A concat typically has multiple inputs. Whenever one of the input values
change the output value of the concat is updated and propagated to its
downstream consumers.
When multiple inputs change within the same cycle each input will cause a
update propagation. Depending of the overall structure of the design this
can cause a significant performance penalty.
E.g. the following synthetic structure has a exponential runtime increase
based on the value of N.
```
reg [N-1:0] x;
generate for (genvar i = 0; i < N - 1; i++)
assign x[i+1] = ^{x[i],x[i]};
endgenerate
```
To improve this defer the value propagation of the concat to the end of the
current cycle, this allows multiple input updates to be included in a
single output update.
For the example in report #1052 this reduced the runtime from 2 minutes to
essentially 0.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cary R
Lars-Peter Clausen