LLVM API Documentation
00001 //===-- LiveVariables.cpp - Live Variable Analysis for Machine Code -------===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This file implements the LiveVariable analysis pass. For each machine 00011 // instruction in the function, this pass calculates the set of registers that 00012 // are immediately dead after the instruction (i.e., the instruction calculates 00013 // the value, but it is never used) and the set of registers that are used by 00014 // the instruction, but are never used after the instruction (i.e., they are 00015 // killed). 00016 // 00017 // This class computes live variables using are sparse implementation based on 00018 // the machine code SSA form. This class computes live variable information for 00019 // each virtual and _register allocatable_ physical register in a function. It 00020 // uses the dominance properties of SSA form to efficiently compute live 00021 // variables for virtual registers, and assumes that physical registers are only 00022 // live within a single basic block (allowing it to do a single local analysis 00023 // to resolve physical register lifetimes in each basic block). If a physical 00024 // register is not register allocatable, it is not tracked. This is useful for 00025 // things like the stack pointer and condition codes. 00026 // 00027 //===----------------------------------------------------------------------===// 00028 00029 #include "llvm/CodeGen/LiveVariables.h" 00030 #include "llvm/CodeGen/MachineInstr.h" 00031 #include "llvm/Target/MRegisterInfo.h" 00032 #include "llvm/Target/TargetInstrInfo.h" 00033 #include "llvm/Target/TargetMachine.h" 00034 #include "llvm/ADT/DepthFirstIterator.h" 00035 #include "llvm/ADT/STLExtras.h" 00036 #include "llvm/Config/alloca.h" 00037 #include <algorithm> 00038 #include <iostream> 00039 using namespace llvm; 00040 00041 static RegisterAnalysis<LiveVariables> X("livevars", "Live Variable Analysis"); 00042 00043 void LiveVariables::VarInfo::dump() const { 00044 std::cerr << "Register Defined by: "; 00045 if (DefInst) 00046 std::cerr << *DefInst; 00047 else 00048 std::cerr << "<null>\n"; 00049 std::cerr << " Alive in blocks: "; 00050 for (unsigned i = 0, e = AliveBlocks.size(); i != e; ++i) 00051 if (AliveBlocks[i]) std::cerr << i << ", "; 00052 std::cerr << "\n Killed by:"; 00053 if (Kills.empty()) 00054 std::cerr << " No instructions.\n"; 00055 else { 00056 for (unsigned i = 0, e = Kills.size(); i != e; ++i) 00057 std::cerr << "\n #" << i << ": " << *Kills[i]; 00058 std::cerr << "\n"; 00059 } 00060 } 00061 00062 LiveVariables::VarInfo &LiveVariables::getVarInfo(unsigned RegIdx) { 00063 assert(MRegisterInfo::isVirtualRegister(RegIdx) && 00064 "getVarInfo: not a virtual register!"); 00065 RegIdx -= MRegisterInfo::FirstVirtualRegister; 00066 if (RegIdx >= VirtRegInfo.size()) { 00067 if (RegIdx >= 2*VirtRegInfo.size()) 00068 VirtRegInfo.resize(RegIdx*2); 00069 else 00070 VirtRegInfo.resize(2*VirtRegInfo.size()); 00071 } 00072 return VirtRegInfo[RegIdx]; 00073 } 00074 00075 bool LiveVariables::KillsRegister(MachineInstr *MI, unsigned Reg) const { 00076 std::map<MachineInstr*, std::vector<unsigned> >::const_iterator I = 00077 RegistersKilled.find(MI); 00078 if (I == RegistersKilled.end()) return false; 00079 00080 // Do a binary search, as these lists can grow pretty big, particularly for 00081 // call instructions on targets with lots of call-clobbered registers. 