Fixing dead code in empty array init.

src/arm/builtins-arm.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 86 matching lines @@
                                  Register scratch3,
                                  Label* gc_required) {
   const int initial_capacity = JSArray::kPreallocatedArrayElements;
   STATIC_ASSERT(initial_capacity >= 0);
   // Load the initial map from the array function.
   __ ldr(scratch1, FieldMemOperand(array_function,
                                    JSFunction::kPrototypeOrInitialMapOffset));
 
   // Allocate the JSArray object together with space for a fixed array with the
   // requested elements.
-  int size = JSArray::kSize + FixedArray::SizeFor(initial_capacity);
+  int size = JSArray::kSize;
+  if (initial_capacity > 0) {
+    size += FixedArray::SizeFor(initial_capacity);
+  }
   __ AllocateInNewSpace(size,
                         result,
                         scratch2,
                         scratch3,
                         gc_required,
                         TAG_OBJECT);
 
   // Allocated the JSArray. Now initialize the fields except for the elements
   // array.
   // result: JSObject
   // scratch1: initial map
   // scratch2: start of next object
   __ str(scratch1, FieldMemOperand(result, JSObject::kMapOffset));
   __ LoadRoot(scratch1, Heap::kEmptyFixedArrayRootIndex);
   __ str(scratch1, FieldMemOperand(result, JSArray::kPropertiesOffset));
   // Field JSArray::kElementsOffset is initialized later.
   __ mov(scratch3,  Operand(0, RelocInfo::NONE));
   __ str(scratch3, FieldMemOperand(result, JSArray::kLengthOffset));
 
+  if (initial_capacity == 0) {
+    __ str(scratch1, FieldMemOperand(result, JSArray::kElementsOffset));
+    return;
+  }
+
   // Calculate the location of the elements array and set elements array member
   // of the JSArray.
   // result: JSObject
   // scratch2: start of next object
   __ add(scratch1, result, Operand(JSArray::kSize));
   __ str(scratch1, FieldMemOperand(result, JSArray::kElementsOffset));
 
   // Clear the heap tag on the elements array.
   STATIC_ASSERT(kSmiTag == 0);

[Kevin Millikin (Chromium), 2011/10/27 10:35:54]

I can't figure out why the following code depends on this assert.  Can you?  If
not, let's just get rid of it.

   __ sub(scratch1, scratch1, Operand(kHeapObjectTag));
 
   // Initialize the FixedArray and fill it with holes. FixedArray length is
   // stored as a smi.
   // result: JSObject
   // scratch1: elements array (untagged)
   // scratch2: start of next object
   __ LoadRoot(scratch3, Heap::kFixedArrayMapRootIndex);
   ASSERT_EQ(0 * kPointerSize, FixedArray::kMapOffset);

[Kevin Millikin (Chromium), 2011/10/27 10:35:54]

This can be a static assert.

   __ str(scratch3, MemOperand(scratch1, kPointerSize, PostIndex));
   __ mov(scratch3,  Operand(Smi::FromInt(initial_capacity)));
   ASSERT_EQ(1 * kPointerSize, FixedArray::kLengthOffset);

[Kevin Millikin (Chromium), 2011/10/27 10:35:54]

This can be a static assert.

   __ str(scratch3, MemOperand(scratch1, kPointerSize, PostIndex));
 
   // Fill the FixedArray with the hole value. Inline the code if short.
-  if (initial_capacity == 0) return;
   ASSERT_EQ(2 * kPointerSize, FixedArray::kHeaderSize);

[Kevin Millikin (Chromium), 2011/10/27 10:35:54]

This can be a static assert.

   __ LoadRoot(scratch3, Heap::kTheHoleValueRootIndex);
   static const int kLoopUnfoldLimit = 4;
   if (initial_capacity <= kLoopUnfoldLimit) {
     for (int i = 0; i < initial_capacity; i++) {
       __ str(scratch3, MemOperand(scratch1, kPointerSize, PostIndex));
     }
   } else {
     Label loop, entry;
     __ add(scratch2, scratch1, Operand(initial_capacity * kPointerSize));
     __ b(&entry);
@@ skipping 1545 matching lines @@
   __ bind(&dont_adapt_arguments);
   __ Jump(r3);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM