llvm/lib/Target/AArch64/AArch64CompressJumpTables.cpp

   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::None())
  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 }