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

[Andy Lutomirski]

On Fri, Mar 6, 2015 at 2:24 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx -- Speaking for myself only



-- 
Andy Lutomirski
AMA Capital Management, LLC
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