ssa_liveness_analysis.cc revision 6058455d486219994921b63a2d774dc9908415a2
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 number_of_forward_predecessors -= block->GetLoopInformation()->NumberOfBackEdges(); 79 } 80 forward_predecessors.Put(block->GetBlockId(), number_of_forward_predecessors); 81 } 82 83 // (2): Following a worklist approach, first start with the entry block, and 84 // iterate over the successors. When all non-back edge predecessors of a 85 // successor block are visited, the successor block is added in the worklist 86 // following an order that satisfies the requirements to build our linear graph. 87 GrowableArray<HBasicBlock*> worklist(graph_->GetArena(), 1); 88 worklist.Add(graph_->GetEntryBlock()); 89 do { 90 HBasicBlock* current = worklist.Pop(); 91 graph_->linear_order_.Add(current); 92 for (HBasicBlock* successor : current->GetSuccessors()) { 93 int block_id = successor->GetBlockId(); 94 size_t number_of_remaining_predecessors = forward_predecessors.Get(block_id); 95 if (number_of_remaining_predecessors == 1) { 96 AddToListForLinearization(&worklist, successor); 97 } 98 forward_predecessors.Put(block_id, number_of_remaining_predecessors - 1); 99 } 100 } while (!worklist.IsEmpty()); 101} 102 103void SsaLivenessAnalysis::NumberInstructions() { 104 int ssa_index = 0; 105 size_t lifetime_position = 0; 106 // Each instruction gets a lifetime position, and a block gets a lifetime 107 // start and end position. Non-phi instructions have a distinct lifetime position than 108 // the block they are in. Phi instructions have the lifetime start of their block as 109 // lifetime position. 110 // 111 // Because the register allocator will insert moves in the graph, we need 112 // to differentiate between the start and end of an instruction. Adding 2 to 113 // the lifetime position for each instruction ensures the start of an 114 // instruction is different than the end of the previous instruction. 115 for (HLinearOrderIterator it(*graph_); !it.Done(); it.Advance()) { 116 HBasicBlock* block = it.Current(); 117 block->SetLifetimeStart(lifetime_position); 118 119 for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) { 120 HInstruction* current = inst_it.Current(); 121 codegen_->AllocateLocations(current); 122 LocationSummary* locations = current->GetLocations(); 123 if (locations != nullptr && locations->Out().IsValid()) { 124 instructions_from_ssa_index_.Add(current); 125 current->SetSsaIndex(ssa_index++); 126 current->SetLiveInterval( 127 LiveInterval::MakeInterval(graph_->GetArena(), current->GetType(), current)); 128 } 129 current->SetLifetimePosition(lifetime_position); 130 } 131 lifetime_position += 2; 132 133 // Add a null marker to notify we are starting a block. 134 instructions_from_lifetime_position_.Add(nullptr); 135 136 for (HInstructionIterator inst_it(block->GetInstructions()); !inst_it.Done(); 137 inst_it.Advance()) { 138 HInstruction* current = inst_it.Current(); 139 codegen_->AllocateLocations(current); 140 LocationSummary* locations = current->GetLocations(); 141 if (locations != nullptr && locations->Out().IsValid()) { 142 instructions_from_ssa_index_.Add(current); 143 current->SetSsaIndex(ssa_index++); 144 current->SetLiveInterval( 145 LiveInterval::MakeInterval(graph_->GetArena(), current->GetType(), current)); 146 } 147 instructions_from_lifetime_position_.Add(current); 148 current->SetLifetimePosition(lifetime_position); 149 lifetime_position += 2; 150 } 151 152 block->SetLifetimeEnd(lifetime_position); 153 } 154 number_of_ssa_values_ = ssa_index; 155} 156 157void SsaLivenessAnalysis::ComputeLiveness() { 158 for (HLinearOrderIterator it(*graph_); !it.Done(); it.Advance()) { 159 HBasicBlock* block = it.Current(); 160 block_infos_.Put( 161 block->GetBlockId(), 162 new (graph_->GetArena()) BlockInfo(graph_->GetArena(), *block, number_of_ssa_values_)); 163 } 164 165 // Compute the live ranges, as well as the initial live_in, live_out, and kill sets. 166 // This method does not handle backward branches for the sets, therefore live_in 167 // and live_out sets are not yet correct. 