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

[Daniel Berlin via llvm-commits]

On Thu, Jul 14, 2016 at 7:49 PM, Ehsan A Amiri <amehsan at ca.ibm.com> wrote:

> I can measure compile time impact of (2) and (3) on power. A couple of
> questions/remarks.
>
> 1- As for (2) I don't think there will be a significant compile time cost.
>

Compile time does not get slower 5% at a time, it gets slower 0.1% or 0.05%
at a time.

It's not "someone adds thing that is super slow", it is "people add lots of
little things to lots of functions called lots of times".

As Hal points out, we never cache that we simplified or didn't, or checked
the phi or didn't, or *anything*, because BasicAA is stateless.

Because BasicAA is stateless, we will do this again and again and again
(both this phi checking and simplifyinstruction and ....)

This adds up.

If we do not reduce the phi to its unique incoming value, we will end up in
> aliasPHI and we do this check there. It is just too late for some cases
> like the problem here, and we get conservative result. Do I miss something
> here?
>
The point is that we are going to try to simplify this, and every phi, a
lot of times, both in AA, and elsewhere.


> 2- I will use Daniel's patch for (3). Depending on how expensive it is, we
> can look at finer grain cache clean ups, as Hal suggests.
>
> 3- I will modify my patch for (2) and modify stripPointerCasts to include
> "seeing through phi". I wanted to separate that move as a next step, but
> Hal disagreed on the code review.
>
> If you disagree with this way of doing the experiments, please let me know.
>
> Thanks
> Ehsan
>
> [image: Inactive hide details for Hal Finkel ---07/14/2016 09:35:07
> PM-------- Original Message ----- > From: "Daniel Berlin" <dberlin@]Hal
> Finkel ---07/14/2016 09:35:07 PM-------- Original Message ----- > From:
> "Daniel Berlin" <dberlin at dberlin.org>
>
> From: Hal Finkel <hfinkel at anl.gov>
> To: Daniel Berlin <dberlin at dberlin.org>
> Cc: llvm-commits <llvm-commits at lists.llvm.org>, <
> reviews+D22305+public+92ca108e50bc4651 at reviews.llvm.org>, Ehsan A
> Amiri/Toronto/IBM at IBMCA
> Date: 07/14/2016 09:35 PM
>
> Subject: Re: [PATCH] D22305: [BasicAA] Strip phi nodes, when all incoming
> values are the same.
> ------------------------------
>
>
>
>
> ------------------------------
>
> *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. 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.
>
> -Hal
>
>
>    Now we actually need to measure the costs.
>
>    -Hal
>
>
>    (Note: i've attached a patch for 3 in case anyone wants to see the
>    compile time cost)
>
>
>
>
>
>
>
>    On Thu, Jul 14, 2016 at 2:12 PM, Daniel Berlin <*dberlin at dberlin.org*
>    <dberlin at dberlin.org>> wrote:
>    Note: This already had GVN run once on it, do you have the one before
>    that?
>
>
>    On Thu, Jul 14, 2016 at 1:53 PM, Ehsan A Amiri <*amehsan at ca.ibm.com*
>    <amehsan at ca.ibm.com>> wrote:
>       I can see the problem with the following command line and attached
>       file
>
>       opt -gvn t.ll
>
> * (See attached file: t.ll)*
>
>       My clang is almost a week old.
>
>       [image: Inactive hide details for Daniel Berlin ---07/14/2016
>       04:27:43 PM---Actually, can you please attach a .ll file and an opt comma]Daniel
>       Berlin ---07/14/2016 04:27:43 PM---Actually, can you please attach a .ll
>       file and an opt command line that reproduces the problem?
>
>       From: Daniel Berlin <*dberlin at dberlin.org* <dberlin at dberlin.org>>
>       To: Ehsan A Amiri/Toronto/IBM at IBMCA
>       Cc: Hal Finkel <*hfinkel at anl.gov* <hfinkel at anl.gov>>, llvm-commits <
>       *llvm-commits at lists.llvm.org* <llvm-commits at lists.llvm.org>>,
>       *reviews+D22305+public+92ca108e50bc4651 at reviews.llvm.org*
>       <reviews%2BD22305%2Bpublic%2B92ca108e50bc4651 at reviews.llvm.org>
>       Date: 07/14/2016 04:27 PM
>       Subject: Re: [PATCH] D22305: [BasicAA] Strip phi nodes, when all
>       incoming values are the same.
>       ------------------------------
>
>
>
>       Actually, can you please attach a .ll file and an opt command line
>       that reproduces the problem?
