object assigned a value but never read - does this effect synthesis?

Here's what I've used:

    // ------------------------------------------------------------
    // SignalTap II counter
    // ------------------------------------------------------------
    //
`ifdef USE_SIGNALTAP
    logic  count /* synthesis noprune */;
    always_ff @ (posedge clkin, negedge pcie_perstN)
    if (~pcie_perstN)
        count <= '0;
    else
        count <= count + 1;
`endif

This preserves a counter that goes nowhere, which I can then probe with SignalTap ... which arguably means it goes somewhere ... however I needed to add this constraint so that Quartus would keep the counter so that I could probe it.

There are a few other synthesis attributes; keep, preserve, etc. Try a few :)

Cheers,

Dave