a question about verilog hdl?

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What is a? Which differences do you expect? I expect no differences, cause both expressions are functionally identical by Verilog specification, either for a bit or a vector.

Discussing possible synthesis results is meaningless without considering the following action, I think. If you set e.g. two registers depending on the comparison result, not all bits of a (assuming it's a vector) may be evaluated by identical LUTs due to optimisation.

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a maybe a input signal ,or a reg variable, or a wire variable.

i think there is no difference between them when i judge whether a variable is equal zero.

THX for reply!