if (I->getOpcode() == PPC::BDZ) {
231 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
232 } else if (I->getOpcode() == PPC::BDZ8) {
233 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
235 llvm_unreachable("Unhandled branch type!");
238 // Uncond branch to the real destination.
239 I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
241 // Remove the old branch from the function.
242 OldBranch.eraseFromParent();
244 // Remember that this instruction is 8-bytes, increase the size of the
245 // block by 4, remember to iterate.
246 BlockSizes[MBB.getNumber()].first += 4;
254 // If we're going to iterate again, make sure we've updated our
255 // padding-based contributions to the block sizes.
256 unsigned Offset = InitialOffset;
257 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
259 MachineBasicBlock *MBB = &*MFI;
261 if (MBB->getNumber() > 0) {
262 auto &BS = BlockSizes[MBB->getNumber()-1];
263 BS.first -= BS.second;
266 unsigned AlignExtra = GetAlignmentAdjustment(*MBB, Offset);
268 BS.first += AlignExtra;
269 BS.second = AlignExtra;
271 Offset += AlignExtra;
274 Offset += BlockSizes[MBB->getNumber()].first;
278 EverMadeChange |= MadeChange;