168 ComputeLiveRanges(); 169 170 // Do a fixed point calculation to take into account backward branches, 171 // that will update live_in of loop headers, and therefore live_out and live_in 172 // of blocks in the loop. 173 ComputeLiveInAndLiveOutSets(); 174} 175 176void SsaLivenessAnalysis::ComputeLiveRanges() { 177 // Do a post order visit, adding inputs of instructions live in the block where 178 // that instruction is defined, and killing instructions that are being visited. 179 for (HLinearPostOrderIterator it(*graph_); !it.Done(); it.Advance()) { 180 HBasicBlock* block = it.Current(); 181 182 BitVector* kill = GetKillSet(*block); 183 BitVector* live_in = GetLiveInSet(*block); 184 185 // Set phi inputs of successors of this block corresponding to this block 186 // as live_in. 187 for (HBasicBlock* successor : block->GetSuccessors()) { 188 live_in->Union(GetLiveInSet(*successor)); 189 size_t phi_input_index = successor->GetPredecessorIndexOf(block); 190 for (HInstructionIterator inst_it(successor->GetPhis()); !inst_it.Done(); inst_it.Advance()) { 191 HInstruction* phi = inst_it.Current(); 192 HInstruction* input = phi->InputAt(phi_input_index); 193 input->GetLiveInterval()->AddPhiUse(phi, phi_input_index, block); 194 // A phi input whose last user is the phi dies at the end of the predecessor block, 195 // and not at the phi's lifetime position. 196 live_in->SetBit(input->GetSsaIndex()); 197 } 198 } 199 200 // Add a range that covers this block to all instructions live_in because of successors. 201 // Instructions defined in this block will have their start of the range adjusted. 202 for (uint32_t idx : live_in->Indexes()) { 203 HInstruction* current = instructions_from_ssa_index_.Get(idx); 204 current->GetLiveInterval()->AddRange(block->GetLifetimeStart(), block->GetLifetimeEnd()); 205 } 206 207 for (HBackwardInstructionIterator back_it(block->GetInstructions()); !back_it.Done(); 208 back_it.Advance()) { 209 HInstruction* current = back_it.Current(); 210 if (current->HasSsaIndex()) { 211 // Kill the instruction and shorten its interval. 212 kill->SetBit(current->GetSsaIndex()); 213 live_in->ClearBit(current->GetSsaIndex()); 214 current->GetLiveInterval()->SetFrom(current->GetLifetimePosition()); 215 } 216 217 // Process the environment first, because we know their uses come after 218 // or at the same liveness position of inputs. 219 for (HEnvironment* environment = current->GetEnvironment(); 220 environment != nullptr; 221 environment = environment->GetParent()) { 222 // Handle environment uses. See statements (b) and (c) of the 223 // SsaLivenessAnalysis. 224 for (size_t i = 0, e = environment->Size(); i < e; ++i) { 225 HInstruction* instruction = environment->GetInstructionAt(i); 226 bool should_be_live = ShouldBeLiveForEnvironment(current, instruction); 227 if (should_be_live) { 228 DCHECK(instruction->HasSsaIndex()); 229 live_in->SetBit(instruction->GetSsaIndex()); 230 } 231 if (instruction != nullptr) { 232 instruction->GetLiveInterval()->AddUse( 233 current, environment, i, should_be_live); 234 } 235 } 236 } 237 238 // All inputs of an instruction must be live. 239 for (size_t i = 0, e = current->InputCount(); i < e; ++i) { 240 HInstruction* input = current->InputAt(i); 241 // Some instructions 'inline' their inputs, that is they do not need 242 // to be materialized. 243 if (input->HasSsaIndex() && current->GetLocations()->InAt(i).IsValid()) { 244 live_in->SetBit(input->GetSsaIndex()); 245 input->GetLiveInterval()->AddUse(current, /* environment */ nullptr, i); 246 } 247 } 248 } 249 250 // Kill phis defined in this block. 251 for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) { 252 HInstruction* current = inst_it.Current(); 253 if (current->HasSsaIndex()) { 254 kill->SetBit(current->GetSsaIndex()); 255 live_in->ClearBit(current->GetSsaIndex()); 256 LiveInterval* interval = current->GetLiveInterval(); 257 DCHECK((interval->GetFirstRange() == nullptr) 258 || (interval->GetStart() == current->GetLifetimePosition())); 259 interval->SetFrom(current->GetLifetimePosition()); 260 } 261 } 262 263 if (block->IsLoopHeader()) { 264 size_t last_position = block->GetLoopInformation()->GetLifetimeEnd(); 265 // For all live_in instructions at the loop header, we need to create a range 266 // that covers the full loop. 267 for (uint32_t idx : live_in->Indexes()) { 268 HInstruction* current = instructions_from_ssa_index_.Get(idx); 269 current->GetLiveInterval()->AddLoopRange(block->GetLifetimeStart(), last_position); 270 } 271 } 272 } 273} 274 275void SsaLivenessAnalysis::ComputeLiveInAndLiveOutSets() { 