LLVM 18.0.0git
MSP430BranchSelector.cpp
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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"
23#include "llvm/Support/Debug.h"
26using namespace llvm;
27
28#define DEBUG_TYPE "msp430-branch-select"
29
30static cl::opt<bool>
31 BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true),
32 cl::desc("Expand out of range branches"));
33
34STATISTIC(NumSplit, "Number of machine basic blocks split");
35STATISTIC(NumExpanded, "Number of branches expanded to long format");
36
37namespace {
38class MSP430BSel : public MachineFunctionPass {
39
40 typedef SmallVector<int, 16> OffsetVector;
41
43 const MSP430InstrInfo *TII;
44
45 unsigned measureFunction(OffsetVector &BlockOffsets,
46 MachineBasicBlock *FromBB = nullptr);
47 bool expandBranches(OffsetVector &BlockOffsets);
48
49public:
50 static char ID;
51 MSP430BSel() : MachineFunctionPass(ID) {}
52
53 bool runOnMachineFunction(MachineFunction &MF) override;
54
57 MachineFunctionProperties::Property::NoVRegs);
58 }
59
60 StringRef getPassName() const override { return "MSP430 Branch Selector"; }
61};
62char MSP430BSel::ID = 0;
63}
64
65static 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
69 // signed integer.
70 const int WordSize = 2;
71
72 assert((DistanceInBytes % WordSize == 0) &&
73 "Branch offset should be word aligned!");
74
75 int Words = DistanceInBytes / WordSize;
76 return isInt<10>(Words);
77}
78
79/// Measure each basic block, fill the BlockOffsets, and return the size of
80/// the function, starting with BB
81unsigned MSP430BSel::measureFunction(OffsetVector &BlockOffsets,
82 MachineBasicBlock *FromBB) {
83 // Give the blocks of the function a dense, in-order, numbering.
84 MF->RenumberBlocks(FromBB);
85
87 if (FromBB == nullptr) {
88 Begin = MF->begin();
89 } else {
90 Begin = FromBB->getIterator();
91 }
92
93 BlockOffsets.resize(MF->getNumBlockIDs());
94
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);
100 }
101 }
102 return TotalSize;
103}
104
105/// Do expand branches and split the basic blocks if necessary.
106/// Returns true if made any change.
107bool 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
110 // like this:
111 // short branch:
112 // bCC MBB
113 // long branch:
114 // b!CC $PC+6
115 // b MBB
116 //
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);
122
123 // If this instruction is not a short branch then skip it.
124 if (MI->getOpcode() != MSP430::JCC && MI->getOpcode() != MSP430::JMP) {
125 continue;
126 }
127
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
131 // instruction.
132 int BlockDistance =
133 BlockOffsets[DestBB->getNumber()] - BlockOffsets[MBB->getNumber()];
134 int BranchDistance = BlockDistance - MBBStartOffset;
135
136 // If this branch is in range, ignore it.
137 if (isInRage(BranchDistance)) {
138 continue;
139 }
140
141 LLVM_DEBUG(dbgs() << " Found a branch that needs expanding, "
142 << printMBBReference(*DestBB) << ", Distance "
143 << BranchDistance << "\n");
144
145 // If JCC is not the last instruction we need to split the MBB.
146 if (MI->getOpcode() == MSP430::JCC && std::next(MI) != EE) {
147
148 LLVM_DEBUG(dbgs() << " Found a basic block that needs to be split, "
149 << printMBBReference(*MBB) << "\n");
150
151 // Create a new basic block.
152 MachineBasicBlock *NewBB =
153 MF->CreateMachineBasicBlock(MBB->getBasicBlock());
154 MF->insert(std::next(MBB), NewBB);
155
156 // Splice the instructions following MI over to the NewBB.
157 NewBB->splice(NewBB->end(), &*MBB, std::next(MI), MBB->end());
158
159 // Update the successor lists.
160 for (MachineBasicBlock *Succ : MBB->successors()) {
161 if (Succ == DestBB) {
162 continue;
163 }
164 MBB->replaceSuccessor(Succ, NewBB);
165 NewBB->addSuccessor(Succ);
166 }
167
168 // We introduced a new MBB so all following blocks should be numbered
169 // and measured again.
170 measureFunction(BlockOffsets, &*MBB);
171
172 ++NumSplit;
173
174 // It may be not necessary to start all over at this point, but it's
175 // safer do this anyway.
176 return true;
177 }
178
179 MachineInstr &OldBranch = *MI;
180 DebugLoc dl = OldBranch.getDebugLoc();
181 int InstrSizeDiff = -TII->getInstSizeInBytes(OldBranch);
182
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!");
187
188 // The BCC operands are:
189 // 0. Target MBB
190 // 1. MSP430 branch predicate
192 Cond.push_back(MI->getOperand(1));
193
194 // Jump over the long branch on the opposite condition
196 MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::JCC))
197 .addMBB(NextMBB)
198 .add(Cond[0]);
199 InstrSizeDiff += TII->getInstSizeInBytes(*MI);
200 ++MI;
201 }
202
203 // Unconditional branch to the real destination.
