Many C algorithms sequentially shift data through arrays. They add a new value to the start of the array, shift the existing data through array, and drop the oldest data value. This operation is implemented in hardware as a shift register.
This most common way to implement a shift register from C into hardware is to completely partition the array into individual elements, and allow the data dependencies between the elements in the RTL to imply a shift register.
Logic synthesis typically implements the RTL shift register into a Xilinx SRL resource, which efficiently implements shift registers. The issue is that sometimes logic synthesis does not implement the RTL shift register using an SRL component:
- When data is accessed in the middle of the shift register, logic synthesis cannot directly infer an SRL.
- Sometimes, even when the SRL is ideal, logic synthesis may implement the shift-resister in flip-flops, due to other factors. (Logic synthesis is also a complex process).
Vitis HLS provides a C++ class (ap_shift_reg) to ensure that the shift register defined in the C code is always implemented using an SRL resource. The ap_shift_reg class has two methods to perform the various read and write accesses supported by an SRL component.