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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