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

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index 54ad4581c5ce632e7c74d7f193e5a627fcdff88f..a60999ba4fb5b86a829856327f930a8c21767422 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -3424,6 +3424,7 @@ void LCodeGen::DoStringCharCodeAt(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()));
@@ -3446,7 +3447,7 @@ void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
   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));
@@ -3458,14 +3459,46 @@ void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
   __ 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);