Reduce splitAt to undefined in illegal contexts

[christiaanb]

Previously, the Clash compiler would try to reduce

splitAt d1 Nil

to something of type

(Vec 1 a, Vec (0-1) a)

by trying to project the head and the tail out of the Nil constructor. This of course does not work resulting in an out-of-bounds indexing error reported in:

#2831

The compiler now reduces above expressions to:

undefined :: (Vec 1 a, Vec (0-1) a)

Which is morally equivalent to the run-time exception Haskell evaluation would have thrown if the circuit description was evaluated like a regular Haskell program.

Fixes #2831

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files

[rowanG077]

Perhaps reduce to clashCompileError? So that if it would latter end up in the circuit it would error out.

[christiaanb]

Perhaps reduce to clashCompileError? So that if it would latter end up in the circuit it would error out.

Why wouldn't we want to generate HDL for something like splitAt d1 (unsafeCoerce Nil :: Vec 1 Bool) ?

[rowanG077]

Because I would say that is nonsensical to generate hardware for. Or rather, my question would be. Why would you want to generate HDL for that? Essentially you have a malformed term, which is something different then something undefined.

[christiaanb]

I do not want the compiler to generate clashCompileError. Let's say the compiler misses some optimization opportunity, and the clashCompileError is not elided before we get to HDL generation. In that case we don't generate the HDL just because of some (performance) bug in the compiler. Anyone hitting that bug then has to wait for someone to fix it and use what's probably a development version of the compiler to continue.

If we just generate undefined, the downstream logic synthesis tools will very likely optimize the logic away and neither the functionality nor the size of the circuit are affected. Users can now just happily synthesize the generated HDL and not bother with having to wait for some fix and run a development version of the compiler.