linux-kernel - Re: [RFC patch] spindep: add cross cache lines checking

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Date:	Thu, 08 Mar 2012 10:30:47 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	gcc@....gnu.org, Ingo Molnar <mingo@...e.hu>, tglx@...utronix.de,
	"mingo@...hat.com" <mingo@...hat.com>, hpa@...or.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, andi.kleen@...el.com, gcc-help@....gnu.org
Subject: Re: [RFC patch] spindep: add cross cache lines checking

> > 1, it is alignof bug for default gcc on my fc15 and Ubuntu 11.10 etc?
> > 
> > struct sub {
> >         int  raw_lock;
> >         char a;
> > };
> > struct foo {
> >         struct sub z;
> >         int slk;
> >         char y;
> > }__attribute__((packed));
> > 
> > struct foo f1;
> > 
> > __alignof__(f1.z.raw_lock) is 4, but its address actually can align on
> > one byte. 
> 
> That looks like correct behavior, because the alignment of raw_lock inside of
> struct sub is still 4. But it does mean that there can be cases where the
> compile-time check is not sufficient, so we might want the run-time check
> as well, at least under some config option.

According to explanation of gcc, seems it should return 1 when it can be
align on char. And then it's useful for design intend. Any comments from
gcc guys? 
====
http://gcc.gnu.org/onlinedocs/gcc/Alignment.html
The keyword __alignof__ allows you to inquire about how an object is
aligned, or the minimum alignment usually required by a type. Its syntax
is just like sizeof.



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