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

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Message-ID: <87h9txahny.fsf@tassilo.jf.intel.com>
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
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