On Sat, Sep 12, 2020 at 10:11:36AM -0700, H.J. Lu via Gcc-patches wrote:
> Clobbering the stack pointer in asm statment has been deprecated. Adding
> the stack pointer register to the clobber list has traditionally had some
> undocumented and somewhat obscure side-effects, including ICE. Issue
> a warning and ignore the clobbered stack pointer in asm statment.
>
> gcc/
>
> PR target/97032
> * cfgexpand.c (asm_clobber_reg_kind): New enum.
> (asm_clobber_reg_is_valid): Renamed to ...
> (get_asm_clobber_reg_kind): This. Ignore the stack pointer.
> (expand_asm_stmt): Replace asm_clobber_reg_is_valid with
> get_asm_clobber_reg_kind. Skip ignored clobbered register.
AFAIK in the PR52813 discussions it was mentioned that the sp clobbers
had some user visible effects like forcing no red-zone, or (at least for
some GCC versions) forcing frame pointer in the function containing the asm.
Such clobbers are deprecated since GCC 9, so I think e.g. this patch isn't
really backportable to 9 branch. Are we ready to stop that behavior?
And if yes, shouldn't we instead just error on this because a warning
clearly doesn't help, as users ignore warnings (at least in firefox this
isn't fixed yet)?
Is there some other way to fix the ICE (mark functions containing that and
perhaps penalize the code, in this case e.g. by perhaps forcing unnecessary
realignment, but not ICE)?
Jakub
