Use a stack-allocated buffer for when computing a small string hash.

Using a 256-byte buffer avoids 99% of allocations across v8's top25
benchmark. This also leads to a significant performance increase on
speedometer, with a ~1.2% improvement on jQuery, ~1.3% on VanillaJS
and an overall ~0.4% improvement on the score.

Bug: v8:7555
Change-Id: Icd6fa07341eb989892431cb1e4995557e35c7a67
Reviewed-on: https://chromium-review.googlesource.com/971837
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52125}

src/objects.cc

@@ -17096,12 +17096,22 @@ class StringTableNoAllocateKey : public StringTableKey {
       special_flattening_ = true;
       uint32_t hash_field = 0;
       if (one_byte_) {
-        one_byte_content_ = new uint8_t[length];
+        if (V8_LIKELY(length <=
+                      static_cast<int>(arraysize(one_byte_buffer_)))) {
+          one_byte_content_ = one_byte_buffer_;
+        } else {
+          one_byte_content_ = new uint8_t[length];
+        }
         String::WriteToFlat(string, one_byte_content_, 0, length);
         hash_field =
             StringHasher::HashSequentialString(one_byte_content_, length, seed);
       } else {
-        two_byte_content_ = new uint16_t[length];
+        if (V8_LIKELY(length <=
+                      static_cast<int>(arraysize(two_byte_buffer_)))) {
+          two_byte_content_ = two_byte_buffer_;
+        } else {
+          two_byte_content_ = new uint16_t[length];
+        }
         String::WriteToFlat(string, two_byte_content_, 0, length);
         hash_field =
             StringHasher::HashSequentialString(two_byte_content_, length, seed);
@@ -17119,9 +17129,9 @@ class StringTableNoAllocateKey : public StringTableKey {
 
   ~StringTableNoAllocateKey() {
     if (one_byte_) {
-      delete[] one_byte_content_;
+      if (one_byte_content_ != one_byte_buffer_) delete[] one_byte_content_;
     } else {
-      delete[] two_byte_content_;
+      if (two_byte_content_ != two_byte_buffer_) delete[] two_byte_content_;
     }
   }
 
@@ -17195,6 +17205,10 @@ class StringTableNoAllocateKey : public StringTableKey {
     uint8_t* one_byte_content_;
     uint16_t* two_byte_content_;
   };
+  union {
+    uint8_t one_byte_buffer_[256];
+    uint16_t two_byte_buffer_[128];
+  };
 };
 
 }  // namespace