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
