Reduce queue size in dead code elimination by eagerly processing live instructions.

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
@@ skipping 13 matching lines @@
 // 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);
+void HDeadCodeEliminationPhase::MarkLive(
+    HValue* instr, ZoneList<HValue*>* worklist) {
+  if (instr->CheckFlag(HValue::kIsLive)) return;  // Already live.
 
-  if (FLAG_trace_dead_code_elimination) {
-    HeapStringAllocator allocator;
-    StringStream stream(&allocator);
-    if (ref != NULL) {
-      ref->PrintTo(&stream);
-    } else {
-      stream.Add("root ");
+  if (FLAG_trace_dead_code_elimination) PrintLive(NULL, instr);
+
+  // Transitively mark all inputs of live instructions live.
+  worklist->Add(instr, zone());
+  while (!worklist->is_empty()) {
+    HValue* instr = worklist->RemoveLast();
+    instr->SetFlag(HValue::kIsLive);
+    for (int i = 0; i < instr->OperandCount(); ++i) {
+      HValue* input = instr->OperandAt(i);
+      if (!input->CheckFlag(HValue::kIsLive)) {
+        input->SetFlag(HValue::kIsLive);
+        worklist->Add(input, zone());
+        if (FLAG_trace_dead_code_elimination) PrintLive(instr, input);
+      }
     }
-    stream.Add(" -> ");
-    instr->PrintTo(&stream);
-    PrintF("[MarkLive %s]\n", *stream.ToCString());
   }
+}
 
-  return true;
+
+void HDeadCodeEliminationPhase::PrintLive(HValue* ref, HValue* instr) {
+  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());
 }
 
 
 void HDeadCodeEliminationPhase::MarkLiveInstructions() {
   ZoneList<HValue*> worklist(graph()->blocks()->length(), zone());
 
-  // Mark initial root instructions for dead code elimination.
+  // Transitively mark all live instructions, starting from roots.
   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());
-      }
+      if (instr->CannotBeEliminated()) MarkLive(instr, &worklist);
     }
     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());
-      }
+      if (phi->CannotBeEliminated()) MarkLive(phi, &worklist);
     }
   }
 
-  // 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());
-      }
-    }
-  }
+  ASSERT(worklist.is_empty());  // Should have processed everything.
 }
 
 
 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()) {
@@ skipping 24 matching lines @@
     HPhi* phi = worklist.RemoveLast();
     HBasicBlock* block = phi->block();
     phi->DeleteAndReplaceWith(NULL);
     if (phi->HasMergedIndex()) {
       block->RecordDeletedPhi(phi->merged_index());
     }
   }
 }
 
 } }  // namespace v8::internal