1 //==-- AArch64CompressJumpTables.cpp - Compress jump tables for AArch64 --====//
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 // This pass looks at the basic blocks each jump-table refers to and works out
8 // whether they can be emitted in a compressed form (with 8 or 16-bit
9 // entries). If so, it changes the opcode and flags them in the associated
10 // AArch64FunctionInfo.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "AArch64.h"
15 #include "AArch64MachineFunctionInfo.h"
16 #include "AArch64Subtarget.h"
17 #include "llvm/ADT/Statistic.h"
18 #include "llvm/CodeGen/MachineFunctionPass.h"
19 #include "llvm/CodeGen/MachineJumpTableInfo.h"
20 #include "llvm/CodeGen/TargetInstrInfo.h"
21 #include "llvm/CodeGen/TargetSubtargetInfo.h"
22 #include "llvm/MC/MCContext.h"
23 #include "llvm/Support/Alignment.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   unsigned Offset = 0;
79   for (MachineBasicBlock &MBB : *MF) {
80     const Align Alignment = MBB.getAlignment();
81     unsigned AlignedOffset;
82     if (Alignment == Align(1))
83       AlignedOffset = Offset;
84     else
85       AlignedOffset = alignTo(Offset, Alignment);
86     BlockInfo[MBB.getNumber()] = AlignedOffset;
87     Offset = AlignedOffset + computeBlockSize(MBB);
88   }
89 }
90 
91 bool AArch64CompressJumpTables::compressJumpTable(MachineInstr &MI,
92                                                   int Offset) {
93   if (MI.getOpcode() != AArch64::JumpTableDest32)
94     return false;
95 
96   int JTIdx = MI.getOperand(4).getIndex();
97   auto &JTInfo = *MF->getJumpTableInfo();
98   const MachineJumpTableEntry &JT = JTInfo.getJumpTables()[JTIdx];
99 
100   // The jump-table might have been optimized away.
101   if (JT.MBBs.empty())
102     return false;
103 
104   int MaxOffset = std::numeric_limits<int>::min(),
105       MinOffset = std::numeric_limits<int>::max();
106   MachineBasicBlock *MinBlock = nullptr;
107   for (auto Block : JT.MBBs) {
108     int BlockOffset = BlockInfo[Block->getNumber()];
109     assert(BlockOffset % 4 == 0 && "misaligned basic block");
110 
111     MaxOffset = std::max(MaxOffset, BlockOffset);
112     if (BlockOffset <= MinOffset) {
113       MinOffset = BlockOffset;
114       MinBlock = Block;
115     }
116   }
117   assert(MinBlock && "Failed to find minimum offset block");
118 
119   // The ADR instruction needed to calculate the address of the first reachable
120   // basic block can address +/-1MB.
121   if (!isInt<21>(MinOffset - Offset)) {
122     ++NumJT32;
123     return false;
124   }
125 
126   int Span = MaxOffset - MinOffset;
127   auto AFI = MF->getInfo<AArch64FunctionInfo>();
128   if (isUInt<8>(Span / 4)) {
129     AFI->setJumpTableEntryInfo(JTIdx, 1, MinBlock->getSymbol());
130     MI.setDesc(TII->get(AArch64::JumpTableDest8));
131     ++NumJT8;
132     return true;
133   } else if (isUInt<16>(Span / 4)) {
134     AFI->setJumpTableEntryInfo(JTIdx, 2, MinBlock->getSymbol());
135     MI.setDesc(TII->get(AArch64::JumpTableDest16));
136     ++NumJT16;
137     return true;
138   }
139 
140   ++NumJT32;
141   return false;
142 }
143 
144 bool AArch64CompressJumpTables::runOnMachineFunction(MachineFunction &MFIn) {
145   bool Changed = false;
146   MF = &MFIn;
147 
148   const auto &ST = MF->getSubtarget<AArch64Subtarget>();
149   TII = ST.getInstrInfo();
150 
151   if (ST.force32BitJumpTables() && !MF->getFunction().hasMinSize())
152     return false;
153 
154   scanFunction();
155 
156   for (MachineBasicBlock &MBB : *MF) {
157     int Offset = BlockInfo[MBB.getNumber()];
158     for (MachineInstr &MI : MBB) {
159       Changed |= compressJumpTable(MI, Offset);
160       Offset += TII->getInstSizeInBytes(MI);
161     }
162   }
163 
164   return Changed;
165 }
166 
167 FunctionPass *llvm::createAArch64CompressJumpTablesPass() {
168   return new AArch64CompressJumpTables();
169 }
170