[AARCH64] question about RELA relocations

Segher Boessenkool segher@kernel.crashing.org
Sun Jan 31 17:04:00 GMT 2016


On Tue, Jan 12, 2016 at 11:56:43AM +0000, Jiong Wang wrote:
> gcc combine should have combined:
> 
>  add  x0, x0, :lo12:.LANCHOR0
>  ldr  x0, [x0,8] 
> 
> into:
> 
>  ldr     x0, [x0,#:lo12:.LANCHOR0+8]
> 
> It's not happen because combine pass invoked "init_recog_no_volatile" to
> prevent volatile memory to be combined. I guess combine is too
> conservative on this, purely combine contant offsets inside volatile
> memory should be safe. (CCed Segher).

[ For some reason I didn't see this mail before, sorry about that. ]

This is a well-known issue, see PR60749, PR50677, PR49807, likely more.
Simply allowing volatile refs is not conservative enough.  Backends can
use a workaround like the msp430 port does, for those cases where it knows
it will work.


Segher



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