5.2.2. Short Path/Long Path
When the critical chain has related paths with conflicting characteristics, where one path can improve performance with more registers, and another path has no place for additional registers, the limiting reason reported is Short Path/Long Path.
A critical chain is categorized as short path/long path when there are conflicting optimization goals for Hyper-Retiming. Short paths and long paths are always connected in some way, with at least one common node. Retimed registers must maintain functional correctness and ensure identical relative latency through both critical chains. This requirement can result in conflicting optimization goals. Therefore, one segment (the long path) can accept the retiming move, but the other segment (the short path) cannot accept the retiming move. The retiming move is typically retiming an additional register into the short and long paths. Figure 1 illustrates this concept.
Critical chains are categorized as short path/long path for the following reasons:
- When Hyper-Register locations are not available on the short path to retime into.
- When retiming a register into both paths to improve the performance of the long path does not meet hold time requirement on the short path. Sometimes, short path/long path critical chains exist as a result of the circuit structures used in a design, such as broadcast control signals, synchronous clears, and clock enables.
Short path/long path critical chains are a new optimization focus associated with post-fit retiming. In conventional retiming, the structure of the netlist can be changed during synthesis or placement and routing. However, during Hyper-Retiming, short path/long path can occur because the netlist structure, and the placement and routing cannot be changed.
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