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      1 //==- llvm/Support/ArrayRecycler.h - Recycling of Arrays ---------*- C++ -*-==//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file defines the ArrayRecycler class template which can recycle small
     11 // arrays allocated from one of the allocators in Allocator.h
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #ifndef LLVM_SUPPORT_ARRAYRECYCLER_H
     16 #define LLVM_SUPPORT_ARRAYRECYCLER_H
     17 
     18 #include "llvm/ADT/SmallVector.h"
     19 #include "llvm/Support/Allocator.h"
     20 #include "llvm/Support/MathExtras.h"
     21 
     22 namespace llvm {
     23 
     24 /// Recycle small arrays allocated from a BumpPtrAllocator.
     25 ///
     26 /// Arrays are allocated in a small number of fixed sizes. For each supported
     27 /// array size, the ArrayRecycler keeps a free list of available arrays.
     28 ///
     29 template <class T, size_t Align = alignof(T)> class ArrayRecycler {
     30   // The free list for a given array size is a simple singly linked list.
     31   // We can't use iplist or Recycler here since those classes can't be copied.
     32   struct FreeList {
     33     FreeList *Next;
     34   };
     35 
     36   static_assert(Align >= alignof(FreeList), "Object underaligned");
     37   static_assert(sizeof(T) >= sizeof(FreeList), "Objects are too small");
     38 
     39   // Keep a free list for each array size.
     40   SmallVector<FreeList*, 8> Bucket;
     41 
     42   // Remove an entry from the free list in Bucket[Idx] and return it.
     43   // Return NULL if no entries are available.
     44   T *pop(unsigned Idx) {
     45     if (Idx >= Bucket.size())
     46       return nullptr;
     47     FreeList *Entry = Bucket[Idx];
     48     if (!Entry)
     49       return nullptr;
     50     Bucket[Idx] = Entry->Next;
     51     return reinterpret_cast<T*>(Entry);
     52   }
     53 
     54   // Add an entry to the free list at Bucket[Idx].
     55   void push(unsigned Idx, T *Ptr) {
     56     assert(Ptr && "Cannot recycle NULL pointer");
     57     FreeList *Entry = reinterpret_cast<FreeList*>(Ptr);
     58     if (Idx >= Bucket.size())
     59       Bucket.resize(size_t(Idx) + 1);
     60     Entry->Next = Bucket[Idx];
     61     Bucket[Idx] = Entry;
     62   }
     63 
     64 public:
     65   /// The size of an allocated array is represented by a Capacity instance.
     66   ///
     67   /// This class is much smaller than a size_t, and it provides methods to work
     68   /// with the set of legal array capacities.
     69   class Capacity {
     70     uint8_t Index;
     71     explicit Capacity(uint8_t idx) : Index(idx) {}
     72 
     73   public:
     74     Capacity() : Index(0) {}
     75 
     76     /// Get the capacity of an array that can hold at least N elements.
     77     static Capacity get(size_t N) {
     78       return Capacity(N ? Log2_64_Ceil(N) : 0);
     79     }
     80 
     81     /// Get the number of elements in an array with this capacity.
     82     size_t getSize() const { return size_t(1u) << Index; }
     83 
     84     /// Get the bucket number for this capacity.
     85     unsigned getBucket() const { return Index; }
     86 
     87     /// Get the next larger capacity. Large capacities grow exponentially, so
     88     /// this function can be used to reallocate incrementally growing vectors
     89     /// in amortized linear time.
     90     Capacity getNext() const { return Capacity(Index + 1); }
     91   };
     92 
     93   ~ArrayRecycler() {
     94     // The client should always call clear() so recycled arrays can be returned
     95     // to the allocator.
     96     assert(Bucket.empty() && "Non-empty ArrayRecycler deleted!");
     97   }
     98 
     99   /// Release all the tracked allocations to the allocator. The recycler must
    100   /// be free of any tracked allocations before being deleted.
    101   template<class AllocatorType>
    102   void clear(AllocatorType &Allocator) {
    103     for (; !Bucket.empty(); Bucket.pop_back())
    104       while (T *Ptr = pop(Bucket.size() - 1))
    105         Allocator.Deallocate(Ptr);
    106   }
    107 
    108   /// Special case for BumpPtrAllocator which has an empty Deallocate()
    109   /// function.
    110   ///
    111   /// There is no need to traverse the free lists, pulling all the objects into
    112   /// cache.
    113   void clear(BumpPtrAllocator&) {
    114     Bucket.clear();
    115   }
    116 
    117   /// Allocate an array of at least the requested capacity.
    118   ///
    119   /// Return an existing recycled array, or allocate one from Allocator if
    120   /// none are available for recycling.
    121   ///
    122   template<class AllocatorType>
    123   T *allocate(Capacity Cap, AllocatorType &Allocator) {
    124     // Try to recycle an existing array.
    125     if (T *Ptr = pop(Cap.getBucket()))
    126       return Ptr;
    127     // Nope, get more memory.
    128     return static_cast<T*>(Allocator.Allocate(sizeof(T)*Cap.getSize(), Align));
    129   }
    130 
    131   /// Deallocate an array with the specified Capacity.
    132   ///
    133   /// Cap must be the same capacity that was given to allocate().
    134   ///
    135   void deallocate(Capacity Cap, T *Ptr) {
    136     push(Cap.getBucket(), Ptr);
    137   }
    138 };
    139 
    140 } // end llvm namespace
    141 
    142 #endif
    143