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 3119 matching lines @@
 void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
   class DeferredStringCharCodeAt: public LDeferredCode {
    public:
     DeferredStringCharCodeAt(LCodeGen* codegen, LStringCharCodeAt* instr)
         : LDeferredCode(codegen), instr_(instr) { }
     virtual void Generate() { codegen()->DoDeferredStringCharCodeAt(instr_); }
    private:
     LStringCharCodeAt* instr_;
   };
 
   Register string = ToRegister(instr->string());

[Vitaly Repeshko, 2011/08/12 19:01:00]

Simplified version using three (writable) registers (untested):

  Register string = ToRegister(instr->string());
  Register index = ToRegister(instr->index);
  Register result = ToRegister(instr->result());

  DeferredStringCharCodeAt* deferred =
      new DeferredStringCharCodeAt(this, instr);

  // 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 indirect strings.
  Label check_sequential;
  __ test(result, Immediate(kIsIndirectStringMask));
  __ j(zero, &check_sequential, Label::kNear);

  // Dispatch on the indirect string shape: slice or cons.
  Label cons_string;
  uint32_t sliced_not_cons_mask = kSlicedStringTag & ~kConsStringTag;
  ASSERT(IsPowerOf2(sliced_not_cons_mask) && sliced_not_cons_mask != 0);
  __ test(result, Immediate(sliced_not_cons_mask));
  __ j(zero, &cons_string, Label::kNear);

  // Handle slices.
  Label indirect_string_loaded;
  __ mov(result, FieldOperand(string, SlicedString::kOffsetOffset));
  __ SmiUntag(result);
  __ add(index, Operand(result));
  __ mov(string, FieldOperand(string, SlicedString::kParentOffset));
  __ jmp(&indirect_string_loaded, Label::kNear);

  // Handle conses.
  // 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());
  __ mov(string, FieldOperand(string, ConsString::kFirstOffset));

  __ bind(&indirect_string_loaded);
  __ mov(result, FieldOperand(string, HeapObject::kMapOffset));
  __ movzx_b(result, FieldOperand(result, Map::kInstanceTypeOffset));

  // Check whether the string is sequential. The only non-sequential
  // shapes we support have just been unwrapped above.
  __ bind(&check_sequential);
  STATIC_ASSERT(kSeqStringTag == 0);
  __ test(result, Immediate(kStringRepresentationMask));
  __ j(not_zero, deferred->entry());

  // Dispatch on the encoding: ASCII or two-byte.
  Label ascii_string;
  STATIC_ASSERT(kAsciiStringTag != 0);
  __ test(result, Immediate(kStringEncodingMask));
  __ j(not_zero, &ascii_string, Label::kNear);

  // Two-byte string.
  // Load the two-byte character code into the result register.
  Label done;
  STATIC_ASSERT(kSmiTag == 0 && kSmiTagSize == 1);
  __ movzx_w(result, FieldOperand(string,
                                  index,
                                  times_2,
                                  SeqTwoByteString::kHeaderSize));
  __ jmp(&done, Label::kNear);

  // ASCII string.
  // Load the byte into the result register.
  __ bind(&ascii_string);
  __ movzx_b(result, FieldOperand(string,
                                  index,
                                  times_1,
                                  SeqAsciiString::kHeaderSize));
  __ bind(&done);
  __ bind(deferred->exit());

   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, two_byte_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);
+  __ 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);
+  __ mov(string, FieldOperand(string, SlicedString::kParentOffset));
+  __ mov(result, FieldOperand(string, HeapObject::kMapOffset));
+  __ movzx_b(result, FieldOperand(result, Map::kInstanceTypeOffset));
+  __ add(string, Operand(offset));
+  // Check for ASCII or two-byte string.
+  STATIC_ASSERT(kAsciiStringTag != 0);
+  __ test(result, Immediate(kStringEncodingMask));
+  __ j(not_zero, &ascii_string, Label::kNear);
+  __ add(string, Operand(offset));
+  __ jmp(&two_byte_string, 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));
   __ j(not_zero, deferred->entry());
 
   // Check for ASCII or two-byte string.
   __ bind(&flat_string);
   STATIC_ASSERT(kAsciiStringTag != 0);
   __ test(result, Immediate(kStringEncodingMask));
   __ j(not_zero, &ascii_string, Label::kNear);
 
   // Two-byte string.
   // Load the two-byte character code into the result register.
+  __ bind(&two_byte_string);
   STATIC_ASSERT(kSmiTag == 0 && kSmiTagSize == 1);
   if (instr->index()->IsConstantOperand()) {
     __ movzx_w(result,
                FieldOperand(string,
                             SeqTwoByteString::kHeaderSize +
                             (kUC16Size * const_index)));
   } else {
     __ movzx_w(result, FieldOperand(string,
                                     index,
                                     times_2,
@@ skipping 1094 matching lines @@
                                          env->deoptimization_index());
   __ InvokeBuiltin(Builtins::IN, CALL_FUNCTION, safepoint_generator);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32