Re: [PATCH 1/1] x86: Change mov $0, %reg with xor %reg, %reg

[H. Peter Anvin]

On August 8, 2022 9:45:45 AM PDT, Kanna Scarlet <knscarlet@xxxxxxxxxxx> 
wrote:
> On 8/5/22 4:42 PM, Joerg Roedel wrote:
> > On Fri, Aug 05, 2022 at 09:26:02AM +0000, David Laight wrote:
> > > Use "xor %rax,%rax" instead of "xor %eax,%eax" to save
> > > the 'reg' prefix.
> >
> > Also, some places explicitly use the mov variant to zero a register
> > without touching rflags. Please be careful to not change those.
>
> thank you for reminder, i will check again to make myself more sure
> the patch doesn't break this %rflags dependency situation
>
> Regards,

In some cases you can hoist the zeroing to avoid that (and sometimes 
improve performance in the process), but be very careful in general when 
messing with hand-optimized assembly code like crypto; for those pieces 
of code benchmarking the change is mandatory.