AstValueFactory: add a cache for one-character strings.

Lowercase 1 character strings occur frequently in minified code. Add a
cache for them, so that we don't need to compute the hash + do the hash
table lookup for each occurrence.

BUG=

Review-Url: https://codereview.chromium.org/2541353002
Cr-Commit-Position: refs/heads/master@{#41597}

src/ast/ast-value-factory.cc

@@ -28,6 +28,7 @@
 #include "src/ast/ast-value-factory.h"
 
 #include "src/api.h"
+#include "src/char-predicates-inl.h"
 #include "src/objects.h"
 #include "src/utils.h"
 
@@ -222,6 +223,15 @@ void AstValue::Internalize(Isolate* isolate) {
 
 AstRawString* AstValueFactory::GetOneByteStringInternal(
     Vector<const uint8_t> literal) {
+  if (literal.length() == 1 && IsInRange(literal[0], 'a', 'z')) {
+    int key = literal[0] - 'a';
+    if (one_character_strings_[key] == nullptr) {
+      uint32_t hash = StringHasher::HashSequentialString<uint8_t>(
+          literal.start(), literal.length(), hash_seed_);
+      one_character_strings_[key] = GetString(hash, true, literal);
+    }
+    return one_character_strings_[key];
+  }
   uint32_t hash = StringHasher::HashSequentialString<uint8_t>(
       literal.start(), literal.length(), hash_seed_);
   return GetString(hash, true, literal);

src/ast/ast-value-factory.h

@@ -333,7 +333,6 @@ class AstValueFactory {
   AstValueFactory(Zone* zone, uint32_t hash_seed)
       : string_table_(AstRawStringCompare),
         values_(nullptr),
-        smis_(),
         strings_(nullptr),
         strings_end_(&strings_),
         zone_(zone),
@@ -345,6 +344,9 @@ class AstValueFactory {
     OTHER_CONSTANTS(F)
 #undef F
     std::fill(smis_, smis_ + arraysize(smis_), nullptr);
+    std::fill(one_character_strings_,
+              one_character_strings_ + arraysize(one_character_strings_),
+              nullptr);
   }
 
   Zone* zone() const { return zone_; }
@@ -422,11 +424,16 @@ class AstValueFactory {
   // they can be internalized later).
   AstValue* values_;
 
-  AstValue* smis_[kMaxCachedSmi + 1];
   // We need to keep track of strings_ in order since cons strings require their
   // members to be internalized first.
   AstString* strings_;
   AstString** strings_end_;
+
+  // Caches for faster access: small numbers, one character lowercase strings
+  // (for minified code).
+  AstValue* smis_[kMaxCachedSmi + 1];
+  AstRawString* one_character_strings_[26];
+
   Zone* zone_;
 
   uint32_t hash_seed_;