On Thu, Oct 29, 2020 at 09:41:13PM +0100, Thomas Gleixner wrote:

On Thu, Oct 29 2020 at 17:59, Paolo Bonzini wrote:

quoted

On 29/10/20 17:56, Arvind Sankar wrote:

quoted

quoted

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

It's not quite the same.

https://godbolt.org/z/4dzPbM

With -fno-strict-aliasing, the compiler reloads the pointer if you write
to the start of what it points to, but not if you write to later
elements.