add fast path for hashing small cons strings

R=yangguo@chromium.org
LOG=N

BUG=437280

Review URL: https://codereview.chromium.org/769453002

Cr-Commit-Position: refs/heads/master@{#25562}

src/objects-inl.h

@@ -6828,9 +6828,8 @@ uint32_t IteratingStringHasher::Hash(String* string, uint32_t seed) {
   // Nothing to do.
   if (hasher.has_trivial_hash()) return hasher.GetHashField();
   ConsString* cons_string = String::VisitFlat(&hasher, string);
-  if (cons_string != nullptr) {
-    hasher.VisitConsString(cons_string);
-  }
+  if (cons_string == nullptr) return hasher.GetHashField();
+  hasher.VisitConsString(cons_string);
   return hasher.GetHashField();
 }
 

src/objects.cc

@@ -9298,6 +9298,18 @@ uint32_t StringHasher::ComputeUtf8Hash(Vector<const char> chars,
 
 
 void IteratingStringHasher::VisitConsString(ConsString* cons_string) {
+  // Run small ConsStrings through ConsStringIterator.
+  if (cons_string->length() < 64) {
+    ConsStringIterator iter(cons_string);
+    int offset;
+    String* string;
+    while (nullptr != (string = iter.Next(&offset))) {
+      DCHECK_EQ(0, offset);
+      String::VisitFlat(this, string, 0);
+    }
+    return;
+  }
+  // Slow case.
   const int max_length = String::kMaxHashCalcLength;
   int length = std::min(cons_string->length(), max_length);
   if (cons_string->HasOnlyOneByteChars()) {