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 "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"
29 #define DEBUG_TYPE "ppc-branch-select"
31 STATISTIC(NumExpanded, "Number of branches expanded to long format");
34 void initializePPCBSelPass(PassRegistry&);
38 struct PPCBSel : public MachineFunctionPass {
40 PPCBSel() : MachineFunctionPass(ID) {
41 initializePPCBSelPass(*PassRegistry::getPassRegistry());
44 /// BlockSizes - The sizes of the basic blocks in the function.
45 std::vector<unsigned> BlockSizes;
47 bool runOnMachineFunction(MachineFunction &Fn) override;
49 MachineFunctionProperties getRequiredProperties() const override {
50 return MachineFunctionProperties().set(
51 MachineFunctionProperties::Property::AllVRegsAllocated);
54 const char *getPassName() const override {
55 return "PowerPC Branch Selector";
61 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
64 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
67 FunctionPass *llvm::createPPCBranchSelectionPass() {
71 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
72 const PPCInstrInfo *TII =
73 static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
74 // Give the blocks of the function a dense, in-order, numbering.
76 BlockSizes.resize(Fn.getNumBlockIDs());
78 auto GetAlignmentAdjustment =
79 [TII](MachineBasicBlock &MBB, unsigned Offset) -> unsigned {
80 unsigned Align = MBB.getAlignment();
84 unsigned AlignAmt = 1 << Align;
85 unsigned ParentAlign = MBB.getParent()->getAlignment();
87 if (Align <= ParentAlign)
88 return OffsetToAlignment(Offset, AlignAmt);
90 // The alignment of this MBB is larger than the function's alignment, so we
91 // can't tell whether or not it will insert nops. Assume that it will.
92 return AlignAmt + OffsetToAlignment(Offset, AlignAmt);
95 // Measure each MBB and compute a size for the entire function.
96 unsigned FuncSize = 0;
97 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
99 MachineBasicBlock *MBB = &*MFI;
101 // The end of the previous block may have extra nops if this block has an
102 // alignment requirement.
103 if (MBB->getNumber() > 0) {
104 unsigned AlignExtra = GetAlignmentAdjustment(*MBB, FuncSize);
105 BlockSizes[MBB->getNumber()-1] += AlignExtra;
106 FuncSize += AlignExtra;
109 unsigned BlockSize = 0;
110 for (MachineInstr &MI : *MBB)
111 BlockSize += TII->GetInstSizeInBytes(MI);
113 BlockSizes[MBB->getNumber()] = BlockSize;
114 FuncSize += BlockSize;
117 // If the entire function is smaller than the displacement of a branch field,
118 // we know we don't need to shrink any branches in this function. This is a
120 if (FuncSize < (1 << 15)) {
125 // For each conditional branch, if the offset to its destination is larger
126 // than the offset field allows, transform it into a long branch sequence
134 bool MadeChange = true;
135 bool EverMadeChange = false;
137 // Iteratively expand branches until we reach a fixed point.
140 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
142 MachineBasicBlock &MBB = *MFI;
143 unsigned MBBStartOffset = 0;
144 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
146 MachineBasicBlock *Dest = nullptr;
147 if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
148 Dest = I->getOperand(2).getMBB();
149 else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
150 !I->getOperand(1).isImm())
151 Dest = I->getOperand(1).getMBB();
152 else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
153 I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
154 !I->getOperand(0).isImm())
155 Dest = I->getOperand(0).getMBB();
158 MBBStartOffset += TII->GetInstSizeInBytes(*I);
162 // Determine the offset from the current branch to the destination
165 if (Dest->getNumber() <= MBB.getNumber()) {
166 // If this is a backwards branch, the delta is the offset from the
167 // start of this block to this branch, plus the sizes of all blocks
168 // from this block to the dest.
169 BranchSize = MBBStartOffset;
171 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
172 BranchSize += BlockSizes[i];
174 // Otherwise, add the size of the blocks between this block and the
175 // dest to the number of bytes left in this block.
176 BranchSize = -MBBStartOffset;
178 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
179 BranchSize += BlockSizes[i];
182 // If this branch is in range, ignore it.
183 if (isInt<16>(BranchSize)) {
188 // Otherwise, we have to expand it to a long branch.
189 MachineInstr *OldBranch = I;
190 DebugLoc dl = OldBranch->getDebugLoc();
192 if (I->getOpcode() == PPC::BCC) {
193 // The BCC operands are:
194 // 0. PPC branch predicate
197 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
198 unsigned CRReg = I->getOperand(1).getReg();
200 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
201 BuildMI(MBB, I, dl, TII->get(PPC::BCC))
202 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
203 } else if (I->getOpcode() == PPC::BC) {
204 unsigned CRBit = I->getOperand(0).getReg();
205 BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
206 } else if (I->getOpcode() == PPC::BCn) {
207 unsigned CRBit = I->getOperand(0).getReg();
208 BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
209 } else if (I->getOpcode() == PPC::BDNZ) {
210 BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
211 } else if (I->getOpcode() == PPC::BDNZ8) {
212 BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
213 } else if (I->getOpcode() == PPC::BDZ) {
214 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
215 } else if (I->getOpcode() == PPC::BDZ8) {
216 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
218 llvm_unreachable("Unhandled branch type!");
221 // Uncond branch to the real destination.
222 I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
224 // Remove the old branch from the function.
225 OldBranch->eraseFromParent();
227 // Remember that this instruction is 8-bytes, increase the size of the
228 // block by 4, remember to iterate.
229 BlockSizes[MBB.getNumber()] += 4;
235 EverMadeChange |= MadeChange;