- Add a size field to the header.

vm/intrinsifier_ia32.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // The intrinsic code below is executed before a method has built its frame.
 // The return address is on the stack and the arguments below it.
 // Registers EDX (arguments descriptor) and ECX (function) must be preserved.
 // Each intrinsification method returns true if the corresponding
 // Dart method was intrinsified.
 
@@ skipping 80 matching lines @@
   const intptr_t kTypeArgumentsOffset = 2 * kWordSize;
   const intptr_t kArrayLengthOffset = 1 * kWordSize;
   Label fall_through;
 
   // Compute the size to be allocated, it is based on the array length
   // and it computed as:
   // RoundedAllocationSize((array_length * kwordSize) + sizeof(RawArray)).
   __ movl(EDI, Address(ESP, kArrayLengthOffset));  // Array Length.
   // Assert that length is a Smi.
   __ testl(EDI, Immediate(kSmiTagSize));
-  __ j(NOT_ZERO, &fall_through, Assembler::kNearJump);
+  __ j(NOT_ZERO, &fall_through);
   intptr_t fixed_size = sizeof(RawArray) + kObjectAlignment - 1;
   __ leal(EDI, Address(EDI, TIMES_2, fixed_size));  // EDI is a Smi.
   ASSERT(kSmiTagShift == 1);
   __ andl(EDI, Immediate(-kObjectAlignment));
 
   Heap* heap = Isolate::Current()->heap();
 
   // EDI: size to allocate.
   __ movl(EAX, Address::Absolute(heap->TopAddress()));
   __ leal(EBX, Address(EAX, EDI, TIMES_1, 0));
 
   // Check if the allocation fits into the remaining space.
   // EAX: potential new object start.
   // EBX: potential next object start.
   __ cmpl(EBX, Address::Absolute(heap->EndAddress()));
-  __ j(ABOVE_EQUAL, &fall_through, Assembler::kNearJump);
+  __ j(ABOVE_EQUAL, &fall_through);
 
   // Successfully allocated the object(s), now update top to point to
   // next object start and initialize the object.
   __ movl(Address::Absolute(heap->TopAddress()), EBX);
   __ addl(EAX, Immediate(kHeapObjectTag));
 
   // EAX: new object start as a tagged pointer.
   // EBX: new object end address.
   // Store class value for array.
   __ movl(EDI, FieldAddress(CTX, Context::isolate_offset()));
   __ movl(EDI, Address(EDI, Isolate::object_store_offset()));
   __ movl(EDI, Address(EDI, ObjectStore::array_class_offset()));
   __ StoreIntoObject(EAX, FieldAddress(EAX, Array::class_offset()), EDI);
 
   // Initialize the tags.
-  __ movl(FieldAddress(EAX, Array::tags_offset()), Immediate(0));
+  // EAX: new object start as a tagged pointer.
+  // EBX: new object end address.
+  {
+    Label size_tag_overflow, done;
+    __ movl(EDI, Address(ESP, kArrayLengthOffset));  // Array length.
+    __ leal(EDI, Address(EDI, TIMES_2, fixed_size));  // EDX is Smi.

[siva, 2012/01/04 01:56:03]

EDI is Smi?

Also if you moved this block of code to initialize tags above the section that
stores the class value, EDI would not have been clobbered and you could save on
these two instructions which are setting up EDI again. 

[Ivan Posva, 2012/01/04 07:59:42]

Rearranged setting of class and tags to avoid recalculating allocation size.

+    ASSERT(kSmiTagShift == 1);
+    __ andl(EDI, Immediate(-kObjectAlignment));
+    __ cmpl(EDI,
+            Immediate(RawObject::SizeTag::kMaxSizeTag));

[siva, 2012/01/04 01:56:03]

Can this fit in one line?

[Ivan Posva, 2012/01/04 07:59:42]

Done.

+    __ j(ABOVE, &size_tag_overflow, Assembler::kNearJump);
+    __ shll(EDI, Immediate(RawObject::kSizeTagBit - kObjectAlignmentLog2));
+    __ movl(FieldAddress(EAX, Array::tags_offset()), EDI);  // Tags.
+    __ jmp(&done);
+
+    __ Bind(&size_tag_overflow);
+    __ movl(FieldAddress(EAX, Array::tags_offset()), Immediate(0));
+    __ Bind(&done);
+  }
 
   // Store the type argument field.
   __ movl(EDI, Address(ESP, kTypeArgumentsOffset));  // type argument.
   __ StoreIntoObject(EAX,
                      FieldAddress(EAX, Array::type_arguments_offset()),
                      EDI);
 
   // Set the length field.
   __ movl(EDI, Address(ESP, kArrayLengthOffset));  // Array Length.
   __ StoreIntoObject(EAX, FieldAddress(EAX, Array::length_offset()), EDI);
@@ skipping 901 matching lines @@
   }                                                                            \
 
 INTRINSIC_LIST(FIND_INTRINSICS);
 #undef FIND_INTRINSICS
   return false;
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32