I am evaluating a PCI core from another vendor.
The .ucf file which came with the core attached the BYPASS constraint to all of the core's external I/O pins.
Implementing the core, I could not get it to work.
Using Chipscope, I found that no external pin stimulus was getting through the IBUFs on any of the I/O pins.
I verified that signals were changing on external pins using an o-scope, but Chipscope showed nothing changing inside.
All output buffers were tri-stated, as verified by Chipscope on the buffer's control pins.
The map report did not indicate that any of this logic was removed.
Anyway, I commented out the BYPASS constraints in the ucf, and everything worked fine after that.
The only info I could find on BYPASS was AR# 22137, which was confusing to me.
If BYPASS does not apply to Spartan 6, the mapper should ignore it.