ssa_liveness_analysis.cc revision 46e2a3915aa68c77426b71e95b9f3658250646b7
1/* 2 * Copyright (C) 2014 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17#include "ssa_liveness_analysis.h" 18 19#include "base/bit_vector-inl.h" 20#include "code_generator.h" 21#include "nodes.h" 22 23namespace art { 24 25void SsaLivenessAnalysis::Analyze() { 26 LinearizeGraph(); 27 NumberInstructions(); 28 ComputeLiveness(); 29} 30 31static bool IsLoop(HLoopInformation* info) { 32 return info != nullptr; 33} 34 35static bool InSameLoop(HLoopInformation* first_loop, HLoopInformation* second_loop) { 36 return first_loop == second_loop; 37} 38 39static bool IsInnerLoop(HLoopInformation* outer, HLoopInformation* inner) { 40 return (inner != outer) 41 && (inner != nullptr) 42 && (outer != nullptr) 43 && inner->IsIn(*outer); 44} 45 46static void AddToListForLinearization(GrowableArray<HBasicBlock*>* worklist, HBasicBlock* block) { 47 size_t insert_at = worklist->Size(); 48 HLoopInformation* block_loop = block->GetLoopInformation(); 49 for (; insert_at > 0; --insert_at) { 50 HBasicBlock* current = worklist->Get(insert_at - 1); 51 HLoopInformation* current_loop = current->GetLoopInformation(); 52 if (InSameLoop(block_loop, current_loop) 53 || !IsLoop(current_loop) 54 || IsInnerLoop(current_loop, block_loop)) { 55 // The block can be processed immediately. 56 break; 57 } 58 } 59 worklist->InsertAt(insert_at, block); 60} 61 62void SsaLivenessAnalysis::LinearizeGraph() { 63 // Create a reverse post ordering with the following properties: 64 // - Blocks in a loop are consecutive, 65 // - Back-edge is the last block before loop exits. 66 67 // (1): Record the number of forward predecessors for each block. This is to 68 // ensure the resulting order is reverse post order. We could use the 69 // current reverse post order in the graph, but it would require making 70 // order queries to a GrowableArray, which is not the best data structure 71 // for it. 72 GrowableArray<uint32_t> forward_predecessors(graph_.GetArena(), graph_.GetBlocks().Size()); 73 forward_predecessors.SetSize(graph_.GetBlocks().Size()); 74 for (HReversePostOrderIterator it(graph_); !it.Done(); it.Advance()) { 75 HBasicBlock* block = it.Current(); 76 size_t number_of_forward_predecessors = block->GetPredecessors().Size(); 77 if (block->IsLoopHeader()) { 78 // We rely on having simplified the CFG. 79 DCHECK_EQ(1u, block->GetLoopInformation()->NumberOfBackEdges()); 80 number_of_forward_predecessors--; 81 } 82 forward_predecessors.Put(block->GetBlockId(), number_of_forward_predecessors); 83 } 84 85 // (2): Following a worklist approach, first start with the entry block, and 86 // iterate over the successors. When all non-back edge predecessors of a 87 // successor block are visited, the successor block is added in the worklist 88 // following an order that satisfies the requirements to build our linear graph. 89 GrowableArray<HBasicBlock*> worklist(graph_.GetArena(), 1); 90 worklist.Add(graph_.GetEntryBlock()); 91 do { 92 HBasicBlock* current = worklist.Pop(); 93 linear_order_.Add(current); 94 for (size_t i = 0, e = current->GetSuccessors().Size(); i < e; ++i) { 95 HBasicBlock* successor = current->GetSuccessors().Get(i); 96 int block_id = successor->GetBlockId(); 97 size_t number_of_remaining_predecessors = forward_predecessors.Get(block_id); 98 if (number_of_remaining_predecessors == 1) { 99 AddToListForLinearization(&worklist, successor); 100 } 101 forward_predecessors.Put(block_id, number_of_remaining_predecessors - 1); 102 } 103 } while (!worklist.IsEmpty()); 104} 105 106void SsaLivenessAnalysis::NumberInstructions() { 107 int ssa_index = 0; 108 size_t lifetime_position = 0; 109 // Each instruction gets a lifetime position, and a block gets a lifetime 110 // start and end position. Non-phi instructions have a distinct lifetime position than 111 // the block they are in. Phi instructions have the lifetime start of their block as 112 // lifetime position. 