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

[David Laight]

From: Arvind Sankar
> Sent: 29 October 2020 21:35
> 
> 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.

I guess it assumes that global data doesn't overlap.

But in general they are sort of opposites:

With the local variable it can reload if it knows the write
cannot have affected the global - but is unlikely to do so.

Using the global it must reload if it is possible the write
might have affected the global.

	David

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