Synthesis Comparison

I like Fmax, LUTs, and Flops.

-- Mike Treseler

Be careful when measuring 'Number of Slices' or 'number of flip flops' or any other internal resource count, they can be misleading when used as a metric for design comparisons. The problem with these is that synthesis will often replicate logic or not optomize things as small as they 'could' be (generally they do this in order to get better clock speed performance), because they don't need to in order to meet the design requirements. From the perspective of synthesis software, meeting design requirements means simply:

- It fits in the selected device

- It runs at the selected clock speed(s).

One way to get a reasonable estimate of internal resource usage is to target the design to something intentionally too small so that, no matter how hard synthesis works, the design won't fit. Then the software will tell you how many slices/flops/etc. that it is trying to fit, this number is frequently different than the numbers you will gets when it targets a larger device that the design actually will fit into. In one recent test case, the difference was 16% of the design.

Kevin Jennings

Yes, slices is meaningless. The number of slices can vary by a factor of two for the same synthesis result.

Instead report DFFs and LUTs, and also note the number of inputs to a lut your target technology uses.

Kolja Sulimma

DFFs and LUTs are not necessarily good metrics either since they can be affected quite a bit by efforts to meet timing (i.e. register replication) and/or lack of efforts to optomize logic (if it doesn't entire fill up the device, synthesis might not try to do any better unless forced to in order to improve timing).

KJ