276 bool changed; 277 do { 278 changed = false; 279 280 for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) { 281 const HBasicBlock& block = *it.Current(); 282 283 // The live_in set depends on the kill set (which does not 284 // change in this loop), and the live_out set. If the live_out 285 // set does not change, there is no need to update the live_in set. 286 if (UpdateLiveOut(block) && UpdateLiveIn(block)) { 287 changed = true; 288 } 289 } 290 } while (changed); 291} 292 293bool SsaLivenessAnalysis::UpdateLiveOut(const HBasicBlock& block) { 294 BitVector* live_out = GetLiveOutSet(block); 295 bool changed = false; 296 // The live_out set of a block is the union of live_in sets of its successors. 297 for (HBasicBlock* successor : block.GetSuccessors()) { 298 if (live_out->Union(GetLiveInSet(*successor))) { 299 changed = true; 300 } 301 } 302 return changed; 303} 304 305 306bool SsaLivenessAnalysis::UpdateLiveIn(const HBasicBlock& block) { 307 BitVector* live_out = GetLiveOutSet(block); 308 BitVector* kill = GetKillSet(block); 309 BitVector* live_in = GetLiveInSet(block); 310 // If live_out is updated (because of backward branches), we need to make 311 // sure instructions in live_out are also in live_in, unless they are killed 312 // by this block. 313 return live_in->UnionIfNotIn(live_out, kill); 314} 315 316static int RegisterOrLowRegister(Location location) { 317 return location.IsPair() ? location.low() : location.reg(); 318} 319 320int LiveInterval::FindFirstRegisterHint(size_t* free_until, 321 const SsaLivenessAnalysis& liveness) const { 322 DCHECK(!IsHighInterval()); 323 if (IsTemp()) return kNoRegister; 324 325 if (GetParent() == this && defined_by_ != nullptr) { 326 // This is the first interval for the instruction. Try to find 327 // a register based on its definition. 328 DCHECK_EQ(defined_by_->GetLiveInterval(), this); 329 int hint = FindHintAtDefinition(); 330 if (hint != kNoRegister && free_until[hint] > GetStart()) { 331 return hint; 332 } 333 } 334 335 if (IsSplit() && liveness.IsAtBlockBoundary(GetStart() / 2)) { 336 // If the start of this interval is at a block boundary, we look at the 337 // location of the interval in blocks preceding the block this interval 338 // starts at. If one location is a register we return it as a hint. This 339 // will avoid a move between the two blocks. 340 HBasicBlock* block = liveness.GetBlockFromPosition(GetStart() / 2); 341 size_t next_register_use = FirstRegisterUse(); 342 for (HBasicBlock* predecessor : block->GetPredecessors()) { 343 size_t position = predecessor->GetLifetimeEnd() - 1; 344 // We know positions above GetStart() do not have a location yet. 345 if (position < GetStart()) { 346 LiveInterval* existing = GetParent()->GetSiblingAt(position); 347 if (existing != nullptr 348 && existing->HasRegister() 349 // It's worth using that register if it is available until 350 // the next use. 351 && (free_until[existing->GetRegister()] >= next_register_use)) { 352 return existing->GetRegister(); 353 } 354 } 355 } 356 } 357 358 UsePosition* use = first_use_; 359 size_t start = GetStart(); 360 size_t end = GetEnd(); 361 while (use != nullptr && use->GetPosition() <= end) { 362 size_t use_position = use->GetPosition(); 363 if (use_position >= start && !use->IsSynthesized()) { 364 HInstruction* user = use->GetUser(); 365 size_t input_index = use->GetInputIndex(); 366 if (user->IsPhi()) { 367 // If the phi has a register, try to use the same. 368 Location phi_location = user->GetLiveInterval()->ToLocation(); 369 if (phi_location.IsRegisterKind()) { 370 DCHECK(SameRegisterKind(phi_location)); 371 int reg = RegisterOrLowRegister(phi_location); 372 if (free_until[reg] >= use_position) { 373 return reg; 374 } 375 } 376 // If the instruction dies at the phi assignment, we can try having the 377 // same register. 