include/llvm/Support/ArrayRecycler.h

   1 //==- llvm/Support/ArrayRecycler.h - Recycling of Arrays ---------*- C++ -*-==//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file defines the ArrayRecycler class template which can recycle small
  10 // arrays allocated from one of the allocators in Allocator.h
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #ifndef LLVM_SUPPORT_ARRAYRECYCLER_H
  15 #define LLVM_SUPPORT_ARRAYRECYCLER_H
  16
  17 #include "llvm/ADT/SmallVector.h"
  18 #include "llvm/Support/Allocator.h"
  19 #include "llvm/Support/MathExtras.h"
  20
  21 namespace llvm {
  22
  23 /// Recycle small arrays allocated from a BumpPtrAllocator.
  24 ///
  25 /// Arrays are allocated in a small number of fixed sizes. For each supported
  26 /// array size, the ArrayRecycler keeps a free list of available arrays.
  27 ///
  28 template <class T, size_t Align = alignof(T)> class ArrayRecycler {
  29   // The free list for a given array size is a simple singly linked list.
  30   // We can't use iplist or Recycler here since those classes can't be copied.
  31   struct FreeList {
  32     FreeList *Next;
  33   };
  34
  35   static_assert(Align >= alignof(FreeList), "Object underaligned");
  36   static_assert(sizeof(T) >= sizeof(FreeList), "Objects are too small");
  37
  38   // Keep a free list for each array size.
  39   SmallVector<FreeList*, 8> Bucket;
  40
  41   // Remove an entry from the free list in Bucket[Idx] and return it.
  42   // Return NULL if no entries are available.
  43   T *pop(unsigned Idx) {
  44     if (Idx >= Bucket.size())
  45       return nullptr;
  46     FreeList *Entry = Bucket[Idx];
  47     if (!Entry)
  48       return nullptr;
  49     __asan_unpoison_memory_region(Entry, Capacity::get(Idx).getSize());
  50     Bucket[Idx] = Entry->Next;
  51     __msan_allocated_memory(Entry, Capacity::get(Idx).getSize());
  52     return reinterpret_cast<T*>(Entry);
  53   }
  54
  55   // Add an entry to the free list at Bucket[Idx].
  56   void push(unsigned Idx, T *Ptr) {
  57     assert(Ptr && "Cannot recycle NULL pointer");
  58     FreeList *Entry = reinterpret_cast<FreeList*>(Ptr);
  59     if (Idx >= Bucket.size())
  60       Bucket.resize(size_t(Idx) + 1);
  61     Entry->Next = Bucket[Idx];
  62     Bucket[Idx] = Entry;
  63     __asan_poison_memory_region(Ptr, Capacity::get(Idx).getSize());
  64   }
  65
  66 public:
  67   /// The size of an allocated array is represented by a Capacity instance.
  68   ///
  69   /// This class is much smaller than a size_t, and it provides methods to work
  70   /// with the set of legal array capacities.
  71   class Capacity {
  72     uint8_t Index;
  73     explicit Capacity(uint8_t idx) : Index(idx) {}
  74
  75   public:
  76     Capacity() : Index(0) {}
  77
  78     /// Get the capacity of an array that can hold at least N elements.
  79     static Capacity get(size_t N) {
  80       return Capacity(N ? Log2_64_Ceil(N) : 0);
  81     }
  82
  83     /// Get the number of elements in an array with this capacity.
  84     size_t getSize() const { return size_t(1u) << Index; }
  85
  86     /// Get the bucket number for this capacity.
  87     unsigned getBucket() const { return Index; }
  88
  89     /// Get the next larger capacity. Large capacities grow exponentially, so
  90     /// this function can be used to reallocate incrementally growing vectors
  91     /// in amortized linear time.
  92     Capacity getNext() const { return Capacity(Index + 1); }
  93   };
  94
  95   ~ArrayRecycler() {
  96     // The client should always call clear() so recycled arrays can be returned
  97     // to the allocator.
  98     assert(Bucket.empty() && "Non-empty ArrayRecycler deleted!");
  99   }
 100
 101   /// Release all the tracked allocations to the allocator. The recycler must
 102   /// be free of any tracked allocations before being deleted.
 103   template<class AllocatorType>
 104   void clear(AllocatorType &Allocator) {
 105     for (; !Bucket.empty(); Bucket.pop_back())
 106       while (T *Ptr = pop(Bucket.size() - 1))
 107         Allocator.Deallocate(Ptr);
 108   }
 109
 110   /// Special case for BumpPtrAllocator which has an empty Deallocate()
 111   /// function.
 112   ///
 113   /// There is no need to traverse the free lists, pulling all the objects into
 114   /// cache.
 115   void clear(BumpPtrAllocator&) {
 116     Bucket.clear();
 117   }
 118
 119   /// Allocate an array of at least the requested capacity.
 120   ///
 121   /// Return an existing recycled array, or allocate one from Allocator if
 122   /// none are available for recycling.
 123   ///
 124   template<class AllocatorType>
 125   T *allocate(Capacity Cap, AllocatorType &Allocator) {
 126     // Try to recycle an existing array.
 127     if (T *Ptr = pop(Cap.getBucket()))
 128       return Ptr;
 129     // Nope, get more memory.
 130     return static_cast<T*>(Allocator.Allocate(sizeof(T)*Cap.getSize(), Align));
 131   }
 132
 133   /// Deallocate an array with the specified Capacity.
 134   ///
 135   /// Cap must be the same capacity that was given to allocate().
 136   ///
 137   void deallocate(Capacity Cap, T *Ptr) {
 138     push(Cap.getBucket(), Ptr);
 139   }
 140 };
 141
 142 } // end llvm namespace
 143
 144 #endif