lib/Target/PowerPC/PPCEarlyReturn.cpp

   1 //===------------- PPCEarlyReturn.cpp - Form Early Returns ----------------===//
   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 // A pass that form early (predicated) returns. If-conversion handles some of
  10 // this, but this pass picks up some remaining cases.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "MCTargetDesc/PPCPredicates.h"
  15 #include "PPC.h"
  16 #include "PPCInstrBuilder.h"
  17 #include "PPCInstrInfo.h"
  18 #include "PPCMachineFunctionInfo.h"
  19 #include "PPCTargetMachine.h"
  20 #include "llvm/ADT/STLExtras.h"
  21 #include "llvm/ADT/Statistic.h"
  22 #include "llvm/CodeGen/MachineFrameInfo.h"
  23 #include "llvm/CodeGen/MachineFunctionPass.h"
  24 #include "llvm/CodeGen/MachineInstrBuilder.h"
  25 #include "llvm/CodeGen/MachineMemOperand.h"
  26 #include "llvm/CodeGen/MachineRegisterInfo.h"
  27 #include "llvm/MC/MCAsmInfo.h"
  28 #include "llvm/Support/Debug.h"
  29 #include "llvm/Support/ErrorHandling.h"
  30 #include "llvm/Support/TargetRegistry.h"
  31 #include "llvm/Support/raw_ostream.h"
  32
  33 using namespace llvm;
  34
  35 #define DEBUG_TYPE "ppc-early-ret"
  36 STATISTIC(NumBCLR, "Number of early conditional returns");
  37 STATISTIC(NumBLR,  "Number of early returns");
  38
  39 namespace {
  40   // PPCEarlyReturn pass - For simple functions without epilogue code, move
  41   // returns up, and create conditional returns, to avoid unnecessary
  42   // branch-to-blr sequences.
  43   struct PPCEarlyReturn : public MachineFunctionPass {
  44     static char ID;
  45     PPCEarlyReturn() : MachineFunctionPass(ID) {
  46       initializePPCEarlyReturnPass(*PassRegistry::getPassRegistry());
  47     }
  48
  49     const TargetInstrInfo *TII;
  50
  51 protected:
  52     bool processBlock(MachineBasicBlock &ReturnMBB) {
  53       bool Changed = false;
  54
  55       MachineBasicBlock::iterator I = ReturnMBB.begin();
  56       I = ReturnMBB.SkipPHIsLabelsAndDebug(I);
  57
  58       // The block must be essentially empty except for the blr.
  59       if (I == ReturnMBB.end() ||
  60           (I->getOpcode() != PPC::BLR && I->getOpcode() != PPC::BLR8) ||
  61           I != ReturnMBB.getLastNonDebugInstr())
  62         return Changed;
  63
  64       SmallVector<MachineBasicBlock*, 8> PredToRemove;
  65       for (MachineBasicBlock::pred_iterator PI = ReturnMBB.pred_begin(),
  66            PIE = ReturnMBB.pred_end(); PI != PIE; ++PI) {
  67         bool OtherReference = false, BlockChanged = false;
  68
  69         if ((*PI)->empty())
  70           continue;
  71
  72         for (MachineBasicBlock::iterator J = (*PI)->getLastNonDebugInstr();;) {
  73           if (J == (*PI)->end())
  74             break;
  75
  76           if (J->getOpcode() == PPC::B) {
  77             if (J->getOperand(0).getMBB() == &ReturnMBB) {
  78               // This is an unconditional branch to the return. Replace the
  79               // branch with a blr.
  80               BuildMI(**PI, J, J->getDebugLoc(), TII->get(I->getOpcode()))
  81                   .copyImplicitOps(*I);
  82               MachineBasicBlock::iterator K = J--;
  83               K->eraseFromParent();
  84               BlockChanged = true;
  85               ++NumBLR;
  86               continue;
  87             }
  88           } else if (J->getOpcode() == PPC::BCC) {
  89             if (J->getOperand(2).getMBB() == &ReturnMBB) {
  90               // This is a conditional branch to the return. Replace the branch
  91               // with a bclr.