113 // 114 // Because the register allocator will insert moves in the graph, we need 115 // to differentiate between the start and end of an instruction. Adding 2 to 116 // the lifetime position for each instruction ensures the start of an 117 // instruction is different than the end of the previous instruction. 118 for (HLinearOrderIterator it(*this); !it.Done(); it.Advance()) { 119 HBasicBlock* block = it.Current(); 120 block->SetLifetimeStart(lifetime_position); 121 122 for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) { 123 HInstruction* current = inst_it.Current(); 124 codegen_->AllocateLocations(current); 125 LocationSummary* locations = current->GetLocations(); 126 if (locations != nullptr && locations->Out().IsValid()) { 127 instructions_from_ssa_index_.Add(current); 128 current->SetSsaIndex(ssa_index++); 129 current->SetLiveInterval( 130 LiveInterval::MakeInterval(graph_.GetArena(), current->GetType(), current)); 131 } 132 current->SetLifetimePosition(lifetime_position); 133 } 134 lifetime_position += 2; 135 136 // Add a null marker to notify we are starting a block. 137 instructions_from_lifetime_position_.Add(nullptr); 138 139 for (HInstructionIterator inst_it(block->GetInstructions()); !inst_it.Done(); 140 inst_it.Advance()) { 141 HInstruction* current = inst_it.Current(); 142 codegen_->AllocateLocations(current); 143 LocationSummary* locations = current->GetLocations(); 144 if (locations != nullptr && locations->Out().IsValid()) { 145 instructions_from_ssa_index_.Add(current); 146 current->SetSsaIndex(ssa_index++); 147 current->SetLiveInterval( 148 LiveInterval::MakeInterval(graph_.GetArena(), current->GetType(), current)); 149 } 150 instructions_from_lifetime_position_.Add(current); 151 current->SetLifetimePosition(lifetime_position); 152 lifetime_position += 2; 153 } 154 155 block->SetLifetimeEnd(lifetime_position); 156 } 157 number_of_ssa_values_ = ssa_index; 158} 159 160void SsaLivenessAnalysis::ComputeLiveness() { 161 for (HLinearOrderIterator it(*this); !it.Done(); it.Advance()) { 162 HBasicBlock* block = it.Current(); 163 block_infos_.Put( 164 block->GetBlockId(), 165 new (graph_.GetArena()) BlockInfo(graph_.GetArena(), *block, number_of_ssa_values_)); 166 } 167 168 // Compute the live ranges, as well as the initial live_in, live_out, and kill sets. 169 // This method does not handle backward branches for the sets, therefore live_in 170 // and live_out sets are not yet correct. 171 ComputeLiveRanges(); 172 173 // Do a fixed point calculation to take into account backward branches, 174 // that will update live_in of loop headers, and therefore live_out and live_in 175 // of blocks in the loop. 176 ComputeLiveInAndLiveOutSets(); 177} 178 179void SsaLivenessAnalysis::ComputeLiveRanges() { 180 // Do a post order visit, adding inputs of instructions live in the block where 181 // that instruction is defined, and killing instructions that are being visited. 