[PATCH] D22305: [BasicAA] Strip phi nodes, when all incoming values are the same.
Daniel Berlin via llvm-commits
llvm-commits at lists.llvm.org
Thu Jul 14 15:11:20 PDT 2016
Sigh. Even with all the bugs this discovers, this is unsolvable in the
general case with the current way GVN is set up.
First, this bug definitely gets fixed by making sure the first GVN
simplifies the phi so the second GVN run doesn't see it.
There are bugs in GVN that cause this to not happen right now, and fixing
them makes it work.
The game turns out to be over as soon as you have this phi node as input to
GVN. However, it's sadly worse than that.
The iteration orders of memdep and GVN are basically the exact opposite, so
GVN will always have memdep cache things that it will later simplify, and
as we discussed on the bug, we can't ever get them out of the memdep cache.
This is about more than PRE or phis, it literally affects every memdep
query it makes where the path involves something GVN will later simplify,
at all, and that simplification makes it able to see a no-alias (or in some
cases, a partial/must-alias).
For example, if you add a pointer that alias queries follow along a
backedge that requires constant folding, or *anything* basicaa does not
itself process but gvn/simplifyinstruction does, it will fail in precisely
the same way.
Worse than that, without the other patches (from above) to GVN to make it
stop generating trivially bad IR, *GVN* itself will generate these kinds of
cases.
So even with your alias patch, i can trivially modify the testcase in a
number of ways to make it break again.
This means while your alias patch will fix the current case, there is
literally no way to fix all the cases (again, in current GVN. New GVN +
MemSSA will not have these issues) you are going to find that doesn't
involve something like blowing away memdep cache entirely on each iteration
of GVN.
I'm not sure how we want to paper over this bug:
1. Making first run of GVN not generate IR the second GVN will have to
trivially simplify (which will fix some cases but not others)
2. Make BasicAA see through phi nodes (ditto, though chandler points out
there are plenty of normal programs with thousands of phi nodes that have
the same arguments, and since basicaa does not cache, there is a compile
time cost to doing this )
3. Add an API and use it to blow away memdep cache entirely between GVN
iterations (which will fix all such cases at some possible compile time
cost)
Some combination of the above?
(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> 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> 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>
>> To: Ehsan A Amiri/Toronto/IBM at IBMCA
>> Cc: Hal Finkel <hfinkel at anl.gov>, llvm-commits <
>> llvm-commits at lists.llvm.org>,
>> reviews+D22305+public+92ca108e50bc4651 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)
>>
>>
>>
>>
>>
>
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diff --git a/include/llvm/Analysis/MemoryDependenceAnalysis.h b/include/llvm/Analysis/MemoryDependenceAnalysis.h
index b19dabb..6d7dd39 100644
--- a/include/llvm/Analysis/MemoryDependenceAnalysis.h
+++ b/include/llvm/Analysis/MemoryDependenceAnalysis.h
@@ -403,6 +403,7 @@ public:
/// critical edges.
void invalidateCachedPredecessors();
+ void invalidateCache();
/// Returns the instruction on which a memory location depends.
///
/// If isLoad is true, this routine ignores may-aliases with read-only
diff --git a/lib/Analysis/MemoryDependenceAnalysis.cpp b/lib/Analysis/MemoryDependenceAnalysis.cpp
index 3349933..bf8cc10 100644
--- a/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -1439,6 +1439,15 @@ void MemoryDependenceResults::RemoveCachedNonLocalPointerDependencies(
// Remove P from NonLocalPointerDeps (which deletes NonLocalDepInfo).
NonLocalPointerDeps.erase(It);
}
+void MemoryDependenceResults::invalidateCache() {
+ NonLocalPointerDeps.clear();
+ ReverseNonLocalPtrDeps.clear();
+ NonLocalDeps.clear();
+ LocalDeps.clear();
+ ReverseLocalDeps.clear();
+ ReverseNonLocalDeps.clear();
+}
+
void MemoryDependenceResults::invalidateCachedPointerInfo(Value *Ptr) {
// If Ptr isn't really a pointer, just ignore it.
diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index a963b2f..1b3d2d2 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -2214,6 +2214,7 @@ bool GVN::runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT,
while (ShouldContinue) {
DEBUG(dbgs() << "GVN iteration: " << Iteration << "\n");
ShouldContinue = iterateOnFunction(F);
+ MD->invalidateCache();
Changed |= ShouldContinue;
++Iteration;
}
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