VM: Micro-optimization of inline array allocation code.

runtime/vm/intermediate_language_arm.cc

Index: runtime/vm/intermediate_language_arm.cc
===================================================================
--- runtime/vm/intermediate_language_arm.cc	(revision 38967)
+++ runtime/vm/intermediate_language_arm.cc	(working copy)
@@ -2257,15 +2257,17 @@
   // R8: iterator which initially points to the start of the variable
   // data area to be initialized.
   // R3: null
-  __ LoadImmediate(R3, reinterpret_cast<intptr_t>(Object::null()));
-  __ AddImmediate(R8, R0, sizeof(RawArray) - kHeapObjectTag);
+  if (num_elements > 0) {
+    __ LoadImmediate(R3, reinterpret_cast<intptr_t>(Object::null()));
+    __ AddImmediate(R8, R0, sizeof(RawArray) - kHeapObjectTag);
 
-  Label init_loop;
-  __ Bind(&init_loop);
-  __ cmp(R8, Operand(R7));
-  __ str(R3, Address(R8, 0), CC);
-  __ AddImmediate(R8, kWordSize, CC);
-  __ b(&init_loop, CC);
+    Label init_loop;
+    __ Bind(&init_loop);
+    __ cmp(R8, Operand(R7));
+    __ str(R3, Address(R8, 0), CC);
+    __ AddImmediate(R8, kWordSize, CC);
+    __ b(&init_loop, CC);
+  }
   __ b(done);
 }