llvm/lib/CodeGen/ProcessImplicitDefs.cpp

   1 //===---------------------- ProcessImplicitDefs.cpp -----------------------===//
   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 #include "llvm/ADT/SetVector.h"
  10 #include "llvm/Analysis/AliasAnalysis.h"
  11 #include "llvm/CodeGen/MachineFunctionPass.h"
  12 #include "llvm/CodeGen/MachineInstr.h"
  13 #include "llvm/CodeGen/MachineRegisterInfo.h"
  14 #include "llvm/CodeGen/TargetInstrInfo.h"
  15 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  16 #include "llvm/InitializePasses.h"
  17 #include "llvm/Pass.h"
  18 #include "llvm/PassRegistry.h"
  19 #include "llvm/Support/Debug.h"
  20 #include "llvm/Support/raw_ostream.h"
  21
  22 using namespace llvm;
  23
  24 #define DEBUG_TYPE "processimpdefs"
  25
  26 namespace {
  27 /// Process IMPLICIT_DEF instructions and make sure there is one implicit_def
  28 /// for each use. Add isUndef marker to implicit_def defs and their uses.
  29 class ProcessImplicitDefs : public MachineFunctionPass {
  30   const TargetInstrInfo *TII = nullptr;
  31   const TargetRegisterInfo *TRI = nullptr;
  32   MachineRegisterInfo *MRI = nullptr;
  33
  34   SmallSetVector<MachineInstr*, 16> WorkList;
  35
  36   void processImplicitDef(MachineInstr *MI);
  37   bool canTurnIntoImplicitDef(MachineInstr *MI);
  38
  39 public:
  40   static char ID;
  41
  42   ProcessImplicitDefs() : MachineFunctionPass(ID) {
  43     initializeProcessImplicitDefsPass(*PassRegistry::getPassRegistry());
  44   }
  45
  46   void getAnalysisUsage(AnalysisUsage &au) const override;
  47
  48   bool runOnMachineFunction(MachineFunction &MF) override;
  49
  50   MachineFunctionProperties getRequiredProperties() const override {
  51     return MachineFunctionProperties().set(
  52         MachineFunctionProperties::Property::IsSSA);
  53   }
  54 };
  55 } // end anonymous namespace
  56
  57 char ProcessImplicitDefs::ID = 0;
  58 char &llvm::ProcessImplicitDefsID = ProcessImplicitDefs::ID;
  59
  60 INITIALIZE_PASS(ProcessImplicitDefs, DEBUG_TYPE,
  61                 "Process Implicit Definitions", false, false)
  62
  63 void ProcessImplicitDefs::getAnalysisUsage(AnalysisUsage &AU) const {
  64   AU.setPreservesCFG();
  65   AU.addPreserved<AAResultsWrapperPass>();
  66   MachineFunctionPass::getAnalysisUsage(AU);
  67 }
  68
  69 bool ProcessImplicitDefs::canTurnIntoImplicitDef(MachineInstr *MI) {
  70   if (!MI->isCopyLike() &&
  71       !MI->isInsertSubreg() &&
  72       !MI->isRegSequence() &&
  73       !MI->isPHI())
  74     return false;
  75   for (const MachineOperand &MO : MI->all_uses())
  76     if (MO.readsReg())
  77       return false;
  78   return true;
  79 }
  80
  81 void ProcessImplicitDefs::processImplicitDef(MachineInstr *MI) {
  82   LLVM_DEBUG(dbgs() << "Processing " << *MI);
  83   Register Reg = MI->getOperand(0).getReg();
  84
  85   if (Reg.isVirtual()) {
  86     // For virtual registers, mark all uses as <undef>, and convert users to
  87     // implicit-def when possible.
  88     for (MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
  89       MO.setIsUndef();
  90       MachineInstr *UserMI = MO.getParent();
  91       if (!canTurnIntoImplicitDef(UserMI))
  92         continue;
  93       LLVM_DEBUG(dbgs() << "Converting to IMPLICIT_DEF: " << *UserMI);
  94       UserMI->setDesc(TII->get(TargetOpcode::IMPLICIT_DEF));
  95       WorkList.insert(UserMI);
  96     }
  97     MI->eraseFromParent();
  98     return;
  99   }
 100
 101   // This is a physreg implicit-def.
 102   // Look for the first instruction to use or define an alias.
 103   MachineBasicBlock::instr_iterator UserMI = MI->getIterator();
 104   MachineBasicBlock::instr_iterator UserE = MI->getParent()->instr_end();
 105   bool Found = false;
 106   for (++UserMI; UserMI != UserE; ++UserMI) {
 107     for (MachineOperand &MO : UserMI->operands()) {
 108       if (!MO.isReg())
 109         continue;
 110       Register UserReg = MO.getReg();
 111       if (!UserReg.isPhysical() || !TRI->regsOverlap(Reg, UserReg))
 112         continue;
 113       // UserMI uses or redefines Reg. Set <undef> flags on all uses.
 114       Found = true;
 115       if (MO.isUse())
 116         MO.setIsUndef();
 117     }
 118     if (Found)
 119       break;
 120   }
 121
 122   // If we found the using MI, we can erase the IMPLICIT_DEF.
 123   if (Found) {
 124     LLVM_DEBUG(dbgs() << "Physreg user: " << *UserMI);
 125     MI->eraseFromParent();
 126     return;
 127   }
 128
 129   // Using instr wasn't found, it could be in another block.
 130   // Leave the physreg IMPLICIT_DEF, but trim any extra operands.
 131   for (unsigned i = MI->getNumOperands() - 1; i; --i)
 132     MI->removeOperand(i);
 133   LLVM_DEBUG(dbgs() << "Keeping physreg: " << *MI);
 134 }
 135
 136 /// processImplicitDefs - Process IMPLICIT_DEF instructions and turn them into
 137 /// <undef> operands.
 138 bool ProcessImplicitDefs::runOnMachineFunction(MachineFunction &MF) {
 139
 140   LLVM_DEBUG(dbgs() << "********** PROCESS IMPLICIT DEFS **********\n"
 141                     << "********** Function: " << MF.getName() << '\n');
 142
 143   bool Changed = false;
 144
 145   TII = MF.getSubtarget().getInstrInfo();
 146   TRI = MF.getSubtarget().getRegisterInfo();
 147   MRI = &MF.getRegInfo();
 148   assert(WorkList.empty() && "Inconsistent worklist state");
 149
 150   for (MachineBasicBlock &MBB : MF) {
 151     // Scan the basic block for implicit defs.
 152     for (MachineInstr &MI : MBB)
 153       if (MI.isImplicitDef())
 154         WorkList.insert(&MI);
 155
 156     if (WorkList.empty())
 157       continue;
 158
 159     LLVM_DEBUG(dbgs() << printMBBReference(MBB) << " has " << WorkList.size()
 160                       << " implicit defs.\n");
 161     Changed = true;
 162
 163     // Drain the WorkList to recursively process any new implicit defs.
 164     do processImplicitDef(WorkList.pop_back_val());
 165     while (!WorkList.empty());
 166   }
 167   return Changed;
 168 }