VM: Micro-optimization of inline array allocation code.

runtime/vm/intermediate_language_arm.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"  // Needed here to get TARGET_ARCH_ARM.
 #if defined(TARGET_ARCH_ARM)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/cpu.h"
@@ skipping 2239 matching lines @@
   __ StoreIntoObjectNoBarrier(R0,
                               FieldAddress(R0, Array::length_offset()),
                               kLengthReg);
 
   // Initialize all array elements to raw_null.
   // R0: new object start as a tagged pointer.
   // R7: new object end address.
   // 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);
 }
 
 
 void CreateArrayInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   const Register kLengthReg = R2;
   const Register kElemTypeReg = R1;
   const Register kResultReg = R0;
 
   ASSERT(locs()->in(kElementTypePos).reg() == kElemTypeReg);
@@ skipping 4333 matching lines @@
   compiler->GenerateCall(token_pos(), &label, stub_kind_, locs());
 #if defined(DEBUG)
   __ LoadImmediate(R4, kInvalidObjectPointer);
   __ LoadImmediate(R5, kInvalidObjectPointer);
 #endif
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM