[GHC] #14123: Figure out invariants surrounding ticks in Core

[GHC]

#14123: Figure out invariants surrounding ticks in Core
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  highest           |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * cc: simonpj, JaffaCake (added)


Old description:

> In the past we have had a number of issues stemming from ticks breaking
> various Core analyses in GHC,
>
>  * #13233: a tick separates a levity-polymorphic tycon (e.g. an unboxed
> tuple tycon) from its levity arguments, resulting in a `typePrimRep`
> panic.
>  * ticket:8472#comment:22: a tick wraps a primitive string literal,
> causing `CoreToStg` to fail to notice it is a string resulting in a panic
>  * #14122: Another literal string issue which I have yet to debug but
> seems very likely to be tick-related.
>
> We have merged a stop-gap solution/hack
> (f5b275a239d2554c4da0b7621211642bf3b10650) to the `ghc-8.2` branch
> however need a more principled solution in the long-term (e.g. before
> 8.4). We will need to start by defining precisely where ticks are allowed
> and add appropriate logic to CoreLint to verify that this invariant is
> upheld.

New description:

 In the past we have had a number of issues stemming from ticks breaking
 various Core analyses in GHC,

  * #13233: a tick separates a levity-polymorphic tycon (e.g. an unboxed
 tuple tycon) from its levity arguments, resulting in a `typePrimRep`
 panic.
  * ticket:8472#comment:22: a tick wraps a primitive string literal,
 causing `CoreToStg` to fail to notice it is a string resulting in a panic
  * #14122: Another literal string issue which I have yet to debug but
 seems very likely to be tick-related.

 We have merged a stop-gap solution/hack
 (f5b275a239d2554c4da0b7621211642bf3b10650) to the `master` branch (present
 in GHC 8.2.1) however we will need a more principled solution in the long-
 term (e.g. before 8.4). We will need to start by defining precisely where
 ticks are allowed and add appropriate logic to `CoreLint` to verify that
 this invariant is upheld.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14123#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler