VM: Micro-optimization of inline array allocation code.

runtime/vm/intermediate_language_x64.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_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 1918 matching lines @@
   // Set the length field.
   __ StoreIntoObjectNoBarrier(RAX,
                               FieldAddress(RAX, Array::length_offset()),
                               kLengthReg);
 
   // Initialize all array elements to raw_null.
   // RAX: new object start as a tagged pointer.
   // RCX: new object end address.
   // RDI: iterator which initially points to the start of the variable
   // data area to be initialized.
-  __ LoadObject(R12, Object::null_object(), PP);
-  __ leaq(RDI, FieldAddress(RAX, sizeof(RawArray)));
-  Label init_loop;
-  __ Bind(&init_loop);
-  __ cmpq(RDI, RCX);
-  __ j(ABOVE_EQUAL, done, Assembler::kNearJump);
-  __ movq(Address(RDI, 0), R12);
-  __ addq(RDI, Immediate(kWordSize));
-  __ jmp(&init_loop, Assembler::kNearJump);
+  if (num_elements > 0) {
+    __ LoadObject(R12, Object::null_object(), PP);
+    __ leaq(RDI, FieldAddress(RAX, sizeof(RawArray)));
+    Label init_loop;
+    __ Bind(&init_loop);
+    __ movq(Address(RDI, 0), R12);
+    __ addq(RDI, Immediate(kWordSize));
+    __ cmpq(RDI, RCX);
+    __ j(BELOW, &init_loop, Assembler::kNearJump);
+  }
+  __ jmp(done, Assembler::kNearJump);
 }
 
 
 void CreateArrayInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   // Allocate the array.  R10 = length, RBX = element type.
   const Register kLengthReg = R10;
   const Register kElemTypeReg = RBX;
   const Register kResultReg = RAX;
   ASSERT(locs()->in(0).reg() == kElemTypeReg);
   ASSERT(locs()->in(1).reg() == kLengthReg);
@@ skipping 3811 matching lines @@
   __ movq(R10, Immediate(kInvalidObjectPointer));
   __ movq(RBX, Immediate(kInvalidObjectPointer));
 #endif
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64