Shorter TryAllocate instruction sequence on ARM/ARM64/MIPS.

runtime/vm/assembler_arm64.cc

Index: runtime/vm/assembler_arm64.cc
diff --git a/runtime/vm/assembler_arm64.cc b/runtime/vm/assembler_arm64.cc
index 6da9770ad9986838a68ca00b97b51422eae62494..7911896dc55ebb6558c148f423c88e404e67c01e 100644
--- a/runtime/vm/assembler_arm64.cc
+++ b/runtime/vm/assembler_arm64.cc
@@ -1376,26 +1376,25 @@ void Assembler::UpdateAllocationStatsWithSize(intptr_t cid,
 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(