182 for (HLinearPostOrderIterator it(*this); !it.Done(); it.Advance()) { 183 HBasicBlock* block = it.Current(); 184 185 BitVector* kill = GetKillSet(*block); 186 BitVector* live_in = GetLiveInSet(*block); 187 188 // Set phi inputs of successors of this block corresponding to this block 189 // as live_in. 190 for (size_t i = 0, e = block->GetSuccessors().Size(); i < e; ++i) { 191 HBasicBlock* successor = block->GetSuccessors().Get(i); 192 live_in->Union(GetLiveInSet(*successor)); 193 size_t phi_input_index = successor->GetPredecessorIndexOf(block); 194 for (HInstructionIterator inst_it(successor->GetPhis()); !inst_it.Done(); inst_it.Advance()) { 195 HInstruction* phi = inst_it.Current(); 196 HInstruction* input = phi->InputAt(phi_input_index); 197 input->GetLiveInterval()->AddPhiUse(phi, phi_input_index, block); 198 // A phi input whose last user is the phi dies at the end of the predecessor block, 199 // and not at the phi's lifetime position. 200 live_in->SetBit(input->GetSsaIndex()); 201 } 202 } 203 204 // Add a range that covers this block to all instructions live_in because of successors. 205 // Instructions defined in this block will have their start of the range adjusted. 206 for (uint32_t idx : live_in->Indexes()) { 207 HInstruction* current = instructions_from_ssa_index_.Get(idx); 208 current->GetLiveInterval()->AddRange(block->GetLifetimeStart(), block->GetLifetimeEnd()); 209 } 210 211 for (HBackwardInstructionIterator back_it(block->GetInstructions()); !back_it.Done(); 212 back_it.Advance()) { 213 HInstruction* current = back_it.Current(); 214 if (current->HasSsaIndex()) { 215 // Kill the instruction and shorten its interval. 216 kill->SetBit(current->GetSsaIndex()); 217 live_in->ClearBit(current->GetSsaIndex()); 218 current->GetLiveInterval()->SetFrom(current->GetLifetimePosition()); 219 } 220 221 // All inputs of an instruction must be live. 222 for (size_t i = 0, e = current->InputCount(); i < e; ++i) { 223 HInstruction* input = current->InputAt(i); 224 // Some instructions 'inline' their inputs, that is they do not need 225 // to be materialized. 226 if (input->HasSsaIndex()) { 227 live_in->SetBit(input->GetSsaIndex()); 228 input->GetLiveInterval()->AddUse(current, i, false); 229 } 230 } 231 232 if (current->HasEnvironment()) { 233 // Handle environment uses. See statements (b) and (c) of the 234 // SsaLivenessAnalysis. 235 HEnvironment* environment = current->GetEnvironment(); 236 for (size_t i = 0, e = environment->Size(); i < e; ++i) { 237 HInstruction* instruction = environment->GetInstructionAt(i); 238 if (ShouldBeLiveForEnvironment(instruction)) { 239 DCHECK(instruction->HasSsaIndex()); 240 live_in->SetBit(instruction->GetSsaIndex()); 241 instruction->GetLiveInterval()->AddUse(current, i, true); 242 } 243 } 244 } 245 } 246 247 // Kill phis defined in this block. 248 for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) { 249 HInstruction* current = inst_it.Current(); 250 if (current->HasSsaIndex()) { 251 kill->SetBit(current->GetSsaIndex()); 252 live_in->ClearBit(current->GetSsaIndex()); 253 LiveInterval* interval = current->GetLiveInterval(); 254 DCHECK((interval->GetFirstRange() == nullptr) 255 || (interval->GetStart() == current->GetLifetimePosition())); 256 interval->SetFrom(current->GetLifetimePosition()); 257 } 258 } 259 260 if (block->IsLoopHeader()) { 261 HBasicBlock* back_edge = block->GetLoopInformation()->GetBackEdges().Get(0); 262 // For all live_in instructions at the loop header, we need to create a range 263 // that covers the full loop. 264 for (uint32_t idx : live_in->Indexes()) { 265 HInstruction* current = instructions_from_ssa_index_.Get(idx); 266 current->GetLiveInterval()->AddLoopRange(block->GetLifetimeStart(), 267 back_edge->GetLifetimeEnd()); 268 } 269 } 270 } 271} 272 273void SsaLivenessAnalysis::ComputeLiveInAndLiveOutSets() { 274 bool changed; 275 do { 276 changed = false; 277 278 for (HPostOrderIterator it(graph_); !it.Done(); it.Advance()) { 279 const HBasicBlock& block = *it.Current(); 280 281 // The live_in set depends on the kill set (which does not 282 // change in this loop), and the live_out set. If the live_out 283 // set does not change, there is no need to update the live_in set. 284 if (UpdateLiveOut(block) && UpdateLiveIn(block)) { 285 changed = true; 286 } 287 } 288 } while (changed); 289} 290 291bool SsaLivenessAnalysis::UpdateLiveOut(const HBasicBlock& block) { 292 BitVector* live_out = GetLiveOutSet(block); 293 bool changed = false; 294 // The live_out set of a block is the union of live_in sets of its successors. 