IntRep etc

[Simon Peyton Jones]

You successfully persuaded me, in #17026, that getting rid of Int and IntRep in a platform-dependent way is very awkward to implement.

My proposal, made in #17026 is:

* Keep Int# distinct from Int32#, Int64#
* Keep IntRep distinct from Int32Rep, Int64Rep.
* Have primops to convert between Ints of various widths. 
  (Need to lay out exactly what these are.)

* And make the core-to-STG (only) implement some of these
  primops as no-ops, depending on the architecture; 
  e.g. Int# -> Int64# is a no-op on a 64 bit machine.

  So STG would "know" that Int# and Int64# are the same.
  I say STG because the unarise pass needs to know the
  width of each type for implementing sum types, I think.

* So yes, some tests in the STG and Cmm part would need
  platform information.

Does that make sense?  If so that would unblock !1381.

Simon

| -----Original Message-----
| From: Ömer Sinan Ağacan <omer at well-typed.com>
| Sent: 04 September 2019 14:29
| To: Simon Peyton Jones <simonpj at microsoft.com>; Ben Gamari <ben at well-
| typed.com>
| Cc: ghc-devs at haskell.org; Simon Marlow <marlowsd at gmail.com>
| Subject: Re: IntRep etc
| 
| Hi all,
| 
|  > !1381/#16893: inline unsafeCoerce# in CorePrep
| 
| The problem with !1381 is that it reveals a coercion accepted by CoreLint
| but rejected by an assertion later in the compilation. The coercion casts
| an Int# to Int64#, which is fine on a 64-bit system. One way to fix this
| is by fixing #17026, but as explained in the issue it's too much work and
| it'll either require huge amount of work (if we implement some kind of
| Type/TyCon caching) or make everything slower (if we make top-level
| intPrimTy/intPrimTyCon which are currently constants, functions).
| 
|  > #17041: coercion safety in STG Lint
| 
| This basically asks for a type checker that works on PrimReps instead of
| Types.
| I was too busy with #9718/!1304 and #17088 so couldn't make progress here.
| Perhaps I can work on this at MuniHac.
| 
|  > !1552: tidy up linting on unsafe coercions
| 
| I'll address your comments and test this more. My main concern (and the
| reason why I added "DO NOT MERGE") was that I wasn't 100% sure that this
| change is sound (though I don't remember what I was concerned about
| specifically). I'll try to test this more on my desktop (where I can more
| efficiently make release builds and test) when I'm back home.
| 
|  > #16964: definition of Int#
| 
| The part related to GHC internals is moved to #17026. Regarding definition
| of Int#, I'm not sure what to do about this,
| 
|  > #17026: definition of IntRep
| 
| Simon argues that if we keep Int and Int# distinct types (rather than
| synonyms to Int32/Int64 and Int32#/Int64# depending on platform word size,
| this is discussed in #16964) then we should keep RuntimeReps for them
| distinct too.
| I think this is mostly fine but a problem with this that blocked !1381 is
| IntRep and Int64Rep are not equal (according to the Eq instance) on a 64-
| bit system so a simple code that checks whether two types are represented
| the same fails.
| 
| If we keep IntRep/WordRep then I think we should remove the Eq instance of
| PrimRep and and provide a `primRepEq :: PlatformWordSize -> PrimRep ->
| PrimRep
| -> Eq` for checking whether types are represented the same on the target
| platform (or maybe name it `primRepCoercible`).
| 
| 
| So in summary; I think I'll make progress in this order:
| 
| - Read the changes in !1552, test it more.
|      - If merged then !1381 will be unblocked, merge it.
| - Remove Eq instance of PrimRep, implement primRepEq (or primRepCoercible)
| that
|    takes target platform size as argument. Use this in coercion checks.
|      - This also unblocks !1381, merge it
| - Implement type checker in StgLint that works on PrimReps (instead of on
|    actual types).
| 
| I'm not sure about how to proceed in #16964. I'm inclined to just document
| the status quo more accurately, but apparently there's also a MR that's
| supposed to improve/fix it (!1102), though I'm having trouble seeing how
| it is even related.
| 
| Ömer
| 
| On 3.09.2019 00:45, Simon Peyton Jones wrote:
| > Omer, Ben
| >
| > There's a little cluster of tickets that it'd be really good to nail.
| >
| >   * !1381/#16893: inline unsafeCoerce# in CorePrep
| >   * #17041: coercion safety in STG Lint
| >   * !1552: tidy up linting on unsafe coercions
| >   * #16964: definition of Int#
| >   * #17026: definition of IntRep
| >
| > They are all somehow tied up together.  Can we kill them off together,
| > soon?  I have left comments...
| >
| > Simon
| >