Re: [gimple] assignments to volatile

[IainS <developer@sandoe-acoustics.co.uk>]

On 21 Jun 2010, at 14:30, Michael Matz wrote:
> On Mon, 21 Jun 2010, Richard Guenther wrote:
>
>>> I've come across inconsistent and surprising behaviour when  
>>> assigning
>>> to volatile lvalues.
>>>
>>> Sometimes we re-read the assigned-to object, and sometimes we do  
>>> not.  For
>>> instance,
>>>   return vobj = data;
>>> will cause a reread of vobj, IF data is not a constant.
>
> I'd consider the latter condition a bug.  I.e. the rereading must  
> happen
> in every case.  It must happen because in C the value of "lhs = rhs"  
> is
> that of reading 'lhs' ("An assignment expression has the value of  
> the left
> operand after the assignment, but is not an lvalue." 6.5.16), and if  
> lhs
> happens to be volatile than the abstract machine implies two  
> accesses to
> lhs, one write (of the assignment) and one read (to get to the value  
> of
> lhs).

that answers the "what" but not the "when".

Suppose that vobj was a hardware register ...

... I would expect that the assignment would write the value of expr  
to the register...

... but I would _not_ expect the register to be re-read (and, indeed,  
in many cases in hardware interaction - that could cause an issue).

or did I miss sth?
Iain