对于.v文件在vivado中是不支持，但是可以修改为.sv或更改文件属性使用sytemverilog来支持。

 /*** Math function: $clog2 as specified in Verilog-2005** clog2 =          0        for value == 0*         ceil(log2(value)) for value >= 1** This implementation is a synthesizable variant of the $clog2 function as* specified in the Verilog-2005 standard (IEEE 1364-2005).** To quote the standard:*   The system function $clog2 shall return the ceiling of the log*   base 2 of the argument (the log rounded up to an integer*   value). The argument can be an integer or an arbitrary sized*   vector value. The argument shall be treated as an unsigned*   value, and an argument value of 0 shall produce a result of 0.*/function automatic integer clog2;input integer value;beginvalue = value - 1;for (clog2 = 0; value > 0; clog2 = clog2 + 1) beginvalue = value >> 1;endendendfunction/*** Math function: enhanced clog2 function**                        0        for value == 0* clog2_width =          1        for value == 1*               ceil(log2(value)) for value > 1*** This function is a variant of the clog2() function, which returns 1 if the* input value is 1. In all other cases it behaves exactly like clog2().* This is useful to define registers which are wide enough to contain* "value" values.** Example 1:*   parameter ITEMS = 1;*   localparam ITEMS_WIDTH = clog2_width(ITEMS); // 1*   reg [ITEMS_WIDTH-1:0] item_register; // items_register is now [0:0]** Example 2:*   parameter ITEMS = 64;*   localparam ITEMS_WIDTH = clog2_width(ITEMS); // 6*   reg [ITEMS_WIDTH-1:0] item_register; // items_register is now [5:0]** Note: I if you want to store the number "value" inside a* register, you need a register with size clog2(value + 1), since* you also need to store the number 0.** Example 3:*   reg [clog2_width(64) - 1 : 0]     store_64_items;  // width is [5:0]*   reg [clog2_width(64 + 1) - 1 : 0] store_number_64; // width is [6:0]*/function automatic integer clog2_width;input integer value;beginif (value == 1) beginclog2_width = 1;end else beginclog2_width = clog2(value);endendendfunction

有一种说法时clog在仿真中以2为对数，而在综合时以e为对数。（暂时认为是一种错误的看法，如果这样那对vivado来说将是非常严重的错误）

参考：

xilinx的$clog2函数_hhh_fpga的博客-CSDN博客