compiler/optimizing/prepare_for_register_allocation.cc

/*
 * Copyright (C) 2014 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "prepare_for_register_allocation.h"

#include "dex_file_types.h"
#include "jni_internal.h"
#include "optimizing_compiler_stats.h"
#include "well_known_classes.h"

namespace art {

void PrepareForRegisterAllocation::Run() {
  // Order does not matter.
  for (HBasicBlock* block : GetGraph()->GetReversePostOrder()) {
    // No need to visit the phis.
    for (HInstructionIterator inst_it(block->GetInstructions()); !inst_it.Done();
         inst_it.Advance()) {
      inst_it.Current()->Accept(this);
    }
  }
}

void PrepareForRegisterAllocation::VisitNullCheck(HNullCheck* check) {
  check->ReplaceWith(check->InputAt(0));
}

void PrepareForRegisterAllocation::VisitDivZeroCheck(HDivZeroCheck* check) {
  check->ReplaceWith(check->InputAt(0));
}

void PrepareForRegisterAllocation::VisitDeoptimize(HDeoptimize* deoptimize) {
  if (deoptimize->GuardsAnInput()) {
    // Replace the uses with the actual guarded instruction.
    deoptimize->ReplaceWith(deoptimize->GuardedInput());
    deoptimize->RemoveGuard();
  }
}

void PrepareForRegisterAllocation::VisitBoundsCheck(HBoundsCheck* check) {
  check->ReplaceWith(check->InputAt(0));
  if (check->IsStringCharAt()) {
    // Add a fake environment for String.charAt() inline info as we want
    // the exception to appear as being thrown from there.
    ArtMethod* char_at_method = jni::DecodeArtMethod(WellKnownClasses::java_lang_String_charAt);
    ArenaAllocator* arena = GetGraph()->GetArena();
    HEnvironment* environment = new (arena) HEnvironment(arena,
                                                         /* number_of_vregs */ 0u,
                                                         char_at_method,
                                                         /* dex_pc */ dex::kDexNoIndex,
                                                         check);
    check->InsertRawEnvironment(environment);
  }
}

void PrepareForRegisterAllocation::VisitBoundType(HBoundType* bound_type) {
  bound_type->ReplaceWith(bound_type->InputAt(0));
  bound_type->GetBlock()->RemoveInstruction(bound_type);
}

void PrepareForRegisterAllocation::VisitArraySet(HArraySet* instruction) {
  HInstruction* value = instruction->GetValue();
  // PrepareForRegisterAllocation::VisitBoundType may have replaced a
  // BoundType (as value input of this ArraySet) with a NullConstant.
  // If so, this ArraySet no longer needs a type check.
  if (value->IsNullConstant()) {
    DCHECK_EQ(value->GetType(), DataType::Type::kReference);
    if (instruction->NeedsTypeCheck()) {
      instruction->ClearNeedsTypeCheck();
    }
  }
}

void PrepareForRegisterAllocation::VisitClinitCheck(HClinitCheck* check) {
  // Try to find a static invoke or a new-instance from which this check originated.
  HInstruction* implicit_clinit = nullptr;
  for (const HUseListNode<HInstruction*>& use : check->GetUses()) {
    HInstruction* user = use.GetUser();
    if ((user->IsInvokeStaticOrDirect() || user->IsNewInstance()) &&
        CanMoveClinitCheck(check, user)) {
      implicit_clinit = user;
      if (user->IsInvokeStaticOrDirect()) {
        DCHECK(user->AsInvokeStaticOrDirect()->IsStaticWithExplicitClinitCheck());
        user->AsInvokeStaticOrDirect()->RemoveExplicitClinitCheck(
            HInvokeStaticOrDirect::ClinitCheckRequirement::kImplicit);
      } else {
        DCHECK(user->IsNewInstance());
        // We delegate the initialization duty to the allocation.
        if (user->AsNewInstance()->GetEntrypoint() == kQuickAllocObjectInitialized) {
          user->AsNewInstance()->SetEntrypoint(kQuickAllocObjectResolved);
        }
      }
      break;
    }
  }
  // If we found a static invoke or new-instance for merging, remove the check
  // from dominated static invokes.
  if (implicit_clinit != nullptr) {
    const HUseList<HInstruction*>& uses = check->GetUses();
    for (auto it = uses.begin(), end = uses.end(); it != end; /* ++it below */) {
      HInstruction* user = it->GetUser();
      // All other uses must be dominated.
      DCHECK(implicit_clinit->StrictlyDominates(user) || (implicit_clinit == user));
      ++it;  // Advance before we remove the node, reference to the next node is preserved.
      if (user->IsInvokeStaticOrDirect()) {
        user->AsInvokeStaticOrDirect()->RemoveExplicitClinitCheck(
            HInvokeStaticOrDirect::ClinitCheckRequirement::kNone);
      }
    }
  }

  HLoadClass* load_class = check->GetLoadClass();
  bool can_merge_with_load_class = CanMoveClinitCheck(load_class, check);

  check->ReplaceWith(load_class);

  if (implicit_clinit != nullptr) {
    // Remove the check from the graph. It has been merged into the invoke or new-instance.
    check->GetBlock()->RemoveInstruction(check);
    // Check if we can merge the load class as well.
    if (can_merge_with_load_class && !load_class->HasUses()) {
      load_class->GetBlock()->RemoveInstruction(load_class);
    }
  } else if (can_merge_with_load_class && !load_class->NeedsAccessCheck()) {
    // Pass the initialization duty to the `HLoadClass` instruction,
    // and remove the instruction from the graph.
    DCHECK(load_class->HasEnvironment());
    load_class->SetMustGenerateClinitCheck(true);
    check->GetBlock()->RemoveInstruction(check);
  }
}

bool PrepareForRegisterAllocation::CanEmitConditionAt(HCondition* condition,
                                                      HInstruction* user) const {
  if (condition->GetNext() != user) {
    return false;
  }

  if (user->IsIf() || user->IsDeoptimize()) {
    return true;
  }

  if (user->IsSelect() && user->AsSelect()->GetCondition() == condition) {
    return true;
  }

  return false;
}

void PrepareForRegisterAllocation::VisitCondition(HCondition* condition) {
  if (condition->HasOnlyOneNonEnvironmentUse()) {
    HInstruction* user = condition->GetUses().front().GetUser();
    if (CanEmitConditionAt(condition, user)) {
      condition->MarkEmittedAtUseSite();
    }
  }
}

void PrepareForRegisterAllocation::VisitConstructorFence(HConstructorFence* constructor_fence) {
  // Trivially remove redundant HConstructorFence when it immediately follows an HNewInstance
  // to an uninitialized class. In this special case, the art_quick_alloc_object_resolved
  // will already have the 'dmb' which is strictly stronger than an HConstructorFence.
  //
  // The instruction builder always emits "x = HNewInstance; HConstructorFence(x)" so this
  // is effectively pattern-matching that particular case and undoing the redundancy the builder
  // had introduced.
  //
  // TODO: Move this to a separate pass.
  HInstruction* allocation_inst = constructor_fence->GetAssociatedAllocation();
  if (allocation_inst != nullptr && allocation_inst->IsNewInstance()) {
    HNewInstance* new_inst = allocation_inst->AsNewInstance();
    // This relies on the entrypoint already being set to the more optimized version;
    // as that happens in this pass, this redundancy removal also cannot happen any earlier.
    if (new_inst != nullptr && new_inst->GetEntrypoint() == kQuickAllocObjectResolved) {
      // If this was done in an earlier pass, we would want to match that `previous` was an input
      // to the `constructor_fence`. However, since this pass removes the inputs to the fence,
      // we can ignore the inputs and just remove the instruction from its block.
      DCHECK_EQ(1u, constructor_fence->InputCount());
      // TODO: GetAssociatedAllocation should not care about multiple inputs
      // if we are in prepare_for_register_allocation pass only.
      constructor_fence->GetBlock()->RemoveInstruction(constructor_fence);
      MaybeRecordStat(stats_,
                      MethodCompilationStat::kConstructorFenceRemovedPFRA);
      return;
    }

    // HNewArray does not need this check because the art_quick_alloc_array does not itself
    // have a dmb in any normal situation (i.e. the array class is never exactly in the
    // "resolved" state). If the array class is not yet loaded, it will always go from
    // Unloaded->Initialized state.
  }

  // Remove all the inputs to the constructor fence;
  // they aren't used by the InstructionCodeGenerator and this lets us avoid creating a
  // LocationSummary in the LocationsBuilder.
  constructor_fence->RemoveAllInputs();
}

void PrepareForRegisterAllocation::VisitInvokeStaticOrDirect(HInvokeStaticOrDirect* invoke) {
  if (invoke->IsStaticWithExplicitClinitCheck()) {
    HInstruction* last_input = invoke->GetInputs().back();
    DCHECK(last_input->IsLoadClass())
        << "Last input is not HLoadClass. It is " << last_input->DebugName();

    // Detach the explicit class initialization check from the invoke.
    // Keeping track of the initializing instruction is no longer required
    // at this stage (i.e., after inlining has been performed).
    invoke->RemoveExplicitClinitCheck(HInvokeStaticOrDirect::ClinitCheckRequirement::kNone);

    // Merging with load class should have happened in VisitClinitCheck().
    DCHECK(!CanMoveClinitCheck(last_input, invoke));
  }
}

bool PrepareForRegisterAllocation::CanMoveClinitCheck(HInstruction* input,
                                                      HInstruction* user) const {
  // Determine if input and user come from the same dex instruction, so that we can move
  // the clinit check responsibility from one to the other, i.e. from HClinitCheck (user)
  // to HLoadClass (input), or from HClinitCheck (input) to HInvokeStaticOrDirect (user),
  // or from HLoadClass (input) to HNewInstance (user).

  // Start with a quick dex pc check.
  if (user->GetDexPc() != input->GetDexPc()) {
    return false;
  }

  // Now do a thorough environment check that this is really coming from the same instruction in
  // the same inlined graph. Unfortunately, we have to go through the whole environment chain.
  HEnvironment* user_environment = user->GetEnvironment();
  HEnvironment* input_environment = input->GetEnvironment();
  while (user_environment != nullptr || input_environment != nullptr) {
    if (user_environment == nullptr || input_environment == nullptr) {
      // Different environment chain length. This happens when a method is called
      // once directly and once indirectly through another inlined method.
      return false;
    }
    if (user_environment->GetDexPc() != input_environment->GetDexPc() ||
        user_environment->GetMethod() != input_environment->GetMethod()) {
      return false;
    }
    user_environment = user_environment->GetParent();
    input_environment = input_environment->GetParent();
  }

  // Check for code motion taking the input to a different block.
  if (user->GetBlock() != input->GetBlock()) {
    return false;
  }

  // In debug mode, check that we have not inserted a throwing instruction
  // or an instruction with side effects between input and user.
  if (kIsDebugBuild) {
    for (HInstruction* between = input->GetNext(); between != user; between = between->GetNext()) {
      CHECK(between != nullptr);  // User must be after input in the same block.
      CHECK(!between->CanThrow());
      CHECK(!between->HasSideEffects());
    }
  }
  return true;
}

}  // namespace art