Remove StgLint type checks
Copying from the mailing list thread:
I've been looking into some StgLint-related tickets:
- #13994 (closed): Found a StgLint problem and fixed, there's another problem
waiting to be fixed. Both related with the fact that after
unarisation we lose even more typing information and type
checks needs to be relaxed.
- #14116 (closed): StgLint failed to look through newtypes, and because coercions
are removed at that point it failed to type check. Solution
was to relax type checks.
#5345 (closed): Because `unsafeCoerce# is operationally no-op, and we don't have coercions in STG, StgLint can't type check at all. The commit message notes:
> Fundamentally STG Lint is impossible, because unsafeCoerce# > can randomise all the types. > This patch does a bit of fiddle faddling in StgLint which > makes it a bit better, but it's a losing battle.
#14117 (closed): Related with StgLint not keeping up with recent changes (join points), because it's not enabled by default in tests/validate.
#14118 (closed): Related with the fact that pre- and post-unarise we have different invariants in STG. Solution was to add a "unarise" parameter and do different checks based on that.
#14120 (closed): Again type checking errors. Commit for #14116 (closed) also fixes this. The commits compares
typePrimReps of types instead of comparing actual types (even this is not enough, see #13994 (closed)).
All this of course took time to debug.
In addition, the new
StgCSE pass makes transformations that trigger
case alternative checks (and probably some other checks) because
scrutinee and result won't have same types after the transformation
Note \[Case 2: CSEing case binders\]\.
There's also this comment in StgLint.hs
This module has suffered bit-rot; it is likely to yield lint errors
for Stg code that is currently perfectly acceptable for code
generation. Solution: don't use it! (KSW 2000-05).
It seems like it hasn't been used since 2000.
All this suggests that
- Checks related to types are impossible in StgLint. (see e.g. commit
- It's not enabled since 2000, which I think means that it's not
This makes me question whether it's worth maintaining. Maybe we should just remove it.
If we still want to keep we should decide on what it's supposed to do. Only invariants I can think of are:
- After unarise there should be no unboxed tuple and sum binders.
unarise is a simple pass and does same thing to all binders, there are no tricky cases so I'm not sure if we need to check this.
- Variables should be defined before use. I again don't know if this
should be checked, could this be useful for StgCSE?
So I think we should do one of these:
- Remove StgLint.
- Rewrite it to only check these two and nothing else, enable it in
validate (and in other build flavours that enable CoreLint).
What do you think? If you think we should keep StgLint, can you think of any other checks? If we could reach a consensus I'm hoping to update StgLint (or remove it).
Simon suggested creating a ticket and keeping StgLint. This is the tracking ticket.