[U-Boot] Query on CONFIG_SYS_THUMB_BUILD

[Victor Ascroft <victorascroft@gmail.com>]

On Monday 17 November 2014 11:58 AM, Simon Glass wrote:
> Hi Albert,
>
> On 16 November 2014 07:50, Albert ARIBAUD <albert.u.boot@aribaud.net> wrote:
>> Hello Simon,
>>
>> On Sat, 15 Nov 2014 15:10:47 -0700, Simon Glass <sjg@chromium.org>
>> wrote:
>>> Hi Albert,
>>>
>>> On 15 November 2014 05:30, Albert ARIBAUD <albert.u.boot@aribaud.net> wrote:
>>>> Hello Simon,
>>>>
>>>> On Fri, 14 Nov 2014 18:56:07 -0700, Simon Glass <sjg@chromium.org>
>>>> wrote:
>>>>
>>>>>> I believe you've built crt0.S for ARM, not Thumb.
>>>>> Yes, but I suspect that is a function of the build system. I checked
>>>>> the rest of U-Boot and most of it (including SPL) is Thumb 2. I
>>>>> suppose we could use Thumb 2 for crt0.S if all the instructions are
>>>>> supported.
>>>> Ok. Just in case, I'll run a check on whether crt0.S can be assembled
>>>> for Thumb and still wrk as expected. :)
>>>>
>>>> Do you have a list of source files which still build for ARM under
>>>> CONFIG_SYS_THUMB_BUILD? I' would prefer all of the code to be thumb for
>>>> consistence, except probably... exception :) entry points -- and even
>>>> these should be able to run in full Thumb 2.
>>> No I don't have a list, but it might be all assembler files. I don't
>>> see why cro0.S would be special.
>> Ok, so after some research, .S files voluntarily not assembled in Thumb
>> mode when -mthumb is defined in gcc because of this:
>>
>> Answer: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=27237
>>
>> (summary: -mthumb for gcc means 'use thumb2', while it means 'use
>> dumb, 16-bit, thumb1' for GNU as, so this option is voluntarily not
>> passed on to GNU as. You have to use .thumb in the .S file instead.)
>>
>> Second: getting a successful, though quick'n'dirty, build with vectors.S
>> assembled in Thumb-2 mode needed surprisingly little change in
>> vectors.S. I tried this with mx53loco, and it only required:
>>
>>         - adding '.syntax unified';
>>
>>         - adding a .thumb directive -- *after* the vectors per se, which
>>           must still be assembled in ARM mode because current hardware
>>           always executes exceptions vectors in ARM mode (1);
>>
>>         - using '.balign' instead of '.balignl' which causes the
>>           assembler to complain that it cannot fit an integer
>>           number of '0xdeadbeef' in the filling space;
>>
>>         - making macro get_bad_stack use lr instead of r13, which
>>           Thumb does not allow in 'msr spsr,' instructions;
>>
>>         - adding '.thumb_func' to all routines so that the linker makes
>>           all references to them odd and therefore, cause the CPU to
>>           enforce Thumb mode when branching to them.
>>
>>         (1) although you *could* produce an ARM-based SoC that runs in
>>         Thumb mode by default. In this case, you'd have to make the
>>         vectors themselves Thumb too.
>>
>> Third: getting a successful *run* of the resulting file will require
>> some work which I'm not going to do without a good incentive :) -- and
>> so does producing a clean vectors.S, i.e. one which will assemble
>> correctly for both ARM and Thumb.
So to use the thumb build correctly, all the .S files have to be changed to use
thumb instructions, by specifying the .thumb option?

-Regards,
Victor.

> That doesn't sound too trciky, but I agree it's not a huge deal. I
> suppose the main benefit is consistency, since the code size
> improvement would be small.
>
> Regards,
> Simon