Re: [ARM] Wrong BLX destination

[Simon Marchi <simon dot marchi at polymtl dot ca>]

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