[mereel]

Sadly you can't synthesize any old piece of C++, there are a ridiculous amount of constraints and you need to write the code in a very specific way.

Also there's not necessarily much of a performance gain, some tasks are not good candidates for being implemented in hardware (as a general rule you need a lot of parallelism to make it worth it). As an overly simple example, implementing hardware RSA and hoping for a significant speedup doesn't make sense because there isn't really much parallelism and it's usually only used to encrypt keys, but something like AES or SHA might benefit from a good hardware implementation because there is much more parallelism to be had and they are used to encrypt much larger amounts of data.

To add even more complexity, the compilers can be obscenely finicky with optimizations.

[derefr]

So, it's less that you're writing C++ and synthesizing it, and more like you're maintaining a VHDL codebase that happens to be presented/edited "through" C++.

[jlokier]

That's a fair description. Even so, with VHDL being rather verbose you might prefer the "C++ skin on VHDL" version for some things, especially algorithmic things.