ssa_liveness_analysis.cc revision 102cbed1e52b7c5f09458b44903fe97bb3e14d5f
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 IsLoopExit(HLoopInformation* current, HLoopInformation* to) { 32 // `to` is either not part of a loop, or `current` is an inner loop of `to`. 33 return to == nullptr || (current != to && current->IsIn(*to)); 34} 35 36static bool IsLoop(HLoopInformation* info) { 37 return info != nullptr; 38} 39 40static bool InSameLoop(HLoopInformation* first_loop, HLoopInformation* second_loop) { 41 return first_loop == second_loop; 42} 43 44static bool IsInnerLoop(HLoopInformation* outer, HLoopInformation* inner) { 45 return (inner != outer) 46 && (inner != nullptr) 47 && (outer != nullptr) 48 && inner->IsIn(*outer); 49} 50 51static void VisitBlockForLinearization(HBasicBlock* block, 52 GrowableArray<HBasicBlock*>* order, 53 ArenaBitVector* visited) { 54 if (visited->IsBitSet(block->GetBlockId())) { 55 return; 56 } 57 visited->SetBit(block->GetBlockId()); 58 size_t number_of_successors = block->GetSuccessors().Size(); 59 if (number_of_successors == 0) { 60 // Nothing to do. 61 } else if (number_of_successors == 1) { 62 VisitBlockForLinearization(block->GetSuccessors().Get(0), order, visited); 63 } else { 64 DCHECK_EQ(number_of_successors, 2u); 65 HBasicBlock* first_successor = block->GetSuccessors().Get(0); 66 HBasicBlock* second_successor = block->GetSuccessors().Get(1); 67 HLoopInformation* my_loop = block->GetLoopInformation(); 68 HLoopInformation* first_loop = first_successor->GetLoopInformation(); 69 HLoopInformation* second_loop = second_successor->GetLoopInformation(); 70 71 if (!IsLoop(my_loop)) { 72 // Nothing to do. Current order is fine. 73 } else if (IsLoopExit(my_loop, second_loop) && InSameLoop(my_loop, first_loop)) { 74 // Visit the loop exit first in post order. 75 std::swap(first_successor, second_successor); 76 } else if (IsInnerLoop(my_loop, first_loop) && !IsInnerLoop(my_loop, second_loop)) { 77 // Visit the inner loop last in post order. 78 std::swap(first_successor, second_successor); 79 } 80 VisitBlockForLinearization(first_successor, order, visited); 81 VisitBlockForLinearization(second_successor, order, visited); 82 } 83 order->Add(block); 84} 85 86void SsaLivenessAnalysis::LinearizeGraph() { 87 // For simplicity of the implementation, we create post linear order. The order for 88 // computing live ranges is the reverse of that order. 89 ArenaBitVector visited(graph_.GetArena(), graph_.GetBlocks().Size(), false); 90 VisitBlockForLinearization(graph_.GetEntryBlock(), &linear_post_order_, &visited); 91} 92 93void SsaLivenessAnalysis::NumberInstructions() { 94 int ssa_index = 0; 95 size_t lifetime_position = 0; 96 // Each instruction gets a lifetime position, and a block gets a lifetime 97 // start and end position. Non-phi instructions have a distinct lifetime position than 98 // the block they are in. Phi instructions have the lifetime start of their block as 99 // lifetime position. 100 // 101 // Because the register allocator will insert moves in the graph, we need 102 // to differentiate between the start and end of an instruction. Adding 2 to 103 // the lifetime position for each instruction ensures the start of an 104 // instruction is different than the end of the previous instruction. 105 HGraphVisitor* location_builder = codegen_->GetLocationBuilder(); 106 for (HLinearOrderIterator it(*this); !it.Done(); it.Advance()) { 107 HBasicBlock* block = it.Current(); 108 block->SetLifetimeStart(lifetime_position); 109 110 for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) { 111 HInstruction* current = it.Current(); 112 current->Accept(location_builder); 113 LocationSummary* locations = current->GetLocations(); 114 if (locations != nullptr && locations->Out().IsValid()) { 115 instructions_from_ssa_index_.Add(current); 116 current->SetSsaIndex(ssa_index++); 117 current->SetLiveInterval( 118 new (graph_.GetArena()) LiveInterval(graph_.GetArena(), current->GetType(), current)); 119 } 120 current->SetLifetimePosition(lifetime_position); 121 } 122 lifetime_position += 2; 123 124 // Add a null marker to notify we are starting a block. 