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

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Date:	Fri, 6 Mar 2015 23:37:23 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
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>,
	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 ML <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: clean up unnecessarily wide TEST insns

On Fri, Mar 6, 2015 at 11:24 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.

Length-changing prefix (LCP) stalls result from 0x66 prefix.

(See https://software.intel.com/en-us/forums/topic/328256).

Basically, LCP happens because adding 66 byte before this instruction:

[test_opcode] [modrm] [imm32]

changes it to

[66] [test_opcode] [modrm] [imm16]

where  [imm16] has *different length* now: 2 bytes instead of 4.
This confuses decoder.


REX prefixes were carefully designed to almost never hit this case:
adding REX prefix does not change instruction length except MOVABS
and MOV [addr],RAX insn.

My patch does not add optimizations which would use 0x66 prefix.
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