[cxx-mem-model] bitfield tests

[Richard Guenther]

On Thu, Mar 31, 2011 at 4:47 PM, Jeff Law <law@redhat.com> wrote:
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> On 03/30/11 08:19, Richard Guenther wrote:
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>>
>> Well, I'm not sure that strict-align targets that provide byte access do
>> not simply hide the issue inside the CPU (thus, perform the read-modify-write
>> there and do not guarantee any atomicity unless you ask for it).
> Certainly some do this internally, but that's clearly out of our
> control.

Sure.  My argument is that the memory model which guarantees
this kind of things for _any_ memory access is fundamentally flawed.
They should have simply required annotating objects which should
behave that way (and then only behave that way "per object", not
for any concurrent field accesses).

Richard.

> However, some really do sub-word accesses.
>
> I even vaguely remember this being controllable by bits in page table
> entries on one architecture.  You could set the bit which meant if I ask
> for a byte access, then do it byte-wise, otherwise the processor would
> do a read-modify-write.  Clearly this was meant to make it easier for
> dealing with memory mapped devices.
>
> Jeff
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