Shorter TryAllocate instruction sequence on ARM/ARM64/MIPS.

runtime/vm/assembler_arm64.cc

 // Copyright (c) 2014, 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_ARM64)
 
 #include "vm/assembler.h"
 #include "vm/cpu.h"
 #include "vm/longjump.h"
@@ skipping 1358 matching lines @@
     ldr(TMP, Address(TMP2, size_field_offset));
     add(TMP, TMP, Operand(size_reg));
     str(TMP, Address(TMP2, size_field_offset));
   }
 }
 
 
 void Assembler::TryAllocate(const Class& cls,
                             Label* failure,
                             Register instance_reg,
+                            Register temp_reg,

[regis, 2014/07/23 22:29:33]

Use TMP2.

[Vyacheslav Egorov (Google), 2014/07/24 11:24:48]

AddImmediate is using TMP2 as a scratch register on some code paths, so while
the code path that uses TMP2 is unlikely to be hit I can't strictly speaking use
TMP2 here.

[regis, 2014/07/24 17:06:07]

Acknowledged.

                             Register pp) {
   ASSERT(failure != NULL);
   if (FLAG_inline_alloc) {
     Heap* heap = Isolate::Current()->heap();
     const intptr_t instance_size = cls.instance_size();
-    LoadImmediate(instance_reg, heap->TopAddress(), pp);
-    ldr(instance_reg, Address(instance_reg));
+    LoadImmediate(temp_reg, heap->NewSpaceAddress(), pp);
+    ldr(instance_reg, Address(temp_reg, Scavenger::top_offset()));
     AddImmediate(instance_reg, instance_reg, instance_size, pp);
 
     // instance_reg: potential next object start.
-    LoadImmediate(TMP, heap->EndAddress(), pp);
-    ldr(TMP, Address(TMP));
+    ldr(TMP, Address(temp_reg, Scavenger::end_offset()));
     CompareRegisters(TMP, instance_reg);
     // fail if heap end unsigned less than or equal to instance_reg.
     b(failure, LS);
 
     // Successfully allocated the object, now update top to point to
     // next object start and store the class in the class field of object.
-    LoadImmediate(TMP, heap->TopAddress(), pp);
-    str(instance_reg, Address(TMP));
+    str(instance_reg, Address(temp_reg, Scavenger::top_offset()));
 
     ASSERT(instance_size >= kHeapObjectTag);
     AddImmediate(
         instance_reg, instance_reg, -instance_size + kHeapObjectTag, pp);
     UpdateAllocationStats(cls.id(), pp);
 
     uword tags = 0;
     tags = RawObject::SizeTag::update(instance_size, tags);
     ASSERT(cls.id() != kIllegalCid);
     tags = RawObject::ClassIdTag::update(cls.id(), tags);
@@ skipping 41 matching lines @@
     add(base, array, Operand(index, LSL, shift));
   }
   const OperandSize size = Address::OperandSizeFor(cid);
   ASSERT(Address::CanHoldOffset(offset, Address::Offset, size));
   return Address(base, offset, Address::Offset, size);
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64