00082 return std::binary_search(I->second.begin(), I->second.end(), Reg); 00083 } 00084 00085 bool LiveVariables::RegisterDefIsDead(MachineInstr *MI, unsigned Reg) const { 00086 std::map<MachineInstr*, std::vector<unsigned> >::const_iterator I = 00087 RegistersDead.find(MI); 00088 if (I == RegistersDead.end()) return false; 00089 00090 // Do a binary search, as these lists can grow pretty big, particularly for 00091 // call instructions on targets with lots of call-clobbered registers. 00092 return std::binary_search(I->second.begin(), I->second.end(), Reg); 00093 } 00094 00095 00096 void LiveVariables::MarkVirtRegAliveInBlock(VarInfo &VRInfo, 00097 MachineBasicBlock *MBB) { 00098 unsigned BBNum = MBB->getNumber(); 00099 00100 // Check to see if this basic block is one of the killing blocks. If so, 00101 // remove it... 00102 for (unsigned i = 0, e = VRInfo.Kills.size(); i != e; ++i) 00103 if (VRInfo.Kills[i]->getParent() == MBB) { 00104 VRInfo.Kills.erase(VRInfo.Kills.begin()+i); // Erase entry 00105 break; 00106 } 00107 00108 if (MBB == VRInfo.DefInst->getParent()) return; // Terminate recursion 00109 00110 if (VRInfo.AliveBlocks.size() <= BBNum) 00111 VRInfo.AliveBlocks.resize(BBNum+1); // Make space... 00112 00113 if (VRInfo.AliveBlocks[BBNum]) 00114 return; // We already know the block is live 00115 00116 // Mark the variable known alive in this bb 00117 VRInfo.AliveBlocks[BBNum] = true; 00118 00119 for (MachineBasicBlock::const_pred_iterator PI = MBB->pred_begin(), 00120 E = MBB->pred_end(); PI != E; ++PI) 00121 MarkVirtRegAliveInBlock(VRInfo, *PI); 00122 } 00123 00124 void LiveVariables::HandleVirtRegUse(VarInfo &VRInfo, MachineBasicBlock *MBB, 00125 MachineInstr *MI) { 00126 assert(VRInfo.DefInst && "Register use before def!"); 00127 00128 // Check to see if this basic block is already a kill block... 00129 if (!VRInfo.Kills.empty() && VRInfo.Kills.back()->getParent() == MBB) { 00130 // Yes, this register is killed in this basic block already. Increase the 00131 // live range by updating the kill instruction. 00132 VRInfo.Kills.back() = MI; 00133 return; 00134 } 00135 00136 #ifndef NDEBUG 00137 for (unsigned i = 0, e = VRInfo.Kills.size(); i != e; ++i) 00138 assert(VRInfo.Kills[i]->getParent() != MBB && "entry should be at end!"); 00139 #endif 00140 00141 assert(MBB != VRInfo.DefInst->getParent() && 00142 "Should have kill for defblock!"); 00143 00144 // Add a new kill entry for this basic block. 00145 VRInfo.Kills.push_back(MI); 00146 00147 // Update all dominating blocks to mark them known live. 00148 for (MachineBasicBlock::const_pred_iterator PI = MBB->pred_begin(), 00149 E = MBB->pred_end(); PI != E; ++PI) 00150 MarkVirtRegAliveInBlock(VRInfo, *PI); 00151 } 00152 00153 void LiveVariables::HandlePhysRegUse(unsigned Reg, MachineInstr *MI) { 00154 PhysRegInfo[Reg] = MI; 00155 PhysRegUsed[Reg] = true; 00156 00157 for (const unsigned *AliasSet = RegInfo->getAliasSet(Reg); 00158 unsigned Alias = *AliasSet; ++AliasSet) { 00159 PhysRegInfo[Alias] = MI; 00160 PhysRegUsed[Alias] = true; 00161 } 00162 } 00163 00164 void LiveVariables::HandlePhysRegDef(unsigned Reg, MachineInstr *MI) { 00165 // Does this kill a previous version of this register? 