StgLint worth maintaining?

[Simon Peyton Jones]

Good summary!  I suggest that you open a ticket with this email as the Description.  Then we can point to it later.

I agree that there is little point in flogging a dead horse.  But there are /some/ invariants, so I vote for
|  2. Rewrite it to only check these two and nothing else, enable it in
|     validate (and in other build flavours that enable CoreLint).
|  

Simon

|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ömer
|  Sinan Agacan
|  Sent: 09 February 2018 08:42
|  To: ghc-devs <ghc-devs at haskell.org>
|  Subject: StgLint worth maintaining?
|  
|  Hi,
|  
|  I've been looking into some StgLint-related tickets:
|  
|  - #13994: 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: 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:  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: Related with StgLint not keeping up with recent changes
|  (join
|            points), because it's not enabled by default in
|            tests/validate.
|  
|  - #14118: 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: Again type checking errors. Commit for #14116 also fixes
|  this.
|            The commits compares `typePrimRep`s of types instead of
|            comparing actual types (even this is not enough, see
|  #13994).
|  
|  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
|  described in `Note [Case 2: CSEing case binders]`.
|  
|  There's also this comment in StgLint.hs
|  
|      WARNING:
|      ~~~~~~~~
|  
|      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
|    messages in #5345, #1420, transformations done by unariser and
|    StgCSE)
|  
|  - It's not enabled since 2000, which I think means that it's not
|    needed.
|  
|  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:
|  
|  1. Remove StgLint.
|  
|  2. 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).
|  
|  Thanks,
|  
|  Ömer
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