[ARM] Wrong BLX destination

[Christophe Lyon]

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.

> Thanks for the tip!
>
> Simon