Bug#870622: ffmpeg: autopkgtest SIGBUS on armhf with binutils 2.29

[Edmund Grimley Evans]

On 5 August 2017 at 17:33, James Cowgill <jcowgill at debian.org> wrote:
> Hi,
>
> On 04/08/17 09:58, Jiong Wang wrote:
>> Change
>>
>>   "adreq lr,X(ff_h264_idct_add_neon) +CONFIG_THUMB"
>>
>> Into:
>>
>> .eqv ff_h264_idct_add_neon_without_func_type, X(ff_h264_idct_add_neon)
>> adreq lr,  ff_h264_idct_add_neon_without_func_type +CONFIG_THUMB
>>
>> might be a solution.  The idea is we use .eqv to remove the function
>> attribute, so the assembler won't set LSB in any case.

> On the technical side, does having the LSB clear when executing a blx
> instruction cause a mode change out of Thumb, or does it retain the
> mode? I think all the code in that file is compiled in the same mode, so
> if the mode is retained then simply dropping +CONFIG_THUMB might work.

All the usual register branch instructions set or reset T32 (Thumb)
mode according to bit 0 of the target address. This is to enable
"interworking": an A32 function can call a T32 function and vice
versa.

What Jiong suggested, above, looks like the right solution for now.