[GHC] #11578: Fix GHC on AArch64/Arm64

[GHC]

#11578: Fix GHC on AArch64/Arm64
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        Reporter:  erikd             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:  aarch64
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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@@ -4,1 +4,0 @@
-
@@ -6,1 +5,0 @@
-

New description:

 Raising this to track discussion about GHC on Arm64. There is already some
 discussion in ticket #11206 (ARM is generally a disaster), but that ticket
 is about 32 bit Arm which is vastly different and is now mostly fixed.
 Information worth saving from #11206:
 * GHC 7.8.4 from Debian can build the ghc-8.0 branch, but cannot build
 HEAD because HEAD requires ghc-7.10 to build.
 * GHC 7.10 from Debian is broken because of an unfixed bug in llvm-3.5.2.
 * ~~User @mfox reports that ghc-7.10 does build against llvm-3.7 and
 results in a compiler that can build HEAD.~~
 * ~~That GHC 7.10 can be build against llvm-3.7 is a happy accident.
 Versions of GHC are usually locked to a specific version of LLVM.~~

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Comment (by erikd):

 Ah now I understand what is going on. When you built your compiler, you
 didn't have LLVM-3.5 so the compiler was build as `Unregisterised` (See
 your `ghc --info` output in ticket:11206#comment:35) which uses the C
 backend instead of the LLVM backend, bypassing the LLVM-3.5 bug.

 When I took your bindist and installed it on my machine, the configure
 script detected the presense of llvm-3.5 and used it resulting in a broken
 compiler.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11578#comment:9>
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