Improve string runtime compare performance for flat strings.

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index 2fcd241fd5350082f60e243e17b906f0904b822c..2cad4c12e4a8f93ee46cd4d2214b8583d8f09dbb 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -528,11 +528,11 @@ static inline void CopyChars(sinkchar* dest, const sourcechar* src, int chars) {
   sinkchar* limit = dest + chars;
 #ifdef V8_HOST_CAN_READ_UNALIGNED
   if (sizeof(*dest) == sizeof(*src)) {
-    // Number of characters in a uint32_t.
-    static const int kStepSize = sizeof(uint32_t) / sizeof(*dest);  // NOLINT
+    // Number of characters in a uintptr_t.
+    static const int kStepSize = sizeof(uintptr_t) / sizeof(*dest);  // NOLINT
     while (dest <= limit - kStepSize) {
-      *reinterpret_cast<uint32_t*>(dest) =
-          *reinterpret_cast<const uint32_t*>(src);
+      *reinterpret_cast<uintptr_t*>(dest) =
+          *reinterpret_cast<const uintptr_t*>(src);
       dest += kStepSize;
       src += kStepSize;
     }
@@ -544,6 +544,34 @@ static inline void CopyChars(sinkchar* dest, const sourcechar* src, int chars) {
 }
 
 
+// Compare ASCII/16bit chars to ASCII/16bit chars.
+template <typename lchar, typename rchar>
+static inline int CompareChars(const lchar* lhs, const rchar* rhs, int chars) {
+  const lchar* limit = lhs + chars;
+#ifdef V8_HOST_CAN_READ_UNALIGNED
+  if (sizeof(*lhs) == sizeof(*rhs)) {
+    // Number of characters in a uintptr_t.
+    static const int kStepSize = sizeof(uintptr_t) / sizeof(*lhs);  // NOLINT
+    while (lhs <= limit - kStepSize) {
+      if (*reinterpret_cast<const uintptr_t*>(lhs) !=
+          *reinterpret_cast<const uintptr_t*>(rhs)) {
+        break;
+      }
+      lhs += kStepSize;
+      rhs += kStepSize;
+    }
+  }
+#endif
+  while (lhs < limit) {
+    int r = static_cast<int>(*lhs) - static_cast<int>(*rhs);
+    if (r != 0) return r;
+    ++lhs;
+    ++rhs;
+  }
+  return 0;
+}
+
+
 // Calculate 10^exponent.
 int TenToThe(int exponent);