[GHC] #9142: LLVM HEAD rejects aliases used by LLVM codegen

[GHC]

#9142: LLVM HEAD rejects aliases  used by LLVM codegen
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              Reporter:  bgamari     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  high        |         Milestone:  7.10.1
             Component:  Compiler    |           Version:  7.8.2
            Resolution:              |          Keywords:
Differential Revisions:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |         Test Case:
            Difficulty:  Unknown     |          Blocking:  4213
            Blocked By:              |
       Related Tickets:              |
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Comment (by bgamari):

 This morning I approached this issue with a fresh perspective and realized
 that the solutions is quite simple (unless I've missed something). The
 issue with new LLVMs is that aliases can't point to things that aren't
 definitions.

 As it turns out, the only case where we emit aliases pointing to non-
 definitions is for external symbols where we emit an extern declaration
 and a corresponding alias. Unless I'm mistaken, there is no reason why
 both of these are necessary: we only use the alias. So, instead of
 emitting,

 {{{
 @stg_BCO_info = external global i8
 @stg_BCO_info$alias = alias private i8* @stg_BCO_info
 }}}

 we should be emitting,

 {{{
 @stg_BCO_info$alias = external global i8
 }}}

 I've implemented this change in my [llvm-3.5-new branch
 https://github.com/bgamari/ghc/compare/llvm-3.5-new and it appears to work
 (or rather, the GHC build hasn't failed yet; I also have yet to verify
 that LLVM 3.4 still works).

 Of course, there are still nice opportunities for cleaning up TNTC (as
 discussed in #4213), but at least this fixes the immediate problem.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9142#comment:15>
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