lib/Target/BPF/BPFMIPeephole.cpp

   1 //===-------------- BPFMIPeephole.cpp - MI Peephole Cleanups  -------------===//
   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 pass performs peephole optimizations to cleanup ugly code sequences at
  10 // MachineInstruction layer.
  11 //
  12 // Currently, there are two optimizations implemented:
  13 //  - One pre-RA MachineSSA pass to eliminate type promotion sequences, those
  14 //    zero extend 32-bit subregisters to 64-bit registers, if the compiler
  15 //    could prove the subregisters is defined by 32-bit operations in which
  16 //    case the upper half of the underlying 64-bit registers were zeroed
  17 //    implicitly.
  18 //
  19 //  - One post-RA PreEmit pass to do final cleanup on some redundant
  20 //    instructions generated due to bad RA on subregister.
  21 //===----------------------------------------------------------------------===//
  22
  23 #include "BPF.h"
  24 #include "BPFInstrInfo.h"
  25 #include "BPFTargetMachine.h"
  26 #include "llvm/ADT/Statistic.h"
  27 #include "llvm/CodeGen/MachineInstrBuilder.h"
  28 #include "llvm/CodeGen/MachineRegisterInfo.h"
  29
  30 using namespace llvm;
  31
  32 #define DEBUG_TYPE "bpf-mi-zext-elim"
  33
  34 STATISTIC(ZExtElemNum, "Number of zero extension shifts eliminated");
  35
  36 namespace {
  37
  38 struct BPFMIPeephole : public MachineFunctionPass {
  39
  40   static char ID;
  41   const BPFInstrInfo *TII;
  42   MachineFunction *MF;
  43   MachineRegisterInfo *MRI;
  44
  45   BPFMIPeephole() : MachineFunctionPass(ID) {
  46     initializeBPFMIPeepholePass(*PassRegistry::getPassRegistry());
  47   }
  48
  49 private:
  50   // Initialize class variables.
  51   void initialize(MachineFunction &MFParm);
  52
  53   bool isMovFrom32Def(MachineInstr *MovMI);
  54   bool eliminateZExtSeq(void);
  55
  56 public:
  57
  58   // Main entry point for this pass.
  59   bool runOnMachineFunction(MachineFunction &MF) override {
  60     if (skipFunction(MF.getFunction()))
  61       return false;
  62
  63     initialize(MF);
  64
  65     return eliminateZExtSeq();
  66   }
  67 };
  68
  69 // Initialize class variables.
  70 void BPFMIPeephole::initialize(MachineFunction &MFParm) {
  71   MF = &MFParm;
  72   MRI = &MF->getRegInfo();
  73   TII = MF->getSubtarget<BPFSubtarget>().getInstrInfo();
  74   LLVM_DEBUG(dbgs() << "*** BPF MachineSSA peephole pass ***\n\n");
  75 }
  76
  77 bool BPFMIPeephole::isMovFrom32Def(MachineInstr *MovMI)
  78 {
  79   MachineInstr *DefInsn = MRI->getVRegDef(MovMI->getOperand(1).getReg());
  80
  81   LLVM_DEBUG(dbgs() << "  Def of Mov Src:");
  82   LLVM_DEBUG(DefInsn->dump());
  83
  84   if (!DefInsn)
  85     return false;
  86
  87   if (DefInsn->isPHI()) {
  88     for (unsigned i = 1, e = DefInsn->getNumOperands(); i < e; i += 2) {
  89       MachineOperand &opnd = DefInsn->getOperand(i);
  90
  91       if (!opnd.isReg())
  92         return false;
  93
  94       MachineInstr *PhiDef = MRI->getVRegDef(opnd.getReg());
  95       // quick check on PHI incoming definitions.
  96       if (!PhiDef || PhiDef->isPHI() || PhiDef->getOpcode() == BPF::COPY)
  97         return false;
  98     }
  99   }
 100
 101   if (DefInsn->getOpcode() == BPF::COPY) {
 102     MachineOperand &opnd = DefInsn->getOperand(1);
 103
 104     if (!opnd.isReg())
 105       return false;
 106
 107     unsigned Reg = opnd.getReg();
 108     if ((TargetRegisterInfo::isVirtualRegister(Reg) &&
 109          MRI->getRegClass(Reg) == &BPF::GPRRegClass))
 110        return false;
 111   }
 112
 113   LLVM_DEBUG(dbgs() << "  One ZExt elim sequence identified.\n");
 114
 115   return true;
 116 }
 117
 118 bool BPFMIPeephole::eliminateZExtSeq(void) {
 119   MachineInstr* ToErase = nullptr;
 120   bool Eliminated = false;
 121
 122   for (MachineBasicBlock &MBB : *MF) {
 123     for (MachineInstr &MI : MBB) {
 124       // If the previous instruction was marked for elimination, remove it now.
 125       if (ToErase) {
 126         ToErase->eraseFromParent();
 127         ToErase = nullptr;
 128       }
 129
 130       // Eliminate the 32-bit to 64-bit zero extension sequence when possible.
