status of template haskell + cross compiling plans for 7.8?

[Simon Marlow]

The issue that we hit sooner than this is that when we build a 
cross-compiler, we build all the libs for the target platform and not 
the host platform (see 
http://hackage.haskell.org/trac/ghc/wiki/CrossCompilation).  So to make 
TH work, we have to change the way we build cross compilers, and make 
the build system able to build and install two sets of libs.  This is 
probably a big job.

I think it might be possible to avoid needing a multi-target GHC, as 
long as you have two identical GHCs, one that produces code for the host 
platform (host-GHC) and one for the target platform (cross-GHC).  You 
compile all the code once with host-GHC, and then again with the 
cross-GHC, the second time using the .o files produced by host-GHC to 
run the TH code.

Cheers,
	Simon

On 17/06/13 14:52, Simon Peyton-Jones wrote:
> I have not been following the details of this debate, but there is a
> good reason why TH doesn’t work on a cross compiler. Specifically,
> suppose module M imports module X, which defines a function mkD that M
> calls in a splice, thus $(mkD “wobble”).
>
> Currently, we compile X to X.o, and when compiling M we dynamically link
> X.o (and all the other libraries it relies on) into GHC so that we can
> call foo.  Obviously X.o has to be runnable on the machine doing the
> compiling, so if X.o is for some other architecture that’s not going to
> work.
>
> There is no reason in principle why one could not compile X into
> X.bytecode (along with all its dependencies), where X.bytecode is
> architecture independent. Then X.bytecode could be interpreted on any
> platform.  But GHC has no mechanism for doing this at all.   I’ve always
> take the view that if you want X.bytecode, you may as well load X.hs and
> translate it into bytecode.  A bit slower, to be sure, but maybe fast
> enough.  But don’t forget those libraries.
>
> Anyway that’s the state of play.   Have fun!
>
> Simon
>
> *From:*ghc-devs-bounces at haskell.org
> [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Carter Schonwald
> *Sent:* 13 June 2013 22:44
> *To:* ghc-devs at haskell.org
> *Subject:* status of template haskell + cross compiling plans for 7.8?
>
> Hey All,
>
> Whats the story planned for template haskell + cross compiler support
> come ghc 7.8?
>
> I understand theres a lot of Template Haskell design underway, some of
> which will help support tools like Manuel's Inline-Objective-C work.
> Does this mean that factored out within this reorganization is a better
> story for cross compilation?
>
> Especially since one kill "app" for the Inline-Objective-C template
> haskell work would be writing IOS applications. Though I guess that also
> touches on the need to sort out supporting "FAT" ARM binaries too, right?
>
> This intersects with a few different large subsets of tickets, so i'm
> not sure if any single ticket is the right fora for this question.
>
> thanks!
>
> -Carter
>
>
>
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