Keep track of the first non-ascii word/char to avoid redoing the work.

src/objects.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 7395 matching lines @@
                                         unsigned* remaining,
                                         unsigned* offset);
 
   // Helper function for flattening strings.
   template <typename sinkchar>
   static void WriteToFlat(String* source,
                           sinkchar* sink,
                           int from,
                           int to);
 
-  static inline bool IsAscii(const char* chars, int length) {
+  // The return value may point to the first aligned word containing the
+  // first non-ascii character, rather than directly to the non-ascii character.
+  // If the return value is >= the passed length, the entire string was ASCII.
+  static inline int NonAsciiStart(const char* chars, int length) {
+    const char* start = chars;
     const char* limit = chars + length;
 #ifdef V8_HOST_CAN_READ_UNALIGNED
     ASSERT(kMaxAsciiCharCode == 0x7F);
     const uintptr_t non_ascii_mask = kUintptrAllBitsSet / 0xFF * 0x80;
     while (chars + sizeof(uintptr_t) <= limit) {
       if (*reinterpret_cast<const uintptr_t*>(chars) & non_ascii_mask) {
-        return false;
+        return chars - start;
       }
       chars += sizeof(uintptr_t);
     }
 #endif
     while (chars < limit) {
-      if (static_cast<uint8_t>(*chars) > kMaxAsciiCharCodeU) return false;
+      if (static_cast<uint8_t>(*chars) > kMaxAsciiCharCodeU) {
+        return chars - start;
+      }
       ++chars;
     }
-    return true;
+    return chars - start;
+  }
+
+  static inline bool IsAscii(const char* chars, int length) {
+    return NonAsciiStart(chars, length) >= length;
+  }
+
+  static inline int NonAsciiStart(const uc16* chars, int length) {
+    const uc16* limit = chars + length;
+    const uc16* start = chars;
+    while (chars < limit) {
+      if (*chars > kMaxAsciiCharCodeU) return chars - start;
+      ++chars;
+    }
+    return chars - start;
   }
 
   static inline bool IsAscii(const uc16* chars, int length) {
-    const uc16* limit = chars + length;
-    while (chars < limit) {
-      if (*chars > kMaxAsciiCharCodeU) return false;
-      ++chars;
-    }
-    return true;
+    return NonAsciiStart(chars, length) >= length;
   }
 
  protected:
   class ReadBlockBuffer {
    public:
     ReadBlockBuffer(unibrow::byte* util_buffer_,
                     unsigned cursor_,
                     unsigned capacity_,
                     unsigned remaining_) :
       util_buffer(util_buffer_),
@@ skipping 1486 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_