CHA guard optimization (elimination/hoisting).

Test: manual by checking the dump-cfg output.

Change-Id: I254e168b9a85d2d3d23e02eea7e129c1bc9ab920

1 files changed, 253 insertions, 0 deletions

diff --git a/compiler/optimizing/cha_guard_optimization.cc b/compiler/optimizing/cha_guard_optimization.cc
new file mode 100644
index 0000000000..fe423012ca
--- /dev/null
+++ b/compiler/optimizing/cha_guard_optimization.cc
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+/*
+ * Copyright (C) 2016 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 "cha_guard_optimization.h"
+
+namespace art {
+
+// Note we can only do CHA guard elimination/motion in a single pass, since
+// if a guard is not removed, another guard might be removed due to
+// the existence of the first guard. The first guard should not be further
+// removed in another pass. For example, due to further optimizations,
+// a receiver of a guard might turn out to be a parameter value, or defined at
+// a different site, which makes the guard removable as a result. However
+// it's not safe to remove the guard in another pass since another guard might
+// have been removed due to the existence of this guard.
+//
+// As a consequence, we decided not to rely on other passes to remove them
+// (such as GVN or instruction simplifier).
+
+class CHAGuardVisitor : HGraphVisitor {
+ public:
+  explicit CHAGuardVisitor(HGraph* graph)
+      : HGraphVisitor(graph),
+        block_has_cha_guard_(GetGraph()->GetBlocks().size(),
+                             0,
+                             graph->GetArena()->Adapter(kArenaAllocCHA)) {
+    number_of_guards_to_visit_ = GetGraph()->GetNumberOfCHAGuards();
+    DCHECK_NE(number_of_guards_to_visit_, 0u);
+    // Will recount number of guards during guard optimization.
+    GetGraph()->SetNumberOfCHAGuards(0);
+  }
+
+  void VisitShouldDeoptimizeFlag(HShouldDeoptimizeFlag* flag) OVERRIDE;
+
+  void VisitBasicBlock(HBasicBlock* block) OVERRIDE;
+
+ private:
+  void RemoveGuard(HShouldDeoptimizeFlag* flag);
+  // Return true if `flag` is removed.
+  bool OptimizeForParameter(HShouldDeoptimizeFlag* flag, HInstruction* receiver);
+  // Return true if `flag` is removed.
+  bool OptimizeWithDominatingGuard(HShouldDeoptimizeFlag* flag, HInstruction* receiver);
+  // Return true if `flag` is hoisted.
+  bool HoistGuard(HShouldDeoptimizeFlag* flag, HInstruction* receiver);
+
+  // Record if each block has any CHA guard. It's updated during the
+  // reverse post order visit. Use int instead of bool since ArenaVector
+  // does not support bool.
+  ArenaVector<int> block_has_cha_guard_;
+
+  // The iterator that's being used for this visitor. Need it to manually
+  // advance the iterator due to removing/moving more than one instruction.
+  HInstructionIterator* instruction_iterator_;
+
+  // Used to short-circuit the pass when there is no more guards left to visit.
+  uint32_t number_of_guards_to_visit_;
+
+  DISALLOW_COPY_AND_ASSIGN(CHAGuardVisitor);
+};
+
+void CHAGuardVisitor::VisitBasicBlock(HBasicBlock* block) {
+  if (number_of_guards_to_visit_ == 0) {
+    return;
+  }
+  // Skip phis, just iterate through instructions.
+  HInstructionIterator it(block->GetInstructions());
+  instruction_iterator_ = &it;
+  for (; !it.Done(); it.Advance()) {
+    DCHECK(it.Current()->IsInBlock());
+    it.Current()->Accept(this);
+  }
+}
+
+void CHAGuardVisitor::RemoveGuard(HShouldDeoptimizeFlag* flag) {
+  HBasicBlock* block = flag->GetBlock();
+  HInstruction* compare = flag->GetNext();
+  DCHECK(compare->IsNotEqual());
+  HInstruction* deopt = compare->GetNext();
+  DCHECK(deopt->IsDeoptimize());
+
+  // Advance instruction iterator first before we remove the guard.
+  // We need to do it twice since we remove three instructions and the
+  // visitor is responsible for advancing it once.
+  instruction_iterator_->Advance();
+  instruction_iterator_->Advance();
+  block->RemoveInstruction(deopt);
+  block->RemoveInstruction(compare);
+  block->RemoveInstruction(flag);
+}
+
+bool CHAGuardVisitor::OptimizeForParameter(HShouldDeoptimizeFlag* flag,
+                                           HInstruction* receiver) {
+  // If some compiled code is invalidated by CHA due to class loading, the
+  // compiled code will not be entered anymore. So the very fact that the
+  // compiled code is invoked guarantees that a parameter receiver conforms
+  // to all the CHA devirtualization assumptions made by the compiled code,
+  // since all parameter receivers pre-exist any (potential) invalidation of
+  // the compiled code.
+  //
+  // TODO: allow more cases such as a phi whose inputs are all parameters.
+  if (receiver->IsParameterValue()) {
+    RemoveGuard(flag);
+    return true;
+  }
+  return false;
+}
+
+bool CHAGuardVisitor::OptimizeWithDominatingGuard(HShouldDeoptimizeFlag* flag,
+                                                  HInstruction* receiver) {
+  // If there is another guard that dominates the current guard, and
+  // that guard is dominated by receiver's definition, then the current
+  // guard can be eliminated, since receiver must pre-exist that other
+  // guard, and passing that guard guarantees that receiver conforms to
+  // all the CHA devirtualization assumptions.
+  HBasicBlock* dominator = flag->GetBlock();
+  HBasicBlock* receiver_def_block = receiver->GetBlock();
+
+  // Complexity of the following algorithm:
+  // We potentially need to traverse the full dominator chain to receiver_def_block,
+  // plus a (partial) linear search within one block for each guard.
+  // So the worst case for each guard is bounded by the size of the
+  // biggest block plus the depth of the dominating tree.
+
+  while (dominator != receiver_def_block) {
+    if (block_has_cha_guard_[dominator->GetBlockId()] == 1) {
+      RemoveGuard(flag);
+      return true;
+    }
+    dominator = dominator->GetDominator();
+  }
+
+  // At this point dominator is the block where receiver is defined.
+  // We do a linear search within dominator to see if there is a guard after
+  // receiver's definition.
+  HInstruction* instruction;
+  if (dominator == flag->GetBlock()) {
+    // Flag and receiver are defined in the same block. Search backward from
+    // the current guard.
+    instruction = flag->GetPrevious();
+  } else {
+    // Search backward from the last instruction of that dominator.
+    instruction = dominator->GetLastInstruction();
+  }
+  while (instruction != receiver) {
+    if (instruction == nullptr) {
+      // receiver must be defined in this block, we didn't find it
+      // in the instruction list, so it must be a Phi.
+      DCHECK(receiver->IsPhi());
+      break;
+    }
+    if (instruction->IsShouldDeoptimizeFlag()) {
+      RemoveGuard(flag);
+      return true;
+    }
+    instruction = instruction->GetPrevious();
+  }
+  return false;
+}
+
+bool CHAGuardVisitor::HoistGuard(HShouldDeoptimizeFlag* flag,
+                                 HInstruction* receiver) {
+  // If receiver is loop invariant, we can hoist the guard out of the
+  // loop since passing a guard before entering the loop guarantees that
+  // receiver conforms to all the CHA devirtualization assumptions.
+  // We only hoist guards out of the inner loop since that offers most of the
+  // benefit and it might help remove other guards in the inner loop.
+  HBasicBlock* block = flag->GetBlock();
+  HLoopInformation* loop_info = block->GetLoopInformation();
+  if (loop_info != nullptr &&
+      !loop_info->IsIrreducible() &&
+      loop_info->IsDefinedOutOfTheLoop(receiver)) {
+    HInstruction* compare = flag->GetNext();
+    DCHECK(compare->IsNotEqual());
+    HInstruction* deopt = compare->GetNext();
+    DCHECK(deopt->IsDeoptimize());
+
+    // Advance instruction iterator first before we move the guard.
+    // We need to do it twice since we move three instructions and the
+    // visitor is responsible for advancing it once.
+    instruction_iterator_->Advance();
+    instruction_iterator_->Advance();
+
+    HBasicBlock* pre_header = loop_info->GetPreHeader();
+    flag->MoveBefore(pre_header->GetLastInstruction());
+    compare->MoveBefore(pre_header->GetLastInstruction());
+
+    block->RemoveInstruction(deopt);
+    HInstruction* suspend = loop_info->GetSuspendCheck();
+    // Need a new deoptimize instruction that copies the environment
+    // of the suspend instruction for the loop.
+    HDeoptimize* deoptimize =
+        new (GetGraph()->GetArena()) HDeoptimize(compare, suspend->GetDexPc());
+    pre_header->InsertInstructionBefore(deoptimize, pre_header->GetLastInstruction());
+    deoptimize->CopyEnvironmentFromWithLoopPhiAdjustment(
+        suspend->GetEnvironment(), loop_info->GetHeader());
+    block_has_cha_guard_[pre_header->GetBlockId()] = 1;
+    GetGraph()->IncrementNumberOfCHAGuards();
+    return true;
+  }
+  return false;
+}
+
+void CHAGuardVisitor::VisitShouldDeoptimizeFlag(HShouldDeoptimizeFlag* flag) {
+  number_of_guards_to_visit_--;
+  HInstruction* receiver = flag->InputAt(0);
+  // Don't need the receiver anymore.
+  flag->RemoveInputAt(0);
+  if (receiver->IsNullCheck()) {
+    receiver = receiver->InputAt(0);
+  }
+
+  if (OptimizeForParameter(flag, receiver)) {
+    DCHECK(!flag->IsInBlock());
+    return;
+  }
+  if (OptimizeWithDominatingGuard(flag, receiver)) {
+    DCHECK(!flag->IsInBlock());
+    return;
+  }
+  if (HoistGuard(flag, receiver)) {
+    DCHECK(flag->IsInBlock());
+    return;
+  }
+
+  // Need to keep the CHA guard in place.
+  block_has_cha_guard_[flag->GetBlock()->GetBlockId()] = 1;
+  GetGraph()->IncrementNumberOfCHAGuards();
+}
+
+void CHAGuardOptimization::Run() {
+  if (graph_->GetNumberOfCHAGuards() == 0) {
+    return;
+  }
+  CHAGuardVisitor visitor(graph_);
+  for (HBasicBlock* block : graph_->GetReversePostOrder()) {
+    visitor.VisitBasicBlock(block);
+  }
+}
+
+}  // namespace art