1 //===-- MSP430BranchSelector.cpp - Emit long conditional branches ---------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file contains a pass that scans a machine function to determine which
10 // conditional branches need more than 10 bits of displacement to reach their
11 // target basic block. It does this in two passes; a calculation of basic block
12 // positions pass, and a branch pseudo op to machine branch opcode pass. This
13 // pass should be run last, just before the assembly printer.
14 //
15 //===----------------------------------------------------------------------===//
16
17 #include "MSP430.h"
18 #include "MSP430InstrInfo.h"
19 #include "MSP430Subtarget.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/CodeGen/MachineFunctionPass.h"
22 #include "llvm/CodeGen/MachineInstrBuilder.h"
23 #include "llvm/Support/MathExtras.h"
24 #include "llvm/Target/TargetMachine.h"
25 using namespace llvm;
26
27 #define DEBUG_TYPE "msp430-branch-select"
28
29 static cl::opt<bool>
30 BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true),
31 cl::desc("Expand out of range branches"));
32
33 STATISTIC(NumSplit, "Number of machine basic blocks split");
34 STATISTIC(NumExpanded, "Number of branches expanded to long format");
35
36 namespace {
37 class MSP430BSel : public MachineFunctionPass {
38
39 typedef SmallVector<int, 16> OffsetVector;
40
41 MachineFunction *MF;
42 const MSP430InstrInfo *TII;
43
44 unsigned measureFunction(OffsetVector &BlockOffsets,
45 MachineBasicBlock *FromBB = nullptr);
46 bool expandBranches(OffsetVector &BlockOffsets);
47
48 public:
49 static char ID;
MSP430BSel()50 MSP430BSel() : MachineFunctionPass(ID) {}
51
52 bool runOnMachineFunction(MachineFunction &MF) override;
53
getRequiredProperties() const54 MachineFunctionProperties getRequiredProperties() const override {
55 return MachineFunctionProperties().set(
56 MachineFunctionProperties::Property::NoVRegs);
57 }
58
getPassName() const59 StringRef getPassName() const override { return "MSP430 Branch Selector"; }
60 };
61 char MSP430BSel::ID = 0;
62 }
63
isInRage(int DistanceInBytes)64 static bool isInRage(int DistanceInBytes) {
65 // According to CC430 Family User's Guide, Section 4.5.1.3, branch
66 // instructions have the signed 10-bit word offset field, so first we need to
67 // convert the distance from bytes to words, then check if it fits in 10-bit
68 // signed integer.
69 const int WordSize = 2;
70
71 assert((DistanceInBytes % WordSize == 0) &&
72 "Branch offset should be word aligned!");
73
74 int Words = DistanceInBytes / WordSize;
75 return isInt<10>(Words);
76 }
77
78 /// Measure each basic block, fill the BlockOffsets, and return the size of
79 /// the function, starting with BB
measureFunction(OffsetVector & BlockOffsets,MachineBasicBlock * FromBB)80 unsigned MSP430BSel::measureFunction(OffsetVector &BlockOffsets,
81 MachineBasicBlock *FromBB) {
82 // Give the blocks of the function a dense, in-order, numbering.
83 MF->RenumberBlocks(FromBB);
84
85 MachineFunction::iterator Begin;
86 if (FromBB == nullptr) {
87 Begin = MF->begin();
88 } else {
89 Begin = FromBB->getIterator();
90 }
91
92 BlockOffsets.resize(MF->getNumBlockIDs());
93
94 unsigned TotalSize = BlockOffsets[Begin->getNumber()];
95 for (auto &MBB : make_range(Begin, MF->end())) {
96 BlockOffsets[MBB.getNumber()] = TotalSize;
97 for (MachineInstr &MI : MBB) {
98 TotalSize += TII->getInstSizeInBytes(MI);
99 }
100 }
101 return TotalSize;
102 }
103
104 /// Do expand branches and split the basic blocks if necessary.
105 /// Returns true if made any change.
expandBranches(OffsetVector & BlockOffsets)106 bool MSP430BSel::expandBranches(OffsetVector &BlockOffsets) {
107 // For each conditional branch, if the offset to its destination is larger
108 // than the offset field allows, transform it into a long branch sequence
109 // like this:
110 // short branch:
111 // bCC MBB
112 // long branch:
113 // b!CC $PC+6
114 // b MBB
115 //
116 bool MadeChange = false;
117 for (auto MBB = MF->begin(), E = MF->end(); MBB != E; ++MBB) {
118 unsigned MBBStartOffset = 0;
119 for (auto MI = MBB->begin(), EE = MBB->end(); MI != EE; ++MI) {
120 MBBStartOffset += TII->getInstSizeInBytes(*MI);
121
122 // If this instruction is not a short branch then skip it.
123 if (MI->getOpcode() != MSP430::JCC && MI->getOpcode() != MSP430::JMP) {
124 continue;
125 }
126
127 MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
128 // Determine the distance from the current branch to the destination
129 // block. MBBStartOffset already includes the size of the current branch
130 // instruction.