204 MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::Bi)).addMBB(DestBB);
205 InstrSizeDiff += TII->getInstSizeInBytes(*MI);
206
207 // Remove the old branch from the function.
208 OldBranch.eraseFromParent();
209
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;
214 }
215 MBBStartOffset += InstrSizeDiff;
216
217 ++NumExpanded;
218 MadeChange = true;
219 }
220 }
221 return MadeChange;
222}
223
224bool MSP430BSel::runOnMachineFunction(MachineFunction &mf) {
225 MF = &mf;
226 TII = static_cast<const MSP430InstrInfo *>(MF->getSubtarget().getInstrInfo());
227
228 // If the pass is disabled, just bail early.
230 return false;
231
232 LLVM_DEBUG(dbgs() << "\n********** " << getPassName() << " **********\n");
233
234 // BlockOffsets - Contains the distance from the beginning of the function to
235 // the beginning of each basic block.
236 OffsetVector BlockOffsets;
237
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)) {
243 return false;
244 }
245
246 // Iteratively expand branches until we reach a fixed point.
247 bool MadeChange = false;
248 while (expandBranches(BlockOffsets))
249 MadeChange = true;
250
251 return MadeChange;
252}
253
254/// Returns an instance of the Branch Selection Pass
256 return new MSP430BSel();
257}
MachineBasicBlock & MBB
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
#define LLVM_DEBUG(X)
Definition: Debug.h:101
const HexagonInstrInfo * TII
IRTranslator LLVM IR MI
static bool isInRage(int DistanceInBytes)
static cl::opt< bool > BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true), cl::desc("Expand out of range branches"))
const SmallVectorImpl< MachineOperand > & Cond
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
This file defines the 'Statistic' class, which is designed to be an easy way to expose various metric...
#define STATISTIC(VARNAME, DESC)
Definition: Statistic.h:167
A debug info location.
Definition: DebugLoc.h:33
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:311
bool reverseBranchCondition(SmallVectorImpl< MachineOperand > &Cond) const override
Reverses the branch condition of the specified condition list, returning false on success and true if...
void replaceSuccessor(MachineBasicBlock *Old, MachineBasicBlock *New)
Replace successor OLD with NEW and update probability info.
instr_iterator insert(instr_iterator I, MachineInstr *M)
Insert MI into the instruction list before I, possibly inside a bundle.
int getNumber() const
MachineBasicBlocks are uniquely numbered at the function level, unless they're not in a MachineFuncti...
const BasicBlock * getBasicBlock() const
Return the LLVM basic block that this instance corresponded to originally.
void addSuccessor(MachineBasicBlock *Succ, BranchProbability Prob=BranchProbability::getUnknown())
Add Succ as a successor of this MachineBasicBlock.
iterator_range< succ_iterator > successors()
bool isSuccessor(const MachineBasicBlock *MBB) const
Return true if the specified MBB is a successor of this block.
void splice(iterator Where, MachineBasicBlock *Other, iterator From)
Take an instruction from MBB 'Other' at the position From, and insert it into this MBB right before '...
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
virtual bool runOnMachineFunction(MachineFunction &MF)=0
runOnMachineFunction - This method must be overloaded to perform the desired machine code transformat...
virtual MachineFunctionProperties getRequiredProperties() const
Properties which a MachineFunction may have at a given point in time.
MachineFunctionProperties & set(Property P)
const MachineInstrBuilder & add(const MachineOperand &MO) const
const MachineInstrBuilder & addMBB(MachineBasicBlock *MBB, unsigned TargetFlags=0) const
Representation of each machine instruction.
Definition: MachineInstr.h:68
const DebugLoc & getDebugLoc() const
Returns the debug location id of this MachineInstr.
Definition: MachineInstr.h:472
void eraseFromParent()
Unlink 'this' from the containing basic block and delete it.
virtual StringRef getPassName() const
getPassName - Return a nice clean name for a pass.
Definition: Pass.cpp:81
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1200
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:50
self_iterator getIterator()
Definition: ilist_node.h:109
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:445
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
MachineInstrBuilder BuildMI(MachineFunction &MF, const MIMetadata &MIMD, const MCInstrDesc &MCID)
Builder interface. Specify how to create the initial instruction itself.
iterator_range< T > make_range(T x, T y)
Convenience function for iterating over sub-ranges.
FunctionPass * createMSP430BranchSelectionPass()
Returns an instance of the Branch Selection Pass.
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
Printable printMBBReference(const MachineBasicBlock &MBB)
Prints a machine basic block reference.