Mapping Directly into SRL Resources - 2023.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2023.2 English

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 an AMD 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.