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]

Date:	Fri, 06 Mar 2015 14:24:17 -0800
From:	Andi Kleen <andi@...stfloor.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	"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>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: clean up unnecessarily wide TEST insns

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.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
--
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/