131 int BlockDistance =
132 BlockOffsets[DestBB->getNumber()] - BlockOffsets[MBB->getNumber()];
133 int BranchDistance = BlockDistance - MBBStartOffset;
134
135 // If this branch is in range, ignore it.
136 if (isInRage(BranchDistance)) {
137 continue;
138 }
139
140 LLVM_DEBUG(dbgs() << " Found a branch that needs expanding, "
141 << printMBBReference(*DestBB) << ", Distance "
142 << BranchDistance << "\n");
143
144 // If JCC is not the last instruction we need to split the MBB.
145 if (MI->getOpcode() == MSP430::JCC && std::next(MI) != EE) {
146
147 LLVM_DEBUG(dbgs() << " Found a basic block that needs to be split, "
148 << printMBBReference(*MBB) << "\n");
149
150 // Create a new basic block.
151 MachineBasicBlock *NewBB =
152 MF->CreateMachineBasicBlock(MBB->getBasicBlock());
153 MF->insert(std::next(MBB), NewBB);
154
155 // Splice the instructions following MI over to the NewBB.
156 NewBB->splice(NewBB->end(), &*MBB, std::next(MI), MBB->end());
157
158 // Update the successor lists.
159 for (MachineBasicBlock *Succ : MBB->successors()) {
160 if (Succ == DestBB) {
161 continue;
162 }
163 MBB->replaceSuccessor(Succ, NewBB);
164 NewBB->addSuccessor(Succ);
165 }
166
167 // We introduced a new MBB so all following blocks should be numbered
168 // and measured again.
169 measureFunction(BlockOffsets, &*MBB);
170
171 ++NumSplit;
172
173 // It may be not necessary to start all over at this point, but it's
174 // safer do this anyway.
175 return true;
176 }
177
178 MachineInstr &OldBranch = *MI;
179 DebugLoc dl = OldBranch.getDebugLoc();
180 int InstrSizeDiff = -TII->getInstSizeInBytes(OldBranch);
181
182 if (MI->getOpcode() == MSP430::JCC) {
183 MachineBasicBlock *NextMBB = &*std::next(MBB);
184 assert(MBB->isSuccessor(NextMBB) &&
185 "This block must have a layout successor!");
186
187 // The BCC operands are:
188 // 0. Target MBB
189 // 1. MSP430 branch predicate
190 SmallVector<MachineOperand, 1> Cond;
191 Cond.push_back(MI->getOperand(1));
192
193 // Jump over the long branch on the opposite condition
194 TII->reverseBranchCondition(Cond);
195 MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::JCC))
196 .addMBB(NextMBB)
197 .add(Cond[0]);
198 InstrSizeDiff += TII->getInstSizeInBytes(*MI);
199 ++MI;
200 }
201
202 // Unconditional branch to the real destination.
203 MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::Bi)).addMBB(DestBB);
204 InstrSizeDiff += TII->getInstSizeInBytes(*MI);
205
206 // Remove the old branch from the function.
207 OldBranch.eraseFromParent();
208
209 // The size of a new instruction is different from the old one, so we need
210 // to correct all block offsets.
211 for (int i = MBB->getNumber() + 1, e = BlockOffsets.size(); i < e; ++i) {
212 BlockOffsets[i] += InstrSizeDiff;
213 }
214 MBBStartOffset += InstrSizeDiff;
215
216 ++NumExpanded;
217 MadeChange = true;
218 }
219 }
220 return MadeChange;
221 }
222
runOnMachineFunction(MachineFunction & mf)223 bool MSP430BSel::runOnMachineFunction(MachineFunction &mf) {
224 MF = &mf;
225 TII = static_cast<const MSP430InstrInfo *>(MF->getSubtarget().getInstrInfo());
226
227 // If the pass is disabled, just bail early.
228 if (!BranchSelectEnabled)
229 return false;
230
231 LLVM_DEBUG(dbgs() << "\n********** " << getPassName() << " **********\n");
232
233 // BlockOffsets - Contains the distance from the beginning of the function to
234 // the beginning of each basic block.
235 OffsetVector BlockOffsets;
236
237 unsigned FunctionSize = measureFunction(BlockOffsets);
238 // If the entire function is smaller than the displacement of a branch field,
239 // we know we don't need to expand any branches in this
240 // function. This is a common case.
241 if (isInRage(FunctionSize)) {
242 return false;
243 }
244
245 // Iteratively expand branches until we reach a fixed point.
246 bool MadeChange = false;
247 while (expandBranches(BlockOffsets))
248 MadeChange = true;
249
250 return MadeChange;
251 }
252
253 /// Returns an instance of the Branch Selection Pass
createMSP430BranchSelectionPass()254 FunctionPass *llvm::createMSP430BranchSelectionPass() {
255 return new MSP430BSel();
256 }
257