[GHC] #14982: LLVM default -mcpu setting inhibits customization

[GHC]

#14982: LLVM default -mcpu setting inhibits customization
-------------------------------------+-------------------------------------
           Reporter:  tommd          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.1
  (LLVM)                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 With GHC 8.4.1 we are now passing a default `-mcpu` selection to llvm.
 This default is passed regardless of any explicit command line option such
 as `-optlc=-mcpu=native`.  If/when the user explicitly specifies the
 architecture we now get a failed build.  The error message from LLVM is:

 {{{
 llc: for the -mcpu option: may only occur zero or one times!
 `llc' failed in phase `LLVM Compiler'. (Exit code: 1)
 }}}

 This is when compiling with GHC 8.4.1 as such:

 {{{
 ghc-8.4.1 -fforce-recomp -fllvm -optlc=-mcpu=native -O2 tt.hs
 }}

 I suggest we either check for a user option of `mcpu` before adding the
 default or revert this behavior and allow llvm to take it's (optimistic
 and less portable) default.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14982>
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