[C11-atomic] [patch] gimple atomic statements

[Richard Sandiford]

Richard Guenther <richard.guenther@gmail.com> writes:
> On Fri, Apr 6, 2012 at 10:13 AM, Richard Sandiford
> <rdsandiford@googlemail.com> wrote:
>> Richard Guenther <richard.guenther@gmail.com> writes:
>>>> They can affect shared memory in some ways like a call, but don't have many
>>>> of the other attributes of call.  They are really more like an assignment or
>>>> other operation with arbitrary shared memory side effects.  I do hope to be
>>>> able to teach the optimizers the directionality of the memory model
>>>> restrictions.  ie, ACQUIRE is only a barrier to hoisting shared memory
>>>> code...  stores can be moved downward past this mode. RELEASE is only a
>>>> barrier to sinking code.   RELAXED is no barrier at all to code motion.  In
>>>> fact, a relaxed store is barely different than a real store... but there is
>>>> a slight difference so we can't make it a normal store :-P.
>>>>
>>>> By teaching the other parts of the compiler about a GIMPLE_ ATOMIC, we could
>>>> hopefully lessen their impact eventually.
>>>
>>> Ok.  I suppose having a GIMPLE_ATOMIC is fine then.
>>
>> Just for my own education really, but: does this mean that there'd
>> be unnecessary pessimisation in representing the thing as a call?
>
> No, there are not.  They are more pessimized than GIMPLE_ASSIGNs
> (unless you handle them specially in a few places).  But the same is
> true for GIMPLE_ATOMIC.  The question is one of convenience as
> far as I understand.  In general I would like to avoid new GIMPLE codes,
> especially "random" ones.  You can do everything with builtins or
> internal functions just fine.
>
>> The interleaved load/store internal fns are really assignments too,
>> so if calls aren't right for that kind of operation, maybe we need
>> to replace the internal fns with something else.  Or at least come
>> up with some new call properties.
>
> What missed optimizations do you see?

None. :-)  But...

>> Which is a roundabout way of wondering what the main difficulties
>> would be in attaching things like directionality to a call.
>
> Directionality?

[See above.]

...I was asking in the context quoted above, which seemed to be the bit
that convinced you GIMPLE_ATOMIC would be OK after all.  And the two
main reasons in the bit quoted above seemed to be that GIMPLE_ATOMIC
was more like GIMPLE_ASSIGN (which is true of the current internal
fns too, and was why I was interested) and that we wanted to add the
directionality of the memory model (which seemed at face value like
something that could be attached to a call).

>> Not arguing for anything here, just an onlooker wanting to understand. :-)
>>
>> (BTW, it sounds like restricting memory accesses to GIMPLE_ASSIGN
>> might cause trouble for the interleave load/store stuff too.)
>
> Well.  In the end my plan was to have a GIMPLE_LOAD and GIMPLE_STORE
> stmt, where the load would load to an SSA name and the store would store from
> a constant or an SSA name.  Advantages would be simplified data-flow analysis
> and things like aggregate copy propagation and value-numbering for free.  It
> would also be easier to attach special data / properties to the now
> single load / store
> sites (where in calls you can have an arbitrary number of loads at
> least).  Whatever
> special property the interleaved load/store has would be stored there, too.  The
> idea was to be able to fold most of the implicit information about
> loads/stores that
> are in the MEM_REF /COMPONENT_REF / ARRAY_REF trees into proper "gimple"
> level information, like having an array of index, stride tuples for
> (multidimensional)
> array accesses.  Think of it like a place where we can properly embed the
> MEM_ATTRs we have on the RTL side for example.  That's much easier if
> there were loads/stores only in very defined places.

Ah, OK, so there'd still be a single gimple stmt (GIMPLE_LOAD or GIMPLE_STORE),
and that stmt would do the interleaving too?  Sounds good.  I was worried at
first that we'd have two separate stmts (e.g. a load and an interleave)
which was what the internal fns were supposed to avoid.

Thanks,
Richard