Experimental: add a pre-merge step to perform some frame moves that do...

src/virtual-frame.cc

 // Copyright 2009 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 211 matching lines @@
 
 
 // Make the type of all elements be MEMORY.
 void VirtualFrame::SpillAll() {
   for (int i = 0; i < elements_.length(); i++) {
     SpillElementAt(i);
   }
 }
 
 
+void VirtualFrame::PrepareMergeTo(VirtualFrame* expected) {
+  // No code needs to be generated to invalidate valid elements.  No
+  // code needs to be generated to move values to memory if they are
+  // already synced.
+  for (int i = 0; i < elements_.length(); i++) {
+    FrameElement source = elements_[i];
+    FrameElement target = expected->elements_[i];
+    if (!target.is_valid() ||
+        (target.is_memory() && !source.is_memory() && source.is_synced())) {
+      if (source.is_register()) {
+        // If the frame is the code generator's current frame, we have
+        // to decrement both the frame-internal and global register
+        // counts.
+        if (cgen_->frame() == this) {
+          Unuse(source.reg());
+        } else {
+          frame_registers_.Unuse(source.reg());
+        }
+      }
+      elements_[i] = target;
+    }
+  }
+}
+
+
 void VirtualFrame::PrepareForCall(int spilled_args, int dropped_args) {
   ASSERT(height() >= dropped_args);
   ASSERT(height() >= spilled_args);
   ASSERT(dropped_args <= spilled_args);
 
   int arg_base_index = elements_.length() - spilled_args;
   // Spill the arguments.  We spill from the top down so that the
   // backing stores of register copies will be spilled only after all
   // the copies are spilled---it is better to spill via a
   // register-to-memory move than a memory-to-memory move.
@@ skipping 266 matching lines @@
   for (int i = 0; i < kNumRegisters; i++) {
     if (frame_registers_.count(i) != other->frame_registers_.count(i)) {
       return false;
     }
   }
 
   return true;
 }
 
 } }  // namespace v8::internal