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

[Jiong Wang]

(Just for the record to avoid misleading)

I think the .eqv should fully work, but need to correct the reason for why.

It is not because it turned the new symbol into non-function type, but 
because .eqv
is expect to always evaulate the snapshot value of the equaled 
expression at each
place the new symbol is used, you don't know it's final value and the 
associated
section, therefore it is associated with undefined section until the 
final stage of
assembling.  So, when doing ADR instruction assembling, at that stage, 
the new
symbol is still without definition section assigned, therefore is able 
to skip the LSB
setting which only happens for thumb function symbol with defined section.

binutils-gdb/gas/config/tc-arm.c: do_t_adr
...

   && S_IS_DEFINED (inst.reloc.exp.X_add_symbol)
   && THUMB_IS_FUNC (inst.reloc.exp.X_add_symbol))
     inst.reloc.exp.X_add_number += 1;

Use .equ, the magic disappear.

Regards,
Jiong

On 04/08/17 14:58, 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 04/08/17 12:39, Jiong Wang wrote:
>> Hi,
>>
>>   This issue is caused by a recent change in ARM assembler included 
>> since Binutils 2.29.
>>
>>   The details of that change can be found at 
>> https://sourceware.org/bugzilla/show_bug.cgi?id=21458
>>
>>   The semantics of ADR has changed.  In general, the address 
>> generated by ADR will guarantee the LSB be set if it's a thumb 
>> function address.
>>
>>    I noticed h264idct_neon.S is using something like:
>>
>> adreq lr,X(ff_h264_idct_add_neon) +CONFIG_THUMB
>>
>>    As ADR now will set the LSB automatically, you don't need 
>> CONFIG_THUMB any more.
>>
>>    I think h264idct_neon.S needs to be updated, and the modification 
>> should make sure it works with both old Binutils and the new one.
>>
>> Regards,
>> Jiong
>>
>

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