What to look for in risers that support bifurcation?

[Alpha17x]

So, I understand what bifurcation is but I'm new to making it physically work.
 

I've got an X399 Phantom Gaming 6 board, there are 3 physical X16 slots that can be set to either x16 or x4x4x4. I've got a potential 64 lanes, and I'm using maybe 36, I don't think I'll 'cap' that out if I try.  However in my voyage across google I've learned that while you can technically just use a 4-way riser/splitter performance will be poor unless the riser/splitter supports bifurcation via some extra component that remained nebulous the entire time I was researching the process.

Have any of you done this?  At this point I'm just trying to find a riser/splitter that does it, Or information that will help me determine if said riser/splitter is viable purchase.
 

[AbydosOne]

17 minutes ago, Alpha17x said:

However in my voyage across google I've learned that while you can technically just use a 4-way riser/splitter performance will be poor unless the riser/splitter supports bifurcation via some extra component that remained nebulous the entire time I was researching the process.

There are two types of PCIe bifurcation:

1. PCIe host bifurcation: the PCIe host handled the dividing of lanes through the motherboard or a special riser (Supermicro's are the ones I know about). These are what those M.2 accelerator cards (usually) use — x16 >> x4/x4/x4/x4. Most motherboards with two CPU x16 slots will split them out using this, too.
2. PLX bifurcation: this is more akin to an Ethernet switch than true bifurcation. Splits the input bandwidth out to output ports. Can be split out to more lanes than are put in, but will be limited to the uplink bandwidth between all attached devices. Usually used to get Extreme™ PCIe slot arrangements on high-end motherboards. Gigabyte Z97X-Gaming GT is a good example.

I'm not aware of any mass-market PLX risers (PLX chips are power-hungry and expensive anyways), so basically if the motherboard you have supports it (which can be hit-or-miss) and you can find a riser that meets your needs it (should) work.