llvm/lib/CodeGen/MachineLoopInfo.cpp

   1 //===- MachineLoopInfo.cpp - Natural Loop Calculator ----------------------===//
   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 file defines the MachineLoopInfo class that is used to identify natural
  10 // loops and determine the loop depth of various nodes of the CFG.  Note that
  11 // the loops identified may actually be several natural loops that share the
  12 // same header node... not just a single natural loop.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/CodeGen/MachineLoopInfo.h"
  17 #include "llvm/Analysis/LoopInfoImpl.h"
  18 #include "llvm/CodeGen/MachineDominators.h"
  19 #include "llvm/CodeGen/MachineRegisterInfo.h"
  20 #include "llvm/CodeGen/Passes.h"
  21 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  22 #include "llvm/Config/llvm-config.h"
  23 #include "llvm/InitializePasses.h"
  24 #include "llvm/Support/Debug.h"
  25 #include "llvm/Support/raw_ostream.h"
  26
  27 using namespace llvm;
  28
  29 // Explicitly instantiate methods in LoopInfoImpl.h for MI-level Loops.
  30 template class llvm::LoopBase<MachineBasicBlock, MachineLoop>;
  31 template class llvm::LoopInfoBase<MachineBasicBlock, MachineLoop>;
  32
  33 char MachineLoopInfo::ID = 0;
  34 MachineLoopInfo::MachineLoopInfo() : MachineFunctionPass(ID) {
  35   initializeMachineLoopInfoPass(*PassRegistry::getPassRegistry());
  36 }
  37 INITIALIZE_PASS_BEGIN(MachineLoopInfo, "machine-loops",
  38                 "Machine Natural Loop Construction", true, true)
  39 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
  40 INITIALIZE_PASS_END(MachineLoopInfo, "machine-loops",
  41                 "Machine Natural Loop Construction", true, true)
  42
  43 char &llvm::MachineLoopInfoID = MachineLoopInfo::ID;
  44
  45 bool MachineLoopInfo::runOnMachineFunction(MachineFunction &) {
  46   calculate(getAnalysis<MachineDominatorTree>());
  47   return false;
  48 }
  49
  50 void MachineLoopInfo::calculate(MachineDominatorTree &MDT) {
  51   releaseMemory();
  52   LI.analyze(MDT.getBase());
  53 }
  54
  55 void MachineLoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
  56   AU.setPreservesAll();
  57   AU.addRequired<MachineDominatorTree>();
  58   MachineFunctionPass::getAnalysisUsage(AU);
  59 }
  60
  61 MachineBasicBlock *MachineLoop::getTopBlock() {
  62   MachineBasicBlock *TopMBB = getHeader();
  63   MachineFunction::iterator Begin = TopMBB->getParent()->begin();
  64   if (TopMBB->getIterator() != Begin) {
  65     MachineBasicBlock *PriorMBB = &*std::prev(TopMBB->getIterator());
  66     while (contains(PriorMBB)) {
  67       TopMBB = PriorMBB;
  68       if (TopMBB->getIterator() == Begin)
  69         break;
  70       PriorMBB = &*std::prev(TopMBB->getIterator());
  71     }
  72   }
  73   return TopMBB;
  74 }
  75
  76 MachineBasicBlock *MachineLoop::getBottomBlock() {
  77   MachineBasicBlock *BotMBB = getHeader();
  78   MachineFunction::iterator End = BotMBB->getParent()->end();
  79   if (BotMBB->getIterator() != std::prev(End)) {
  80     MachineBasicBlock *NextMBB = &*std::next(BotMBB->getIterator());
  81     while (contains(NextMBB)) {
  82       BotMBB = NextMBB;
  83       if (BotMBB == &*std::next(BotMBB->getIterator()))
  84         break;
  85       NextMBB = &*std::next(BotMBB->getIterator());
  86     }
  87   }
  88   return BotMBB;
  89 }
  90
  91 MachineBasicBlock *MachineLoop::findLoopControlBlock() {
  92   if (MachineBasicBlock *Latch = getLoopLatch()) {
  93     if (isLoopExiting(Latch))
  94       return Latch;
  95     else
  96       return getExitingBlock();
  97   }
  98   return nullptr;
  99 }
 100
 101 DebugLoc MachineLoop::getStartLoc() const {
 102   // Try the pre-header first.
