• darat (7/7) Apr 10 2016 Is there a particular reason explaining why mov reg,0 is used and
• =?UTF-8?Q?Ali_=c3=87ehreli?= (5/12) Apr 10 2016 I don't know the answer but Google finds interesting discussions about t...
• darat (5/11) Apr 10 2016 I've read them all before posting here. Nothing answers the
• Kagamin (1/5) Apr 11 2016

darat <darat darat.web.cr> writes:

Is there a particular reason explaining why mov reg,0 is used and 
not xor reg,reg ?
(or even and reg, 0)

for example here:

https://github.com/D-Programming-Language/druntime/blob/master/src/core/atomic.d#L1009

It seems that in Go too mov is used:
https://github.com/golang/go/blob/0104a31b8fbcbe52728a08867b26415d282c35d2/src/runtime/internal/atomic/asm_amd64p32.s#L15

=?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:

On 04/10/2016 04:00 AM, darat wrote:
 Is there a particular reason explaining why mov reg,0 is used and not
 xor reg,reg ?
 (or even and reg, 0)

 for example here:

 https://github.com/D-Programming-Language/druntime/blob/master/src/core/atomic.d#L1009


 It seems that in Go too mov is used:
 https://github.com/golang/go/blob/0104a31b8fbcbe52728a08867b26415d282c35d2/src/runtime/internal/atomic/asm_amd64p32.s#L15

I don't know the answer but Google finds interesting discussions about that:

 
https://www.google.com/search?q=mov+0+versus+xor+cycles&ie=utf-8&oe=utf-8#q=mov+0+versus+xor+cpu+cycle

Ali

darat <darat online.de> writes:

On Monday, 11 April 2016 at 05:45:17 UTC, Ali Çehreli wrote:
 On 04/10/2016 04:00 AM, darat wrote:
 Is there a particular reason explaining why mov reg,0 is used 
 and not
 xor reg,reg ?

 https://www.google.com/search?q=mov+0+versus+xor+cycles&ie=utf-8&oe=utf-8#q=mov+0+versus+xor+cpu+cycle

 Ali

I've read them all before posting here. Nothing answers the 
question. The only clue is maybe that xor modifies the EFLAG 
register, but since inline asm is not inlined (there's always a 
CALL) I'm not 100% sure this is relevant.

Kagamin <spam here.lot> writes:

https://randomascii.wordpress.com/2012/12/29/the-surprising-subtleties-of-zeroing-a-register/
 But there was at least one out-of-order design that did not 
 recognize xor reg, reg as a special case: the Pentium Pro. The 
 Intel Optimization manuals for the Pentium Pro recommended 
 “mov” to zero a register.