Re: StgLint worth maintaining?

Created #14787 as tracking ticket. Patch is at D4404. Ömer 2018-02-09 12:22 GMT+03:00 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@haskell.org] On Behalf Of Ömer | Sinan Agacan | Sent: 09 February 2018 08:42 | To: ghc-devs | 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 | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.h | askell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc- | devs&data=04%7C01%7Csimonpj%40microsoft.com%7C9d9affa423c84c84a25908d5 | 6f992d87%7Cee3303d7fb734b0c8589bcd847f1c277%7C1%7C0%7C6365376260479856 | 64%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI | 6Ik1haWwifQ%3D%3D%7C- | 1&sdata=GZ4xMoVQGyQFZxhlODBqMnWoiZrV82pqOn2ZrbvDo4U%3D&reserved=0