INPUT_STRIDES and OUTPUT_STRIDES

The FFT interface provides configuration parameters that define the layout of multidimensional data in the computer memory. For d-dimensional data set, \(X\), defined by dimensions \(n_1\times n_2 \times\dots\times n_d\) , the layout describes where a particular element \(X(k_1, k_2, \dots, k_d )\) of the data set is located. The memory address of the element \(X(k_1, k_2, \dots, k_d )\) is expressed by the formula: \(X(k_1, k_2, \dots, k_d ) =\) the \(+ s_0 + k_1*s_1 + k_2*s_2 + \dots + k_d*s_d\)-th element of the container (sycl::buffer or USM pointer) provided to the compute function, where \(s_0\) is the displacement and \(s_1 , \dots, s_d\) are generalized strides. The configuration parameters config_param::INPUT_STRIDES and config_param::OUTPUT_STRIDES enable you to get and set these values. The configuration value is a \(d+1`\) lengthed std::vector<std::int64_t> of values \((s_0, s_1, \dots, s_d )\).

The offset is counted in elements of the data type (complex or real) defined by the descriptor configuration as tabulated below.

The computation functions take containers(sycl::buffer or USM pointer) which are typed according to the descriptor configuration parameters. Specifically, the forward domain which defines the type of transformation and the storage format configuration parameters: config_param::COMPLEX_STORAGE, config_param::REAL_STORAGE and config_param::CONJUGATE_EVEN_STORAGE define the type of the elements as shown here:

Assumed Element Types using complex-to-complex transform and config_param::COMPLEX_STORAGE:

COMPLEX_STORAGE	Element type of forward data	Element type of backward data
COMPLEX_COMPLEX	Complex	Complex
REAL_REAL	Real	Real

Assumed Element Types using real-to-complex transform and config_param::REAL_STORAGE:

REAL_STORAGE	Element type of forward data	Element type of backward data
REAL_REAL	Real	Real

Assumed Element Types using real-to-complex transform and config_param::CONJUGATE_EVEN_STORAGE:

CONJUGATE_EVEN_STORAGE	Element type of forward data	Element type of backward data
COMPLEX_COMPLEX	Real	Complex

The config_param::INPUT_STRIDES configuration parameter defines the layout of the input data, while the element type is defined by the forward domain for the compute_forward function and by the backward domain for the compute_backward function. The config_param::OUTPUT_STRIDES configuration parameter defines the layout of the output data, while the element type is defined by the backward domain for the compute_forward function and by the forward domain for compute_backward function.

For in-place transforms ( config_param::PLACEMENT=config_value::INPLACE ), the configuration set by config_param::OUTPUT_STRIDES is ignored when the element types in the forward and backward domains are the same. If they are different, set config_param::OUTPUT_STRIDES explicitly (even though the transform is in-place). Ensure a consistent configuration for in-place transforms, that is, the locations of the first elements on input and output must coincide in each dimension.

Parent topic Configuration Parameters and Enums