lib/CodeGen/OptimizeExts.cpp

   1 //===-- OptimizeExts.cpp - Optimize sign / zero extension instrs -----===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass performs optimization of sign / zero extension instructions. It
  11 // may be extended to handle other instructions of similar property.
  12 //
  13 // On some targets, some instructions, e.g. X86 sign / zero extension, may
  14 // leave the source value in the lower part of the result. This pass will
  15 // replace (some) uses of the pre-extension value with uses of the sub-register
  16 // of the results.
  17 //
  18 //===----------------------------------------------------------------------===//
  19
  20 #define DEBUG_TYPE "ext-opt"
  21 #include "llvm/CodeGen/Passes.h"
  22 #include "llvm/CodeGen/MachineDominators.h"
  23 #include "llvm/CodeGen/MachineInstrBuilder.h"
  24 #include "llvm/CodeGen/MachineRegisterInfo.h"
  25 #include "llvm/Target/TargetInstrInfo.h"
  26 #include "llvm/Target/TargetRegisterInfo.h"
  27 #include "llvm/Support/CommandLine.h"
  28 #include "llvm/ADT/SmallPtrSet.h"
  29 #include "llvm/ADT/Statistic.h"
  30 using namespace llvm;
  31
  32 static cl::opt<bool> Aggressive("aggressive-ext-opt", cl::Hidden,
  33                                 cl::desc("Aggressive extension optimization"));
  34
  35 STATISTIC(NumReuse, "Number of extension results reused");
  36
  37 namespace {
  38   class OptimizeExts : public MachineFunctionPass {
  39     const TargetMachine   *TM;
  40     const TargetInstrInfo *TII;
  41     MachineRegisterInfo *MRI;
  42     MachineDominatorTree *DT;   // Machine dominator tree
  43
  44   public:
  45     static char ID; // Pass identification
  46     OptimizeExts() : MachineFunctionPass(&ID) {}
  47
  48     virtual bool runOnMachineFunction(MachineFunction &MF);
  49
  50     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  51       AU.setPreservesCFG();
  52       MachineFunctionPass::getAnalysisUsage(AU);
  53       if (Aggressive) {
  54         AU.addRequired<MachineDominatorTree>();
  55         AU.addPreserved<MachineDominatorTree>();
  56       }
  57     }
  58
  59   private:
  60     bool OptimizeInstr(MachineInstr *MI, MachineBasicBlock *MBB,
  61                        SmallPtrSet<MachineInstr*, 8> &LocalMIs);
  62   };
  63 }
  64
  65 char OptimizeExts::ID = 0;
  66 static RegisterPass<OptimizeExts>
  67 X("opt-exts", "Optimize sign / zero extensions");
  68
  69 FunctionPass *llvm::createOptimizeExtsPass() { return new OptimizeExts(); }
  70
  71 /// OptimizeInstr - If instruction is a copy-like instruction, i.e. it reads
  72 /// a single register and writes a single register and it does not modify
  73 /// the source, and if the source value is preserved as a sub-register of
  74 /// the result, then replace all reachable uses of the source with the subreg
  75 /// of the result.
  76 /// Do not generate an EXTRACT that is used only in a debug use, as this
  77 /// changes the code.  Since this code does not currently share EXTRACTs, just
  78 /// ignore all debug uses.
  79 bool OptimizeExts::OptimizeInstr(MachineInstr *MI, MachineBasicBlock *MBB,
  80                                  SmallPtrSet<MachineInstr*, 8> &LocalMIs) {
  81   bool Changed = false;
  82   LocalMIs.insert(MI);
  83
  84   unsigned SrcReg, DstReg, SubIdx;
  85   if (TII->isCoalescableExtInstr(*MI, SrcReg, DstReg, SubIdx)) {
  86     if (TargetRegisterInfo::isPhysicalRegister(DstReg) ||
  87         TargetRegisterInfo::isPhysicalRegister(SrcReg))
  88       return false;
  89
  90     MachineRegisterInfo::use_nodbg_iterator UI = MRI->use_nodbg_begin(SrcReg);
  91     if (++UI == MRI->use_nodbg_end())
  92       // No other uses.
  93       return false;
  94
  95     // Ok, the source has other uses. See if we can replace the other uses
  96     // with use of the result of the extension.
  97     SmallPtrSet<MachineBasicBlock*, 4> ReachedBBs;
  98     UI = MRI->use_nodbg_begin(DstReg);
  99     for (MachineRegisterInfo::use_nodbg_iterator UE = MRI->use_nodbg_end();
 100          UI != UE; ++UI)
 101       ReachedBBs.insert(UI->getParent());
 102
 103     bool ExtendLife = true;
 104     // Uses that are in the same BB of uses of the result of the instruction.
