[GHC] #15169: SRT offset optimisation breaks on MachO platforms

[GHC]

#15169: SRT offset optimisation breaks on MachO platforms
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        Reporter:  bgamari       |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:  8.6.1
       Component:  Compiler      |              Version:  8.2.2
      Resolution:                |             Keywords:
Operating System:  MacOS X       |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"bf10456edaa03dc010821cd4c3d9f49cb11d89da/ghc" bf10456/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="bf10456edaa03dc010821cd4c3d9f49cb11d89da"
 Disable the SRT offset optimisation on MachO platforms

 Unfortunately, this optimisation is infeasible on MachO platforms (e.g.
 Darwin) due to an object format limitation. Specifically, linking fails
 with errors of the form:

      error: unsupported relocation with subtraction expression, symbol
      '_integerzmgmp_GHCziIntegerziType_quotInteger_closure' can not be
      undefined in a subtraction expression

 Apparently MachO does not permit relocations' subtraction expressions to
 refer to undefined symbols. As far as I can tell this means that it is
 essentially impossible to express an offset between symbols living in
 different compilation units. This means that we lively can't use this
 optimisation on MachO platforms.

 Test Plan: Validate on Darwin

 Reviewers: simonmar, erikd

 Subscribers: rwbarton, thomie, carter, angerman

 GHC Trac Issues: #15169

 Differential Revision: https://phabricator.haskell.org/D4715
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15169#comment:1>
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