Hunting down a compilation performance regression involving type families

[Richard Eisenberg]

> On May 31, 2017, at 5:21 PM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
> Does you know what might be going on here?

I think so, but I don't know how to fix it.

The commit you found (thank you!) makes simple_opt_expr (the "simple optimizer", run directly after desugaring, even with -O0) a little more selective in what `case` expressions it throws away. Previous to that commit, the optimizer would throw away a `case error "deferred type error" of _ -> ...` which is terrible. It seems that you have discovered that we are now too timid in throwing away unhelpful cases. It would be interesting to know what the newly-retained cases look like, so that we might throw them away.

But there remains a mystery: Why do we need this code at all? Reading Note [Getting the map/coerce RULE to work] closely, it seems we need to simplify

  forall a b (co :: a ~R# b).
    let dict = MkCoercible @* @a @b co in
    case Coercible_SCSel @* @a @b dict of
      _ [Dead] -> map @a @b (\(x :: a) -> case dict of
         MkCoercible (co :: a ~R# b) -> x |> co) = let dict = ... in ...

to

  forall a b (co :: a ~R# b).
    map @a @b (\(x :: a) -> x |> co) = \(x :: [a]) -> x |> [co]

Part of doing so is to drop the `case Coercible_SCSel ...`, which gets in the way. The mystery is why this needs special code -- shouldn't the eliminate-case-of-known-constructor do the trick? This would require unfolding Coercible_SCSel. Does that happen? It would seem not... but maybe it should, which would remove the special-case code that I changed in that commit, and quite likely would simplify much more code besides.

So: Is Coercible_SCSel unfolded during simple_opt? If not, what wonderful or terrible things happen if we do? If so, why does case-of-known-constructor not work here? My guess is that answering these questions may solve the original problem, but this guess could be wrong.

Richard