1 //===-- MSP430BranchSelector.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 10 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 //===----------------------------------------------------------------------===//
19 #include "MSP430InstrInfo.h"
20 #include "MSP430Subtarget.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/CodeGen/MachineFunctionPass.h"
23 #include "llvm/CodeGen/MachineInstrBuilder.h"
24 #include "llvm/Support/MathExtras.h"
25 #include "llvm/Target/TargetMachine.h"
28 #define DEBUG_TYPE "msp430-branch-select"
31 BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true),
32 cl::desc("Expand out of range branches"));
34 STATISTIC(NumSplit, "Number of machine basic blocks split");
35 STATISTIC(NumExpanded, "Number of branches expanded to long format");
38 class MSP430BSel : public MachineFunctionPass {
40 typedef SmallVector<int, 16> OffsetVector;
43 const MSP430InstrInfo *TII;
45 unsigned measureFunction(OffsetVector &BlockOffsets,
46 MachineBasicBlock *FromBB = nullptr);
47 bool expandBranches(OffsetVector &BlockOffsets);
51 MSP430BSel() : MachineFunctionPass(ID) {}
53 bool runOnMachineFunction(MachineFunction &MF) override;
55 MachineFunctionProperties getRequiredProperties() const override {
56 return MachineFunctionProperties().set(
57 MachineFunctionProperties::Property::NoVRegs);
60 StringRef getPassName() const override { return "MSP430 Branch Selector"; }
62 char MSP430BSel::ID = 0;
65 static bool isInRage(int DistanceInBytes) {
66 // According to CC430 Family User's Guide, Section 4.5.1.3, branch
67 // instructions have the signed 10-bit word offset field, so first we need to
68 // convert the distance from bytes to words, then check if it fits in 10-bit
70 const int WordSize = 2;
72 assert((DistanceInBytes % WordSize == 0) &&
73 "Branch offset should be word aligned!");
75 int Words = DistanceInBytes / WordSize;
76 return isInt<10>(Words);
79 /// Measure each basic block, fill the BlockOffsets, and return the size of
80 /// the function, starting with BB
81 unsigned MSP430BSel::measureFunction(OffsetVector &BlockOffsets,
82 MachineBasicBlock *FromBB) {
83 // Give the blocks of the function a dense, in-order, numbering.
84 MF->RenumberBlocks(FromBB);
86 MachineFunction::iterator Begin;
87 if (FromBB == nullptr) {
90 Begin = FromBB->getIterator();
93 BlockOffsets.resize(MF->getNumBlockIDs());
95 unsigned TotalSize = BlockOffsets[Begin->getNumber()];
96 for (auto &MBB : make_range(Begin, MF->end())) {
97 BlockOffsets[MBB.getNumber()] = TotalSize;
98 for (MachineInstr &MI : MBB) {
99 TotalSize += TII->getInstSizeInBytes(MI);
105 /// Do expand branches and split the basic blocks if necessary.
106 /// Returns true if made any change.
107 bool MSP430BSel::expandBranches(OffsetVector &BlockOffsets) {
108 // For each conditional branch, if the offset to its destination is larger
109 // than the offset field allows, transform it into a long branch sequence
117 bool MadeChange = false;
118 for (auto MBB = MF->begin(), E = MF->end(); MBB != E; ++MBB) {
119 unsigned MBBStartOffset = 0;
120 for (auto MI = MBB->begin(), EE = MBB->end(); MI != EE; ++MI) {
121 MBBStartOffset += TII->getInstSizeInBytes(*MI);
123 // If this instruction is not a short branch then skip it.
124 if (MI->getOpcode() != MSP430::JCC && MI->getOpcode() != MSP430::JMP) {
128 MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
129 // Determine the distance from the current branch to the destination
130 // block. MBBStartOffset already includes the size of the current branch
133 BlockOffsets[DestBB->getNumber()] - BlockOffsets[MBB->getNumber()];
134 int BranchDistance = BlockDistance - MBBStartOffset;
136 // If this branch is in range, ignore it.
