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

src/ia32/lithium-codegen-ia32.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 3121 matching lines @@
    public:
     DeferredStringCharCodeAt(LCodeGen* codegen, LStringCharCodeAt* instr)
         : LDeferredCode(codegen), instr_(instr) { }
     virtual void Generate() { codegen()->DoDeferredStringCharCodeAt(instr_); }
    private:
     LStringCharCodeAt* instr_;
   };
 
   Register string = ToRegister(instr->string());
   Register index = no_reg;
+  Register offset = ToRegister(instr->TempAt(0));
   int const_index = -1;
   if (instr->index()->IsConstantOperand()) {
     const_index = ToInteger32(LConstantOperand::cast(instr->index()));
     STATIC_ASSERT(String::kMaxLength <= Smi::kMaxValue);
     if (!Smi::IsValid(const_index)) {
       // Guaranteed to be out of bounds because of the assert above.
       // So the bounds check that must dominate this instruction must
       // have deoptimized already.
       if (FLAG_debug_code) {
         __ Abort("StringCharCodeAt: out of bounds index.");
       }
       // No code needs to be generated.
       return;
     }
   } else {
     index = ToRegister(instr->index());
   }
   Register result = ToRegister(instr->result());
 
   DeferredStringCharCodeAt* deferred =
       new DeferredStringCharCodeAt(this, instr);
 
-  Label flat_string, ascii_string, done;
+  Label flat_string, ascii_string, cons_string, sliced_ascii_string, done;
 
   // Fetch the instance type of the receiver into result register.
   __ mov(result, FieldOperand(string, HeapObject::kMapOffset));
   __ movzx_b(result, FieldOperand(result, Map::kInstanceTypeOffset));
 
   // We need special handling for non-flat strings.
   STATIC_ASSERT(kSeqStringTag == 0);
   __ test(result, Immediate(kStringRepresentationMask));
   __ j(zero, &flat_string, Label::kNear);
 
   // Handle non-flat strings.
-  __ test(result, Immediate(kIsConsStringMask));
-  __ j(zero, deferred->entry());
+  __ and_(result, kStringRepresentationMask);

[Vitaly Repeshko, 2011/08/05 12:14:14]

Too much code now. Either put it behind the flag or restructure to avoid two
copies of the code that checks encoding and loads chars. If the index is updated
relatively early the latter should be doable.

+  __ cmp(result, kConsStringTag);
+  __ j(equal, &cons_string, Label::kNear);
+  __ cmp(result, kExternalStringTag);
+  __ j(equal, deferred->entry());
+
+  // SlicedString.
+  // Unpack slice, add offset and retrieve the result char.
+  __ mov(offset, FieldOperand(string, SlicedString::kOffsetOffset));
+  __ SmiUntag(offset);
+  if (instr->index()->IsConstantOperand()) {
+    __ add(Operand(offset), Immediate(const_index));
+  } else {
+    __ add(offset, Operand(index));
+  }
+  __ mov(string, FieldOperand(string, SlicedString::kParentOffset));
+  __ mov(result, FieldOperand(string, HeapObject::kMapOffset));
+  __ movzx_b(result, FieldOperand(result, Map::kInstanceTypeOffset));
+  // Check for ASCII or two-byte string.
+  STATIC_ASSERT(kAsciiStringTag != 0);
+  __ test(result, Immediate(kStringEncodingMask));
+  __ j(not_zero, &sliced_ascii_string, Label::kNear);
+  __ movzx_w(result, FieldOperand(string,
+                                  offset,
+                                  times_2,
+                                  SeqTwoByteString::kHeaderSize));
+  __ jmp(&done, Label::kNear);
+  __ bind(&sliced_ascii_string);
+  __ movzx_b(result, FieldOperand(string,
+                                  offset,
+                                  times_1,
+                                  SeqAsciiString::kHeaderSize));
+  __ jmp(&done, Label::kNear);
 
   // 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.
+  __ bind(&cons_string);
   __ cmp(FieldOperand(string, ConsString::kSecondOffset),
          Immediate(factory()->empty_string()));
   __ j(not_equal, deferred->entry());
   // Get the first of the two strings and load its instance type.
   __ mov(string, FieldOperand(string, ConsString::kFirstOffset));
   __ mov(result, FieldOperand(string, HeapObject::kMapOffset));
   __ movzx_b(result, FieldOperand(result, Map::kInstanceTypeOffset));
   // If the first cons component is also non-flat, then go to runtime.
   STATIC_ASSERT(kSeqStringTag == 0);
   __ test(result, Immediate(kStringRepresentationMask));
@@ skipping 1114 matching lines @@
                                          env->deoptimization_index());
   __ InvokeBuiltin(Builtins::IN, CALL_FUNCTION, safepoint_generator);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32