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

src/x64/regexp-macro-assembler-x64.cc

Index: src/x64/regexp-macro-assembler-x64.cc
diff --git a/src/x64/regexp-macro-assembler-x64.cc b/src/x64/regexp-macro-assembler-x64.cc
index 395466e7774a82489c216aec0c64c68fea37d412..6d351d23dd8d60c5727c8b6e33dbb4ff2297195c 100644
--- a/src/x64/regexp-macro-assembler-x64.cc
+++ b/src/x64/regexp-macro-assembler-x64.cc
@@ -1174,8 +1174,21 @@ int RegExpMacroAssemblerX64::CheckStackGuardState(Address* return_address,
   Handle<Code> code_handle(re_code);
 
   Handle<String> subject(frame_entry<String*>(re_frame, kInputString));
+  String* subject_ptr = *subject;
+  int slice_offset = 0;
+
+
+  // Extract the underlying string and the slice offset.
+  if (StringShape(subject_ptr).IsCons()) {
+    subject_ptr = ConsString::cast(subject_ptr)->first();
+  } else if (StringShape(subject_ptr).IsSliced()) {
+    SlicedString* slice = SlicedString::cast(subject_ptr);
+    subject_ptr = slice->parent();
+    slice_offset = slice->offset();
+  }
+
   // Current string.
-  bool is_ascii = subject->IsAsciiRepresentation();
+  bool is_ascii = subject_ptr->IsAsciiRepresentation();
 
   ASSERT(re_code->instruction_start() <= *return_address);
   ASSERT(*return_address <=
@@ -1194,7 +1207,7 @@ int RegExpMacroAssemblerX64::CheckStackGuardState(Address* return_address,
   }
 
   // String might have changed.
-  if (subject->IsAsciiRepresentation() != is_ascii) {
+  if (subject_ptr->IsAsciiRepresentation() != is_ascii) {
     // If we changed between an ASCII and an UC16 string, the specialized
     // code cannot be used, and we need to restart regexp matching from
     // scratch (including, potentially, compiling a new version of the code).
@@ -1205,8 +1218,8 @@ int RegExpMacroAssemblerX64::CheckStackGuardState(Address* return_address,
   // be a sequential or external string with the same content.
   // Update the start and end pointers in the stack frame to the current
   // location (whether it has actually moved or not).
-  ASSERT(StringShape(*subject).IsSequential() ||
-      StringShape(*subject).IsExternal());
+  ASSERT(StringShape(subject_ptr).IsSequential() ||
+      StringShape(subject_ptr).IsExternal());
 
   // The original start address of the characters to match.
   const byte* start_address = frame_entry<const byte*>(re_frame, kInputStart);
@@ -1214,7 +1227,8 @@ int RegExpMacroAssemblerX64::CheckStackGuardState(Address* return_address,
   // Find the current start address of the same character at the current string
   // position.
   int start_index = frame_entry<int>(re_frame, kStartIndex);
-  const byte* new_address = StringCharacterPosition(*subject, start_index);
+  const byte* new_address = StringCharacterPosition(subject_ptr,
+                                                    start_index + slice_offset);
 
   if (start_address != new_address) {
     // If there is a difference, update the object pointer and start and end