00166 if (MachineInstr *LastUse = PhysRegInfo[Reg]) { 00167 if (PhysRegUsed[Reg]) 00168 RegistersKilled[LastUse].push_back(Reg); 00169 else 00170 RegistersDead[LastUse].push_back(Reg); 00171 } 00172 PhysRegInfo[Reg] = MI; 00173 PhysRegUsed[Reg] = false; 00174 00175 for (const unsigned *AliasSet = RegInfo->getAliasSet(Reg); 00176 unsigned Alias = *AliasSet; ++AliasSet) { 00177 if (MachineInstr *LastUse = PhysRegInfo[Alias]) { 00178 if (PhysRegUsed[Alias]) 00179 RegistersKilled[LastUse].push_back(Alias); 00180 else 00181 RegistersDead[LastUse].push_back(Alias); 00182 } 00183 PhysRegInfo[Alias] = MI; 00184 PhysRegUsed[Alias] = false; 00185 } 00186 } 00187 00188 bool LiveVariables::runOnMachineFunction(MachineFunction &MF) { 00189 const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo(); 00190 RegInfo = MF.getTarget().getRegisterInfo(); 00191 assert(RegInfo && "Target doesn't have register information?"); 00192 00193 AllocatablePhysicalRegisters = RegInfo->getAllocatableSet(MF); 00194 00195 // PhysRegInfo - Keep track of which instruction was the last use of a 00196 // physical register. This is a purely local property, because all physical 00197 // register references as presumed dead across basic blocks. 00198 // 00199 PhysRegInfo = (MachineInstr**)alloca(sizeof(MachineInstr*) * 00200 RegInfo->getNumRegs()); 00201 PhysRegUsed = (bool*)alloca(sizeof(bool)*RegInfo->getNumRegs()); 00202 std::fill(PhysRegInfo, PhysRegInfo+RegInfo->getNumRegs(), (MachineInstr*)0); 00203 00204 /// Get some space for a respectable number of registers... 00205 VirtRegInfo.resize(64); 00206 00207 // Mark live-in registers as live-in. 00208 for (MachineFunction::livein_iterator I = MF.livein_begin(), 00209 E = MF.livein_end(); I != E; ++I) { 00210 assert(MRegisterInfo::isPhysicalRegister(I->first) && 00211 "Cannot have a live-in virtual register!"); 00212 HandlePhysRegDef(I->first, 0); 00213 } 00214 00215 // Calculate live variable information in depth first order on the CFG of the 00216 // function. This guarantees that we will see the definition of a virtual 00217 // register before its uses due to dominance properties of SSA (except for PHI 00218 // nodes, which are treated as a special case). 00219 // 00220 MachineBasicBlock *Entry = MF.begin(); 00221 std::set<MachineBasicBlock*> Visited; 00222 for (df_ext_iterator<MachineBasicBlock*> DFI = df_ext_begin(Entry, Visited), 00223 E = df_ext_end(Entry, Visited); DFI != E; ++DFI) { 00224 MachineBasicBlock *MBB = *DFI; 00225 unsigned BBNum = MBB->getNumber(); 00226 00227 // Loop over all of the instructions, processing them. 00228 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 00229 I != E; ++I) { 00230 MachineInstr *MI = I; 00231 const TargetInstrDescriptor &MID = TII.get(MI->getOpcode()); 00232 00233 // Process all of the operands of the instruction... 00234 unsigned NumOperandsToProcess = MI->getNumOperands(); 00235 00236 // Unless it is a PHI node. In this case, ONLY process the DEF, not any 00237 // of the uses. They will be handled in other basic blocks. 