PPCBranchSelector.cpp revision 208954
1//===-- PPCBranchSelector.cpp - Emit long conditional branches-----*- C++ -*-=// 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 psuedo op to machine branch opcode pass. This 14// pass should be run last, just before the assembly printer. 15// 16//===----------------------------------------------------------------------===// 17 18#define DEBUG_TYPE "ppc-branch-select" 19#include "PPC.h" 20#include "PPCInstrBuilder.h" 21#include "PPCInstrInfo.h" 22#include "PPCPredicates.h" 23#include "llvm/CodeGen/MachineFunctionPass.h" 24#include "llvm/Target/TargetMachine.h" 25#include "llvm/ADT/Statistic.h" 26#include "llvm/Support/MathExtras.h" 27using namespace llvm; 28 29STATISTIC(NumExpanded, "Number of branches expanded to long format"); 30 31namespace { 32 struct PPCBSel : public MachineFunctionPass { 33 static char ID; 34 PPCBSel() : MachineFunctionPass(&ID) {} 35 36 /// BlockSizes - The sizes of the basic blocks in the function. 37 std::vector<unsigned> BlockSizes; 38 39 virtual bool runOnMachineFunction(MachineFunction &Fn); 40 41 virtual const char *getPassName() const { 42 return "PowerPC Branch Selector"; 43 } 44 }; 45 char PPCBSel::ID = 0; 46} 47 48/// createPPCBranchSelectionPass - returns an instance of the Branch Selection 49/// Pass 50/// 51FunctionPass *llvm::createPPCBranchSelectionPass() { 52 return new PPCBSel(); 53} 54 55bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) { 56 const TargetInstrInfo *TII = Fn.getTarget().getInstrInfo(); 57 // Give the blocks of the function a dense, in-order, numbering. 58 Fn.RenumberBlocks(); 59 BlockSizes.resize(Fn.getNumBlockIDs()); 60 61 // Measure each MBB and compute a size for the entire function. 62 unsigned FuncSize = 0; 63 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; 64 ++MFI) { 65 MachineBasicBlock *MBB = MFI; 66 67 unsigned BlockSize = 0; 68 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end(); 69 MBBI != EE; ++MBBI) 70 BlockSize += TII->GetInstSizeInBytes(MBBI); 71 72 BlockSizes[MBB->getNumber()] = BlockSize; 73 FuncSize += BlockSize; 74 } 75 76 // If the entire function is smaller than the displacement of a branch field, 77 // we know we don't need to shrink any branches in this function. This is a 78 // common case. 79 if (FuncSize < (1 << 15)) { 80 BlockSizes.clear(); 81 return false; 82 } 83 84 // For each conditional branch, if the offset to its destination is larger 85 // than the offset field allows, transform it into a long branch sequence 86 // like this: 87 // short branch: 88 // bCC MBB 89 // long branch: 90 // b!CC $PC+8 91 // b MBB 92 // 93 bool MadeChange = true; 94 bool EverMadeChange = false; 95 while (MadeChange) { 96 // Iteratively expand branches until we reach a fixed point. 97 MadeChange = false; 98 99 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E; 100 ++MFI) { 101 MachineBasicBlock &MBB = *MFI; 102 unsigned MBBStartOffset = 0; 103 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); 104 I != E; ++I) { 105 if (I->getOpcode() != PPC::BCC || I->getOperand(2).isImm()) { 106 MBBStartOffset += TII->GetInstSizeInBytes(I); 107 continue; 108 } 109 110 // Determine the offset from the current branch to the destination 111 // block. 112 MachineBasicBlock *Dest = I->getOperand(2).getMBB(); 113 114 int BranchSize; 115 if (Dest->getNumber() <= MBB.getNumber()) { 116 // If this is a backwards branch, the delta is the offset from the 117 // start of this block to this branch, plus the sizes of all blocks 118 // from this block to the dest. 119 BranchSize = MBBStartOffset; 120 121 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i) 122 BranchSize += BlockSizes[i]; 123 } else { 124 // Otherwise, add the size of the blocks between this block and the 125 // dest to the number of bytes left in this block. 126 BranchSize = -MBBStartOffset; 127 128 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i) 129 BranchSize += BlockSizes[i]; 130 } 131 132 // If this branch is in range, ignore it. 133 if (isInt<16>(BranchSize)) { 134 MBBStartOffset += 4; 135 continue; 136 } 137 138 // Otherwise, we have to expand it to a long branch. 139 // The BCC operands are: 140 // 0. PPC branch predicate 141 // 1. CR register 142 // 2. Target MBB 143 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm(); 144 unsigned CRReg = I->getOperand(1).getReg(); 145 146 MachineInstr *OldBranch = I; 147 DebugLoc dl = OldBranch->getDebugLoc(); 148 149 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition. 150 BuildMI(MBB, I, dl, TII->get(PPC::BCC)) 151 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2); 152 153 // Uncond branch to the real destination. 154 I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest); 155 156 // Remove the old branch from the function. 157 OldBranch->eraseFromParent(); 158 159 // Remember that this instruction is 8-bytes, increase the size of the 160 // block by 4, remember to iterate. 161 BlockSizes[MBB.getNumber()] += 4; 162 MBBStartOffset += 8; 163 ++NumExpanded; 164 MadeChange = true; 165 } 166 } 167 EverMadeChange |= MadeChange; 168 } 169 170 BlockSizes.clear(); 171 return true; 172} 173 174