Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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

> > > Here, I should obviously admit that the semantics of *(volatile int 
> > > *)&
> > > aren't any neater or well-defined in the _language standard_ at all. 
> > > The
> > > standard does say (verbatim) "precisely what constitutes as access to
> > > object of volatile-qualified type is implementation-defined", but GCC
> > > does help us out here by doing the right thing.
> >
> > Where do you get that idea?
>
> Try a testcase (experimentally verify).

That doesn't prove anything.  Experiments can only disprove
things.

> > GCC manual, section 6.1, "When
> > is a Volatile Object Accessed?" doesn't say anything of the
> > kind.
>
> True, "implementation-defined" as per the C standard _is_ supposed to 
> mean
> "unspecified behaviour where each implementation documents how the 
> choice
> is made". So ok, probably GCC isn't "documenting" this
> implementation-defined behaviour which it is supposed to, but can't 
> really
> fault them much for this, probably.

GCC _is_ documenting this, namely in this section 6.1.  It doesn't
mention volatile-casted stuff.  Draw your own conclusions.


Segher

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