stackBuffer.h

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//
//   Copyright 2015 DreamWorks Animation LLC.
//
//   Licensed under the terms set forth in the LICENSE.txt file available at
//   https://opensubdiv.org/license.
//
#ifndef OPENSUBDIV3_VTR_STACK_BUFFER_H
#define OPENSUBDIV3_VTR_STACK_BUFFER_H
 
#include "../version.h"
 
namespace OpenSubdiv {
namespace OPENSUBDIV_VERSION {
 
namespace Vtr {
namespace internal {
 
//
//  The StackBuffer class is intended solely to take the place of VLAs (Variable
//  Length Arrays) which most compilers support, but are not strictly standard C++.
//  Portability concerns forces us to make use of either alloca() or some other
//  mechanism to create small arrays on the stack that are typically based on the
//  valence of a vertex -- small in general, but occasionally large.
//
//  Note also that since the intent of this is to replace VLAs -- not general
//  std::vectors -- support for std::vector functionality is intentionally limited
//  and STL-like naming is avoided.  Like a VLA there is no incremental growth.
//  Support for resizing is available to reuse an instance at the beginning of a
//  loop with a new size, but resizing in this case reinitializes all elements.
//
 
template <typename TYPE, unsigned int SIZE, bool POD_TYPE = false>
class StackBuffer
{
public:
    typedef unsigned int size_type;
 
public:
    //  Constructors and destructor -- declared inline below:
    StackBuffer();
    StackBuffer(size_type size);
    ~StackBuffer();
 
public:
    //  Note the reliance on implicit casting so that it can be used similar to
    //  a VLA.  This removes the need for operator[] as the resulting TYPE* will
    //  natively support [].  (The presence of both TYPE* and operator[] also
    //  causes an ambiguous overloading error with 32-bit MSVC builds.)
 
    operator TYPE const * () const { return _data; }
    operator TYPE *       ()       { return _data; }
 
    size_type GetSize() const { return _size; }
 
    void SetSize(size_type size);
    void Reserve(size_type capacity);
 
private:
    //  Non-copyable:
    StackBuffer(const StackBuffer<TYPE,SIZE,POD_TYPE> &) { }
    StackBuffer& operator=(const StackBuffer<TYPE,SIZE,POD_TYPE> &) { return *this; }
 
    void allocate(size_type capacity);
    void deallocate();
    void construct();
    void destruct();
 
private:
    TYPE *     _data;
    size_type  _size;
    size_type  _capacity;
 
    //  Is alignment an issue here?  The staticData arena will at least be double-word
    //  aligned within this struct, which meets current and most anticipated needs.
    char   _staticData[SIZE * sizeof(TYPE)];
    char * _dynamicData;
};
 
 
//
//  Core allocation/deallocation methods:
//
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline void
StackBuffer<TYPE,SIZE,POD_TYPE>::allocate(size_type capacity) {
 
    //  Again, is alignment an issue here?  C++ spec says new will return pointer
    //  "suitably aligned" for conversion to pointers of other types, which implies
    //  at least an alignment of 16.
    _dynamicData = static_cast<char*>(::operator new(capacity * sizeof(TYPE)));
 
    _data = reinterpret_cast<TYPE*>(_dynamicData);
    _capacity = capacity;
}
 
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline void
StackBuffer<TYPE,SIZE,POD_TYPE>::deallocate() {
 
    ::operator delete(_dynamicData);
    _dynamicData = nullptr;
 
    _data = reinterpret_cast<TYPE*>(_staticData);
    _capacity = SIZE;
}
 
//
//  Explicit element-wise construction and destruction within allocated memory.
//  Compilers do not always optimize out the iteration here even when there is
//  no construction or destruction, so the POD_TYPE arguement can be used to
//  force this when/if it becomes an issue (and it has been in some cases).
//
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline void
StackBuffer<TYPE,SIZE,POD_TYPE>::construct() {
 
    for (size_type i = 0; i < _size; ++i) {
        (void) new (&_data[i]) TYPE;
    }
}
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline void
StackBuffer<TYPE,SIZE,POD_TYPE>::destruct() {
 
    for (size_type i = 0; i < _size; ++i) {
        _data[i].~TYPE();
    }
}
 
//
//  Inline constructors and destructor:
//
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline
StackBuffer<TYPE,SIZE,POD_TYPE>::StackBuffer() :
    _data(reinterpret_cast<TYPE*>(_staticData)),
    _size(0),
    _capacity(SIZE),
    _dynamicData(nullptr) {
 
}
 
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline
StackBuffer<TYPE,SIZE,POD_TYPE>::StackBuffer(size_type size) :
    _data(reinterpret_cast<TYPE*>(_staticData)),
    _size(size),
    _capacity(SIZE),
    _dynamicData(nullptr) {
 
    if (size > SIZE) {
        allocate(size);
    }
    if (!POD_TYPE) {
        construct();
    }
}
 
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline
StackBuffer<TYPE,SIZE,POD_TYPE>::~StackBuffer() {
 
    if (!POD_TYPE) {
        destruct();
    }
    deallocate();
}
 
//
//  Inline sizing methods:
//
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline void
StackBuffer<TYPE,SIZE,POD_TYPE>::Reserve(size_type capacity) {
 
    if (capacity > _capacity) {
        if (!POD_TYPE) {
            destruct();
        }
        deallocate();
        allocate(capacity);
    }
}
 
template <typename TYPE, unsigned int SIZE, bool POD_TYPE>
inline void
StackBuffer<TYPE,SIZE,POD_TYPE>::SetSize(size_type size)
{
    if (!POD_TYPE) {
        destruct();
    }
    if (size == 0) {
        deallocate();
    } else if (size > _capacity) {
        deallocate();
        allocate(size);
    }
    _size = size;
    if (!POD_TYPE) {
        construct();
    }
}
 
} // end namespace internal
} // end namespace Vtr
 
} // end namespace OPENSUBDIV_VERSION
using namespace OPENSUBDIV_VERSION;
} // end namespace OpenSubdiv
 
#endif /* OPENSUBDIV3_VTR_STACK_BUFFER_H */