295 for (size_t i = 0, e = block.GetSuccessors().Size(); i < e; ++i) { 296 HBasicBlock* successor = block.GetSuccessors().Get(i); 297 if (live_out->Union(GetLiveInSet(*successor))) { 298 changed = true; 299 } 300 } 301 return changed; 302} 303 304 305bool SsaLivenessAnalysis::UpdateLiveIn(const HBasicBlock& block) { 306 BitVector* live_out = GetLiveOutSet(block); 307 BitVector* kill = GetKillSet(block); 308 BitVector* live_in = GetLiveInSet(block); 309 // If live_out is updated (because of backward branches), we need to make 310 // sure instructions in live_out are also in live_in, unless they are killed 311 // by this block. 312 return live_in->UnionIfNotIn(live_out, kill); 313} 314 315static int RegisterOrLowRegister(Location location) { 316 return location.IsPair() ? location.low() : location.reg(); 317} 318 319int LiveInterval::FindFirstRegisterHint(size_t* free_until) const { 320 DCHECK(!IsHighInterval()); 321 if (GetParent() == this && defined_by_ != nullptr) { 322 // This is the first interval for the instruction. Try to find 323 // a register based on its definition. 324 DCHECK_EQ(defined_by_->GetLiveInterval(), this); 325 int hint = FindHintAtDefinition(); 326 if (hint != kNoRegister && free_until[hint] > GetStart()) { 327 return hint; 328 } 329 } 330 331 UsePosition* use = first_use_; 332 size_t start = GetStart(); 333 size_t end = GetEnd(); 334 while (use != nullptr && use->GetPosition() <= end) { 335 size_t use_position = use->GetPosition(); 336 if (use_position >= start && !use->GetIsEnvironment()) { 337 HInstruction* user = use->GetUser(); 338 size_t input_index = use->GetInputIndex(); 339 if (user->IsPhi()) { 340 // If the phi has a register, try to use the same. 341 Location phi_location = user->GetLiveInterval()->ToLocation(); 342 if (phi_location.IsRegisterKind()) { 343 DCHECK(SameRegisterKind(phi_location)); 344 int reg = RegisterOrLowRegister(phi_location); 345 if (free_until[reg] >= use_position) { 346 return reg; 347 } 348 } 349 const GrowableArray<HBasicBlock*>& predecessors = user->GetBlock()->GetPredecessors(); 350 // If the instruction dies at the phi assignment, we can try having the 351 // same register. 352 if (end == predecessors.Get(input_index)->GetLifetimeEnd()) { 353 for (size_t i = 0, e = user->InputCount(); i < e; ++i) { 354 if (i == input_index) { 355 continue; 356 } 357 HInstruction* input = user->InputAt(i); 358 Location location = input->GetLiveInterval()->GetLocationAt( 359 predecessors.Get(i)->GetLifetimeEnd() - 1); 360 if (location.IsRegisterKind()) { 361 int reg = RegisterOrLowRegister(location); 362 if (free_until[reg] >= use_position) { 363 return reg; 364 } 365 } 366 } 367 } 368 } else { 369 // If the instruction is expected in a register, try to use it. 370 LocationSummary* locations = user->GetLocations(); 371 Location expected = locations->InAt(use->GetInputIndex()); 372 // We use the user's lifetime position - 1 (and not `use_position`) because the 373 // register is blocked at the beginning of the user. 374 size_t position = user->GetLifetimePosition() - 1; 375 if (expected.IsRegisterKind()) { 376 DCHECK(SameRegisterKind(expected)); 377 int reg = RegisterOrLowRegister(expected); 378 if (free_until[reg] >= position) { 379 return reg; 380 } 381 } 382 } 383 } 384 use = use->GetNext(); 385 } 386 387 return kNoRegister; 388} 389 390int LiveInterval::FindHintAtDefinition() const { 391 if (defined_by_->IsPhi()) { 392 // Try to use the same register as one of the inputs. 