PING [Patch][Middle-end]Add -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]

[Segher Boessenkool]

On Fri, Sep 11, 2020 at 11:06:03AM +0100, Richard Sandiford wrote:
> This might have already been discussed/answered, sorry, but:
> when there's a choice, is there an obvious winner between:
> 
> (1) clearing call-clobbered registers and then restoring call-preserved ones
> (2) restoring call-preserved registers and then clearing call-clobbered ones
>     
> Is one option more likely to be useful to attackers than the other?
> 
> (For some frames, it might be necessary to use a small number of
> call-clobbered registers to perform the restore sequence, so (1)
> wouldn't be fully achievable in all cases.)

The same is true for what you have to do *after* restoring registers, as
I said before.  Clearing all is not correct in all cases, and also it is
not useful in all cases (code right after it might write the registers
again.

This really is very (sub-)target-specific, it cannot be done by generic
code on its own *at all*.


Segher