  92               BuildMI(**PI, J, J->getDebugLoc(), TII->get(PPC::BCCLR))
  93                   .addImm(J->getOperand(0).getImm())
  94                   .addReg(J->getOperand(1).getReg())
  95                   .copyImplicitOps(*I);
  96               MachineBasicBlock::iterator K = J--;
  97               K->eraseFromParent();
  98               BlockChanged = true;
  99               ++NumBCLR;
 100               continue;
 101             }
 102           } else if (J->getOpcode() == PPC::BC || J->getOpcode() == PPC::BCn) {
 103             if (J->getOperand(1).getMBB() == &ReturnMBB) {
 104               // This is a conditional branch to the return. Replace the branch
 105               // with a bclr.
 106               BuildMI(
 107                   **PI, J, J->getDebugLoc(),
 108                   TII->get(J->getOpcode() == PPC::BC ? PPC::BCLR : PPC::BCLRn))
 109                   .addReg(J->getOperand(0).getReg())
 110                   .copyImplicitOps(*I);
 111               MachineBasicBlock::iterator K = J--;
 112               K->eraseFromParent();
 113               BlockChanged = true;
 114               ++NumBCLR;
 115               continue;
 116             }
 117           } else if (J->isBranch()) {
 118             if (J->isIndirectBranch()) {
 119               if (ReturnMBB.hasAddressTaken())
 120                 OtherReference = true;
 121             } else
 122               for (unsigned i = 0; i < J->getNumOperands(); ++i)
 123                 if (J->getOperand(i).isMBB() &&
 124                     J->getOperand(i).getMBB() == &ReturnMBB)
 125                   OtherReference = true;
 126           } else if (!J->isTerminator() && !J->isDebugInstr())
 127             break;
 128
 129           if (J == (*PI)->begin())
 130             break;
 131
 132           --J;
 133         }
 134
 135         if ((*PI)->canFallThrough() && (*PI)->isLayoutSuccessor(&ReturnMBB))
 136           OtherReference = true;
 137
 138         // Predecessors are stored in a vector and can't be removed here.
 139         if (!OtherReference && BlockChanged) {
 140           PredToRemove.push_back(*PI);
 141         }
 142
 143         if (BlockChanged)
 144           Changed = true;
 145       }
 146
 147       for (unsigned i = 0, ie = PredToRemove.size(); i != ie; ++i)
 148         PredToRemove[i]->removeSuccessor(&ReturnMBB, true);
 149
 150       if (Changed && !ReturnMBB.hasAddressTaken()) {
 151         // We now might be able to merge this blr-only block into its
 152         // by-layout predecessor.
 153         if (ReturnMBB.pred_size() == 1) {
 154           MachineBasicBlock &PrevMBB = **ReturnMBB.pred_begin();
 155           if (PrevMBB.isLayoutSuccessor(&ReturnMBB) && PrevMBB.canFallThrough()) {
 156             // Move the blr into the preceding block.
 157             PrevMBB.splice(PrevMBB.end(), &ReturnMBB, I);
 158             PrevMBB.removeSuccessor(&ReturnMBB, true);
 159           }
 160         }
 161
 162         if (ReturnMBB.pred_empty())
 163           ReturnMBB.eraseFromParent();
 164       }
 165
 166       return Changed;
 167     }
 168
 169 public:
 170     bool runOnMachineFunction(MachineFunction &MF) override {
 171       if (skipFunction(MF.getFunction()))
 172         return false;
 173
 174       TII = MF.getSubtarget().getInstrInfo();
 175
 176       bool Changed = false;
 177
 178       // If the function does not have at least two blocks, then there is
 179       // nothing to do.
 180       if (MF.size() < 2)
 181         return Changed;
 182
 183       // We can't use a range-based for loop due to clobbering the iterator.
 184       for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E;) {
 185         MachineBasicBlock &B = *I++;
 186         Changed |= processBlock(B);
 187       }
 188
 189       return Changed;
 190     }
 191
 192     MachineFunctionProperties getRequiredProperties() const override {
 193       return MachineFunctionProperties().set(
 194           MachineFunctionProperties::Property::NoVRegs);
 195     }
 196
 197     void getAnalysisUsage(AnalysisUsage &AU) const override {
 198       MachineFunctionPass::getAnalysisUsage(AU);
 199     }
 200   };
 201 }
 202
 203 INITIALIZE_PASS(PPCEarlyReturn, DEBUG_TYPE,
 204                 "PowerPC Early-Return Creation", false, false)
 205
 206 char PPCEarlyReturn::ID = 0;
 207 FunctionPass*
 208 llvm::createPPCEarlyReturnPass() { return new PPCEarlyReturn(); }