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

src/arm/lithium-codegen-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 3406 matching lines @@
     DeferredStringCharCodeAt(LCodeGen* codegen, LStringCharCodeAt* instr)
         : LDeferredCode(codegen), instr_(instr) { }
     virtual void Generate() { codegen()->DoDeferredStringCharCodeAt(instr_); }
    private:
     LStringCharCodeAt* instr_;
   };
 
   Register scratch = scratch0();
   Register string = ToRegister(instr->string());
   Register index = no_reg;
+  Register offset = ToRegister(instr->TempAt(0));

[antonm, 2011/07/27 14:04:49]

do you need offset, cannot you live only with scratch?

   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.
   __ ldr(result, FieldMemOperand(string, HeapObject::kMapOffset));
   __ ldrb(result, FieldMemOperand(result, Map::kInstanceTypeOffset));
 
   // We need special handling for non-flat strings.
   STATIC_ASSERT(kSeqStringTag == 0);
   __ tst(result, Operand(kStringRepresentationMask));

[antonm, 2011/07/27 14:04:49]

here you have another scratch---offset.  so optimization should apply

   __ b(eq, &flat_string);
 
   // Handle non-flat strings.
-  __ tst(result, Operand(kIsConsStringMask));
+  __ and_(result, result, Operand(kStringRepresentationMask));
+  __ cmp(result, Operand(kConsStringTag));
+  __ b(eq, &cons_string);
+  __ cmp(result, Operand(kExternalStringTag));
   __ b(eq, deferred->entry());
 
+  // SlicedString.
+  // Unpack slice, add offset and retrieve the result char.
+  __ ldr(result, FieldMemOperand(string, SlicedString::kOffsetOffset));
+  if (instr->index()->IsConstantOperand()) {
+    __ mov(offset, Operand(result, LSR, kSmiTagSize));

[antonm, 2011/07/27 14:04:49]

do you want to untag here?  if you defer it to lookup, it apparently will take
less instructions.

+    __ add(offset, offset, Operand(const_index));
+  } else {
+    __ add(offset, index, Operand(result, LSR, kSmiTagSize));
+  }
+  __ ldr(string, FieldMemOperand(string, SlicedString::kParentOffset));
+  __ ldr(result, FieldMemOperand(string, HeapObject::kMapOffset));
+  __ ldrb(result, FieldMemOperand(result, Map::kInstanceTypeOffset));
+  // Check for 1-byte or 2-byte string.
+  STATIC_ASSERT(kAsciiStringTag != 0);
+  __ tst(result, Operand(kStringEncodingMask));
+  __ b(ne, &sliced_ascii_string);
+  __ add(scratch,
+         string,
+         Operand(SeqTwoByteString::kHeaderSize - kHeapObjectTag));

[antonm, 2011/07/27 14:04:49]

you probably may encode const_index into this constant.

+  __ ldrh(result, MemOperand(scratch, offset, LSL, 1));
+  __ jmp(&done);
+  __ bind(&sliced_ascii_string);
+  __ add(scratch,
+         string,
+         Operand(SeqAsciiString::kHeaderSize - kHeapObjectTag));

[antonm, 2011/07/27 14:04:49]

ditto

+  __ ldrb(result, MemOperand(scratch, offset));
+  __ jmp(&done);
+
   // 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);
   __ ldr(scratch, FieldMemOperand(string, ConsString::kSecondOffset));
   __ LoadRoot(ip, Heap::kEmptyStringRootIndex);
   __ cmp(scratch, ip);
   __ b(ne, deferred->entry());
   // Get the first of the two strings and load its instance type.
   __ ldr(string, FieldMemOperand(string, ConsString::kFirstOffset));
   __ ldr(result, FieldMemOperand(string, 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);
@@ skipping 1006 matching lines @@
   ASSERT(osr_pc_offset_ == -1);
   osr_pc_offset_ = masm()->pc_offset();
 }
 
 
 
 
 #undef __
 
 } }  // namespace v8::internal