Unrolls array initialization loop for small arrays.

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 1902 matching lines @@
   locs->set_out(0, Location::RegisterLocation(RAX));
   return locs;
 }
 
 
 // Inlines array allocation for known constant values.
 static void InlineArrayAllocation(FlowGraphCompiler* compiler,
                                    intptr_t num_elements,
                                    Label* slow_path,
                                    Label* done) {
+  const int kInlineArraySize = 12;  // Same as kInlineInstanceSize.
   const Register kLengthReg = R10;
   const Register kElemTypeReg = RBX;
   const intptr_t instance_size = Array::InstanceSize(num_elements);
 
   __ TryAllocateArray(kArrayCid, instance_size, slow_path, Assembler::kFarJump,
                       RAX,  // instance
                       RCX);  // end address
 
   // RAX: new object start as a tagged pointer.
   // Store the type argument field.
   __ StoreIntoObjectNoBarrier(RAX,
                               FieldAddress(RAX, Array::type_arguments_offset()),
                               kElemTypeReg);
 
   // 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.
   if (num_elements > 0) {
+    const intptr_t array_size = instance_size - sizeof(RawArray);
     __ 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);
+    if (array_size < (kInlineArraySize * kWordSize)) {
+      intptr_t current_offset = 0;
+      while (current_offset < array_size) {
+        __ movq(Address(RDI, current_offset), R12);
+        current_offset += kWordSize;
+      }
+    } else {
+      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;
@@ skipping 3947 matching lines @@
   __ movq(R10, Immediate(kInvalidObjectPointer));
   __ movq(RBX, Immediate(kInvalidObjectPointer));
 #endif
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64