Remove dead constants not in block #0.

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 79 matching lines @@
 
 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);
+        // Instruction has not been marked live, so remove it.
+        if (!instr->IsConstant() || instr->block()->block_id() != 0) {
+          // TODO(titzer): Some global constants in block 0 can be used
+          // again later, and can't currently be removed. Fix that.
+          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);
     if (phi->HasMergedIndex()) {
       block->RecordDeletedPhi(phi->merged_index());
     }
   }
 }
 
 } }  // namespace v8::internal