If you are like me, then when you design logic to interact with a FIFO, you make sure to carefully control your reads and writes. Since I do this for all FIFOs, it is very wasteful to have logic internal to the FIFO that checks whether I'm writing to a full FIFO, or reading from an empty FIFO.
Altera's tools allow you to choose whether or not you want such overflow and underflow logic in the FIFO. Xilinx and Lattice's tools do not. What this means is that every time you create and instantiate a FIFO with Lattice or Xilinx chips, there is logic that will negatively affect timing and use more resources, internal to the FIFO. This logic is so wasteful (and dangerous since it hides bugs), that I feel it important to point this out to the general public.
If you are using Lattice's IPExpress, the logic is shown in the Verilog or VHDL source files. This is great because it allows you to disable the logic with some simple comments.
For Xilinx, there doesn't appear to be a way to get at the source code that creates the logic, so I would strongly advise asking your FAE to request that Xilinx adds an option to Core Generator to allow for lighter weight FIFO creation that excludes this 'safety' feature.
In my system, I found a significant savings by hacking the (Lattice) created Verilog FIFO and disabling the empty and full check. Since my system depends on correct read and write logic, this was a function that was very useless to me. If I was reading when there was no data, or writing when there was no room - the system won't work anyhow - so I MAKE sure that doesn't happen in my logic.