Dear Stefan,

I believe it is currently impossible to build a working cross-compiler
between different word sizes, as the compiler does not differentiate
between the host and the target's native integers.

Best wishes,
Nicolas


On Thu, Oct 8, 2015 at 11:43 AM, Stefan Hellermann <stefan@the2masters.de>
wrote:

> Hello,
>
> currently I'm trying to add OCaml to the openwrt router distribution [1]
> [2]. This would mean you could run your favorite OCaml applications on your
> router. I would like to run unison [3] on my router.
>
> So I first build native OCaml 4.02.3 on my x86_64 linux host, and then
> build a OCaml cross-compiler for the 32 bit target, e.g.
> mips-openwrt-linux. Building OCaml went fine after adding a small patch to
> manually set Endianness and Bitness in configure script, please have a look
> at attached patch.
>
> host OCaml configure line:
> ./configure -prefix somepath -no-pthread -no-debugger -no-ocamldoc
> -no-graph -no-cfi
> cross OCaml configure line:
> ./configure -prefix someotherpath -target-bindir /usr/bin -host
> x86_64-unknown-linux-gnu -target mips-openwrt-linux-musl -cc
> "mips-openwrt-linux-musl-gcc some target cflags" -as
> "mips-openwrt-linux-musl-as some target as flags" -no-pthread
> -no-shared-libs -no-debugger -no-ocamldoc -no-graph -no-cfi -big-endian
>
>
> Now the bug: When running OCaml applicantions (build with -custom), they
> crash on startup on the target:
>
> Fatal error: exception Failure("input_value: integer too large")
>
>
> This is similar to OCaml Bug #5977 [4]
>
> There is a workaround:
> Build the host OCaml as 32 bit OCaml, then build the cross compiler with
> this 32 bit host OCaml. The resulting binaries work on the target.
> This is what I did for openwrt [2], but it fails on 64 bit hosts where no
> 32 bit development tools are installed. It only catches the case when
> building on x86_64 hosts, it will break when building on say powerpc64 or
> 64 bit arm. It's also problematic for openwrt, as only the cross compiler
> but not the host tools are recompiled if the target arch changes, e.g. from
> 64 bit arm to 32 bit mips.
>
> Now my questions:
> - Is building a 32 bit cross compiler on a 64 bit host supposed to work?
> - Is my patch to configure ok, or is there a better way to configure the
> cross-compiler?
> - Should I run some test suite on the target and post the results?
>
> [WARNING] Something went wrong while checking native division and modulus
> please report it at http://http://caml.inria.fr/mantis/
> Do I have to care?
> I tried defining and undefining NONSTANDARD_DIV_MOD in m.h. without any
> visible difference. Running the test-prog divmod.c on a few targets always
> returns 0.
>
> [1] https://openwrt.org/
> [2] https://patchwork.ozlabs.org/patch/518982/
> [3] https://www.cis.upenn.edu/~bcpierce/unison/
> [4] http://caml.inria.fr/mantis/view.php?id=5977
>
>
>