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 "llvm/CodeGen/MachineFunctionPass.h"
22 #include "llvm/Support/Visibility.h"
27 struct VISIBILITY_HIDDEN PPCBSel : public MachineFunctionPass {
28 // OffsetMap - Mapping between BB and byte offset from start of function
29 std::map<MachineBasicBlock*, unsigned> OffsetMap;
31 virtual bool runOnMachineFunction(MachineFunction &Fn);
33 virtual const char *getPassName() const {
34 return "PowerPC Branch Selection";
39 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
42 FunctionPass *llvm::createPPCBranchSelectionPass() {
46 /// getNumBytesForInstruction - Return the number of bytes of code the specified
47 /// instruction may be. This returns the maximum number of bytes.
49 static unsigned getNumBytesForInstruction(MachineInstr *MI) {
50 switch (MI->getOpcode()) {
51 case PPC::COND_BRANCH:
52 // while this will be 4 most of the time, if we emit 8 it is just a
53 // minor pessimization that saves us from having to worry about
54 // keeping the offsets up to date later when we emit long branch glue.
56 case PPC::IMPLICIT_DEF_GPRC: // no asm emitted
57 case PPC::IMPLICIT_DEF_G8RC: // no asm emitted
58 case PPC::IMPLICIT_DEF_F4: // no asm emitted
59 case PPC::IMPLICIT_DEF_F8: // no asm emitted
61 case PPC::INLINEASM: // Inline Asm: Variable size.
62 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i)
63 if (MI->getOperand(i).isExternalSymbol()) {
64 const char *AsmStr = MI->getOperand(i).getSymbolName();
65 // Count the number of newline's in the asm string.
66 unsigned NumInstrs = 0;
67 for (; *AsmStr; ++AsmStr)
68 NumInstrs += *AsmStr == '\n';
71 assert(0 && "INLINEASM didn't have format string??");
73 return 4; // PowerPC instructions are all 4 bytes
78 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
79 // Running total of instructions encountered since beginning of function
80 unsigned ByteCount = 0;
82 // For each MBB, add its offset to the offset map, and count up its
84 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
86 MachineBasicBlock *MBB = MFI;
87 OffsetMap[MBB] = ByteCount;
89 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
91 ByteCount += getNumBytesForInstruction(MBBI);
94 // We're about to run over the MBB's again, so reset the ByteCount
97 // For each MBB, find the conditional branch pseudo instructions, and
98 // calculate the difference between the target MBB and the current ICount
99 // to decide whether or not to emit a short or long branch.
107 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
109 MachineBasicBlock *MBB = MFI;
111 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
112 MBBI != EE; ++MBBI) {
113 // We may end up deleting the MachineInstr that MBBI points to, so
114 // remember its opcode now so we can refer to it after calling erase()
115 unsigned ByteSize = getNumBytesForInstruction(MBBI);
116 if (MBBI->getOpcode() == PPC::COND_BRANCH) {
117 MachineBasicBlock::iterator MBBJ = MBBI;
120 // condbranch operands:
124 // 3. fallthrough MBB
125 MachineBasicBlock *trueMBB =
126 MBBI->getOperand(2).getMachineBasicBlock();
128 int Displacement = OffsetMap[trueMBB] - ByteCount;
129 unsigned Opcode = MBBI->getOperand(1).getImmedValue();
130 unsigned CRReg = MBBI->getOperand(0).getReg();
131 unsigned Inverted = PPCInstrInfo::invertPPCBranchOpcode(Opcode);
133 if (Displacement >= -32768 && Displacement <= 32767) {
134 BuildMI(*MBB, MBBJ, Opcode, 2).addReg(CRReg).addMBB(trueMBB);
136 BuildMI(*MBB, MBBJ, Inverted, 2).addReg(CRReg).addImm(2);
137 BuildMI(*MBB, MBBJ, PPC::B, 1).addMBB(trueMBB);
140 // Erase the psuedo COND_BRANCH instruction, and then back up the
141 // iterator so that when the for loop increments it, we end up in
142 // the correct place rather than iterating off the end.
146 ByteCount += ByteSize;