[PATCH] D22305: [BasicAA] Strip phi nodes, when all incoming values are the same.

[Daniel Berlin via llvm-commits]

On Thu, Jul 14, 2016 at 6:34 PM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> ------------------------------
>
> *From: *"Daniel Berlin" <dberlin at dberlin.org>
> *To: *"Hal Finkel" <hfinkel at anl.gov>
> *Cc: *"llvm-commits" <llvm-commits at lists.llvm.org>,
> reviews+D22305+public+92ca108e50bc4651 at reviews.llvm.org, "Ehsan A Amiri" <
> amehsan at ca.ibm.com>
> *Sent: *Thursday, July 14, 2016 8:30:09 PM
> *Subject: *Re: [PATCH] D22305: [BasicAA] Strip phi nodes, when all
> incoming values are the same.
>
>
>> (2) is, in theory, the right thing to do. Even if we were to consider
>> uniform PHIs to be anti-canonical, and thus something which should be
>> simplified, we can't simplify often enough to prevent these from blocking
>> useful analysis work.
>>
>
> FWIW, i'm fine with this approach if our approach is going to be as you
> say - that we will not simplify often enough.
>
> Right now, as i said, we simplify *everywhere*, and every one of those
> calls will eliminate this phi node.
>
> So it's only *this particular path* that misses all those calls.
>
> For example, if the alias check had gone through a gep of a phi anywhere,
> it would simplify the phi as part of getunderlyingobject, etc.
>
>
>> Arbitrary uses of RAUW can create these PHIs, and we can't (and probably
>> shouldn't) run InstCombine in between every other pass. This is a local
>> pattern that stripPointerCasts, and similar functions, can look through.
>>
>
> Fine with this as long as we maybe stop trying to simplify instructions 8
> or 9 times, and instead do it once (max) per instructions, and make this
> part of it.
>
> (IE This would mean we would have SimplifyAndGetUnderlyingObject and
> GetUnderlyingObject, and we simplify once and call the latter or something,
> or whatever. Not suggesting we decide this second, just suggesting that we
> agree if this is going to be our general approach).
>
> This is hard because we don't cache the simplifications in any way. It is
> not like we're updating the IR when we discover some simplification; we're
> only using the simplified version in place. I'm not sure how to fix this.
> Maybe we should run InstSimplify a lot more often. It is not as expensive
> was InstCombine by a large margin (IIRC).
>
>
>
>>
>> (3) is also, in theory, the right thing to do. The memdep cache, by
>> necessity, caches negative results.
>>
>
BTW, i missed this part.  I haven't looked at the memdep caches in a while,
but if by "caching negative results" you mean it caches anything other than
noalias, that seems .. wrong, since otherwise it would do all the work to
prove things are noalias again and again :)

In that case, you can of course, simply cache that the answer is "no
dependencies", and i thought it did.


> Each GVN iteration, however, performs "information revealing" operations
>> which can make the cached results more conservative than a new query's
>> results.
>>
>
> Yes.
> It's theoretically possible to make it less expensive than blowing away
> the whole cache, but so far, experience has told me that fully maintaining
> the cache becomes slower than redoing the queries :P
>
> I'm not sure how much this helps, but we could/should only clear out the
> MayAlias results.
>
Again, haven't looked at detail at the caches for a while, but my
recollection is this is the vast majority of results.
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