[regexp] Match spec semantics in AdvanceStringIndex

Let AdvanceStringIndex return the incremented index instead of the
increment, and adjust all use sites.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2415383002
Cr-Commit-Position: refs/heads/master@{#40310}

src/builtins/builtins-regexp.cc

@@ -1372,8 +1372,8 @@ BUILTIN(RegExpPrototypeSplit) {
                                                  factory->undefined_value()));
 
     if (result->IsNull(isolate)) {
-      string_index += RegExpUtils::AdvanceStringIndex(isolate, string,
-                                                      string_index, unicode);
+      string_index = RegExpUtils::AdvanceStringIndex(isolate, string,
+                                                     string_index, unicode);
       continue;
     }
 
@@ -1388,8 +1388,8 @@ BUILTIN(RegExpPrototypeSplit) {
 
     const int end = std::min(last_index, length);
     if (end == prev_string_index) {
-      string_index += RegExpUtils::AdvanceStringIndex(isolate, string,
-                                                      string_index, unicode);
+      string_index = RegExpUtils::AdvanceStringIndex(isolate, string,
+                                                     string_index, unicode);
       continue;
     }
 

src/regexp/regexp-utils.cc

@@ -145,23 +145,19 @@ bool RegExpUtils::IsBuiltinExec(Handle<Object> exec) {
   return (code->builtin_index() == Builtins::kRegExpPrototypeExec);
 }
 
-// ES#sec-advancestringindex
-// AdvanceStringIndex ( S, index, unicode )
 int RegExpUtils::AdvanceStringIndex(Isolate* isolate, Handle<String> string,
                                     int index, bool unicode) {
-  int increment = 1;
-
   if (unicode && index < string->length()) {
     const uint16_t first = string->Get(index);
     if (first >= 0xD800 && first <= 0xDBFF && string->length() > index + 1) {
       const uint16_t second = string->Get(index + 1);
       if (second >= 0xDC00 && second <= 0xDFFF) {
-        increment = 2;
+        return index + 2;
       }
     }
   }
 
-  return increment;
+  return index + 1;
 }
 
 MaybeHandle<Object> RegExpUtils::SetAdvancedStringIndex(
@@ -178,7 +174,7 @@ MaybeHandle<Object> RegExpUtils::SetAdvancedStringIndex(
 
   const int last_index = Handle<Smi>::cast(last_index_obj)->value();
   const int new_last_index =
-      last_index + AdvanceStringIndex(isolate, string, last_index, unicode);
+      AdvanceStringIndex(isolate, string, last_index, unicode);
 
   return SetLastIndex(isolate, regexp, new_last_index);
 }

src/regexp/regexp-utils.h

@@ -37,7 +37,8 @@ class RegExpUtils : public AllStatic {
   // Checks whether exec is identical to the initial RegExp.prototype.exec.
   static bool IsBuiltinExec(Handle<Object> exec);
 
-  // Generic index increment methods respecting unicode surrogate pairs.
+  // ES#sec-advancestringindex
+  // AdvanceStringIndex ( S, index, unicode )
   static int AdvanceStringIndex(Isolate* isolate, Handle<String> string,
                                 int index, bool unicode);
   static MaybeHandle<Object> SetAdvancedStringIndex(Isolate* isolate,