393 const GrowableArray<HBasicBlock*>& predecessors = defined_by_->GetBlock()->GetPredecessors(); 394 for (size_t i = 0, e = defined_by_->InputCount(); i < e; ++i) { 395 HInstruction* input = defined_by_->InputAt(i); 396 size_t end = predecessors.Get(i)->GetLifetimeEnd(); 397 const LiveInterval& input_interval = input->GetLiveInterval()->GetIntervalAt(end - 1); 398 if (input_interval.GetEnd() == end) { 399 // If the input dies at the end of the predecessor, we know its register can 400 // be reused. 401 Location input_location = input_interval.ToLocation(); 402 if (input_location.IsRegisterKind()) { 403 DCHECK(SameRegisterKind(input_location)); 404 return RegisterOrLowRegister(input_location); 405 } 406 } 407 } 408 } else { 409 LocationSummary* locations = GetDefinedBy()->GetLocations(); 410 Location out = locations->Out(); 411 if (out.IsUnallocated() && out.GetPolicy() == Location::kSameAsFirstInput) { 412 // Try to use the same register as the first input. 413 const LiveInterval& input_interval = 414 GetDefinedBy()->InputAt(0)->GetLiveInterval()->GetIntervalAt(GetStart() - 1); 415 if (input_interval.GetEnd() == GetStart()) { 416 // If the input dies at the start of this instruction, we know its register can 417 // be reused. 418 Location location = input_interval.ToLocation(); 419 if (location.IsRegisterKind()) { 420 DCHECK(SameRegisterKind(location)); 421 return RegisterOrLowRegister(location); 422 } 423 } 424 } 425 } 426 return kNoRegister; 427} 428 429bool LiveInterval::SameRegisterKind(Location other) const { 430 if (IsFloatingPoint()) { 431 if (IsLowInterval() || IsHighInterval()) { 432 return other.IsFpuRegisterPair(); 433 } else { 434 return other.IsFpuRegister(); 435 } 436 } else { 437 if (IsLowInterval() || IsHighInterval()) { 438 return other.IsRegisterPair(); 439 } else { 440 return other.IsRegister(); 441 } 442 } 443} 444 445bool LiveInterval::NeedsTwoSpillSlots() const { 446 return type_ == Primitive::kPrimLong || type_ == Primitive::kPrimDouble; 447} 448 449Location LiveInterval::ToLocation() const { 450 DCHECK(!IsHighInterval()); 451 if (HasRegister()) { 452 if (IsFloatingPoint()) { 453 if (HasHighInterval()) { 454 return Location::FpuRegisterPairLocation(GetRegister(), GetHighInterval()->GetRegister()); 455 } else { 456 return Location::FpuRegisterLocation(GetRegister()); 457 } 458 } else { 459 if (HasHighInterval()) { 460 return Location::RegisterPairLocation(GetRegister(), GetHighInterval()->GetRegister()); 461 } else { 462 return Location::RegisterLocation(GetRegister()); 463 } 464 } 465 } else { 466 HInstruction* defined_by = GetParent()->GetDefinedBy(); 467 if (defined_by->IsConstant()) { 468 return defined_by->GetLocations()->Out(); 469 } else if (GetParent()->HasSpillSlot()) { 470 if (NeedsTwoSpillSlots()) { 471 return Location::DoubleStackSlot(GetParent()->GetSpillSlot()); 472 } else { 473 return Location::StackSlot(GetParent()->GetSpillSlot()); 474 } 475 } else { 476 return Location(); 477 } 478 } 479} 480 481Location LiveInterval::GetLocationAt(size_t position) { 482 return GetIntervalAt(position).ToLocation(); 483} 484 485const LiveInterval& LiveInterval::GetIntervalAt(size_t position) { 486 LiveInterval* current = this; 487 while (!current->Covers(position)) { 488 current = current->GetNextSibling(); 489 DCHECK(current != nullptr); 490 } 491 return *current; 492} 493 494} // namespace art 495