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

[Andi Kleen]

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.

-Andi

-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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