1 //==-- AArch64CompressJumpTables.cpp - Compress jump tables for AArch64 --====//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 // This pass looks at the basic blocks each jump-table refers to and works out
9 // whether they can be emitted in a compressed form (with 8 or 16-bit
10 // entries). If so, it changes the opcode and flags them in the associated
11 // AArch64FunctionInfo.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "AArch64.h"
16 #include "AArch64MachineFunctionInfo.h"
17 #include "AArch64Subtarget.h"
18 #include "llvm/ADT/Statistic.h"
19 #include "llvm/CodeGen/MachineFunctionPass.h"
20 #include "llvm/CodeGen/MachineJumpTableInfo.h"
21 #include "llvm/CodeGen/TargetInstrInfo.h"
22 #include "llvm/CodeGen/TargetSubtargetInfo.h"
23 #include "llvm/MC/MCContext.h"
24 #include "llvm/Support/Debug.h"
25 
26 using namespace llvm;
27 
28 #define DEBUG_TYPE "aarch64-jump-tables"
29 
30 STATISTIC(NumJT8, "Number of jump-tables with 1-byte entries");
31 STATISTIC(NumJT16, "Number of jump-tables with 2-byte entries");
32 STATISTIC(NumJT32, "Number of jump-tables with 4-byte entries");
33 
34 namespace {
35 class AArch64CompressJumpTables : public MachineFunctionPass {
36   const TargetInstrInfo *TII;
37   MachineFunction *MF;
38   SmallVector<int, 8> BlockInfo;
39 
40   int computeBlockSize(MachineBasicBlock &MBB);
41   void scanFunction();
42 
43   bool compressJumpTable(MachineInstr &MI, int Offset);
44 
45 public:
46   static char ID;
47   AArch64CompressJumpTables() : MachineFunctionPass(ID) {
48     initializeAArch64CompressJumpTablesPass(*PassRegistry::getPassRegistry());
49   }
50 
51   bool runOnMachineFunction(MachineFunction &MF) override;
52 
53   MachineFunctionProperties getRequiredProperties() const override {
54     return MachineFunctionProperties().set(
55         MachineFunctionProperties::Property::NoVRegs);
56   }
57   StringRef getPassName() const override {
58     return "AArch64 Compress Jump Tables";
59   }
60 };
61 char AArch64CompressJumpTables::ID = 0;
62 }
63 
64 INITIALIZE_PASS(AArch64CompressJumpTables, DEBUG_TYPE,
65                 "AArch64 compress jump tables pass", false, false)
66 
67 int AArch64CompressJumpTables::computeBlockSize(MachineBasicBlock &MBB) {
68   int Size = 0;
69   for (const MachineInstr &MI : MBB)
70     Size += TII->getInstSizeInBytes(MI);
71   return Size;
72 }
73 
74 void AArch64CompressJumpTables::scanFunction() {
75   BlockInfo.clear();
76   BlockInfo.resize(MF->getNumBlockIDs());
77 
78   int Offset = 0;
79   for (MachineBasicBlock &MBB : *MF) {
80     BlockInfo[MBB.getNumber()] = Offset;
81     Offset += computeBlockSize(MBB);
82   }
83 }
84 
85 bool AArch64CompressJumpTables::compressJumpTable(MachineInstr &MI,
86                                                   int Offset) {
87   if (MI.getOpcode() != AArch64::JumpTableDest32)
88     return false;
89 
90   int JTIdx = MI.getOperand(4).getIndex();
91   auto &JTInfo = *MF->getJumpTableInfo();
92   const MachineJumpTableEntry &JT = JTInfo.getJumpTables()[JTIdx];
93 
94   // The jump-table might have been optimized away.
95   if (JT.MBBs.empty())
96     return false;
97 
98   int MaxOffset = std::numeric_limits<int>::min(),
99       MinOffset = std::numeric_limits<int>::max();
100   MachineBasicBlock *MinBlock = nullptr;
101   for (auto Block : JT.MBBs) {
102     int BlockOffset = BlockInfo[Block->getNumber()];
103     assert(BlockOffset % 4 == 0 && "misaligned basic block");
104 
105     MaxOffset = std::max(MaxOffset, BlockOffset);
106     if (BlockOffset <= MinOffset) {
107       MinOffset = BlockOffset;
108       MinBlock = Block;
109     }
110   }
111 
112   // The ADR instruction needed to calculate the address of the first reachable
113   // basic block can address +/-1MB.
114   if (!isInt<21>(MinOffset - Offset)) {
115     ++NumJT32;
116     return false;
117   }
118 
119   int Span = MaxOffset - MinOffset;
120   auto AFI = MF->getInfo<AArch64FunctionInfo>();
121   if (isUInt<8>(Span / 4)) {
122     AFI->setJumpTableEntryInfo(JTIdx, 1, MinBlock->getSymbol());
123     MI.setDesc(TII->get(AArch64::JumpTableDest8));
124     ++NumJT8;
125     return true;
126   } else if (isUInt<16>(Span / 4)) {
127     AFI->setJumpTableEntryInfo(JTIdx, 2, MinBlock->getSymbol());
128     MI.setDesc(TII->get(AArch64::JumpTableDest16));
129     ++NumJT16;
130     return true;
131   }
132 
133   ++NumJT32;
134   return false;
135 }
136 
137 bool AArch64CompressJumpTables::runOnMachineFunction(MachineFunction &MFIn) {
138   bool Changed = false;
139   MF = &MFIn;
140 
141   const auto &ST = MF->getSubtarget<AArch64Subtarget>();
142   TII = ST.getInstrInfo();
143 
144   if (ST.force32BitJumpTables() && !MF->getFunction().optForMinSize())
145     return false;
146 
147   scanFunction();
148 
149   for (MachineBasicBlock &MBB : *MF) {
150     int Offset = BlockInfo[MBB.getNumber()];
151     for (MachineInstr &MI : MBB) {
152       Changed |= compressJumpTable(MI, Offset);
153       Offset += TII->getInstSizeInBytes(MI);
154     }
155   }
156 
157   return Changed;
158 }
159 
160 FunctionPass *llvm::createAArch64CompressJumpTablesPass() {
161   return new AArch64CompressJumpTables();
162 }
163