https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92172
--- Comment #6 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Seth LaForge from comment #5)
> GCC 8:
> push {r7, lr}
> sub sp, sp, #8
> add r7, sp, #0
> str r0, [r7, #4]
> ...
>
> Clang 9:
> push {r7, lr}
> mov r7, sp
> sub sp, #8
> str r0, [sp, #4]
> ...
Crazy yes, but it's due to historical reasons. Originally GCC could only emit
code using a frame pointer. Later the frame pointer could be switched off
(hence -fomit-frame-pointer), but you still needed it for debug tables. Then
there was Dwarf which didn't need a frame pointer anymore. And today the frame
pointer is off by default globally in GCC.
> - GCC ARM and Clang ARM use R11 for frame pointer, pointing to the stacked
> R11. Useful.
Well Clang does this:
push {r4, r10, r11, lr}
add r11, sp, #8
but GCC does something different:
push {r4, r5, fp, lr}
add fp, sp, #12
Ie. FP points to saved LR with GCC but saved FP with Clang, so it's not
possible for a generic unwinder to follow the chain, even ignoring Arm/Thumb
interworking (which is a real issue when an application is Thumb-2 but various
library functions use Arm assembly).