>       The clang command line you have is very sensitive to versions, etc.
>
>       I cannot get your issue to reproduce with the clang i have
>       installed that can target powerpc-linux (and the issue does not reproduce
>       with your testcase on x86) :)
>
>       While debugging a bit, note that there is at least one obvious bug
>       in GVN that may affect this, by inspection:
>
>       When GVN splits a critical edge, it never adds the new block to the
>       iteration order (at all), even though it inserts into it.
>       So they will not get processed until the next iteration of GVN on
>       the function, even though they have code in them. While this is okay from a
>       correctness standpoint, it may block optimization of certain things
>       (including the cases you've discovered). In practice, there is no way to
>       perfectly solve that without pre-splitting all critical edges, but you
>       should get the same effect if we throw the critical edge block and then
>       it's successors (including the current blcok) into bbvect after the current
>       block again.
>
>       It is also missing a real phi simplification.
>       While simplifyinstruction will check if all arguments are trivially
>       the same, that is not the real test that should be performed.
>
>       It should be doing VN.lookup on each argument and seeing if they
>       come up with the the same value number.
>
>       Once you attach the .ll file, i'll fix these and see if it fixes
>       your testcase, and if not, debug further.
>
>
>       On Thu, Jul 14, 2016 at 10:11 AM, Daniel Berlin <
>       *dberlin at dberlin.org* <dberlin at dberlin.org>> wrote:
>
>
>             On Thu, Jul 14, 2016 at 7:09 AM, Ehsan A Amiri <
>             *amehsan at ca.ibm.com* <amehsan at ca.ibm.com>> wrote:
>             This is order of events:
>             This order cannot be correct if the solution I gave (or
>             adding simplification to *some place in GVN*) does not work. Or GVN is
>             broken in other ways.
>
>             1) GVN starts looking at the function. At this point the phi
>             node has two different incoming values.
>             2) GVN performs an RAUW. The phi is converted to the one that
>             has two identincal incoming values.
>
>             At this point, it should now process the phi instruction
>             again before it processes the load, because it is doing a reverse postorder
>             traversal.
>             When it did that, the phi should have been simplified
>             So why did that not happen?
>
>             Given the complexity of fixing the real problem,
>
>             Look, i understand why you want to just fix this in AA and be
>             done with it.
>             Really, I do.
>
>             I understand you have spent a lot of time on this bug, and I
>             greatly appreciate that.
>             But I really want to understand what is going on before we
>             try to actually fix it.
>             I have a good understanding of what happens once the bad
>             answer gets into memdep (and thank you for that!), but i still have trouble
>             seeing why it lived long enough to get there.
>
>             To that end, so you don't have to spend more time running
>             around for me, i'll take over this bug, and either figure out why GVN lets
>             this PHI live to the point it gets an AA query about it (and fix it/decide
>             it can't be fixed), or commit the AA patch for you if we decide it can't be
>             fixed.
>
>             My ETA is by friday.
>
>             I assume the testcase in the bug is the one we are still
>             using, right?
>             (If not, if you can attach it, that would be helpful)
>
>
>
>
>
>
>
>    --
>    Hal Finkel
>    Assistant Computational Scientist
>    Leadership Computing Facility
>    Argonne National Laboratory
>
>
>
>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> [attachment "graycol.gif" deleted by Ehsan A Amiri/Toronto/IBM]
>
>
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