Some assembler-level optimizations on ARM.

src/arm/builtins-arm.cc

Index: src/arm/builtins-arm.cc
diff --git a/src/arm/builtins-arm.cc b/src/arm/builtins-arm.cc
index 1e55f7277cb336418de002941b3a36330dd46b78..123f4c79e0c55d812798e5cbbb0b6ec1a62a0e34 100644
--- a/src/arm/builtins-arm.cc
+++ b/src/arm/builtins-arm.cc
@@ -136,10 +136,11 @@ static void AllocateEmptyJSArray(MacroAssembler* masm,
   // 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));
+  // scratch1: empty FixedArray
+  __ str(scratch1, FieldMemOperand(result, JSArray::kPropertiesOffset));
 
   if (initial_capacity == 0) {
     __ str(scratch1, FieldMemOperand(result, JSArray::kElementsOffset));
@@ -153,6 +154,8 @@ static void AllocateEmptyJSArray(MacroAssembler* masm,
   __ add(scratch1, result, Operand(JSArray::kSize));
   __ str(scratch1, FieldMemOperand(result, JSArray::kElementsOffset));
 
+  __ LoadRoot(scratch3, Heap::kFixedArrayMapRootIndex);

[ulan, 2012/01/27 13:19:11]

In this case it seems to reduce readability. I think such optimizations should
be done on hot paths only.

+
   // Clear the heap tag on the elements array.
   __ sub(scratch1, scratch1, Operand(kHeapObjectTag));
 
@@ -161,7 +164,7 @@ static void AllocateEmptyJSArray(MacroAssembler* masm,
   // result: JSObject
   // scratch1: elements array (untagged)
   // scratch2: start of next object
-  __ LoadRoot(scratch3, Heap::kFixedArrayMapRootIndex);
+  // scratch3: FixedArray map
   STATIC_ASSERT(0 * kPointerSize == FixedArray::kMapOffset);
   __ str(scratch3, MemOperand(scratch1, kPointerSize, PostIndex));
   __ mov(scratch3,  Operand(Smi::FromInt(initial_capacity)));
@@ -252,6 +255,8 @@ static void AllocateJSArray(MacroAssembler* masm,
   __ str(elements_array_storage,
          FieldMemOperand(result, JSArray::kElementsOffset));
 
+  __ LoadRoot(scratch1, Heap::kFixedArrayMapRootIndex);

[ulan, 2012/01/27 13:19:11]

Ditto.

+
   // Clear the heap tag on the elements array.
   STATIC_ASSERT(kSmiTag == 0);
   __ sub(elements_array_storage,
@@ -262,7 +267,7 @@ static void AllocateJSArray(MacroAssembler* masm,
   // result: JSObject
   // elements_array_storage: elements array (untagged)
   // array_size: size of array (smi)
-  __ LoadRoot(scratch1, Heap::kFixedArrayMapRootIndex);
+  // scratch1: FixedArray map
   ASSERT_EQ(0 * kPointerSize, FixedArray::kMapOffset);
   __ str(scratch1, MemOperand(elements_array_storage, kPointerSize, PostIndex));
   STATIC_ASSERT(kSmiTag == 0);