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

src/arm/code-stubs-arm.cc

Index: src/arm/code-stubs-arm.cc
diff --git a/src/arm/code-stubs-arm.cc b/src/arm/code-stubs-arm.cc
index eaad9f293b330eb0936a5dceeb45ae51bb5f840b..3889371b82e6ff4be438a71ce73ed028676583cf 100644
--- a/src/arm/code-stubs-arm.cc
+++ b/src/arm/code-stubs-arm.cc
@@ -4775,6 +4775,7 @@ 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_);
@@ -4804,7 +4805,10 @@ void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   __ 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.
@@ -4824,6 +4828,15 @@ void StringCharCodeAtGenerator::GenerateFast(MacroAssembler* masm) {
   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));

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

maybe add an assert that result_ is always a flat string.

 
   // Check for 1-byte or 2-byte string.
   __ bind(&flat_string);
@@ -5380,6 +5393,9 @@ void StringHelper::GenerateHashGetHash(MacroAssembler* masm,
 void SubStringStub::Generate(MacroAssembler* masm) {
   Label runtime;
 
+  if (FLAG_string_slices) {
+    __ jmp(&runtime);
+  }
   // Stack frame on entry.
   //  lr: return address
   //  sp[0]: to
@@ -5918,6 +5934,8 @@ void StringAddStub::Generate(MacroAssembler* masm) {
   __ 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);

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

again, might be worth adding a generated code check, something like if
(FLAG_debug_code) { ... }

   // Now check if both strings have the same encoding (ASCII/Two-byte).
   // r0: first string.
   // r1: second string.