Intel® Hyperflex™ Architecture High-Performance Design Handbook

ID 683353
Date 10/04/2021
Document Table of Contents Duplicate Common Nodes

When the short path/long path critical chain contains common segments originating from same register, you can duplicate the register so one duplicate feeds the short path and one duplicate feeds the long path.

Figure 109. Critical Chain with Alternating Short Path/Long Path
Figure 110. Short Path/Long Path with Two Duplicate Nodes

The Fitter can optimize the newly-independent segments separately. The duplicated registers have common sources themselves, so they are not completely independent, but the optimization is easier with an extra, independent register in each part of the critical chain.

You can apply a maximum fan-out synthesis directive to the common source registers. Use a value of one, because a value greater than one can result in the short and long path segments having the same source node, which you tried to avoid.

Alternately, use a synthesis directive to preserve the duplicate registers if you manually duplicate the common source register in a short path/long path critical chain. Otherwise, the duplicates may get merged during synthesis. Using a synthesis directive to preserve the duplicate registers can cause an unintended retiming restriction. Use a maximum fan-out directive.

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