1 //===-- PPCBranchSelector.cpp - Emit long conditional branches-----*- C++ -*-=//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by Nate Baegeman and is distributed under the
6 // University of Illinois Open Source 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 psuedo op to machine branch opcode pass. This
14 // pass should be run last, just before the assembly printer.
16 //===----------------------------------------------------------------------===//
19 #include "PPCInstrBuilder.h"
20 #include "PPCInstrInfo.h"
21 #include "PPCPredicates.h"
22 #include "llvm/CodeGen/MachineFunctionPass.h"
23 #include "llvm/Target/TargetMachine.h"
24 #include "llvm/Target/TargetAsmInfo.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/Support/Compiler.h"
29 static Statistic<> NumExpanded("ppc-branch-select",
30 "Num branches expanded to long format");
33 struct VISIBILITY_HIDDEN PPCBSel : public MachineFunctionPass {
34 /// OffsetMap - Mapping between BB # and byte offset from start of function.
35 std::vector<unsigned> OffsetMap;
37 virtual bool runOnMachineFunction(MachineFunction &Fn);
39 virtual const char *getPassName() const {
40 return "PowerPC Branch Selection";
45 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
48 FunctionPass *llvm::createPPCBranchSelectionPass() {
52 /// getNumBytesForInstruction - Return the number of bytes of code the specified
53 /// instruction may be. This returns the maximum number of bytes.
55 static unsigned getNumBytesForInstruction(MachineInstr *MI) {
56 switch (MI->getOpcode()) {
58 // while this will be 4 most of the time, if we emit 8 it is just a
59 // minor pessimization that saves us from having to worry about
60 // keeping the offsets up to date later when we emit long branch glue.
62 case PPC::IMPLICIT_DEF_GPRC: // no asm emitted
63 case PPC::IMPLICIT_DEF_G8RC: // no asm emitted
64 case PPC::IMPLICIT_DEF_F4: // no asm emitted
65 case PPC::IMPLICIT_DEF_F8: // no asm emitted
66 case PPC::IMPLICIT_DEF_VRRC: // no asm emitted
68 case PPC::INLINEASM: { // Inline Asm: Variable size.
69 MachineFunction *MF = MI->getParent()->getParent();
70 const char *AsmStr = MI->getOperand(0).getSymbolName();
71 return MF->getTarget().getTargetAsmInfo()->getInlineAsmLength(AsmStr);
74 return 4; // PowerPC instructions are all 4 bytes
79 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
80 // Running total of instructions encountered since beginning of function
81 unsigned ByteCount = 0;
83 OffsetMap.resize(Fn.getNumBlockIDs());
85 // For each MBB, add its offset to the offset map, and count up its
87 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
89 MachineBasicBlock *MBB = MFI;
90 OffsetMap[MBB->getNumber()] = ByteCount;
92 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
94 ByteCount += getNumBytesForInstruction(MBBI);
97 // We're about to run over the MBB's again, so reset the ByteCount
100 // For each MBB, find the conditional branch pseudo instructions, and
101 // calculate the difference between the target MBB and the current ICount
102 // to decide whether or not to emit a short or long branch.
110 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
112 MachineBasicBlock *MBB = MFI;
114 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
115 MBBI != EE; ++MBBI) {
116 // We may end up deleting the MachineInstr that MBBI points to, so
117 // remember its opcode now so we can refer to it after calling erase()
118 unsigned ByteSize = getNumBytesForInstruction(MBBI);
119 if (MBBI->getOpcode() != PPC::BCC) {
120 ByteCount += ByteSize;
124 // condbranch operands:
126 // 1. PPC branch opcode
128 MachineBasicBlock *DestMBB = MBBI->getOperand(2).getMachineBasicBlock();
129 PPC::Predicate Pred = (PPC::Predicate)MBBI->getOperand(1).getImm();
130 unsigned CRReg = MBBI->getOperand(0).getReg();
131 int Displacement = OffsetMap[DestMBB->getNumber()] - ByteCount;
133 bool ShortBranchOk = Displacement >= -32768 && Displacement <= 32767;
135 // Branch on opposite condition if a short branch isn't ok.
137 Pred = PPC::InvertPredicate(Pred);
141 default: assert(0 && "Unknown cond branch predicate!");
142 case PPC::PRED_LT: Opcode = PPC::BLT; break;
143 case PPC::PRED_LE: Opcode = PPC::BLE; break;
144 case PPC::PRED_EQ: Opcode = PPC::BEQ; break;
145 case PPC::PRED_GE: Opcode = PPC::BGE; break;
146 case PPC::PRED_GT: Opcode = PPC::BGT; break;
147 case PPC::PRED_NE: Opcode = PPC::BNE; break;
148 case PPC::PRED_UN: Opcode = PPC::BUN; break;
149 case PPC::PRED_NU: Opcode = PPC::BNU; break;
152 MachineBasicBlock::iterator MBBJ;
154 MBBJ = BuildMI(*MBB, MBBI, Opcode, 2).addReg(CRReg).addMBB(DestMBB);
156 // Long branch, skip next branch instruction (i.e. $PC+8).
158 BuildMI(*MBB, MBBI, Opcode, 2).addReg(CRReg).addImm(2);
159 MBBJ = BuildMI(*MBB, MBBI, PPC::B, 1).addMBB(DestMBB);
162 // Erase the psuedo BCC instruction, and then back up the
163 // iterator so that when the for loop increments it, we end up in
164 // the correct place rather than iterating off the end.
167 ByteCount += ByteSize;