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

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index de8c6fddc4231e5579a8b2afa6dd1a6276c478e9..c638397338c7b6b55a43eca28d69c74fb61d3506 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -2966,35 +2966,30 @@ int Runtime::StringMatch(Isolate* isolate,
   if (!pat->IsFlat()) FlattenString(pat);
 
   AssertNoAllocation no_heap_allocation;  // ensure vectors stay valid
-  // Extract flattened substrings of cons strings before determining asciiness.
-  String* seq_sub = *sub;
-  if (seq_sub->IsConsString()) seq_sub = ConsString::cast(seq_sub)->first();
-  String* seq_pat = *pat;
-  if (seq_pat->IsConsString()) seq_pat = ConsString::cast(seq_pat)->first();
 
   // dispatch on type of strings
-  if (seq_pat->IsAsciiRepresentation()) {
-    Vector<const char> pat_vector = seq_pat->ToAsciiVector();
-    if (seq_sub->IsAsciiRepresentation()) {
+  if (pat->IsAsciiRepresentationUnderneath()) {
+    Vector<const char> pat_vector = pat->ToAsciiVector();
+    if (sub->IsAsciiRepresentationUnderneath()) {
       return SearchString(isolate,
-                          seq_sub->ToAsciiVector(),
+                          sub->ToAsciiVector(),
                           pat_vector,
                           start_index);
     }
     return SearchString(isolate,
-                        seq_sub->ToUC16Vector(),
+                        sub->ToUC16Vector(),
                         pat_vector,
                         start_index);
   }
-  Vector<const uc16> pat_vector = seq_pat->ToUC16Vector();
-  if (seq_sub->IsAsciiRepresentation()) {
+  Vector<const uc16> pat_vector = pat->ToUC16Vector();
+  if (sub->IsAsciiRepresentationUnderneath()) {
     return SearchString(isolate,
-                        seq_sub->ToAsciiVector(),
+                        sub->ToAsciiVector(),
                         pat_vector,
                         start_index);
   }
   return SearchString(isolate,
-                      seq_sub->ToUC16Vector(),
+                      sub->ToUC16Vector(),
                       pat_vector,
                       start_index);
 }
@@ -3579,7 +3574,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_RegExpExecMultiple) {
   HandleScope handles(isolate);
 
   CONVERT_ARG_CHECKED(String, subject, 1);
-  if (!subject->IsFlat()) { FlattenString(subject); }
+  if (!subject->IsFlat()) FlattenString(subject);
   CONVERT_ARG_CHECKED(JSRegExp, regexp, 0);
   CONVERT_ARG_CHECKED(JSArray, last_match_info, 2);
   CONVERT_ARG_CHECKED(JSArray, result_array, 3);