Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

[Segher Boessenkool <segher@kernel.crashing.org>]

> > > > That means GCC cannot compile Linux; it already optimises
> > > > some accesses to scalars to smaller accesses when it knows
> > > > it is allowed to.  Not often though, since it hardly ever
> > > > helps in the cost model it employs.
> > >
> > > Please give an example code snippet + gcc version + arch
> > > to back this up.
> >
> > 	unsigned char f(unsigned long *p)
> > 	{
> > 	        return *p & 1;
> > 	}
>
> This doesn't really matter since we only care about the LSB.

It is exactly what I claimed, and what you asked proof of.

> Do you have an example where gcc reads it non-atmoically and
> we care about all parts?

Like I explained in the original mail; no, I suspect such
a testcase will be really hard to construct, esp. as a small
testcase.  I have no reason to believe it is impossible to
do so though -- maybe someone else can write trickier code
than I can, in which case, please do so.


Segher

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