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 //===----------------------------------------------------------------------===//
18 #define DEBUG_TYPE "ppc-branch-select"
20 #include "MCTargetDesc/PPCPredicates.h"
21 #include "PPCInstrBuilder.h"
22 #include "PPCInstrInfo.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/CodeGen/MachineFunctionPass.h"
25 #include "llvm/Support/MathExtras.h"
26 #include "llvm/Target/TargetMachine.h"
29 STATISTIC(NumExpanded, "Number of branches expanded to long format");
32 void initializePPCBSelPass(PassRegistry&);
36 struct PPCBSel : public MachineFunctionPass {
38 PPCBSel() : MachineFunctionPass(ID) {
39 initializePPCBSelPass(*PassRegistry::getPassRegistry());
42 /// BlockSizes - The sizes of the basic blocks in the function.
43 std::vector<unsigned> BlockSizes;
45 virtual bool runOnMachineFunction(MachineFunction &Fn);
47 virtual const char *getPassName() const {
48 return "PowerPC Branch Selector";
54 INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
57 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
60 FunctionPass *llvm::createPPCBranchSelectionPass() {
64 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
65 const PPCInstrInfo *TII =
66 static_cast<const PPCInstrInfo*>(Fn.getTarget().getInstrInfo());
67 // Give the blocks of the function a dense, in-order, numbering.
69 BlockSizes.resize(Fn.getNumBlockIDs());
71 // Measure each MBB and compute a size for the entire function.
72 unsigned FuncSize = 0;
73 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
75 MachineBasicBlock *MBB = MFI;
77 unsigned BlockSize = 0;
78 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
80 BlockSize += TII->GetInstSizeInBytes(MBBI);
82 BlockSizes[MBB->getNumber()] = BlockSize;
83 FuncSize += BlockSize;
86 // If the entire function is smaller than the displacement of a branch field,
87 // we know we don't need to shrink any branches in this function. This is a
89 if (FuncSize < (1 << 15)) {
94 // For each conditional branch, if the offset to its destination is larger
95 // than the offset field allows, transform it into a long branch sequence
103 bool MadeChange = true;
104 bool EverMadeChange = false;
106 // Iteratively expand branches until we reach a fixed point.
109 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
111 MachineBasicBlock &MBB = *MFI;
112 unsigned MBBStartOffset = 0;
113 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
115 MachineBasicBlock *Dest = 0;
116 if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
117 Dest = I->getOperand(2).getMBB();
118 else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
119 I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
120 !I->getOperand(0).isImm())
121 Dest = I->getOperand(0).getMBB();
124 MBBStartOffset += TII->GetInstSizeInBytes(I);
128 // Determine the offset from the current branch to the destination
131 if (Dest->getNumber() <= MBB.getNumber()) {
132 // If this is a backwards branch, the delta is the offset from the
133 // start of this block to this branch, plus the sizes of all blocks
134 // from this block to the dest.
135 BranchSize = MBBStartOffset;
137 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
138 BranchSize += BlockSizes[i];
140 // Otherwise, add the size of the blocks between this block and the
141 // dest to the number of bytes left in this block.
142 BranchSize = -MBBStartOffset;
144 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
145 BranchSize += BlockSizes[i];
148 // If this branch is in range, ignore it.
149 if (isInt<16>(BranchSize)) {
154 // Otherwise, we have to expand it to a long branch.
155 MachineInstr *OldBranch = I;
156 DebugLoc dl = OldBranch->getDebugLoc();
158 if (I->getOpcode() == PPC::BCC) {
159 // The BCC operands are:
160 // 0. PPC branch predicate
163 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
164 unsigned CRReg = I->getOperand(1).getReg();
166 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
167 BuildMI(MBB, I, dl, TII->get(PPC::BCC))
168 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
169 } else if (I->getOpcode() == PPC::BDNZ) {
170 BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
171 } else if (I->getOpcode() == PPC::BDNZ8) {
172 BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
173 } else if (I->getOpcode() == PPC::BDZ) {
174 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
175 } else if (I->getOpcode() == PPC::BDZ8) {
176 BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
178 llvm_unreachable("Unhandled branch type!");
181 // Uncond branch to the real destination.
182 I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
184 // Remove the old branch from the function.
185 OldBranch->eraseFromParent();
187 // Remember that this instruction is 8-bytes, increase the size of the
188 // block by 4, remember to iterate.
189 BlockSizes[MBB.getNumber()] += 4;
195 EverMadeChange |= MadeChange;