[GHC] #11470: Support changing cross compiler target at runtime

[GHC]

#11470: Support changing cross compiler target at runtime
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11378            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by luite):

 I'd be happy to help with this. Fortunately there is not all that much
 information about the target platform hardcoded in GHC anymore.

 I think a new cross compilation target could be added by pointing GHC to
 some target-specific `settings` file. This file should also specify how to
 run TH and GHCi (for example via the external interpreter for general TH
 and by loading code from another package DB for host-only TH)

 We would create an `initialize-target` tool that:

   * queries the toolchain and generates an appropriate `settings` file
   * (possibly) builds an initial set of packages for the target, similar
 to what ships with GHC

 I think this is still reasonably straightforward, and I think it would be
 a good first step, and would mainly involve separating part of the build
 system and making sure that all relevant configuration options can indeed
 be specified by `settings`. This should allow cross compilers to load
 native code for plugins (the code generator for the target could even be a
 plugin).

 For the next step, my main concern would be how to match up the host and
 target architecture package databases for things like host-only TH. Say a
 cross compiler needs to run some host-only TH, which means it probably
 needs to load some code from a host-arch package DB:

   * Who is responsible for the code to be there? Will `cabal-install` make
 sure that a host version of a package is built and registered before a
 target version?
   * ABI hashes will likely be different between architectures: do we just
 require transitive version numbers to be the same? Or should there be a
 way to link hashes between architectures? (if we match on version numbers,
 i suspect we may lose things like cabal flag settings)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11470#comment:7>
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