llvmGen: Compatibility with LLVM 3.5 (re #9142)

Due to changes in LLVM 3.5 aliases now may only refer to definitions.
Previously to handle symbols defined outside of the current commpilation
unit GHC would emit both an `external` declaration, as well as an alias
pointing to it, e.g.,

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

Where references to `stg_BCO_info` will use the alias
`stg_BCO_info$alias`. This is not permitted under the new alias
behavior, resulting in errors resembling,

    Alias must point to a definition
    i8* @"stg_BCO_info$alias"

To fix this, we invert the naming relationship between aliases and
definitions. That is, now the symbol definition takes the name
`@stg_BCO_info$def` and references use the actual name, `@stg_BCO_info`.
This means the external symbols can be handled by simply emitting an
`external` declaration,

    @stg_BCO_info = external global i8

Whereas in the case of a forward declaration we emit,

    @stg_BCO_info = alias private i8* @stg_BCO_info$def

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D155

1 files changed, 5 insertions, 5 deletions

diff --git a/compiler/llvmGen/LlvmCodeGen/Data.hs b/compiler/llvmGen/LlvmCodeGen/Data.hs
index 1dbfb4b527..90ce44367a 100644
--- a/compiler/llvmGen/LlvmCodeGen/Data.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Data.hs
@@ -39,15 +39,16 @@ genLlvmData (sec, Statics lbl xs) = do
     let types   = map getStatType static
 
         strucTy = LMStruct types
-        alias   = LMAlias ((label `appendFS` structStr), strucTy)
+        tyAlias = LMAlias ((label `appendFS` structStr), strucTy)
 
-        struct         = Just $ LMStaticStruc static alias
+        struct         = Just $ LMStaticStruc static tyAlias
         link           = if (externallyVisibleCLabel lbl)
                             then ExternallyVisible else Internal
         const          = if isSecConstant sec then Constant else Global
-        glob           = LMGlobalVar label alias link Nothing Nothing const
+        varDef         = LMGlobalVar label tyAlias link Nothing Nothing const
+        globDef        = LMGlobal varDef struct
 
-    return ([LMGlobal glob struct], [alias])
+    return ([globDef], [tyAlias])
 
 -- | Should a data in this section be considered constant
 isSecConstant :: Section -> Bool
@@ -134,4 +135,3 @@ genStaticLit (CmmHighStackMark)
 -- | Error Function
 panic :: String -> a
 panic s = Outputable.panic $ "LlvmCodeGen.Data." ++ s
-