378 if (end == user->GetBlock()->GetPredecessor(input_index)->GetLifetimeEnd()) { 379 for (size_t i = 0, e = user->InputCount(); i < e; ++i) { 380 if (i == input_index) { 381 continue; 382 } 383 HInstruction* input = user->InputAt(i); 384 Location location = input->GetLiveInterval()->GetLocationAt( 385 user->GetBlock()->GetPredecessor(i)->GetLifetimeEnd() - 1); 386 if (location.IsRegisterKind()) { 387 int reg = RegisterOrLowRegister(location); 388 if (free_until[reg] >= use_position) { 389 return reg; 390 } 391 } 392 } 393 } 394 } else { 395 // If the instruction is expected in a register, try to use it. 396 LocationSummary* locations = user->GetLocations(); 397 Location expected = locations->InAt(use->GetInputIndex()); 398 // We use the user's lifetime position - 1 (and not `use_position`) because the 399 // register is blocked at the beginning of the user. 400 size_t position = user->GetLifetimePosition() - 1; 401 if (expected.IsRegisterKind()) { 402 DCHECK(SameRegisterKind(expected)); 403 int reg = RegisterOrLowRegister(expected); 404 if (free_until[reg] >= position) { 405 return reg; 406 } 407 } 408 } 409 } 410 use = use->GetNext(); 411 } 412 413 return kNoRegister; 414} 415 416int LiveInterval::FindHintAtDefinition() const { 417 if (defined_by_->IsPhi()) { 418 // Try to use the same register as one of the inputs. 419 const ArenaVector<HBasicBlock*>& predecessors = defined_by_->GetBlock()->GetPredecessors(); 420 for (size_t i = 0, e = defined_by_->InputCount(); i < e; ++i) { 421 HInstruction* input = defined_by_->InputAt(i); 422 DCHECK_LT(i, predecessors.size()); 423 size_t end = predecessors[i]->GetLifetimeEnd(); 424 LiveInterval* input_interval = input->GetLiveInterval()->GetSiblingAt(end - 1); 425 if (input_interval->GetEnd() == end) { 426 // If the input dies at the end of the predecessor, we know its register can 427 // be reused. 428 Location input_location = input_interval->ToLocation(); 429 if (input_location.IsRegisterKind()) { 430 DCHECK(SameRegisterKind(input_location)); 431 return RegisterOrLowRegister(input_location); 432 } 433 } 434 } 435 } else { 436 LocationSummary* locations = GetDefinedBy()->GetLocations(); 437 Location out = locations->Out(); 438 if (out.IsUnallocated() && out.GetPolicy() == Location::kSameAsFirstInput) { 439 // Try to use the same register as the first input. 440 LiveInterval* input_interval = 441 GetDefinedBy()->InputAt(0)->GetLiveInterval()->GetSiblingAt(GetStart() - 1); 442 if (input_interval->GetEnd() == GetStart()) { 443 // If the input dies at the start of this instruction, we know its register can 444 // be reused. 445 Location location = input_interval->ToLocation(); 446 if (location.IsRegisterKind()) { 447 DCHECK(SameRegisterKind(location)); 448 return RegisterOrLowRegister(location); 449 } 450 } 451 } 452 } 453 return kNoRegister; 454} 455 456bool LiveInterval::SameRegisterKind(Location other) const { 457 if (IsFloatingPoint()) { 458 if (IsLowInterval() || IsHighInterval()) { 459 return other.IsFpuRegisterPair(); 460 } else { 461 return other.IsFpuRegister(); 462 } 463 } else { 464 if (IsLowInterval() || IsHighInterval()) { 465 return other.IsRegisterPair(); 466 } else { 467 return other.IsRegister(); 468 } 469 } 470} 471 472bool LiveInterval::NeedsTwoSpillSlots() const { 473 return type_ == Primitive::kPrimLong || type_ == Primitive::kPrimDouble; 474} 475 476Location LiveInterval::ToLocation() const { 477 DCHECK(!IsHighInterval()); 478 if (HasRegister()) { 479 if (IsFloatingPoint()) { 480 if (HasHighInterval()) { 481 return Location::FpuRegisterPairLocation(GetRegister(), GetHighInterval()->GetRegister()); 482 } else { 483 return Location::FpuRegisterLocation(GetRegister()); 484 } 485 } else { 486 if (HasHighInterval()) { 487 return Location::RegisterPairLocation(GetRegister(), GetHighInterval()->GetRegister()); 488 } else { 489 return Location::RegisterLocation(GetRegister()); 490 } 491 } 492 } else { 493 HInstruction* defined_by = GetParent()->GetDefinedBy(); 494 if (defined_by->IsConstant()) { 495 return defined_by->GetLocations()->Out(); 496 } else if (GetParent()->HasSpillSlot()) { 497 if (NeedsTwoSpillSlots()) { 498 return Location::DoubleStackSlot(GetParent()->GetSpillSlot()); 499 } else { 500 return Location::StackSlot(GetParent()->GetSpillSlot()); 501 } 502 } else { 503 return Location(); 504 } 505 } 506} 507 508Location LiveInterval::GetLocationAt(size_t position) { 509 LiveInterval* sibling = GetSiblingAt(position); 510 DCHECK(sibling != nullptr); 511 return sibling->ToLocation(); 512} 513 514LiveInterval* LiveInterval::GetSiblingAt(size_t position) { 515 LiveInterval* current = this; 516 while (current != nullptr && !current->IsDefinedAt(position)) { 517 current = current->GetNextSibling(); 518 } 519 return current; 520} 521 522} // namespace art 523