On Mon, Nov 24, 2014 at 12:19:11PM -0800, Linus Torvalds wrote:
> On Mon, Nov 24, 2014 at 11:42 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > OK, how about the following?
>
> Ugh. Disgusting.
>
> Why the heck isn't it just "sizeof(*__vp) <= sizeof(long)"?
>
> If the architecture has a 3-byte scalar type, then it probably has a
> 3-byte load.
Because I was allowing for the possibility of a 3-byte struct, which
as you point out below...
> > It complains if the variable is too large, for example, long long on
> > 32-bit systems or large structures. It is OK loading from and storing
> > to small structures as well, which I am having a hard time thinking of
> > as a disadvantage.
>
> .. but that's *exactly* the gcc bug in question. It's a word-sized
> struct that gcc loads twice.
...was a stupid thought anyway.
OK, how about the attempt below? The initialization of __p complains
for structures and unions, but gets optimized out.
Thanx, Paul
-------------------------------------------------------------------------
#define get_scalar_volatile_pointer(x) ({ \
typeof(x) __maybe_unused __p = 0; \
volatile typeof(x) *__vp = &(x); \
BUILD_BUG_ON(sizeof(*__vp) > sizeof(long)); \
__vp; })
#define ACCESS_ONCE(x) (*get_scalar_volatile_pointer(x))
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