137 if (isInRage(BranchDistance)) {
141 DEBUG(dbgs() << " Found a branch that needs expanding, BB#"
142 << DestBB->getNumber() << ", Distance " << BranchDistance
145 // If JCC is not the last instruction we need to split the MBB.
146 if (MI->getOpcode() == MSP430::JCC && std::next(MI) != EE) {
148 DEBUG(dbgs() << " Found a basic block that needs to be split, BB#"
149 << MBB->getNumber() << "\n");
151 // Create a new basic block.
152 MachineBasicBlock *NewBB =
153 MF->CreateMachineBasicBlock(MBB->getBasicBlock());
154 MF->insert(std::next(MBB), NewBB);
156 // Splice the instructions following MI over to the NewBB.
157 NewBB->splice(NewBB->end(), &*MBB, std::next(MI), MBB->end());
159 // Update the successor lists.
160 for (MachineBasicBlock *Succ : MBB->successors()) {
161 if (Succ == DestBB) {
164 MBB->replaceSuccessor(Succ, NewBB);
165 NewBB->addSuccessor(Succ);
168 // We introduced a new MBB so all following blocks should be numbered
169 // and measured again.
170 measureFunction(BlockOffsets, &*MBB);
174 // It may be not necessary to start all over at this point, but it's
175 // safer do this anyway.
179 MachineInstr &OldBranch = *MI;
180 DebugLoc dl = OldBranch.getDebugLoc();
181 int InstrSizeDiff = -TII->getInstSizeInBytes(OldBranch);
183 if (MI->getOpcode() == MSP430::JCC) {
184 MachineBasicBlock *NextMBB = &*std::next(MBB);
185 assert(MBB->isSuccessor(NextMBB) &&
186 "This block must have a layout successor!");
188 // The BCC operands are:
190 // 1. MSP430 branch predicate
191 SmallVector<MachineOperand, 1> Cond;
192 Cond.push_back(MI->getOperand(1));
194 // Jump over the long branch on the opposite condition
195 TII->reverseBranchCondition(Cond);
196 MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::JCC))
198 .addOperand(Cond[0]);
199 InstrSizeDiff += TII->getInstSizeInBytes(*MI);
203 // Unconditional branch to the real destination.
204 MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::Bi)).addMBB(DestBB);
205 InstrSizeDiff += TII->getInstSizeInBytes(*MI);
207 // Remove the old branch from the function.
208 OldBranch.eraseFromParent();
210 // The size of a new instruction is different from the old one, so we need
211 // to correct all block offsets.
212 for (int i = MBB->getNumber() + 1, e = BlockOffsets.size(); i < e; ++i) {
213 BlockOffsets[i] += InstrSizeDiff;
215 MBBStartOffset += InstrSizeDiff;
224 bool MSP430BSel::runOnMachineFunction(MachineFunction &mf) {
226 TII = static_cast<const MSP430InstrInfo *>(MF->getSubtarget().getInstrInfo());
228 // If the pass is disabled, just bail early.
229 if (!BranchSelectEnabled)
232 DEBUG(dbgs() << "\n********** " << getPassName() << " **********\n");
234 // BlockOffsets - Contains the distance from the beginning of the function to
235 // the beginning of each basic block.
236 OffsetVector BlockOffsets;
238 unsigned FunctionSize = measureFunction(BlockOffsets);
239 // If the entire function is smaller than the displacement of a branch field,
240 // we know we don't need to expand any branches in this
241 // function. This is a common case.
242 if (isInRage(FunctionSize)) {
246 // Iteratively expand branches until we reach a fixed point.
247 bool MadeChange = false;
248 while (expandBranches(BlockOffsets))
254 /// Returns an instance of the Branch Selection Pass
255 FunctionPass *llvm::createMSP430BranchSelectionPass() {
256 return new MSP430BSel();