Re-use phi reachability information to avoid a fixed-point computation.

src/hydrogen.cc

 // Copyright 2011 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 1672 matching lines @@
         HValue* use = it.value();
         if (use->IsPhi()) {
           int id = HPhi::cast(use)->phi_id();
           if (connected_phis[i]->UnionIsChanged(*connected_phis[id]))
             change = true;
         }
       }
     }
   }
 
-  // (3) Sum up the non-phi use counts of all connected phis.  Don't include
-  // the non-phi uses of the phi itself.
+  // (3) Use the phi reachability information from step 2 to
+  //     (a) sum up the non-phi use counts of all connected phis.
+  //     (b) push information about values which can't be converted to integer
+  //         without deoptimization through the phi use-def chains, avoiding
+  //         unnecessary deoptimizations later.
   for (int i = 0; i < phi_count; ++i) {
     HPhi* phi = phi_list->at(i);
+    bool cti = phi->AllOperandsConvertibleToInteger();
     for (BitVector::Iterator it(connected_phis.at(i));
          !it.Done();
          it.Advance()) {
       int index = it.Current();
-      if (index != i) {
-        HPhi* it_use = phi_list->at(it.Current());
-        phi->AddNonPhiUsesFrom(it_use);
-      }
+      HPhi* it_use = phi_list->at(it.Current());
+      if (index != i) phi->AddNonPhiUsesFrom(it_use);  // Don't count twice!
+      if (!cti) it_use->set_is_convertible_to_integer(false);
     }
   }
 
-  // (4) Compute phis that definitely can't be converted to integer
-  // without deoptimization and mark them to avoid unnecessary deoptimization.
-  change = true;
-  while (change) {
-    change = false;
-    for (int i = 0; i < phi_count; ++i) {
-      HPhi* phi = phi_list->at(i);
-      for (int j = 0; j < phi->OperandCount(); ++j) {
-        if (phi->IsConvertibleToInteger() &&
-            !phi->OperandAt(j)->IsConvertibleToInteger()) {
-          phi->set_is_convertible_to_integer(false);
-          change = true;
-          break;
-        }
-      }
-    }
-  }
-
-
+  // Initialize work list
   for (int i = 0; i < graph_->blocks()->length(); ++i) {
     HBasicBlock* block = graph_->blocks()->at(i);
     const ZoneList<HPhi*>* phis = block->phis();
     for (int j = 0; j < phis->length(); ++j) {
       AddToWorklist(phis->at(j));
     }
 
     HInstruction* current = block->first();
     while (current != NULL) {
       AddToWorklist(current);
       current = current->next();
     }
   }
 
+  // Do a fixed point iteration, trying to improve representations
   while (!worklist_.is_empty()) {
     HValue* current = worklist_.RemoveLast();
     in_worklist_.Remove(current->id());
     InferBasedOnInputs(current);
     InferBasedOnUses(current);
   }
 }
 
 
 void HGraph::InitializeInferredTypes() {
@@ skipping 5059 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify();
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal