Stable Arrays - 2022.1 English - UG1399

Vitis High-Level Synthesis User Guide (UG1399)
Document ID
UG1399
Release Date
2022-06-07
Version
2022.1 English

The stable pragma can be used to mark input or output variables of a dataflow region. Its effect is to remove their corresponding synchronizations, assuming that the user guarantees this removal is indeed correct.

void dataflow_region(int A[...], ...
#pragma HLS stable variable=A
#pragma HLS dataflow
    proc1(...);
    proc2(A, ...);

Without the stable pragma, and assuming that A is read by proc2, then proc2 would be part of the initial synchronization (via ap_start), for the dataflow region where it is located. This means that proc1 would not restart until proc2 is also ready to start again, which would prevent dataflow iterations to be overlapped and induce a possible loss of performance. The stable pragma indicates that this synchronization is not necessary to preserve correctness.

In the previous example, without the stable pragma, and assuming that A is read by proc2 as proc2 is bypassing the tasks, there will be a performance loss.

With the stable pragma, the compiler assumes that:
  • if A is read by proc2, then the memory locations that are read will not be overwritten, by any other process or calling context, while dataflow_region is being executed.
  • if A is written by proc2, then the memory locations written will not be read, before their definition, by any other process or calling context, while dataflow_region is being executed.

A typical scenario is when the caller updates or reads these variables only when the dataflow region has not started yet or has completed execution.