On Replication and Wire Length

September 12, 2008

It is for some reason a common view, that when using replication you also have to pay in increased wire length. It looks reasonable isn’t it? After all, you now have more blocks to wire into and out of and therefore total wire length should increase, right? Well, not really…

In some cases this might be true, but in most cases wire length should decrease. Wiring in a chip obeys taxicab geometry laws, so it is a bit less intuitive than usual.

Here is a simple example showing how wire length can decrease after replication. Sure, I chose the block placements and the replicated block (R) size to be in my favor, but this is not a rigorous math proof.

Before replication

After replication

Notice how blocks (A) and (B) are now actually farther apart. This leaves more room for other critical logic to be placed in the precious place near the center. On the other hand, after replication we now have one really long wire going out of block (C).

Bottom line: don’t be afraid to use replication when you can, it has many advantages and not only for improving timing.

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