On Wed, Jul 12, 2017 at 03:20:40PM -0700, Matthias Kaehlcke wrote:
> > This is admittedly an awkward way of achieving this goal, but it's the
> > only way I know how to do it with GCC.
> >
> > What extra instruction does clang add?
>
> I was looking at the get_user() call in drm_mode_setcrtc(). The code
> generated by clang without the patch is:
>
> if (get_user(out_id, &set_connectors_ptr[i])) {
> ffffffff81386955: 4a 8d 04 bd 00 00 00 lea 0x0(,%r15,4),%rax
> ffffffff8138695c: 00
> ffffffff8138695d: 49 03 06 add (%r14),%rax
> ffffffff81386960: e8 2b a5 f0 ff callq ffffffff81290e90
> <__get_user_4>
>
> And with the patch:
>
> if (get_user(out_id, &set_connectors_ptr[i])) {
> ffffffff81386a56: 4a 8d 04 bd 00 00 00 lea 0x0(,%r15,4),%rax
> ffffffff81386a5d: 00
> ffffffff81386a5e: 49 03 06 add (%r14),%rax
> ffffffff81386a61: 48 8b 64 24 28 mov 0x28(%rsp),%rsp
> ffffffff81386a66: e8 15 a5 f0 ff callq
> ffffffff81290f80 <__get_user_4>
Hm, that seems odd. Can you sure the disassembly for the whole
function?
--
Josh
