Re: [PATCH] [v2] x86: apic: avoid -Wshadow warning in header

[Thomas Gleixner]

On Thu, Oct 29 2020 at 17:59, Paolo Bonzini wrote:
> On 29/10/20 17:56, Arvind Sankar wrote:
>>> For those two just add:
>>> 	struct apic *apic = x86_system_apic;
>>> before all the assignments.
>>> Less churn and much better code.
>>>
>> Why would it be better code?
>> 
>
> I think he means the compiler produces better code, because it won't
> read the global variable repeatedly.  Not sure if that's true,(*) but I
> think I do prefer that version if Arnd wants to do that tweak.

It's not true.

     foo *p = bar;

     p->a = 1;
     p->b = 2;

The compiler is free to reload bar after accessing p->a and with

    bar->a = 1;
    bar->b = 1;

it can either cache bar in a register or reread it after bar->a

The generated code is the same as long as there is no reason to reload,
e.g. register pressure.

Thanks,

        tglx