linux-kernel - Re: [PATCH] x86: clean up unnecessarily wide TEST insns

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CALCETrXtWLbLgRbRhhG1Bg4__t6W1D3doGvQ8pxMq1oRN7WsgQ@mail.gmail.com>
Date:	Fri, 6 Mar 2015 14:33:04 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: clean up unnecessarily wide TEST insns

On Fri, Mar 6, 2015 at 2:24 PM, Andi Kleen <andi@...stfloor.org> wrote:
> Denys Vlasenko <dvlasenk@...hat.com> writes:
>
>> By the nature of TEST operation, it is often possible
>> to test a narrower part of the operand:
>>     "testl $3, mem"  -> "testb $3, mem",
>>     "testq $3, %rcx" -> "testb $3, %cl"
>> This results in shorter insns, because TEST insn has no
>> sign-entending byte-immediate forms unlike other ALU ops.
>
> It also results in expensive LCP stalls. Please don't do it.
> If you feel the need to change instructions around like this read
> the optimization manuals first.

Is this true for register access or just memory?

--Andy

>
> -Andi
>
> --
> ak@...ux.intel.com -- Speaking for myself only



-- 
Andy Lutomirski
AMA Capital Management, LLC
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/