Improve and simplify removal of unreachable code

src/hydrogen-deoptimizing-mark.cc

-// Copyright 2013 the V8 project authors. All rights reserved.
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are
-// met:
-//
-//     * Redistributions of source code must retain the above copyright
-//       notice, this list of conditions and the following disclaimer.
-//     * Redistributions in binary form must reproduce the above
-//       copyright notice, this list of conditions and the following
-//       disclaimer in the documentation and/or other materials provided
-//       with the distribution.
-//     * Neither the name of Google Inc. nor the names of its
-//       contributors may be used to endorse or promote products derived
-//       from this software without specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
-#include "hydrogen-deoptimizing-mark.h"
-
-namespace v8 {
-namespace internal {
-
-void HPropagateDeoptimizingMarkPhase::MarkAsDeoptimizing() {
-  HBasicBlock* block = graph()->entry_block();
-  ZoneList<HBasicBlock*> stack(graph()->blocks()->length(), zone());
-  while (block != NULL) {
-    const ZoneList<HBasicBlock*>* dominated_blocks(block->dominated_blocks());
-    if (!dominated_blocks->is_empty()) {
-      if (block->IsDeoptimizing()) {
-        for (int i = 0; i < dominated_blocks->length(); ++i) {
-          dominated_blocks->at(i)->MarkAsDeoptimizing();
-        }
-      }
-      for (int i = 1; i < dominated_blocks->length(); ++i) {
-        stack.Add(dominated_blocks->at(i), zone());
-      }
-      block = dominated_blocks->at(0);
-    } else if (!stack.is_empty()) {
-      // Pop next block from stack.
-      block = stack.RemoveLast();
-    } else {
-      // All blocks processed.
-      block = NULL;
-    }
-  }
-}
-
-
-void HPropagateDeoptimizingMarkPhase::NullifyUnreachableInstructions() {
-  if (!FLAG_unreachable_code_elimination) return;
-  for (int i = 0; i < graph()->blocks()->length(); ++i) {
-    HBasicBlock* block = graph()->blocks()->at(i);
-    bool nullify = false;
-    const ZoneList<HBasicBlock*>* predecessors = block->predecessors();
-    int predecessors_length = predecessors->length();
-    bool all_predecessors_deoptimizing = (predecessors_length > 0);
-    for (int j = 0; j < predecessors_length; ++j) {
-      if (!predecessors->at(j)->IsDeoptimizing()) {
-        all_predecessors_deoptimizing = false;
-        break;
-      }
-    }
-    if (all_predecessors_deoptimizing) nullify = true;
-    for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
-      HInstruction* instr = it.Current();
-      // Leave the basic structure of the graph intact.
-      if (instr->IsBlockEntry()) continue;
-      if (instr->IsControlInstruction()) continue;
-      if (instr->IsSimulate()) continue;
-      if (instr->IsEnterInlined()) continue;
-      if (instr->IsLeaveInlined()) continue;
-      if (nullify) {
-        HInstruction* last_dummy = NULL;
-        for (int j = 0; j < instr->OperandCount(); ++j) {
-          HValue* operand = instr->OperandAt(j);
-          // Insert an HDummyUse for each operand, unless the operand
-          // is an HDummyUse itself. If it's even from the same block,
-          // remember it as a potential replacement for the instruction.
-          if (operand->IsDummyUse()) {
-            if (operand->block() == instr->block() &&
-                last_dummy == NULL) {
-              last_dummy = HInstruction::cast(operand);
-            }
-            continue;
-          }
-          if (operand->IsControlInstruction()) {
-            // Inserting a dummy use for a value that's not defined anywhere
-            // will fail. Some instructions define fake inputs on such
-            // values as control flow dependencies.
-            continue;
-          }
-          HDummyUse* dummy = new(graph()->zone()) HDummyUse(operand);
-          dummy->InsertBefore(instr);
-          last_dummy = dummy;
-        }
-        if (last_dummy == NULL) last_dummy = graph()->GetConstant1();
-        instr->DeleteAndReplaceWith(last_dummy);
-        continue;
-      }
-      if (instr->IsDeoptimize()) {
-        ASSERT(block->IsDeoptimizing());
-        nullify = true;
-      }
-    }
-  }
-}
-
-
-void HPropagateDeoptimizingMarkPhase::Run() {
-  // Skip this phase if there is nothing to be done anyway.
-  if (!graph()->has_soft_deoptimize()) return;
-  MarkAsDeoptimizing();
-  NullifyUnreachableInstructions();
-}
-
-} }  // namespace v8::internal