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"
27 #include "llvm/Support/MathExtras.h"
30 static Statistic<> NumExpanded("ppc-branch-select",
31 "Num branches expanded to long format");
34 struct VISIBILITY_HIDDEN PPCBSel : public MachineFunctionPass {
35 /// BlockSizes - The sizes of the basic blocks in the function.
36 std::vector<unsigned> BlockSizes;
38 virtual bool runOnMachineFunction(MachineFunction &Fn);
40 virtual const char *getPassName() const {
41 return "PowerPC Branch Selector";
46 /// createPPCBranchSelectionPass - returns an instance of the Branch Selection
49 FunctionPass *llvm::createPPCBranchSelectionPass() {
53 /// getNumBytesForInstruction - Return the number of bytes of code the specified
54 /// instruction may be. This returns the maximum number of bytes.
56 static unsigned getNumBytesForInstruction(MachineInstr *MI) {
57 switch (MI->getOpcode()) {
58 case PPC::IMPLICIT_DEF_GPRC: // no asm emitted
59 case PPC::IMPLICIT_DEF_G8RC: // no asm emitted
60 case PPC::IMPLICIT_DEF_F4: // no asm emitted
61 case PPC::IMPLICIT_DEF_F8: // no asm emitted
62 case PPC::IMPLICIT_DEF_VRRC: // no asm emitted
64 case PPC::INLINEASM: { // Inline Asm: Variable size.
65 MachineFunction *MF = MI->getParent()->getParent();
66 const char *AsmStr = MI->getOperand(0).getSymbolName();
67 return MF->getTarget().getTargetAsmInfo()->getInlineAsmLength(AsmStr);
70 return 4; // PowerPC instructions are all 4 bytes
75 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
76 // Give the blocks of the function a dense, in-order, numbering.
78 BlockSizes.resize(Fn.getNumBlockIDs());
80 // Measure each MBB and compute a size for the entire function.
81 unsigned FuncSize = 0;
82 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
84 MachineBasicBlock *MBB = MFI;
86 unsigned BlockSize = 0;
87 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
89 BlockSize += getNumBytesForInstruction(MBBI);
91 BlockSizes[MBB->getNumber()] = BlockSize;
92 FuncSize += BlockSize;
95 // If the entire function is smaller than the displacement of a branch field,
96 // we know we don't need to shrink any branches in this function. This is a
98 if (FuncSize < (1 << 15)) {
103 // For each conditional branch, if the offset to its destination is larger
104 // than the offset field allows, transform it into a long branch sequence
112 bool MadeChange = true;
113 bool EverMadeChange = false;
115 // Iteratively expand branches until we reach a fixed point.
118 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
120 MachineBasicBlock &MBB = *MFI;
121 unsigned MBBStartOffset = 0;
122 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
124 if (I->getOpcode() != PPC::BCC || I->getOperand(2).isImm()) {
125 MBBStartOffset += getNumBytesForInstruction(I);
129 // Determine the offset from the current branch to the destination
131 MachineBasicBlock *Dest = I->getOperand(2).getMachineBasicBlock();
134 if (Dest->getNumber() <= MBB.getNumber()) {
135 // If this is a backwards branch, the delta is the offset from the
136 // start of this block to this branch, plus the sizes of all blocks
137 // from this block to the dest.
138 BranchSize = MBBStartOffset;
140 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
141 BranchSize += BlockSizes[i];
143 // Otherwise, add the size of the blocks between this block and the
144 // dest to the number of bytes left in this block.
145 BranchSize = -MBBStartOffset;
147 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
148 BranchSize += BlockSizes[i];
151 // If this branch is in range, ignore it.
152 if (isInt16(BranchSize)) {
157 // Otherwise, we have to expand it to a long branch.
158 // The BCC operands are:
159 // 0. PPC branch predicate
162 PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
163 unsigned CRReg = I->getOperand(1).getReg();
165 MachineInstr *OldBranch = I;
167 // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
168 BuildMI(MBB, I, PPC::BCC, 3)
169 .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
171 // Uncond branch to the real destination.
172 I = BuildMI(MBB, I, PPC::B, 1).addMBB(Dest);
174 // Remove the old branch from the function.
175 OldBranch->eraseFromParent();
177 // Remember that this instruction is 8-bytes, increase the size of the
178 // block by 4, remember to iterate.
179 BlockSizes[MBB.getNumber()] += 4;
185 EverMadeChange |= MadeChange;