原因找到了：
Before using an instance of a signal, array, parameter, etc., the instance must first be declared. The XST parser scans a Verilog file from beginning to end, so any usage must be preceded by the declaration of that instance.
晕死，XST跟其他综合器不一样.v中必须先声明再使用，真当是.c文件啊
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