how to build 4 serial inverters

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I want to build 4 serial inverters but at last when programming the FPGA quartus automatically fixes it as if no inverters exists.how can I do some thing to ignore this auto fixing?

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altera has provided various attributes useful in controlling fitter.

for comb. nodes(wires) try this attribute:

signal invert : std_logic;

attribute keep: boolean;

attribute keep of invert: signal is true;

apply to all relevant wires in your chain.