Eliminate Loop-Carried Dependencies - 2021.2 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

Document ID
UG1393
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

The most common case for loop-carried dependencies is when a loop iteration relies on a value computed in a prior iteration. There are differences whether the dependencies are on arrays or on scalar variables. For more information, see Unrolling Loops to Improve Pipelining in the Vitis HLS User Guide (UG1399).

  • Eliminating dependencies on arrays

    The HLS compiler performs index analysis to determine whether array dependencies exist (read-after-write, write-after-read, write-after-write). The tool may not always be able to statically resolve potential dependencies and will in this case report false dependencies.

    Special compiler pragmas can overwrite these dependencies and improve the II of the design. In this situation, be cautious and do not overwrite a valid dependency.

  • Eliminating dependencies on scalars

    In the case of scalar dependencies, there is usually a feedback path with a computation scheduled over multiple clock cycles. Complex arithmetic operations such as multiplications, divisions, or modulus are often found on these feedback paths. The number of cycles in the feedback path directly limits the potential II and should be reduced to improve II and throughput. To do so, analyze the feedback path to determine if and how it can be shortened. This can potentially be done using HLS scheduling constraints or code modifications such as reducing bit widths.