# Synthesis and HLS Backend

> Working with synthesis backends is painful and requires advanced debugging.
> For most things, we recommend not installing these tools yourself.
> If you're a Cornell Student, you can use our lab server, [Gorgonzola][].

`fud` uses [Vivado][] to generate area and resource estimates for designs
generated by Calyx.
It also supports compiling Dahlia programs through the HLS-backend using
[Vivado HLS][vhls].

There are two ways to get `fud` working with Vivado.

## Vivado and Vivado HLS over SSH
`fud` supports invoking these tools over SSH. You have to tell `fud` the username and hostname
for a server that has these tools installed:
```
# vivado
fud config stages.synth-verilog.ssh_host <hostname>
fud config stages.synth-verilog.ssh_username <username>

# vivado hls
fud config stages.vivado-hls.ssh_host <hostname>
fud config stages.vivado-hls.ssh_username <username>
```

**Note:** `vivado` or `vivado_hls` have to be on the path of the remote machine
for this to work.
If you need the names to be something else, file an issue.
`fud` currently does not support other names.

## Vivado/VivadoHLS locally
We don't provide installation instructions for this. However, `fud` will look for
`vivado` and `vivado-hls` binaries on the system. If these are installed, you can
use `fud` to invoke these tools. You can change the paths `fud` looks for with
```
fud config stages.synth-verilog.exec <path> # update vivado path
fud config stages.vivado-hls.exec <path> # update vivado_hls path
```


[vivado]: https://www.xilinx.com/products/design-tools/vivado.html
[vhls]: https://www.xilinx.com/products/design-tools/vivado/integration/esl-design.html
[gorgonzola]: https://capra.cs.cornell.edu/private/gorgonzola.html