125 instructions_from_lifetime_position_.Add(nullptr); 126 127 for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) { 128 HInstruction* current = it.Current(); 129 current->Accept(codegen_->GetLocationBuilder()); 130 LocationSummary* locations = current->GetLocations(); 131 if (locations != nullptr && locations->Out().IsValid()) { 132 instructions_from_ssa_index_.Add(current); 133 current->SetSsaIndex(ssa_index++); 134 current->SetLiveInterval( 135 new (graph_.GetArena()) LiveInterval(graph_.GetArena(), current->GetType(), current)); 136 } 137 instructions_from_lifetime_position_.Add(current); 138 current->SetLifetimePosition(lifetime_position); 139 lifetime_position += 2; 140 } 141 142 block->SetLifetimeEnd(lifetime_position); 143 } 144 number_of_ssa_values_ = ssa_index; 145} 146 147void SsaLivenessAnalysis::ComputeLiveness() { 148 for (HLinearOrderIterator it(*this); !it.Done(); it.Advance()) { 149 HBasicBlock* block = it.Current(); 150 block_infos_.Put( 151 block->GetBlockId(), 152 new (graph_.GetArena()) BlockInfo(graph_.GetArena(), *block, number_of_ssa_values_)); 153 } 154 155 // Compute the live ranges, as well as the initial live_in, live_out, and kill sets. 156 // This method does not handle backward branches for the sets, therefore live_in 157 // and live_out sets are not yet correct. 158 ComputeLiveRanges(); 159 160 // Do a fixed point calculation to take into account backward branches, 161 // that will update live_in of loop headers, and therefore live_out and live_in 162 // of blocks in the loop. 163 ComputeLiveInAndLiveOutSets(); 164} 165 166void SsaLivenessAnalysis::ComputeLiveRanges() { 167 // Do a post order visit, adding inputs of instructions live in the block where 168 // that instruction is defined, and killing instructions that are being visited. 169 for (HLinearPostOrderIterator it(*this); !it.Done(); it.Advance()) { 170 HBasicBlock* block = it.Current(); 171 172 BitVector* kill = GetKillSet(*block); 173 BitVector* live_in = GetLiveInSet(*block); 174 175 // Set phi inputs of successors of this block corresponding to this block 176 // as live_in. 177 for (size_t i = 0, e = block->GetSuccessors().Size(); i < e; ++i) { 178 HBasicBlock* successor = block->GetSuccessors().Get(i); 179 live_in->Union(GetLiveInSet(*successor)); 180 size_t phi_input_index = successor->GetPredecessorIndexOf(block); 181 for (HInstructionIterator it(successor->GetPhis()); !it.Done(); it.Advance()) { 182 HInstruction* phi = it.Current(); 183 HInstruction* input = phi->InputAt(phi_input_index); 184 input->GetLiveInterval()->AddPhiUse(phi, phi_input_index, block); 185 // A phi input whose last user is the phi dies at the end of the predecessor block, 186 // and not at the phi's lifetime position. 187 live_in->SetBit(input->GetSsaIndex()); 188 } 189 } 190 191 // Add a range that covers this block to all instructions live_in because of successors. 192 // Instructions defined in this block will have their start of the range adjusted. 193 for (uint32_t idx : live_in->Indexes()) { 194 HInstruction* current = instructions_from_ssa_index_.Get(idx); 195 current->GetLiveInterval()->AddRange(block->GetLifetimeStart(), block->GetLifetimeEnd()); 196 } 197 198 for (HBackwardInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) { 199 HInstruction* current = it.Current(); 200 if (current->HasSsaIndex()) { 201 // Kill the instruction and shorten its interval. 202 kill->SetBit(current->GetSsaIndex()); 203 live_in->ClearBit(current->GetSsaIndex()); 204 current->GetLiveInterval()->SetFrom(current->GetLifetimePosition()); 205 } 206 207 // All inputs of an instruction must be live. 208 for (size_t i = 0, e = current->InputCount(); i < e; ++i) { 209 HInstruction* input = current->InputAt(i); 210 // Some instructions 'inline' their inputs, that is they do not need 211 // to be materialized. 