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

From: Segher Boessenkool <[email-masked]>
Date: Sat, 18 Aug 2007 00:38:12 +0200

>>> 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