Re-landing of "replace TrySync with Metadata".

runtime/vm/fixed_cache.h

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_VM_FIXED_CACHE_H_
 #define RUNTIME_VM_FIXED_CACHE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 namespace dart {
 
 // A simple sorted fixed size Key-Value storage.
 //
-// Assumes both Key and Value are POD-like objects.
+// Assumes both Key and Value are default-constructible objects.
 //
 // Keys must be comparable with operator<.
 //
 // Duplicates are not allowed - check with Lookup before insertion.
 //
-// Optionally Values may have cleanup function to delete
-// any resources they point to.
 template <class K, class V, intptr_t kCapacity>
 class FixedCache {
  public:
-  typedef void (*Deleter)(V*);
-
   struct Entry {
     K key;
     V value;
   };
 
-  explicit FixedCache(Deleter deleter = NULL) : deleter_(deleter), length_(0) {}
+  FixedCache() : length_(0) {}
 
   ~FixedCache() { Clear(); }
 
   V* Lookup(K key) {
     intptr_t i = LowerBound(key);
     if (i != length_ && pairs_[i].key == key) return &pairs_[i].value;
     return NULL;
   }
 
   void Insert(K key, V value) {
     intptr_t i = LowerBound(key);
 
     if (length_ == kCapacity) {
-      if (deleter_) deleter_(&pairs_[length_ - 1].value);
       length_ = kCapacity - 1;
       if (i == kCapacity) i = kCapacity - 1;
     }
 
     for (intptr_t j = length_ - 1; j >= i; j--) {
       pairs_[j + 1] = pairs_[j];
     }
 
     length_ += 1;
     pairs_[i].key = key;
     pairs_[i].value = value;
   }
 
   void Clear() {
-    if (deleter_) {
-      for (intptr_t i = 0; i < length_; i++) {
-        deleter_(&pairs_[i].value);
-      }
-    }
     length_ = 0;
   }
 
  private:
   intptr_t LowerBound(K key) {
     intptr_t low = 0, high = length_;
     while (low != high) {
       intptr_t mid = low + (high - low) / 2;
       if (key < pairs_[mid].key) {
         high = mid;
       } else if (key > pairs_[mid].key) {
         low = mid + 1;
       } else {
         low = high = mid;
       }
     }
     return low;
   }
 
   Entry pairs_[kCapacity];  // Sorted array of pairs.
-  Deleter deleter_;
   intptr_t length_;
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_FIXED_CACHE_H_