00238 if (MI->getOpcode() == TargetInstrInfo::PHI) 00239 NumOperandsToProcess = 1; 00240 00241 // Loop over implicit uses, using them. 00242 for (const unsigned *ImplicitUses = MID.ImplicitUses; 00243 *ImplicitUses; ++ImplicitUses) 00244 HandlePhysRegUse(*ImplicitUses, MI); 00245 00246 // Process all explicit uses... 00247 for (unsigned i = 0; i != NumOperandsToProcess; ++i) { 00248 MachineOperand &MO = MI->getOperand(i); 00249 if (MO.isUse() && MO.isRegister() && MO.getReg()) { 00250 if (MRegisterInfo::isVirtualRegister(MO.getReg())){ 00251 HandleVirtRegUse(getVarInfo(MO.getReg()), MBB, MI); 00252 } else if (MRegisterInfo::isPhysicalRegister(MO.getReg()) && 00253 AllocatablePhysicalRegisters[MO.getReg()]) { 00254 HandlePhysRegUse(MO.getReg(), MI); 00255 } 00256 } 00257 } 00258 00259 // Loop over implicit defs, defining them. 00260 for (const unsigned *ImplicitDefs = MID.ImplicitDefs; 00261 *ImplicitDefs; ++ImplicitDefs) 00262 HandlePhysRegDef(*ImplicitDefs, MI); 00263 00264 // Process all explicit defs... 00265 for (unsigned i = 0; i != NumOperandsToProcess; ++i) { 00266 MachineOperand &MO = MI->getOperand(i); 00267 if (MO.isDef() && MO.isRegister() && MO.getReg()) { 00268 if (MRegisterInfo::isVirtualRegister(MO.getReg())) { 00269 VarInfo &VRInfo = getVarInfo(MO.getReg()); 00270 00271 assert(VRInfo.DefInst == 0 && "Variable multiply defined!"); 00272 VRInfo.DefInst = MI; 00273 // Defaults to dead 00274 VRInfo.Kills.push_back(MI); 00275 } else if (MRegisterInfo::isPhysicalRegister(MO.getReg()) && 00276 AllocatablePhysicalRegisters[MO.getReg()]) { 00277 HandlePhysRegDef(MO.getReg(), MI); 00278 } 00279 } 00280 } 00281 } 00282 00283 // Handle any virtual assignments from PHI nodes which might be at the 00284 // bottom of this basic block. We check all of our successor blocks to see 00285 // if they have PHI nodes, and if so, we simulate an assignment at the end 00286 // of the current block. 00287 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 00288 E = MBB->succ_end(); SI != E; ++SI) { 00289 MachineBasicBlock *Succ = *SI; 00290 00291 // PHI nodes are guaranteed to be at the top of the block... 00292 for (MachineBasicBlock::iterator MI = Succ->begin(), ME = Succ->end(); 00293 MI != ME && MI->getOpcode() == TargetInstrInfo::PHI; ++MI) { 00294 for (unsigned i = 1; ; i += 2) { 00295 assert(MI->getNumOperands() > i+1 && 00296 "Didn't find an entry for our predecessor??"); 00297 if (MI->getOperand(i+1).getMachineBasicBlock() == MBB) { 00298 MachineOperand &MO = MI->getOperand(i); 00299 if (!MO.getVRegValueOrNull()) { 00300 VarInfo &VRInfo = getVarInfo(MO.getReg()); 00301 assert(VRInfo.DefInst && "Register use before def (or no def)!"); 00302 00303 // Only mark it alive only in the block we are representing. 00304 MarkVirtRegAliveInBlock(VRInfo, MBB); 00305 break; // Found the PHI entry for this block. 00306 } 00307 } 00308 } 00309 } 00310 } 00311 00312 // Finally, if the last block in the function is a return, make sure to mark 00313 // it as using all of the live-out values in the function. 00314 if (!MBB->empty() && TII.isReturn(MBB->back().getOpcode())) { 00315 MachineInstr *Ret = &MBB->back(); 00316 for (MachineFunction::liveout_iterator I = MF.liveout_begin(), 00317 E = MF.liveout_end(); I != E; ++I) { 00318 assert(MRegisterInfo::isPhysicalRegister(*I) && 00319 "Cannot have a live-in virtual register!"); 00320 HandlePhysRegUse(*I, Ret); 00321 } 00322 } 00323 00324 // Loop over PhysRegInfo, killing any registers that are available at the 00325 // end of the basic block. This also resets the PhysRegInfo map. 00326 for (unsigned i = 0, e = RegInfo->getNumRegs(); i != e; ++i) 00327 if (PhysRegInfo[i]) 00328 HandlePhysRegDef(i, 0); 00329 } 00330 00331 // Convert the information we have gathered into VirtRegInfo and transform it 00332 // into a form usable by RegistersKilled. 