[ARM] Wrong BLX destination

[Simon Marchi]

On 2016-03-09 10:48, Christophe Lyon wrote:
> On 9 March 2016 at 16:35, Simon Marchi <simon.marchi@polymtl.ca> wrote:
>> On 2016-03-09 03:07, Christophe Lyon wrote:
>>> 
>>> Hi,
>>> 
>>> I haven't looked at this for a while, but:
>>> - which relocations are encoded by gas? (objdump -r). I suspect you
>>> have the ARM version of call (as opposed to the thumb one)
>> 
>> 
>> Indeed:
>> 
>>   00000000 R_ARM_CALL        hello1
>> 
>>> - I think you have to use the .thumb_func assembly directive right
>>> before helloX definition, and probably give it a function type too
>>> 
>>> The linker will take care of making the call point to the right
>>> address, provided it has the right relocations.
>> 
>> 
>> When adding .thumb_func before the helloX definitions, the branches 
>> are now
>> 
>>     8220:       eb00013c        bl      8718 <__hello1_from_arm>
>>     8224:       eb000135        bl      8700 <__hello2_from_arm>
>>     8228:       eb000137        bl      870c <__hello3_from_arm>
>> 
>> 
>> and this additional code was added somewhere:
>> 
>>   00008700 <__hello2_from_arm>:
>>       8700:       e59fc000        ldr     ip, [pc]        ; 8708
>> <__hello2_from_arm+0x8>
>>       8704:       e12fff1c        bx      ip
>>       8708:       0000827f        .word   0x0000827f
>> 
>>   0000870c <__hello3_from_arm>:
>>       870c:       e59fc000        ldr     ip, [pc]        ; 8714
>> <__hello3_from_arm+0x8>
>>       8710:       e12fff1c        bx      ip
>>       8714:       00008281        .word   0x00008281
>> 
>>   00008718 <__hello1_from_arm>:
>>       8718:       e59fc000        ldr     ip, [pc]        ; 8720
>> <__hello1_from_arm+0x8>
>>       871c:       e12fff1c        bx      ip
>>       8720:       0000827d        .word   0x0000827d
>> 
>> So while it doesn't simply use BLX with the H bit set, it seems that 
>> it
>> would now work properly with this indirection.
>> 
> 
> Did you supply a --cpu flag or a .cpu directive in your asm source?
> If no, then the tools default to some old version (I don't remember
> which) of the arm architecture which did not support the blx
> instruction.
> 
> Hence the linker insert stubs to provide the desired behaviour
> (switching between arm and thumb modes).
> 
> Add .cpu cortex-a9 in your source, and the linker should not insert 
> stubs.

Right!  Adding .cpu cortex-a9 to either assembly source file works.  The 
result is now:

     8220:       fa000015        blx     827c <hello1>
     8224:       fb000014        blx     827e <hello2>
     8228:       fa000014        blx     8280 <hello3>

Adding -mcpu=cortex-a9 to the command line works as well, obviously.

Thanks for your help, I would never have guessed it.

Simon