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 const char *getPassName() const override {
50 return "PowerPC Branch Selector";
56 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
59 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
62 FunctionPass *llvm::createPPCBranchSelectionPass() {
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.
71 BlockSizes.resize(Fn.getNumBlockIDs());
73 // Measure each MBB and compute a size for the entire function.
74 unsigned FuncSize = 0;
75 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
77 MachineBasicBlock *MBB = MFI;
79 unsigned BlockSize = 0;
80 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
82 BlockSize += TII->GetInstSizeInBytes(MBBI);
84 BlockSizes[MBB->getNumber()] = BlockSize;
85 FuncSize += BlockSize;
88 // If the entire function is smaller than the displacement of a branch field,
89 // we know we don't need to shrink any branches in this function. This is a
91 if (FuncSize < (1 << 15)) {
96 // For each conditional branch, if the offset to its destination is larger
97 // than the offset field allows, transform it into a long branch sequence
105 bool MadeChange = true;
106 bool EverMadeChange = false;
108 // Iteratively expand branches until we reach a fixed point.
111 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
113 MachineBasicBlock &MBB = *MFI;
114 unsigned MBBStartOffset = 0;
115 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
117 MachineBasicBlock *Dest = nullptr;
118 if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
119 Dest = I->getOperand(2).getMBB();
120 else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
121 !I->getOperand(1).isImm())
122 Dest = I->getOperand(1).getMBB();
123 else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
124 I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
125 !I->getOperand(0).isImm())
126 Dest = I->getOperand(0).getMBB();
129 MBBStartOffset += TII->GetInstSizeInBytes(I);
133 // Determine the offset from the current branch to the destination
136 if (Dest->getNumber() <= MBB.getNumber()) {
137 // If this is a backwards branch, the delta is the offset from the
138 // start of this block to this branch, plus the sizes of all blocks
139 // from this block to the dest.
140 BranchSize = MBBStartOffset;
142 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
143 BranchSize += BlockSizes[i];
145 // Otherwise, add the size of the blocks between this block and the
146 // dest to the number of bytes left in this block.
147 BranchSize = -MBBStartOffset;
149 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
150 BranchSize += BlockSizes[i];
153 // If this branch is in range, ignore it.
154 if (isInt<16>(BranchSize)) {
159 // Otherwise, we have to expand it to a long branch.
160 MachineInstr *OldBranch = I;
161 DebugLoc dl = OldBranch->getDebugLoc();
163 if (I->getOpcode() == PPC::BCC) {
164 // The BCC operands are:
165 // 0. PPC branch predicate
168 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
169 unsigned CRReg = I->getOperand(1).getReg();
171 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
172 BuildMI(MBB, I, dl, TII->get(PPC::BCC))
173 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
174 } else if (I->getOpcode() == PPC::BC) {
175 unsigned CRBit = I->getOperand(0).getReg();
176 BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
177 } else if (I->getOpcode() == PPC::BCn) {
178 unsigned CRBit = I->getOperand(0).getReg();
179 BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
180 } else if (I->getOpcode() == PPC::BDNZ) {
181 BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
182 } else if (I->getOpcode() == PPC::BDNZ8) {
183 BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
184 } else if (I->getOpcode() == PPC::BDZ) {
185 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
186 } else if (I->getOpcode() == PPC::BDZ8) {
187 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
189 llvm_unreachable("Unhandled branch type!");
192 // Uncond branch to the real destination.
193 I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
195 // Remove the old branch from the function.
196 OldBranch->eraseFromParent();
198 // Remember that this instruction is 8-bytes, increase the size of the
199 // block by 4, remember to iterate.
200 BlockSizes[MBB.getNumber()] += 4;
206 EverMadeChange |= MadeChange;