From: "H. Peter Anvin" <hpa@zytor.com>
To: Kanna Scarlet <knscarlet@gnuweeb.org>, Joerg Roedel <jroedel@suse.de>
Cc: David Laight <David.Laight@aculab.com>,
Borislav Petkov <bp@alien8.de>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
"x86@kernel.org" <x86@kernel.org>,
Ard Biesheuvel <ardb@kernel.org>,
Bill Metzenthen <billm@melbpc.org.au>,
Brijesh Singh <brijesh.singh@amd.com>,
Josh Poimboeuf <jpoimboe@kernel.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Mark Rutland <mark.rutland@arm.com>,
Michael Roth <michael.roth@amd.com>,
Peter Zijlstra <peterz@infradead.org>,
Sean Christopherson <seanjc@google.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ammar Faizi <ammarfaizi2@gnuweeb.org>,
GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/1] x86: Change mov $0, %reg with xor %reg, %reg
Date: Mon, 8 Aug 2022 11:59:17 -0700 [thread overview]
Message-ID: <ad5f732f-778f-a337-38e5-0129d2f49793@zytor.com> (raw)
In-Reply-To: <4764d363-b5a4-0cf5-6ed0-0911c4182b5b@gnuweeb.org>
On August 8, 2022 9:45:45 AM PDT, Kanna Scarlet <knscarlet@gnuweeb.org>
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.
next prev parent reply other threads:[~2022-08-08 19:03 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-04 15:26 [PATCH 0/1] x86 change mov 0, %reg to xor %reg, %reg Kanna Scarlet
2022-08-04 15:26 ` [PATCH 1/1] x86: Change mov $0, %reg with " Kanna Scarlet
2022-08-04 15:53 ` Borislav Petkov
2022-08-04 18:08 ` Kanna Scarlet
2022-08-05 9:26 ` David Laight
2022-08-05 9:42 ` Joerg Roedel
2022-08-08 16:45 ` Kanna Scarlet
2022-08-08 18:59 ` H. Peter Anvin [this message]
2022-08-08 16:38 ` Kanna Scarlet
2022-08-08 18:59 ` H. Peter Anvin
2022-08-09 7:38 ` David Laight
2022-08-05 9:54 ` Borislav Petkov
2022-08-08 16:57 ` Kanna Scarlet
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