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

[H. Peter Anvin]

On August 5, 2022 2:26:02 AM PDT, David Laight <David.Laight@xxxxxxxxxx> 
wrote:
> From: Kanna Scarlet
> > Sent: 04 August 2022 19:08
> >
> > On 8/4/22 10:53 PM, Borislav Petkov wrote:
> > > Bonus points if you find out what other advantage
> > >
> > > XOR reg,reg
> > >
> > > has when it comes to clearing integer registers.
> >
> > Hello sir Borislav,
> >
> > Thank you for your response. I tried to find out other advantages of
> > xor reg,reg on Google and found this:
> > https://stackoverflow.com/a/33668295/7275114
> >
> >   "xor (being a recognized zeroing idiom, unlike mov reg, 0) has some
> >   obvious and some subtle advantages:
> >
> >   1. smaller code-size than mov reg,0. (All CPUs)
> >   2. avoids partial-register penalties for later code.
> >      (Intel P6-family and SnB-family).
> >   3. doesn't use an execution unit, saving power and freeing up
> >      execution resources. (Intel SnB-family)
> >   4. smaller uop (no immediate data) leaves room in the uop cache-line
> >      for nearby instructions to borrow if needed. (Intel SnB-family).
> >   5. doesn't use up entries in the physical register file. (Intel
> >      SnB-family (and P4) at least, possibly AMD as well since they use
> >      a similar PRF design instead of keeping register state in the ROB
> >      like Intel P6-family microarchitectures.)"
>
> You missed one, and an additional change:
>
> Use "xor %rax,%rax" instead of "xor %eax,%eax" to save
> the 'reg' prefix.
>
> 	David
>
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You mean the other way around...