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

From: Denys Vlasenko
Date: Fri Mar 06 2015 - 17:37:50 EST

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

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


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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