 103   if (MachineBasicBlock *PHeadMBB = getLoopPreheader())
 104     if (const BasicBlock *PHeadBB = PHeadMBB->getBasicBlock())
 105       if (DebugLoc DL = PHeadBB->getTerminator()->getDebugLoc())
 106         return DL;
 107
 108   // If we have no pre-header or there are no instructions with debug
 109   // info in it, try the header.
 110   if (MachineBasicBlock *HeadMBB = getHeader())
 111     if (const BasicBlock *HeadBB = HeadMBB->getBasicBlock())
 112       return HeadBB->getTerminator()->getDebugLoc();
 113
 114   return DebugLoc();
 115 }
 116
 117 MachineBasicBlock *
 118 MachineLoopInfo::findLoopPreheader(MachineLoop *L, bool SpeculativePreheader,
 119                                    bool FindMultiLoopPreheader) const {
 120   if (MachineBasicBlock *PB = L->getLoopPreheader())
 121     return PB;
 122
 123   if (!SpeculativePreheader)
 124     return nullptr;
 125
 126   MachineBasicBlock *HB = L->getHeader(), *LB = L->getLoopLatch();
 127   if (HB->pred_size() != 2 || HB->hasAddressTaken())
 128     return nullptr;
 129   // Find the predecessor of the header that is not the latch block.
 130   MachineBasicBlock *Preheader = nullptr;
 131   for (MachineBasicBlock *P : HB->predecessors()) {
 132     if (P == LB)
 133       continue;
 134     // Sanity.
 135     if (Preheader)
 136       return nullptr;
 137     Preheader = P;
 138   }
 139
 140   // Check if the preheader candidate is a successor of any other loop
 141   // headers. We want to avoid having two loop setups in the same block.
 142   if (!FindMultiLoopPreheader) {
 143     for (MachineBasicBlock *S : Preheader->successors()) {
 144       if (S == HB)
 145         continue;
 146       MachineLoop *T = getLoopFor(S);
 147       if (T && T->getHeader() == S)
 148         return nullptr;
 149     }
 150   }
 151   return Preheader;
 152 }
 153
 154 bool MachineLoop::isLoopInvariant(MachineInstr &I) const {
 155   MachineFunction *MF = I.getParent()->getParent();
 156   MachineRegisterInfo *MRI = &MF->getRegInfo();
 157   const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
 158
 159   // The instruction is loop invariant if all of its operands are.
 160   for (const MachineOperand &MO : I.operands()) {
 161     if (!MO.isReg())
 162       continue;
 163
 164     Register Reg = MO.getReg();
 165     if (Reg == 0) continue;
 166
 167     // An instruction that uses or defines a physical register can't e.g. be
 168     // hoisted, so mark this as not invariant.
 169     if (Register::isPhysicalRegister(Reg)) {
 170       if (MO.isUse()) {
 171         // If the physreg has no defs anywhere, it's just an ambient register
 172         // and we can freely move its uses. Alternatively, if it's allocatable,
 173         // it could get allocated to something with a def during allocation.
 174         // However, if the physreg is known to always be caller saved/restored
 175         // then this use is safe to hoist.
 176         if (!MRI->isConstantPhysReg(Reg) &&
 177             !(TRI->isCallerPreservedPhysReg(Reg.asMCReg(), *I.getMF())))
 178           return false;
 179         // Otherwise it's safe to move.
 180         continue;
 181       } else if (!MO.isDead()) {
 182         // A def that isn't dead can't be moved.
 183         return false;
 184       } else if (getHeader()->isLiveIn(Reg)) {
 185         // If the reg is live into the loop, we can't hoist an instruction
 186         // which would clobber it.
 187         return false;
 188       }
 189     }
 190
 191     if (!MO.isUse())
 192       continue;
 193
 194     assert(MRI->getVRegDef(Reg) &&
 195            "Machine instr not mapped for this vreg?!");
 196
 197     // If the loop contains the definition of an operand, then the instruction
 198     // isn't loop invariant.
 199     if (contains(MRI->getVRegDef(Reg)))
 200       return false;
 201   }
 202
 203   // If we got this far, the instruction is loop invariant!
 204   return true;
 205 }
 206
 207 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 208 LLVM_DUMP_METHOD void MachineLoop::dump() const {
 209   print(dbgs());
 210 }
 211 #endif