[llvm-dev] Dereferenceable load semantics & LICM

[Piotr Padlewski via llvm-dev]

2017-04-04 19:15 GMT+02:00 Chandler Carruth <chandlerc at google.com>:

> On Mon, Apr 3, 2017 at 2:01 PM Piotr Padlewski <piotr.padlewski at gmail.com>
> wrote:
>
>> 2017-04-01 15:59 GMT+02:00 Piotr Padlewski <piotr.padlewski at gmail.com>:
>>
>>
>>
>> 2017-03-31 23:20 GMT+02:00 Sanjoy Das <sanjoy at playingwithpointers.com>:
>>
>> Hi Piotr,
>>
>> On March 31, 2017 at 1:07:12 PM, Piotr Padlewski
>> (piotr.padlewski at gmail.com) wrote:
>> > [snip]
>> > Do I understand it correctly, that it is legal to do the hoist because
>> all
>> > of the instructions above %vtable does not throw?
>>
>> Yes, I think you're right.  HeaderMayThrow is a conservative
>> approximation, and the conservativeness is biting us here.
>>
>> > Are there any plans to fix it in the future? The fix doesn't seem hard
>> to
>>
>> Not to my knowledge.
>>
>> > write and I can do it, but I am not sure if it won't be too expensive.
>>
>> Maybe we can do it (relatively) cheaply:
>>
>>  - Remember the first throwing instruction in the header, instead of a
>>    boolean, in LoopSafetyInfo
>>
>>  - In hoistRegion, remember if you've seen the first throwing
>>    instruction yet
>>
>>  - Pass the above as a boolean parameter to isGuaranteedToExecute, and
>>    instead of
>>      if (Inst.getParent() == CurLoop->getHeader())
>>        return !SafetyInfo->HeaderMayThrow;
>>    do something like
>>      if (Inst.getParent() == CurLoop->getHeader())
>>        return IsBeforeThrowingInst;
>>
>> -- Sanjoy
>>
>> I was thinking about something very similar and it seems to be a good
>> approach to me because it has much lower complexity.
>>
>> In the review Sanjoy correctly spoted that I am not discarding
>> invariant.group metadata when hoisting, which is incorrect in general.
>> This generates a big problem to devirtualization, because discarding
>> metadata when hoisting vtable load might be worse than not hoisting when
>> there might be another load/store with !invariant.group dominating it
>> (e.g. after inlining).
>>
>> I don't see any way to model it right now in LLVM, but I see a one simple
>> solution:
>>  add extra information to the metadata nodes indicating that this
>> property is non-local:
>>
>> %0 = load... %p, !invariant.group !1, !dereferenceable !2
>> %1 = load ... %0, !invariant.load !0, !dereferenceable !2
>>
>> !0 = !{!"GlobalProperty"}
>> !1 = !{!"MyType", !"GlobalProperty"}
>> !2 = !{i64 8, !"GlobalProperty}
>>
>> With that we would strip only the metadata not containing this
>> information.
>>
>> For devirtualization it would make sense with invariant.load,
>> invariant.group and dereferenceable.
>>
>> What do you think about this idea?
>>
>> Do you have any comments? I would like to know if the idea does even make
>> sense, so I won't start writing prototype that will be throwed to thrash
>>
>
> I don't really see why you need a separate property....
>
> It seems plausible that rather than having the hoisting logic handle
> invariant metadata generically (by stripping it when hoisting) it could
> instead be aware of invariant metadata and try to avoid hoisting the load.
>
> So when we hoist based on the guarantee that it will be executed it seems
to be safe to keep the metadata. LICM always discard.
So we could only hoist invariant.group loads if they are guarantee to
execute.
Right now LICM will not hoist any invariant.group load, because it doesn't
know it is invariant.

It would be still nice to hoist it when we know that it is safe to load
from vptr and from vtable, but we can't do it if we will discard medata
(for this
case we need dereferenceable)


> But this will end up being yet another way in which enabling invariants
> obstructs normal optimizations to get devirtualization. I'm increasingly
> worried that the cost is going to outweigh the gain.
>
What kind of cost you are reffering?
If we would add a way to say that specific metadata is a global property,
then I don't think the cost of maitanence (understanding idea, fixing
documentation
and fixing passes) would be higher than the gains, because I assume it
would be very low.
It does not seem to be that complicated to understand and use,
and to implement this we would only need function like
`dropAllUnknownLocalMetadata` that would replace `dropAllUnknownMedatata`
in most of the
places. We could even firstly fix LICM, and then fix other passes, because
the global properties would not guarantee that they would be not discarded.

When we add medata in the frontend then in most cases (probably all,
because it is not that easy to add metadata depending on the BB) it is a
non local property. Because of this it seems weird that we can only model
it as a local property.


Piotr
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