llvm dev - Mar 2018 - Cross-compiling for ARM Cortex-M3 on x86

Thanks for your prompt reply.

I didn't build the libc at all, I downloaded a pre-built version of the
whole toolchain from
https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads, so I
guess it is an official version provided by ARM and should work? This
version targets ARM Cortex-M and Cortex-R family of processors...

Son Tuan Vu

On Mon, Mar 12, 2018 at 4:15 PM, Tim Northover <t.p.northover at
gmail.com>
wrote:
> Hi,
>
> On 12 March 2018 at 15:06, Son Tuan VU via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > But libc code is in ARM mode:
>
> That's very bad, because Cortex-M doesn't support ARM mode. I think
> your libc is broken.
>
> > As you can see, the call to srand is just a bl (Branch with Link), not
a
> blx
> > (Branch with Link and Exchange instruction set), so I think something
is
> > going wrong here.
>
> Yes, Cortex-M has no blx instruction.
>
> > Can anyone kindly give me some pointers on how to debug this? Or at
least
> > tell me whether this is a bug?
>
> You don't mention how you build libc, but that's what you need to
fix.
>
> > I am a bit lost now, I've tried to look up
> > for more information on this but cannot find out why Clang doesn't
> generate
> > Thumb code for libc functions.
>
> It'll be part of your libc's build system (Makefile?). Clang should
> simply refuse to produce ARM mode code if it knows it's targeting
> Cortex-M, so my guess is no -mcpu option is being specified. There
> might be subtle differences in how GCC has been compiled that means
> your version defaults to Thumb mode anyway, but Clang doesn't.
>
> Cheers.
>
> Tim.
>
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On 12 March 2018 at 16:35, Son Tuan VU via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Thanks for your prompt reply.
>
> I didn't build the libc at all, I downloaded a pre-built version of the
> whole toolchain from
> https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads, so I
> guess it is an official version provided by ARM and should work? This
> version targets ARM Cortex-M and Cortex-R family of processors...
>
I believe these GCC toolchains are multilib-enabled (that is, the libs are built
multiple times, with different options, including thumb/arm).

Since you already use the gcc driver to link your program,
adding -mcpu=cortex-m3 -mthumb to your link line should do the trick.

To see which multilibs are available in your GCC toolchain, use:
arm-none-eabi-gcc --print-multi-lib

HTH

Christophe

> Son Tuan Vu
>
> On Mon, Mar 12, 2018 at 4:15 PM, Tim Northover <t.p.northover at
gmail.com>
> wrote:
>>
>> Hi,
>>
>> On 12 March 2018 at 15:06, Son Tuan VU via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > But libc code is in ARM mode:
>>
>> That's very bad, because Cortex-M doesn't support ARM mode. I
think
>> your libc is broken.
>>
>> > As you can see, the call to srand is just a bl (Branch with Link),
not a
>> > blx
>> > (Branch with Link and Exchange instruction set), so I think
something is
>> > going wrong here.
>>
>> Yes, Cortex-M has no blx instruction.
>>
>> > Can anyone kindly give me some pointers on how to debug this? Or
at
>> > least
>> > tell me whether this is a bug?
>>
>> You don't mention how you build libc, but that's what you need
to fix.
>>
>> > I am a bit lost now, I've tried to look up
>> > for more information on this but cannot find out why Clang
doesn't
>> > generate
>> > Thumb code for libc functions.
>>
>> It'll be part of your libc's build system (Makefile?). Clang
should
>> simply refuse to produce ARM mode code if it knows it's targeting
>> Cortex-M, so my guess is no -mcpu option is being specified. There
>> might be subtle differences in how GCC has been compiled that means
>> your version defaults to Thumb mode anyway, but Clang doesn't.
>>
>> Cheers.
>>
>> Tim.
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

@ Christophe: Oh wow yes adding these option to link command does indeed
solve the problem. Thank you so much!
@ Peter: Yes I've found the samples. I should have look at these sooner.
Thanks for the pointer!

Son Tuan Vu

On Mon, Mar 12, 2018 at 4:42 PM, Christophe Lyon <christophe.lyon at
linaro.org
> wrote:
> On 12 March 2018 at 16:35, Son Tuan VU via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Thanks for your prompt reply.
> >
> > I didn't build the libc at all, I downloaded a pre-built version
of the
> > whole toolchain from
> > https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads,
> so I
> > guess it is an official version provided by ARM and should work? This
> > version targets ARM Cortex-M and Cortex-R family of processors...
> >
>
> I believe these GCC toolchains are multilib-enabled (that is, the libs are
> built
> multiple times, with different options, including thumb/arm).
>
> Since you already use the gcc driver to link your program,
> adding -mcpu=cortex-m3 -mthumb to your link line should do the trick.
>
> To see which multilibs are available in your GCC toolchain, use:
> arm-none-eabi-gcc --print-multi-lib
>
> HTH
>
> Christophe
>
>
> > Son Tuan Vu
> >
> > On Mon, Mar 12, 2018 at 4:15 PM, Tim Northover <t.p.northover at
gmail.com>
> > wrote:
> >>
> >> Hi,
> >>
> >> On 12 March 2018 at 15:06, Son Tuan VU via llvm-dev
> >> <llvm-dev at lists.llvm.org> wrote:
> >> > But libc code is in ARM mode:
> >>
> >> That's very bad, because Cortex-M doesn't support ARM
mode. I think
> >> your libc is broken.
> >>
> >> > As you can see, the call to srand is just a bl (Branch with
Link),
> not a
> >> > blx
> >> > (Branch with Link and Exchange instruction set), so I think
something
> is
> >> > going wrong here.
> >>
> >> Yes, Cortex-M has no blx instruction.
> >>
> >> > Can anyone kindly give me some pointers on how to debug this?
Or at
> >> > least
> >> > tell me whether this is a bug?
> >>
> >> You don't mention how you build libc, but that's what you
need to fix.
> >>
> >> > I am a bit lost now, I've tried to look up
> >> > for more information on this but cannot find out why Clang
doesn't
> >> > generate
> >> > Thumb code for libc functions.
> >>
> >> It'll be part of your libc's build system (Makefile?).
Clang should
> >> simply refuse to produce ARM mode code if it knows it's
targeting
> >> Cortex-M, so my guess is no -mcpu option is being specified. There
> >> might be subtle differences in how GCC has been compiled that
means
> >> your version defaults to Thumb mode anyway, but Clang doesn't.
> >>
> >> Cheers.
> >>
> >> Tim.
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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