Don't reference transitioned elements in loop pre header

src/hydrogen.cc

Index: src/hydrogen.cc
===================================================================
--- src/hydrogen.cc	(revision 11825)
+++ src/hydrogen.cc	(working copy)
@@ -1512,6 +1512,7 @@
                         GVNFlagSet loop_kills,
                         GVNFlagSet* accumulated_first_time_depends,
                         GVNFlagSet* accumulated_first_time_changes);
+  void PostProcessLoopPreHeader(HBasicBlock* pre_header);
   bool AllowCodeMotion();
   bool ShouldMove(HInstruction* instr, HBasicBlock* loop_header);
 
@@ -1691,6 +1692,12 @@
       }
     }
   }
+  for (int i = graph_->blocks()->length() - 1; i >= 0; --i) {
+    HBasicBlock* block = graph_->blocks()->at(i);
+    if (block->IsLoopHeader()) {
+      PostProcessLoopPreHeader(block->predecessors()->at(0));
+    }
+  }
 }
 
 
@@ -1801,7 +1808,27 @@
   }
 }
 
+void HGlobalValueNumberer::PostProcessLoopPreHeader(HBasicBlock* pre_header) {
+  HInstruction* instr = pre_header->first();
+  while (instr != NULL) {
+    HInstruction* next = instr->next();
+    if (instr->IsTransitionElementsKind()) {
+      HValue* old_elems = instr->OperandAt(0), *new_elems = instr;
+      HInstruction* current = next;
+      while (current != NULL) {
+        for (int i = 0; i < current->OperandCount(); ++i) {
+          if (current->OperandAt(i) == old_elems) {
+            current->SetOperandAt(i, new_elems);
+          }
+        }
+        current = current->next();
+      }
+    }

[danno, 2012/06/19 15:45:05]

This is potentially a n^2 algorithm with respect to the number of instructions
in the pre header. This is not the right way to do this.

+    instr = next;
+  }
+}
 
+
 bool HGlobalValueNumberer::AllowCodeMotion() {
   return info()->shared_info()->opt_count() + 1 < Compiler::kDefaultMaxOptCount;
 }