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Multidimensional arrays and UNOPTFLAT warning #739

veripoolbot opened this issue Apr 10, 2014 · 1 comment

Multidimensional arrays and UNOPTFLAT warning #739

veripoolbot opened this issue Apr 10, 2014 · 1 comment


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@veripoolbot veripoolbot commented Apr 10, 2014

Author Name: Glen Gibb
Original Redmine Issue: 739 from
Original Date: 2014-04-10

I'd like to use a multidimensional array to connect a number of combinational blocks where the output of one block is used as the input to the next block.

To explain what I mean, I've put together a simple test case that sums the number of ones in a signal. I'm using the technique that one would use in a software implementation where it iteratively masks, shifts, and adds blocks of increasing width. (Ignore the fact that there may be better ways to do this in hardware.) A parameterized module performs a single mask-shift-add step. A number of these modules are instantiated and connected in sequence. The first module instance takes row 0 as input and generates row 1 as output; the second module instance takes row 1 as input and generates row 2 as output etc.

If I create distinct signals for each row (e.g., @reg [31:0] row_0, row_1, row_2, ...@) then everything works fine. If I try using a multi-dimensional array (e.g., @reg [31:0] row[6]@) then I receive an UNOPTFLAT warning because of a circular loop:

%Warning-UNOPTFLAT: t/t_multi_dim_array.v:16: Signal unoptimizable: Feedback to clock or circular logic: v.row
%Warning-UNOPTFLAT: Use "/* verilator lint_off UNOPTFLAT */" and lint_on around source to disable this message.

The code does execute correctly with the appropriate lint_off directive. Two questions:

  1. What is the performance impact of disabling the UNOPTFLAT warning and having this signal unoptimized?
  2. I think that this coding style (i.e., using a multidimensional array to propagate a signal through a number of combination processing stages) is a reasonable coding choice. If so, should Verilator be modified to better optimize code using multidimensional arrays in this manner?
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@veripoolbot veripoolbot commented Apr 11, 2014

Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2014-04-11T00:59:57Z

(Unfortunately) There are other bugs about UNOPTFLAT here, for example #� on a version of this with bit numbers. In brief the cost is as much as dividing performance of your whole model by the number of loops; Verilator reevaluates ALL combinatorial code when a change occurs, and there may be a change for each loop.

The fixes include one or more of:

  1. After unrolling converting arrays (or bits in arrays) to non-arrays so code schedules properly. (Presumably only when each element is completely "independent."
  2. Recognizing array elements and ordering separately.
  3. Putting unopt code into a smaller code section so it is less expensive when there is an UNOPTFLAT.
  4. Adding a real event model so such code can self-resolve.

Long term I'd like to do #1 as it is the best performing, AND #4 as it allows for clock generation and other more standard features.

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