 105     SmallVector<MachineOperand*, 8> Uses;
 106     // Uses that the result of the instruction can reach.
 107     SmallVector<MachineOperand*, 8> ExtendedUses;
 108
 109     UI = MRI->use_nodbg_begin(SrcReg);
 110     for (MachineRegisterInfo::use_nodbg_iterator UE = MRI->use_nodbg_end();
 111          UI != UE; ++UI) {
 112       MachineOperand &UseMO = UI.getOperand();
 113       MachineInstr *UseMI = &*UI;
 114       if (UseMI == MI)
 115         continue;
 116       if (UseMI->isPHI()) {
 117         ExtendLife = false;
 118         continue;
 119       }
 120
 121       MachineBasicBlock *UseMBB = UseMI->getParent();
 122       if (UseMBB == MBB) {
 123         // Local uses that come after the extension.
 124         if (!LocalMIs.count(UseMI))
 125           Uses.push_back(&UseMO);
 126       } else if (ReachedBBs.count(UseMBB))
 127         // Non-local uses where the result of extension is used. Always
 128         // replace these unless it's a PHI.
 129         Uses.push_back(&UseMO);
 130       else if (Aggressive && DT->dominates(MBB, UseMBB))
 131         // We may want to extend live range of the extension result in order
 132         // to replace these uses.
 133         ExtendedUses.push_back(&UseMO);
 134       else {
 135         // Both will be live out of the def MBB anyway. Don't extend live
 136         // range of the extension result.
 137         ExtendLife = false;
 138         break;
 139       }
 140     }
 141
 142     if (ExtendLife && !ExtendedUses.empty())
 143       // Ok, we'll extend the liveness of the extension result.
 144       std::copy(ExtendedUses.begin(), ExtendedUses.end(),
 145                 std::back_inserter(Uses));
 146
 147     // Now replace all uses.
 148     if (!Uses.empty()) {
 149       SmallPtrSet<MachineBasicBlock*, 4> PHIBBs;
 150       // Look for PHI uses of the extended result, we don't want to extend the
 151       // liveness of a PHI input. It breaks all kinds of assumptions down
 152       // stream. A PHI use is expected to be the kill of its source values.
 153       UI = MRI->use_nodbg_begin(DstReg);
 154       for (MachineRegisterInfo::use_nodbg_iterator UE = MRI->use_nodbg_end();
 155            UI != UE; ++UI)
 156         if (UI->isPHI())
 157           PHIBBs.insert(UI->getParent());
 158
 159       const TargetRegisterClass *RC = MRI->getRegClass(SrcReg);
 160       for (unsigned i = 0, e = Uses.size(); i != e; ++i) {
 161         MachineOperand *UseMO = Uses[i];
 162         MachineInstr *UseMI = UseMO->getParent();
 163         MachineBasicBlock *UseMBB = UseMI->getParent();
 164         if (PHIBBs.count(UseMBB))
 165           continue;
 166         unsigned NewVR = MRI->createVirtualRegister(RC);
 167         BuildMI(*UseMBB, UseMI, UseMI->getDebugLoc(),
 168                 TII->get(TargetOpcode::EXTRACT_SUBREG), NewVR)
 169           .addReg(DstReg).addImm(SubIdx);
 170         UseMO->setReg(NewVR);
 171         ++NumReuse;
 172         Changed = true;
 173       }
 174     }
 175   }
 176
 177   return Changed;
 178 }
 179
 180 bool OptimizeExts::runOnMachineFunction(MachineFunction &MF) {
 181   TM = &MF.getTarget();
 182   TII = TM->getInstrInfo();
 183   MRI = &MF.getRegInfo();
 184   DT = Aggressive ? &getAnalysis<MachineDominatorTree>() : 0;
 185
 186   bool Changed = false;
 187
 188   SmallPtrSet<MachineInstr*, 8> LocalMIs;
 189   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
 190     MachineBasicBlock *MBB = &*I;
 191     LocalMIs.clear();
 192     for (MachineBasicBlock::iterator MII = I->begin(), ME = I->end(); MII != ME;
 193          ++MII) {
 194       MachineInstr *MI = &*MII;
 195       Changed |= OptimizeInstr(MI, MBB, LocalMIs);
 196     }
 197   }
 198
 199   return Changed;
 200 }