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