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

[Qing Zhao via Gcc-patches]

> On Aug 24, 2020, at 12:59 PM, Segher Boessenkool <seg...@kernel.crashing.org> 
> wrote:
> 
> [ Please quote correctly.  I fixed this up a bit. ]
> 
> On Mon, Aug 24, 2020 at 02:47:22PM +0000, Rodriguez Bahena, Victor wrote:
>>> The call-clobbered regs are the only ones you *can* touch.  That does
>>> not mean you should clear them all (it doesn't help much at all in some
>>> cases).  Only the backend knows.
>> 
>> I think that for ROP mitigation purpose, we only need to clear the call-used 
>> (i.e, call-clobbered) registers that are used in the current routine and
>> can pass parameters.
> 
> Which is more than you *can* do as well (consider return value registers
> for example; there are more cases, in general; only the backend code can
> know what is safe to do).

Yes, So, we agreed to move the code generation implementation part into backend.

In Middle-end, we will only compute the hard register set based on call abi 
information and data flow information, also handle the command line option.

Qing
> 
> 
> Segher