Correct, it's unsupported. Unfortunately, there is no check in the
configure script whether the word sizes match (it only checks for the
OCaml version).

It's not only the value marshaller that can fail, but I've also seen
illegal assembler code. The core of the problem is that the generic parts
of the native-code backend use nativeint for integer calculations, and the
code emitters for 32 bit platforms then just assume nativeint has only 32
bits. I guess the solution would be to introduce a new integer type
targetint into the compiler that is either int32 or int64, depending on
what the cross-compile target is.

Gerd

> 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
>>
>>
>>
>

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--
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
My OCaml site:          http://www.camlcity.org
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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