Intel® Quartus® Prime Standard Edition User Guide: Design Recommendations

ID 683323
Date 9/24/2018
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2.6.5.3. Implicit Defaults in IF Statements

IF statements in Verilog HDL and VHDL can simplify expressing conditions that do not easily lend themselves to a CASE-type approach. However, IF statements can result in complex multiplexer trees that are not easy for synthesis tools to optimize. In particular, all IF statements have an ELSE condition, even when not specified in the code. These implicit defaults can cause additional complexity in multiplexed designs.

You can simplify multiplexed logic and remove unneeded defaults with multiple methods. The optimal method is recoding the design, so the logic takes the structure of a 4:1 CASE statement. Alternatively, if priority is important, you can restructure the code to reduce default cases and flatten the multiplexer. Examine whether the default "ELSE IF" conditions are don’t care cases. You can add a default ELSE statement to make the behavior explicit. Avoid unnecessary default conditions in the multiplexer logic to reduce the complexity and logic utilization that the design implementation requires.