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There are a lot more than 2 modes. You can do 4 and 4, 5 and 3, some special 7LUT functions, etc. They're all dependent on your logic which is better. It's very difficult to control, and probably the last thing you want to be doing. (Stratix size devices would never get done if you had to worry about what size LUT all your logic was going into). There are occasional one-off reasons a user might need to force it, but in general let the tools take care of it for you. Instead worry about high-level settings like synthesis for Area, Speed, or Balanced, for example.
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but such as in stratix : i write a addr
for example
input a,b,c,d,e ;
output wire [2:0]cout ;
wire [1:0]sum1,sum2 ;
wire [2:0]sum3 ;
wire [1:0]sum4,sum5 ;
////////1 //////////////
sum1 = a +b ;
sum2 = c +d ;
sum3 = e +sum1 ;
cout = sum2 + sum3 ;
////////2 ///////////
sum4 = a +b+c ;
cout = d+e+sum4
then
when 1 : then need 4 2-input addr ,
but when 2 then only need 2 3-input addr .
and the cost of LE ,2 is less than 1 ，
can you tell me why ？
thank you very much