212 if (input->HasSsaIndex()) { 213 live_in->SetBit(input->GetSsaIndex()); 214 input->GetLiveInterval()->AddUse(current, i, false); 215 } 216 } 217 218 if (current->HasEnvironment()) { 219 // All instructions in the environment must be live. 220 GrowableArray<HInstruction*>* environment = current->GetEnvironment()->GetVRegs(); 221 for (size_t i = 0, e = environment->Size(); i < e; ++i) { 222 HInstruction* instruction = environment->Get(i); 223 if (instruction != nullptr) { 224 DCHECK(instruction->HasSsaIndex()); 225 live_in->SetBit(instruction->GetSsaIndex()); 226 instruction->GetLiveInterval()->AddUse(current, i, true); 227 } 228 } 229 } 230 } 231 232 // Kill phis defined in this block. 233 for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) { 234 HInstruction* current = it.Current(); 235 if (current->HasSsaIndex()) { 236 kill->SetBit(current->GetSsaIndex()); 237 live_in->ClearBit(current->GetSsaIndex()); 238 LiveInterval* interval = current->GetLiveInterval(); 239 DCHECK((interval->GetFirstRange() == nullptr) 240 || (interval->GetStart() == current->GetLifetimePosition())); 241 interval->SetFrom(current->GetLifetimePosition()); 242 } 243 } 244 245 if (block->IsLoopHeader()) { 246 HBasicBlock* back_edge = block->GetLoopInformation()->GetBackEdges().Get(0); 247 // For all live_in instructions at the loop header, we need to create a range 248 // that covers the full loop. 249 for (uint32_t idx : live_in->Indexes()) { 250 HInstruction* current = instructions_from_ssa_index_.Get(idx); 251 current->GetLiveInterval()->AddLoopRange(block->GetLifetimeStart(), 252 back_edge->GetLifetimeEnd()); 253 } 254 } 255 } 256} 257 258void SsaLivenessAnalysis::ComputeLiveInAndLiveOutSets() { 259 bool changed; 260 do { 261 changed = false; 262 263 for (HPostOrderIterator it(graph_); !it.Done(); it.Advance()) { 264 const HBasicBlock& block = *it.Current(); 265 266 // The live_in set depends on the kill set (which does not 267 // change in this loop), and the live_out set. If the live_out 268 // set does not change, there is no need to update the live_in set. 269 if (UpdateLiveOut(block) && UpdateLiveIn(block)) { 270 changed = true; 271 } 272 } 273 } while (changed); 274} 275 276bool SsaLivenessAnalysis::UpdateLiveOut(const HBasicBlock& block) { 277 BitVector* live_out = GetLiveOutSet(block); 278 bool changed = false; 279 // The live_out set of a block is the union of live_in sets of its successors. 280 for (size_t i = 0, e = block.GetSuccessors().Size(); i < e; ++i) { 281 HBasicBlock* successor = block.GetSuccessors().Get(i); 282 if (live_out->Union(GetLiveInSet(*successor))) { 283 changed = true; 284 } 285 } 286 return changed; 287} 288 289 290bool SsaLivenessAnalysis::UpdateLiveIn(const HBasicBlock& block) { 291 BitVector* live_out = GetLiveOutSet(block); 292 BitVector* kill = GetKillSet(block); 293 BitVector* live_in = GetLiveInSet(block); 294 // If live_out is updated (because of backward branches), we need to make 295 // sure instructions in live_out are also in live_in, unless they are killed 296 // by this block. 297 return live_in->UnionIfNotIn(live_out, kill); 298} 299 300int LiveInterval::FindFirstRegisterHint(size_t* free_until) const { 301 if (GetParent() == this && defined_by_ != nullptr) { 302 // This is the first interval for the instruction. Try to find 303 // a register based on its definition. 304 DCHECK_EQ(defined_by_->GetLiveInterval(), this); 305 int hint = FindHintAtDefinition(); 306 if (hint != kNoRegister && free_until[hint] > GetStart()) { 307 return hint; 308 } 309 } 310 311 UsePosition* use = first_use_; 312 size_t start = GetStart(); 313 size_t end = GetEnd(); 314 while (use != nullptr && use->GetPosition() <= end) { 315 size_t use_position = use->GetPosition(); 316 if (use_position >= start && !use->GetIsEnvironment()) { 317 HInstruction* user = use->GetUser(); 318 size_t input_index = use->GetInputIndex(); 319 if (user->IsPhi()) { 320 // If the phi has a register, try to use the same. 321 Location phi_location = user->GetLiveInterval()->ToLocation(); 322 if (SameRegisterKind(phi_location) && free_until[phi_location.reg()] >= use_position) { 323 return phi_location.reg(); 324 } 325 const GrowableArray<HBasicBlock*>& predecessors = user->GetBlock()->GetPredecessors(); 326 // If the instruction dies at the phi assignment, we can try having the 327 // same register. 