[GHC] #4213: LLVM: Add support for TNTC to LLVM compiler suite

[GHC]

#4213: LLVM: Add support for TNTC to LLVM compiler suite
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    Reporter:  dterei            |       Owner:  dterei          
        Type:  feature request   |      Status:  new             
    Priority:  normal            |   Milestone:  7.4.1           
   Component:  Compiler (LLVM)   |     Version:  6.13            
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:                    |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by dterei):

 No clear way forward. I have two ideas on how to approach it

 1. Add a new special llvm variable 'llvm.order' similar to how LLVM has
 the [http://llvm.org/docs/LangRef.html#intg_used 'llvm.used'] variable.
 This variable would be a array type, containing function and global
 pointers where their order in the array is the order they should be
 printed to assembly in. This should be enough to get us TNTC but it's
 perhaps a little of a hacky way to achieve it. The upside is it should be
 pretty easy to implement and get accepted upstream. It basically
 corresponds to the GCC option 'fno-toplevel-reorder'.

 2. Add support for attaching a global variable to a function. So this
 would be a fairly direct TNTC mapping to LLVM IR. Upside is the direct
 conveying of TNTC semantics may make it safer (maintained in the face of
 change) and pen up opportunities for optimisation. Downside is I think
 this would be a fair amount of work and may be too specific to GHC for
 getting accepted by LLVM.

 If I ever get the time to hack on this I would give 1 a go first. I think
 it could potentially be done in a day or two by someone who knew LLVM
 already.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4213#comment:6>
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