llvm dev - Nov 2015 - beginner problem with cross compilation

Hi everyone,
I am a new commer and I 'm tying to use LLVM to cross compile a code. I
want to cross compile to MIPS and the host is the default x86. The more I
read the document the more confused I get. I see documents on
cross-compiling to ARM. But some install some gcc packages, while some use
clang. I don't know where to start and how to do it.
Is this just enough to run a command line like this:

llc -march=mips test.bc -o testmips

will the testmips file be executable for mips?
I will appreciate if anyone help me with this problem.
Thanks,
Fami
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llc will make an object file with MIPS machine code for only the functions
in your test.bc. You then need to link it with any MIPS libraries it needs
(usually at least the C runtime library that calls main() for you etc).

The generated code will have exactly the data types (e.g. integer sizes)
and struct layouts specified in your test.bc. These might or might not be
correct for MIPS. If you're running on bare hardware then it's probably
ok,
but if you call library functions then you need to be laying out the data
in the correct way. If you wrote test.bc yourself then that's your job, but
usually it's the job of e.g. clang, and you need to also tell clang that
you're compiling for MIPS (and which ABI) so it does that correctly.

A random .bc file for a similar enough machine might work -- same sizes,
same padding rules and so forth. But unless you know for sure those are the
same then chances are good that it won't work.


On Mon, Nov 2, 2015 at 11:37 PM, fateme Hoseini via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi everyone,
> I am a new commer and I 'm tying to use LLVM to cross compile a code. I
> want to cross compile to MIPS and the host is the default x86. The more I
> read the document the more confused I get. I see documents on
> cross-compiling to ARM. But some install some gcc packages, while some use
> clang. I don't know where to start and how to do it.
> Is this just enough to run a command line like this:
>
> llc -march=mips test.bc -o testmips
>
> will the testmips file be executable for mips?
> I will appreciate if anyone help me with this problem.
> Thanks,
> Fami
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Bruce, Thank you so much for your help. Now it's clear to me. I am stuck in
implementation phase of my project and I have many questions about LLVM. I
apologize for my long and beginner level.questions.

I've seen many documents on cross-compiling. for example, I have a book
named"Getting Started with LLVM
Core Libraries" It has described several pages on how to cross-compile and
there is a document on http://llvm.org/docs/HowToCrossCompileLLVM.html.
Which document and steps do you recommend? I had many problems in
installing LLVM3.7 and I am worried that following wrong steps might mess
with the LLVM tool and I have to go through installation again.

Also, I have another question. In my project I want to do some optimization
on machine code, i.e, I want to write a pass before code emission phase.
This pass reads machine instructions and do some optimizations on it. for
example it tries to find all register dependencies. I want the input of my
pass to be machine instructions in MIPS. Now my question is that, if I
cross compile llvm for mips, then when I am running pass, will the input of
my pass be in MIPS? I mean we run a pass on a bitcode (whithout running the
llc mrach=MIPS), and bitcode is target independant. How LLVM knows it
should be cross compile to MIPS not x86?

Regards,
Fami

On Tue, Nov 3, 2015 at 3:48 AM, Bruce Hoult <bruce at hoult.org> wrote:
> llc will make an object file with MIPS machine code for only the functions
> in your test.bc. You then need to link it with any MIPS libraries it needs
> (usually at least the C runtime library that calls main() for you etc).
>
> The generated code will have exactly the data types (e.g. integer sizes)
> and struct layouts specified in your test.bc. These might or might not be
> correct for MIPS. If you're running on bare hardware then it's
probably ok,
> but if you call library functions then you need to be laying out the data
> in the correct way. If you wrote test.bc yourself then that's your job,
but
> usually it's the job of e.g. clang, and you need to also tell clang
that
> you're compiling for MIPS (and which ABI) so it does that correctly.
>
> A random .bc file for a similar enough machine might work -- same sizes,
> same padding rules and so forth. But unless you know for sure those are the
> same then chances are good that it won't work.
>
>
> On Mon, Nov 2, 2015 at 11:37 PM, fateme Hoseini via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi everyone,
>> I am a new commer and I 'm tying to use LLVM to cross compile a
code. I
>> want to cross compile to MIPS and the host is the default x86. The more
I
>> read the document the more confused I get. I see documents on
>> cross-compiling to ARM. But some install some gcc packages, while some
use
>> clang. I don't know where to start and how to do it.
>> Is this just enough to run a command line like this:
>>
>> llc -march=mips test.bc -o testmips
>>
>> will the testmips file be executable for mips?
>> I will appreciate if anyone help me with this problem.
>> Thanks,
>> Fami
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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