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struct dnnl::memory::desc

Overview

A memory descriptor. More…

#include <dnnl.hpp>

struct desc
{
    // fields

    dnnl_memory_desc_t data;

    // construction

    desc();

    desc(
        const dims& adims,
        data_type adata_type,
        format_tag aformat_tag,
        bool allow_empty = false
        );

    desc(
        const dims& adims,
        data_type adata_type,
        const dims& strides,
        bool allow_empty = false
        );

    desc(const dnnl_memory_desc_t& data);

    // methods

    desc submemory_desc(
        const dims& adims,
        const dims& offsets,
        bool allow_empty = false
        ) const;

    desc reshape(const dims& adims, bool allow_empty = false) const;
    desc permute_axes(const std::vector<int>& permutation, bool allow_empty = false) const;
    memory::data_type data_type() const;
    memory::dims dims() const;
    size_t get_size() const;
    bool is_zero() const;
    bool operator == (const desc& other) const;
    bool operator != (const desc& other) const;
    operator bool () const;
};

Detailed Documentation

A memory descriptor.

Fields

dnnl_memory_desc_t data

The underlying C API data structure.

Construction

desc()

Constructs a zero (empty) memory descriptor.

Such a memory descriptor can be used to indicate absence of an argument.

desc(
    const dims& adims,
    data_type adata_type,
    format_tag aformat_tag,
    bool allow_empty = false
    )

Constructs a memory descriptor.

Note

The logical order of dimensions corresponds to the abc... format tag, and the physical meaning of the dimensions depends both on the primitive that would operate on this memory and the operation context.

Parameters:

adims	Tensor dimensions.
adata_type	Data precision/type.
aformat_tag	Memory format tag.
allow_empty	A flag signifying whether construction is allowed to fail without throwing an exception. In this case a zero memory descriptor will be constructed. This flag is optional and defaults to false.

desc(
    const dims& adims,
    data_type adata_type,
    const dims& strides,
    bool allow_empty = false
    )

Constructs a memory descriptor by strides.

Note

The logical order of dimensions corresponds to the abc... format tag, and the physical meaning of the dimensions depends both on the primitive that would operate on this memory and the operation context.

Parameters:

adims	Tensor dimensions.
adata_type	Data precision/type.
strides	Strides for each dimension.
allow_empty	A flag signifying whether construction is allowed to fail without throwing an exception. In this case a zero memory descriptor will be constructed. This flag is optional and defaults to false.

desc(const dnnl_memory_desc_t& data)

Constructs a memory descriptor from a C API data structure.

Parameters:

data	A C API dnnl_memory_desc_t structure.

Methods

desc submemory_desc(
    const dims& adims,
    const dims& offsets,
    bool allow_empty = false
    ) const

Constructs a memory descriptor for a region inside an area described by this memory descriptor.

Parameters:

adims	Sizes of the region.
offsets	Offsets to the region from the encompassing memory object in each dimension.
allow_empty	A flag signifying whether construction is allowed to fail without throwing an exception. In this case a zero memory descriptor will be returned. This flag is optional and defaults to false.

Returns:

A memory descriptor for the region.

desc reshape(const dims& adims, bool allow_empty = false) const

Constructs a memory descriptor by reshaping an existing one.

The new memory descriptor inherits the data type. This operation is valid only for memory descriptors that have format_kind set to dnnl::memory::format_kind::blocked or dnnl::memory::format_kind::any.

The operation ensures that the transformation of the physical memory format corresponds to the transformation of the logical dimensions. If such transformation is impossible, the function either throws an exception (default) or returns a zero memory descriptor depending on the allow_empty flag.

The reshape operation can be described as a combination of the following basic operations:

1. Add a dimension of size 1. This is always possible.

2. Remove a dimension of size 1. This is possible only if the dimension has no padding (i.e. padded_dims[dim] == dims[dim] && dims[dim] == 1).

3. Split a dimension into multiple ones. This is possible only if the product of all tensor dimensions stays constant and the dimension being split does not have padding (i.e. padded_dims[dim] = dims[dim]).

4. Join multiple consecutive dimensions into a single one. As in the cases above, this requires that the dimensions do not have padding and that the memory format is such that in physical memory these dimensions are dense and have the same order as their logical counterparts. This also assumes that these dimensions are not blocked.

   • Here, ‘dense’ means: stride for dim[i] == (stride for dim[i + 1]) * dim[i + 1];

   • And ‘same order’ means: i < j if and only if stride for dim[j] <= stride for dim[i].

Warning

Some combinations of physical memory layout and/or offsets or dimensions may result in a failure to make a reshape.

Parameters:

adims	New dimensions. The product of dimensions must remain constant.
allow_empty	A flag signifying whether construction is allowed to fail without throwing an exception. In this case a zero memory descriptor will be returned. This flag is optional and defaults to false.

Returns:

A new memory descriptor with new dimensions.

desc permute_axes(const std::vector<int>& permutation, bool allow_empty = false) const

Constructs a memory descriptor by permuting axes in an existing one.

The physical memory layout representation is adjusted accordingly to maintain the consistency between the logical and physical parts of the memory descriptor. The new memory descriptor inherits the data type.

The new memory descriptor inherits the data type. This operation is valid only for memory descriptors that have format_kind set to dnnl::memory::format_kind::blocked or dnnl::memory::format_kind::any.

The logical axes will be permuted in the following manner:

for (i = 0; i < ndims(); i++)
    new_desc.dims()[permutation[i]] = dims()[i];

Example:

std::vector<int> permutation = {1, 0}; // swap the first and
                                       // the second axes
dnnl::memory::desc in_md(
        {2, 3}, data_type, memory::format_tag::ab);
dnnl::memory::desc expect_out_md(
        {3, 2}, data_type, memory::format_tag::ba);

assert(in_md.permute_axes(permutation) == expect_out_md);

Parameters:

permutation	Axes permutation.
allow_empty	A flag signifying whether construction is allowed to fail without throwing an exception. In this case a zero memory descriptor will be returned. This flag is optional and defaults to false.

Returns:

A new memory descriptor with new dimensions.

memory::data_type data_type() const

Returns the data type of the memory descriptor.

Returns:

The data type.

memory::dims dims() const

Returns dimensions of the memory descriptor.

Potentially expensive due to the data copy involved.

Returns:

A copy of the dimensions vector.

size_t get_size() const

Returns size of the memory descriptor in bytes.

Returns:

The number of bytes required to allocate a memory buffer for the memory object described by this memory descriptor including the padding area.

bool is_zero() const

Checks whether the memory descriptor is zero (empty).

Returns:

true if the memory descriptor describes an empty memory and false otherwise.

bool operator == (const desc& other) const

An equality operator.

Parameters:

other

Another memory descriptor.

Returns:

Whether this and the other memory descriptors have the same format tag, dimensions, strides, blocking, etc.

bool operator != (const desc& other) const

An inequality operator.

Parameters:

other

Another memory descriptor.

Returns:

Whether this and the other memory descriptors describe different memory.

operator bool () const

Checks whether the object is not empty.

Returns:

Whether the object is not empty.

enum dnnl::memory::format_tag Engine