Tentative implementation of string slices (hidden under the flag --string-slices).

src/arm/code-stubs-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 4757 matching lines @@
          | IncludeNumberCompareField::encode(include_number_compare_)
          | IncludeSmiCompareField::encode(include_smi_compare_);
 }
 
 
 // StringCharCodeAtGenerator
 void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   Label flat_string;
   Label ascii_string;
   Label got_char_code;
+  Label sliced_string;
 
   // If the receiver is a smi trigger the non-string case.
   __ JumpIfSmi(object_, receiver_not_string_);
 
   // Fetch the instance type of the receiver into result register.
   __ ldr(result_, FieldMemOperand(object_, HeapObject::kMapOffset));
   __ ldrb(result_, FieldMemOperand(result_, Map::kInstanceTypeOffset));
   // If the receiver is not a string trigger the non-string case.
   __ tst(result_, Operand(kIsNotStringMask));
   __ b(ne, receiver_not_string_);
 
   // If the index is non-smi trigger the non-smi case.
   __ JumpIfNotSmi(index_, &index_not_smi_);
 
   // Put smi-tagged index into scratch register.
   __ mov(scratch_, index_);
   __ bind(&got_smi_index_);
 
   // Check for index out of range.
   __ ldr(ip, FieldMemOperand(object_, String::kLengthOffset));
   __ cmp(ip, Operand(scratch_));
   __ b(ls, index_out_of_range_);
 
   // We need special handling for non-flat strings.
   STATIC_ASSERT(kSeqStringTag == 0);
   __ tst(result_, Operand(kStringRepresentationMask));
   __ b(eq, &flat_string);
 
   // Handle non-flat strings.
-  __ tst(result_, Operand(kIsConsStringMask));
+  __ and_(result_, result_, Operand(kStringRepresentationMask));
+  __ cmp(result_, Operand(kSlicedStringTag));
+  __ b(eq, &sliced_string);
+  __ cmp(result_, Operand(kExternalStringTag));
   __ b(eq, &call_runtime_);
 
   // ConsString.
   // Check whether the right hand side is the empty string (i.e. if
   // this is really a flat string in a cons string). If that is not
   // the case we would rather go to the runtime system now to flatten
   // the string.
   __ ldr(result_, FieldMemOperand(object_, ConsString::kSecondOffset));
   __ LoadRoot(ip, Heap::kEmptyStringRootIndex);
   __ cmp(result_, Operand(ip));
   __ b(ne, &call_runtime_);
   // Get the first of the two strings and load its instance type.
   __ ldr(object_, FieldMemOperand(object_, ConsString::kFirstOffset));
   __ ldr(result_, FieldMemOperand(object_, HeapObject::kMapOffset));
   __ ldrb(result_, FieldMemOperand(result_, Map::kInstanceTypeOffset));
   // If the first cons component is also non-flat, then go to runtime.
   STATIC_ASSERT(kSeqStringTag == 0);
   __ tst(result_, Operand(kStringRepresentationMask));
   __ b(ne, &call_runtime_);
+  __ jmp(&flat_string);
+
+  // SlicedString, unpack and add offset.
+  __ bind(&sliced_string);
+  __ ldr(result_, FieldMemOperand(object_, SlicedString::kOffsetOffset));
+  __ add(scratch_, scratch_, result_);
+  __ ldr(object_, FieldMemOperand(object_, SlicedString::kParentOffset));
+  __ ldr(result_, FieldMemOperand(object_, HeapObject::kMapOffset));
+  __ ldrb(result_, FieldMemOperand(result_, Map::kInstanceTypeOffset));
 
   // Check for 1-byte or 2-byte string.
   __ bind(&flat_string);
   STATIC_ASSERT(kAsciiStringTag != 0);
   __ tst(result_, Operand(kStringEncodingMask));
   __ b(ne, &ascii_string);
 
   // 2-byte string.
   // Load the 2-byte character code into the result register. We can
   // add without shifting since the smi tag size is the log2 of the
@@ skipping 535 matching lines @@
   // hash += hash << 15;
   __ add(hash, hash, Operand(hash, LSL, 15), SetCC);
 
   // if (hash == 0) hash = 27;
   __ mov(hash, Operand(27), LeaveCC, ne);
 }
 
 
 void SubStringStub::Generate(MacroAssembler* masm) {
   Label runtime;
 

[Yang, 2011/07/27 11:51:58]

Do not create string slice unless experimental flag is set.

+  if (FLAG_string_slices) {
+    __ jmp(&runtime);
+  }
   // Stack frame on entry.
   //  lr: return address
   //  sp[0]: to
   //  sp[4]: from
   //  sp[8]: string
 
   // This stub is called from the native-call %_SubString(...), so
   // nothing can be assumed about the arguments. It is tested that:
   //  "string" is a sequential string,
   //  both "from" and "to" are smis, and
@@ skipping 518 matching lines @@
     __ ldr(r4, FieldMemOperand(r0, HeapObject::kMapOffset));
     __ ldr(r5, FieldMemOperand(r1, HeapObject::kMapOffset));
     __ ldrb(r4, FieldMemOperand(r4, Map::kInstanceTypeOffset));
     __ ldrb(r5, FieldMemOperand(r5, Map::kInstanceTypeOffset));
   }
   // Check that both strings are sequential.
   STATIC_ASSERT(kSeqStringTag == 0);
   __ tst(r4, Operand(kStringRepresentationMask));
   __ tst(r5, Operand(kStringRepresentationMask), eq);
   __ b(ne, &string_add_runtime);
+  // We cannot encounter sliced strings here since:
+  STATIC_ASSERT(SlicedString::kMinLength >= String::kMinNonFlatLength);
   // Now check if both strings have the same encoding (ASCII/Two-byte).
   // r0: first string.
   // r1: second string.
   // r2: length of first string.
   // r3: length of second string.
   // r6: sum of lengths..
   Label non_ascii_string_add_flat_result;
   ASSERT(IsPowerOf2(kStringEncodingMask));  // Just one bit to test.
   __ eor(r7, r4, Operand(r5));
   __ tst(r7, Operand(kStringEncodingMask));
@@ skipping 604 matching lines @@
   __ mov(result, Operand(0));
   __ Ret();
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM