Turn dead code elimination into a proper HPhase.

src/hydrogen-dce.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-dce.h"
+#include "v8.h"
+
+namespace v8 {
+namespace internal {
+
+bool HDeadCodeEliminationPhase::MarkLive(HValue* ref, HValue* instr) {
+  if (instr->CheckFlag(HValue::kIsLive)) return false;
+  instr->SetFlag(HValue::kIsLive);
+
+  if (FLAG_trace_dead_code_elimination) {
+    HeapStringAllocator allocator;
+    StringStream stream(&allocator);
+    if (ref != NULL) {
+      ref->PrintTo(&stream);
+    } else {
+      stream.Add("root ");
+    }
+    stream.Add(" -> ");
+    instr->PrintTo(&stream);
+    PrintF("[MarkLive %s]\n", *stream.ToCString());
+  }
+
+  return true;
+}
+
+
+void HDeadCodeEliminationPhase::MarkLiveInstructions() {
+  ZoneList<HValue*> worklist(graph()->blocks()->length(), zone());
+
+  // Mark initial root instructions for dead code elimination.
+  for (int i = 0; i < graph()->blocks()->length(); ++i) {
+    HBasicBlock* block = graph()->blocks()->at(i);
+    for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
+      HInstruction* instr = it.Current();
+      if (instr->CannotBeEliminated() && MarkLive(NULL, instr)) {
+        worklist.Add(instr, zone());
+      }
+    }
+    for (int j = 0; j < block->phis()->length(); j++) {
+      HPhi* phi = block->phis()->at(j);
+      if (phi->CannotBeEliminated() && MarkLive(NULL, phi)) {
+        worklist.Add(phi, zone());
+      }
+    }
+  }
+
+  // Transitively mark all inputs of live instructions live.
+  while (!worklist.is_empty()) {
+    HValue* instr = worklist.RemoveLast();
+    for (int i = 0; i < instr->OperandCount(); ++i) {
+      if (MarkLive(instr, instr->OperandAt(i))) {
+        worklist.Add(instr->OperandAt(i), zone());
+      }
+    }
+  }
+}
+
+
+void HDeadCodeEliminationPhase::RemoveDeadInstructions() {
+  ZoneList<HPhi*> worklist(graph()->blocks()->length(), zone());
+
+  // Remove any instruction not marked kIsLive.
+  for (int i = 0; i < graph()->blocks()->length(); ++i) {
+    HBasicBlock* block = graph()->blocks()->at(i);
+    for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
+      HInstruction* instr = it.Current();
+      if (!instr->CheckFlag(HValue::kIsLive)) {
+        // Instruction has not been marked live; assume it is dead and remove.
+        // TODO(titzer): we don't remove constants because some special ones
+        // might be used by later phases and are assumed to be in the graph
+        if (!instr->IsConstant()) instr->DeleteAndReplaceWith(NULL);
+      } else {
+        // Clear the liveness flag to leave the graph clean for the next DCE.
+        instr->ClearFlag(HValue::kIsLive);
+      }
+    }
+    // Collect phis that are dead and remove them in the next pass.
+    for (int j = 0; j < block->phis()->length(); j++) {
+      HPhi* phi = block->phis()->at(j);
+      if (!phi->CheckFlag(HValue::kIsLive)) {
+        worklist.Add(phi, zone());
+      } else {
+        phi->ClearFlag(HValue::kIsLive);
+      }
+    }
+  }
+
+  // Process phis separately to avoid simultaneously mutating the phi list.
+  while (!worklist.is_empty()) {
+    HPhi* phi = worklist.RemoveLast();
+    HBasicBlock* block = phi->block();
+    phi->DeleteAndReplaceWith(NULL);
+    block->RecordDeletedPhi(phi->merged_index());
+  }
+}
+
+} }  // namespace v8::internal