Intel® High Level Synthesis Compiler Pro Edition: Reference Manual

ID 683349
Date 10/04/2021

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Document Table of Contents Stall-Free RTL

The Intel® HLS Compiler can optimize hardware resource usage and performance by not placing stall logic around an RTL module with fixed latency.

If you have an RTL module with a fixed latency that you want integrated into your component pipeline without surrounding stall logic, ensure that you set attributes in the object manifest file (.xml) as follows:

  1. Specify a value for the EXPECTED_LATENCY attribute (under the FUNCTION element) so that the latency equals the number of pipeline stages in the module.
    Important: An inaccurate EXPECTED_LATENCY value causes the RTL module to be out of sync with the rest of the pipeline, and can lead to functionally incorrect results.
  2. Set the IS_STALL_FREE attribute under the FUNCTION element to "yes".

    This setting instructs the Intel® HLS Compiler to avoid placing stall logic around the RTL module. This setting also tells the compiler that the RTL module produces a result after the number cycles specified in the EXPECTED_LATENCY attribute after accepting input values. The stall free logic produces a result every cycle but the result is delayed by the number cycles specified in the EXPECTED_LATENCY attribute.

For RTL modules with a fixed latency, the output signals ( ovalid and oready) can have constant high values, and the input ready signal ( iready) can be ignored.

A stall-free RTL module might receive an invalid input signal (ivalid is low). In this case, the module must produce invalid data on the output EXPECTED_LATENCY cycles after the cycle in which the input was invalid. For a stall-free RTL module without an internal state, you might find it convenient to propagate the invalid input through the module. If the module has an internal state, that state should not be affect by data inputs that are not accompanied by ivalid = 1.