328 if (end == predecessors.Get(input_index)->GetLifetimeEnd()) { 329 for (size_t i = 0, e = user->InputCount(); i < e; ++i) { 330 if (i == input_index) { 331 continue; 332 } 333 HInstruction* input = user->InputAt(i); 334 Location location = input->GetLiveInterval()->GetLocationAt( 335 predecessors.Get(i)->GetLifetimeEnd() - 1); 336 if (location.IsRegister() && free_until[location.reg()] >= use_position) { 337 return location.reg(); 338 } 339 } 340 } 341 } else { 342 // If the instruction is expected in a register, try to use it. 343 LocationSummary* locations = user->GetLocations(); 344 Location expected = locations->InAt(use->GetInputIndex()); 345 // We use the user's lifetime position - 1 (and not `use_position`) because the 346 // register is blocked at the beginning of the user. 347 size_t position = user->GetLifetimePosition() - 1; 348 if (SameRegisterKind(expected) && free_until[expected.reg()] >= position) { 349 return expected.reg(); 350 } 351 } 352 } 353 use = use->GetNext(); 354 } 355 356 return kNoRegister; 357} 358 359int LiveInterval::FindHintAtDefinition() const { 360 if (defined_by_->IsPhi()) { 361 // Try to use the same register as one of the inputs. 362 const GrowableArray<HBasicBlock*>& predecessors = defined_by_->GetBlock()->GetPredecessors(); 363 for (size_t i = 0, e = defined_by_->InputCount(); i < e; ++i) { 364 HInstruction* input = defined_by_->InputAt(i); 365 size_t end = predecessors.Get(i)->GetLifetimeEnd(); 366 const LiveInterval& input_interval = input->GetLiveInterval()->GetIntervalAt(end - 1); 367 if (input_interval.GetEnd() == end) { 368 // If the input dies at the end of the predecessor, we know its register can 369 // be reused. 370 Location input_location = input_interval.ToLocation(); 371 if (SameRegisterKind(input_location)) { 372 return input_location.reg(); 373 } 374 } 375 } 376 } else { 377 LocationSummary* locations = GetDefinedBy()->GetLocations(); 378 Location out = locations->Out(); 379 if (out.IsUnallocated() && out.GetPolicy() == Location::kSameAsFirstInput) { 380 // Try to use the same register as the first input. 381 const LiveInterval& input_interval = 382 GetDefinedBy()->InputAt(0)->GetLiveInterval()->GetIntervalAt(GetStart() - 1); 383 if (input_interval.GetEnd() == GetStart()) { 384 // If the input dies at the start of this instruction, we know its register can 385 // be reused. 386 Location location = input_interval.ToLocation(); 387 if (SameRegisterKind(location)) { 388 return location.reg(); 389 } 390 } 391 } 392 } 393 return kNoRegister; 394} 395 396bool LiveInterval::SameRegisterKind(Location other) const { 397 return IsFloatingPoint() 398 ? other.IsFpuRegister() 399 : other.IsRegister(); 400} 401 402bool LiveInterval::NeedsTwoSpillSlots() const { 403 return type_ == Primitive::kPrimLong || type_ == Primitive::kPrimDouble; 404} 405 406Location LiveInterval::ToLocation() const { 407 if (HasRegister()) { 408 return IsFloatingPoint() 409 ? Location::FpuRegisterLocation(GetRegister()) 410 : Location::RegisterLocation(GetRegister()); 411 } else { 412 HInstruction* defined_by = GetParent()->GetDefinedBy(); 413 if (defined_by->IsConstant()) { 414 return defined_by->GetLocations()->Out(); 415 } else if (GetParent()->HasSpillSlot()) { 416 if (NeedsTwoSpillSlots()) { 417 return Location::DoubleStackSlot(GetParent()->GetSpillSlot()); 418 } else { 419 return Location::StackSlot(GetParent()->GetSpillSlot()); 420 } 421 } else { 422 return Location(); 423 } 424 } 425} 426 427Location LiveInterval::GetLocationAt(size_t position) const { 428 return GetIntervalAt(position).ToLocation(); 429} 430 431const LiveInterval& LiveInterval::GetIntervalAt(size_t position) const { 432 const LiveInterval* current = this; 433 while (!current->Covers(position)) { 434 current = current->GetNextSibling(); 435 DCHECK(current != nullptr); 436 } 437 return *current; 438} 439 440} // namespace art 441