VM: Fix allocation size stats for contexts on ia32 and x64.

runtime/vm/stub_code_x64.cc

 // Copyright (c) 2013, 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.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/compiler.h"
 #include "vm/dart_entry.h"
@@ skipping 896 matching lines @@
       __ j(ABOVE_EQUAL, &slow_case);
     }
 
     // Successfully allocated the object, now update top to point to
     // next object start and initialize the object.
     // RAX: new object.
     // R13: next object start.
     // R10: number of context variables.
     // RCX: heap.
     __ movq(Address(RCX, Heap::TopOffset(space)), R13);
-    __ addq(RAX, Immediate(kHeapObjectTag));
     // R13: Size of allocation in bytes.
     __ subq(R13, RAX);
+    __ addq(RAX, Immediate(kHeapObjectTag));
     // Generate isolate-independent code to allow sharing between isolates.
     __ UpdateAllocationStatsWithSize(cid, R13, space,
                                      /* inline_isolate */ false);
 
     // Calculate the size tag.
     // RAX: new object.
     // R10: number of context variables.
     {
       Label size_tag_overflow, done;
       __ leaq(R13, Address(R10, TIMES_8, fixed_size));
@@ skipping 1254 matching lines @@
 // Result:
 //  RCX: entry point.
 void StubCode::GenerateMegamorphicLookupStub(Assembler* assembler) {
   EmitMegamorphicLookup(assembler, RDI, RBX, RCX);
   __ ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64