00333 // 00334 for (unsigned i = 0, e = VirtRegInfo.size(); i != e; ++i) 00335 for (unsigned j = 0, e = VirtRegInfo[i].Kills.size(); j != e; ++j) { 00336 if (VirtRegInfo[i].Kills[j] == VirtRegInfo[i].DefInst) 00337 RegistersDead[VirtRegInfo[i].Kills[j]].push_back( 00338 i + MRegisterInfo::FirstVirtualRegister); 00339 00340 else 00341 RegistersKilled[VirtRegInfo[i].Kills[j]].push_back( 00342 i + MRegisterInfo::FirstVirtualRegister); 00343 } 00344 00345 // Walk through the RegistersKilled/Dead sets, and sort the registers killed 00346 // or dead. This allows us to use efficient binary search for membership 00347 // testing. 00348 for (std::map<MachineInstr*, std::vector<unsigned> >::iterator 00349 I = RegistersKilled.begin(), E = RegistersKilled.end(); I != E; ++I) 00350 std::sort(I->second.begin(), I->second.end()); 00351 for (std::map<MachineInstr*, std::vector<unsigned> >::iterator 00352 I = RegistersDead.begin(), E = RegistersDead.end(); I != E; ++I) 00353 std::sort(I->second.begin(), I->second.end()); 00354 00355 // Check to make sure there are no unreachable blocks in the MC CFG for the 00356 // function. If so, it is due to a bug in the instruction selector or some 00357 // other part of the code generator if this happens. 00358 #ifndef NDEBUG 00359 for(MachineFunction::iterator i = MF.begin(), e = MF.end(); i != e; ++i) 00360 assert(Visited.count(&*i) != 0 && "unreachable basic block found"); 00361 #endif 00362 00363 return false; 00364 } 00365 00366 /// instructionChanged - When the address of an instruction changes, this 00367 /// method should be called so that live variables can update its internal 00368 /// data structures. This removes the records for OldMI, transfering them to 00369 /// the records for NewMI. 00370 void LiveVariables::instructionChanged(MachineInstr *OldMI, 00371 MachineInstr *NewMI) { 00372 // If the instruction defines any virtual registers, update the VarInfo for 00373 // the instruction. 00374 for (unsigned i = 0, e = OldMI->getNumOperands(); i != e; ++i) { 00375 MachineOperand &MO = OldMI->getOperand(i); 00376 if (MO.isRegister() && MO.getReg() && 00377 MRegisterInfo::isVirtualRegister(MO.getReg())) { 00378 unsigned Reg = MO.getReg(); 00379 VarInfo &VI = getVarInfo(Reg); 00380 if (MO.isDef()) { 00381 // Update the defining instruction. 00382 if (VI.DefInst == OldMI) 00383 VI.DefInst = NewMI; 00384 } 00385 if (MO.isUse()) { 00386 // If this is a kill of the value, update the VI kills list. 00387 if (VI.removeKill(OldMI)) 00388 VI.Kills.push_back(NewMI); // Yes, there was a kill of it 00389 } 00390 } 00391 } 00392 00393 // Move the killed information over... 00394 killed_iterator I, E; 00395 tie(I, E) = killed_range(OldMI); 00396 if (I != E) { 00397 std::vector<unsigned> &V = RegistersKilled[NewMI]; 00398 bool WasEmpty = V.empty(); 00399 V.insert(V.end(), I, E); 00400 if (!WasEmpty) 00401 std::sort(V.begin(), V.end()); // Keep the reg list sorted. 00402 RegistersKilled.erase(OldMI); 00403 } 00404 00405 // Move the dead information over... 00406 tie(I, E) = dead_range(OldMI); 00407 if (I != E) { 00408 std::vector<unsigned> &V = RegistersDead[NewMI]; 00409 bool WasEmpty = V.empty(); 00410 V.insert(V.end(), I, E); 00411 if (!WasEmpty) 00412 std::sort(V.begin(), V.end()); // Keep the reg list sorted. 00413 RegistersDead.erase(OldMI); 00414 } 00415 }