What does it mean by place-n-route in FPGA?

[shemo]

The transistors inside the FPGA cannot physically move around!

Yes, you can make connection but you can't physcially move the logic around.

Why it's calling place and route? like placing the transistor anywhere?

 

[vGoodtimes]

FPGA designs are based on lookup tables (and a few other basic elements). The lookup tables are small RAMs, and can be configured to perform a specific logic function after the device has powered up. Connections to these LUTs connects to various muxing logic, which can also be configured after the device has powered up.

Placement is selecting which LUT on the FPGA will perform a specific logic function. Routing is configuring the muxes so the LUTs take their input from the correct source.

 

[shemo]

FPGA designs are based on lookup tables (and a few other basic elements). The lookup tables are small RAMs, and can be configured to perform a specific logic function after the device has powered up. Connections to these LUTs connects to various muxing logic, which can also be configured after the device has powered up.

Placement is selecting which LUT on the FPGA will perform a specific logic function. Routing is configuring the muxes so the LUTs take their input from the correct source.

That means not physically moving transistors around then?

 

[vGoodtimes]

no, transistors are not moved around. just bits within a configuration file.

 

[Dan Mills]

Place is deciding which of the many available LUTs should house a particular piece of logic, routing is deciding how to configure the interconnects between those logic elements.

It is hard because both things impact timing closure and producing a fully optimal solution is NP complete and thus intractable. The best you can do is to apply various heuristic methods like simulated annealing to get a fit that is good enough. This is why the time taken tends to go non linear as the size of the design encroaches on the maximum the fabric can accomodate.

In no case does any transistor migrate around the fabric!