VM: Micro-optimization of inline array allocation code.

runtime/vm/intermediate_language_mips.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_MIPS.
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 2025 matching lines @@
   // Store the type argument field.
   __ StoreIntoObjectNoBarrier(V0,
                               FieldAddress(V0, Array::type_arguments_offset()),
                               kElemTypeReg);
 
   // Set the length field.
   __ StoreIntoObjectNoBarrier(V0,
                               FieldAddress(V0, Array::length_offset()),
                               kLengthReg);
 
-  __ LoadImmediate(T7, reinterpret_cast<int32_t>(Object::null()));
   // Initialize all array elements to raw_null.
   // V0: new object start as a tagged pointer.
   // T1: new object end address.
   // T2: iterator which initially points to the start of the variable
   // data area to be initialized.
   // T7: null.
-  __ AddImmediate(T2, V0, sizeof(RawArray) - kHeapObjectTag);
+  if (num_elements > 0) {
+    __ LoadImmediate(T7, reinterpret_cast<int32_t>(Object::null()));
+    __ AddImmediate(T2, V0, sizeof(RawArray) - kHeapObjectTag);
 
-  Label init_loop;
-  __ Bind(&init_loop);
-  __ BranchUnsignedGreaterEqual(T2, T1, done);
-  __ sw(T7, Address(T2, 0));
-  __ b(&init_loop);
-  __ delay_slot()->addiu(T2, T2, Immediate(kWordSize));
+    Label init_loop;
+    __ Bind(&init_loop);
+    __ sw(T7, Address(T2, 0));
+    __ addiu(T2, T2, Immediate(kWordSize));
+    __ BranchUnsignedLess(T2, T1, &init_loop);
+  }
+  __ b(done);
 }
 
 
 void CreateArrayInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   __ TraceSimMsg("CreateArrayInstr");
   const Register kLengthReg = A1;
   const Register kElemTypeReg = A0;
   const Register kResultReg = V0;
   ASSERT(locs()->in(0).reg() == kElemTypeReg);
   ASSERT(locs()->in(1).reg() == kLengthReg);
@@ skipping 2705 matching lines @@
   compiler->GenerateCall(token_pos(), &label, stub_kind_, locs());
 #if defined(DEBUG)
   __ LoadImmediate(S4, kInvalidObjectPointer);
   __ LoadImmediate(S5, kInvalidObjectPointer);
 #endif
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS