40nm spacing rules ..........

The reason I found the following problem is that I have metal1 gap between standard cell row and the dro cell which is not really a macro, which is built from standard cell.
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Filler insertion. the 40nm minimum spacing  rule is  2 placement grid. which is 0.28.

The problem is when the space is say, 4 x 0.14, you have 3x filler cell in your list, 3x is chosen first, then only 1x space is left. You have a gap left.  This part is relative easy to resolve by controlling the size of standard rows, make the size always even number of 0.14. But it will become impossible after you place the standard cell, you can't control the gap between standard cells, it could be 2,3,4,5,6.

Because of the above reason, Synopsys creates this option -no_x1, which is to resolve the issue I mentioned above. It could resolve like 4x gap, it will try 3x filler, then use two 2x filler to fill the gap.