do aligned reads in ContainsOnlyOneByte

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index ed1e26525aa9a75dfb1f0d3e1731020c607c179f..3c1a003809bbb4efd0021c1bc8c45e5144b5a051 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4315,17 +4315,49 @@ class ContainsOnlyOneByteHelper {
   void VisitOneByteString(const uint8_t* chars, int length) {
     // Nothing to do.
   }
-  // TODO(dcarney): do word aligned read.
   void VisitTwoByteString(const uint16_t* chars, int length) {
-    // Check whole string without breaking.
-    uint16_t total = 0;
-    for (int i = 0; i < length; i++) {
-      total |= chars[i] >> 8;
+    // Accumulated bits.
+    uintptr_t acc = 0;
+    // Align to uintptr_t.
+    const uint16_t* end = chars + length;
+    while (Unaligned(chars) && chars != end) {
+        acc |= *chars++;
     }
-    if (total != 0) is_one_byte_ = false;
+    // Read word aligned in blocks,
+    // checking the return value at the end of each block.
+    const uint16_t* aligned_end = Align(end);
+    const int increment = sizeof(uintptr_t)/sizeof(uint16_t);
+    const int inner_loops = 16;
+    while (chars + inner_loops*increment < aligned_end) {
+      for (int i = 0; i < inner_loops; i++) {
+        acc |= *reinterpret_cast<const uintptr_t*>(chars);
+        chars += increment;
+      }
+      // Check for early return.
+      if ((acc & kOneByteMask) != 0) {
+        is_one_byte_ = false;
+        return;
+      }
+    }
+    // Read the rest.
+    while (chars != end) {
+      acc |= *chars++;
+    }
+    // Check result.
+    if ((acc & kOneByteMask) != 0) is_one_byte_ = false;
   }
 
  private:
+  static const uintptr_t kOneByteMask =
+      static_cast<uintptr_t>(0xFF00FF00FF00FF00ULL);
+  static const uintptr_t kAlignmentMask = sizeof(uintptr_t) - 1;
+  static inline bool Unaligned(const uint16_t* chars) {
+    return reinterpret_cast<const uintptr_t>(chars) & kAlignmentMask;
+  }
+  static inline const uint16_t* Align(const uint16_t* chars) {
+    return reinterpret_cast<uint16_t*>(
+        reinterpret_cast<uintptr_t>(chars) & ~kAlignmentMask);
+  }
   bool CheckCons(i::ConsString* cons_string) {
     while (true) {
       // Check left side if flat.