 131       //
 132       //   MOV_32_64 rB, wA
 133       //   SLL_ri    rB, rB, 32
 134       //   SRL_ri    rB, rB, 32
 135       if (MI.getOpcode() == BPF::SRL_ri &&
 136           MI.getOperand(2).getImm() == 32) {
 137         unsigned DstReg = MI.getOperand(0).getReg();
 138         unsigned ShfReg = MI.getOperand(1).getReg();
 139         MachineInstr *SllMI = MRI->getVRegDef(ShfReg);
 140
 141         LLVM_DEBUG(dbgs() << "Starting SRL found:");
 142         LLVM_DEBUG(MI.dump());
 143
 144         if (!SllMI ||
 145             SllMI->isPHI() ||
 146             SllMI->getOpcode() != BPF::SLL_ri ||
 147             SllMI->getOperand(2).getImm() != 32)
 148           continue;
 149
 150         LLVM_DEBUG(dbgs() << "  SLL found:");
 151         LLVM_DEBUG(SllMI->dump());
 152
 153         MachineInstr *MovMI = MRI->getVRegDef(SllMI->getOperand(1).getReg());
 154         if (!MovMI ||
 155             MovMI->isPHI() ||
 156             MovMI->getOpcode() != BPF::MOV_32_64)
 157           continue;
 158
 159         LLVM_DEBUG(dbgs() << "  Type cast Mov found:");
 160         LLVM_DEBUG(MovMI->dump());
 161
 162         unsigned SubReg = MovMI->getOperand(1).getReg();
 163         if (!isMovFrom32Def(MovMI)) {
 164           LLVM_DEBUG(dbgs()
 165                      << "  One ZExt elim sequence failed qualifying elim.\n");
 166           continue;
 167         }
 168
 169         BuildMI(MBB, MI, MI.getDebugLoc(), TII->get(BPF::SUBREG_TO_REG), DstReg)
 170           .addImm(0).addReg(SubReg).addImm(BPF::sub_32);
 171
 172         SllMI->eraseFromParent();
 173         MovMI->eraseFromParent();
 174         // MI is the right shift, we can't erase it in it's own iteration.
 175         // Mark it to ToErase, and erase in the next iteration.
 176         ToErase = &MI;
 177         ZExtElemNum++;
 178         Eliminated = true;
 179       }
 180     }
 181   }
 182
 183   return Eliminated;
 184 }
 185
 186 } // end default namespace
 187
 188 INITIALIZE_PASS(BPFMIPeephole, DEBUG_TYPE,
 189                 "BPF MachineSSA Peephole Optimization", false, false)
 190
 191 char BPFMIPeephole::ID = 0;
 192 FunctionPass* llvm::createBPFMIPeepholePass() { return new BPFMIPeephole(); }
 193
 194 STATISTIC(RedundantMovElemNum, "Number of redundant moves eliminated");
 195
 196 namespace {
 197
 198 struct BPFMIPreEmitPeephole : public MachineFunctionPass {
 199
 200   static char ID;
 201   MachineFunction *MF;
 202   const TargetRegisterInfo *TRI;
 203
 204   BPFMIPreEmitPeephole() : MachineFunctionPass(ID) {
 205     initializeBPFMIPreEmitPeepholePass(*PassRegistry::getPassRegistry());
 206   }
 207
 208 private:
 209   // Initialize class variables.
 210   void initialize(MachineFunction &MFParm);
 211
 212   bool eliminateRedundantMov(void);
 213
 214 public:
 215
 216   // Main entry point for this pass.
 217   bool runOnMachineFunction(MachineFunction &MF) override {
 218     if (skipFunction(MF.getFunction()))
 219       return false;
 220
 221     initialize(MF);
 222
 223     return eliminateRedundantMov();
 224   }
 225 };
 226
 227 // Initialize class variables.
 228 void BPFMIPreEmitPeephole::initialize(MachineFunction &MFParm) {
 229   MF = &MFParm;
 230   TRI = MF->getSubtarget<BPFSubtarget>().getRegisterInfo();
 231   LLVM_DEBUG(dbgs() << "*** BPF PreEmit peephole pass ***\n\n");
 232 }
 233
 234 bool BPFMIPreEmitPeephole::eliminateRedundantMov(void) {
 235   MachineInstr* ToErase = nullptr;
 236   bool Eliminated = false;
 237
 238   for (MachineBasicBlock &MBB : *MF) {
 239     for (MachineInstr &MI : MBB) {
 240       // If the previous instruction was marked for elimination, remove it now.
 241       if (ToErase) {
 242         LLVM_DEBUG(dbgs() << "  Redundant Mov Eliminated:");
 243         LLVM_DEBUG(ToErase->dump());
 244         ToErase->eraseFromParent();
 245         ToErase = nullptr;
 246       }
 247
 248       // Eliminate identical move:
 249       //
 250       //   MOV rA, rA
 251       //
 252       // This is particularly possible to happen when sub-register support
 253       // enabled. The special type cast insn MOV_32_64 involves different
 254       // register class on src (i32) and dst (i64), RA could generate useless
 255       // instruction due to this.
 256       if (MI.getOpcode() == BPF::MOV_32_64) {
 257         unsigned dst = MI.getOperand(0).getReg();
 258         unsigned dst_sub = TRI->getSubReg(dst, BPF::sub_32);
 259         unsigned src = MI.getOperand(1).getReg();
 260
 261         if (dst_sub != src)
 262           continue;
 263
 264         ToErase = &MI;
 265         RedundantMovElemNum++;
 266         Eliminated = true;
 267       }
 268     }
 269   }
 270
 271   return Eliminated;
 272 }
 273
 274 } // end default namespace
 275
 276 INITIALIZE_PASS(BPFMIPreEmitPeephole, "bpf-mi-pemit-peephole",
 277                 "BPF PreEmit Peephole Optimization", false, false)
 278
 279 char BPFMIPreEmitPeephole::ID = 0;
 280 FunctionPass* llvm::createBPFMIPreEmitPeepholePass()
 281 {
 282   return new BPFMIPreEmitPeephole();
 283 }