This is the mail archive of the
mailing list for the GCC project.
Re: [C11-atomic] [patch] gimple atomic statements
On Tue, Apr 10, 2012 at 3:14 PM, Richard Sandiford
> Richard Guenther <email@example.com> writes:
>> On Fri, Apr 6, 2012 at 10:13 AM, Richard Sandiford
>> <firstname.lastname@example.org> wrote:
>>> Richard Guenther <email@example.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.
> [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
>> 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?
Well, whatever we'd like to add (part of the atomic stuff would fit here, too,
just not the operation part like increment).
> ?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.