[patch][rtl-optimization][i386][pr97777] Fix a reg-stack df maintenance bug triggered by zero-call-used-regs pass.

[Qing Zhao]

Thanks a lot for your review.

I will commit the patch soon.

Qing

> On Dec 2, 2020, at 12:27 PM, Jeff Law <law@redhat.com> wrote:
> 
> 
> 
> On 12/2/20 9:32 AM, Qing Zhao wrote:
>> 
>>> So we are clearing the x87 registers with that option.  Hence the change
>>> in reg-stack behavior.  I'm a bit surprised by this as I don't see
>>> clearing the x87 registers as particularly helpful from a security
>>> standpoint.  But I haven't followed that discussion closely.
>> Even with the option -fzero-call-used-regs=used-gpr (without clearing any x87 registers), 
>> We have the same compiler time error. 
>> 
>> The first thing that the new pass “zero_call_used_regs” does is:
>> 
>> df_analyze();
> OK.  So that's the key issue, until now nothing has really cared about
> the DF state after reg-stack, but the pass to zero registers does.  I
> had the order backwards in my head with register clearing happening
> before regstack.  Hence my significant confusion about why things were
> suddenly going wrong.
> 
> The patch is OK.  Thanks for patiently walking me through this.
> 
> jeff
>