[GHC] #8472: Primitive string literals prevent optimization

[GHC]

#8472: Primitive string literals prevent optimization
-------------------------------------+-------------------------------------
        Reporter:  akio              |                Owner:  gridaphobe
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.3
      Resolution:  fixed             |             Keywords:  newcomer
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Runtime           |  (amd64)
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2554,
       Wiki Page:                    |  Phab:D2605
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"f5b275a239d2554c4da0b7621211642bf3b10650/ghc"
 f5b275a2/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f5b275a239d2554c4da0b7621211642bf3b10650"
 Don't tick top-level string literals

 This fixes a regression due to D2605 (see #8472) wherein top-level
 primitive
 strings would fail to be noticed by CoreToStg as they were wrapped in a
 tick. This resulted in a panic in CoreToStg due to inconsistent CAF
 information
 (or a Core Lint failure, if enabled). Here we document the invariant that
 unlifted expressions can only sit at top-level if of the form `Lit
 (MachStr
 ...)` with no ticks or other embellishments. Moreover, we fix instance of
 this in `Simplify.prepareRhs` and `FloatOut.wrapTick` where this
 invariant was being broken.

 Test Plan: Validate with `-g`. Run testsuite with `WAY=ghci`.

 Reviewers: austin, simonpj

 Reviewed By: simonpj

 Subscribers: simonpj, akio, scpmw, thomie

 Differential Revision: https://phabricator.haskell.org/D3051
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8472#comment:22>
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