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 $n_1\times n_2\times \cdots \times n_d$ , 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 $+s_0+k_1\ast s_1+k_2\ast s_2+\cdots +k_d\ast s_d$-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