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

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 56b54323f0cf59711e6ee9047eb5524b29ca0020..f6fb8d23d12d811aad2ce0d4d79d4a945950f3e4 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -7413,32 +7413,47 @@ class String: public HeapObject {
                           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 uc16* chars, int length) {
+  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 false;
+      if (*chars > kMaxAsciiCharCodeU) return chars - start;
       ++chars;
     }
-    return true;
+    return chars - start;
+  }
+
+  static inline bool IsAscii(const uc16* chars, int length) {
+    return NonAsciiStart(chars, length) >= length;
   }
 
  protected: