[RFC][SCEV] Behavior of AddRec in CompareSCEVComplexity

Hi,

Recently I got a crash when I tried to analysis a program with ScalarEvolution AliasAnalysis(SCEV-AA for short). It turns out to be a (possibly) incorrect assertion inside the CompareSCEVComplexity routine.
The simplest solution would be just remove that assertion but I also found that the surrounding logics on calculating SCEV cost seems to be incorrect either. Thus I want to discuss with you folks about the best way to solve this.
Here are the details:

Setup
Off-the-tip llvm-project, including clang and libcxx, built in full Debug build

Input Program
Both the original C file and IR file are enclosed in the attachment. The IR generation command is clang -O1 -emit-llvm -S ptr_test.c -o ptr_test.O1.ll

Crashed Command
opt -S -disable-output -basicaa -scev-aa -aa-eval -print-no-aliases ptr_test.O1.ll
The core dump message is also in the attachments.

Investigations

  1. SCEV-AA try to ‘minus’ the SCEV expressions of the given two pointers(lib/Analysis/ScalarEvolutionAliasAnalysis.cpp:64)
  2. ScalarEvolution::getMinusSCEV will boil down into ScalarEvolution::getAddExpr. On line 2383 of lib/Analysis/ScalarEvolution.cpp, GroupByComplexity is invoked.
  3. CompareSCEVComplexity is eventually called to give a partial order between two SCEV expression.
  4. If there are two SCEVAddExpr that are located in different loops which don’t have any hierarchical relation, just like pointers in line 6 and line 10 in the input program(i.e. ptr_test.c), it will violate the assertion in line 705 in lib/Analysis/ScalarEvolution.cpp.

The point is that the assertion in line 705 doesn’t make sense in most of the cases: I don’t think there is any limitation imposed on arbitrary SCEV expressions to make the enclosing SCEVAddRec to be in the same loop(Or should we?).
As I mentioned earlier, the simplest solution is to remove this assertion, but still, the very assumption is still encoded in the surrounding code.
So I want to hear from you folks whether we should calculate the complexity of SCEVAddRec located in different loops. If yes, what’s the best way? For the latter question, currently I have an idea in my mind to compare their loop trip counts before doing the following lexicographic comparison.

Thank you,

B.R.

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ptr_test.c (225 Bytes)

ptr_test.O1.ll (3.23 KB)

core_dump_msg.txt (8.29 KB)

CC the code owner and the last author touching the affected part.

Hi,

if I understand CompareSCEVComplexity correctly, we just need some
deterministic order, not that important which one. We have three
cases:

1. LHead dominates RHead
2. RHead dominates LHead
3. There is no dominance relationship between the loops

LHead dominating RHead can either mean that R is nested inside L, or
the entire loop of L dominated R. From ptr_test.c that latter seems
the case, but LoopRotation might change this relation.

For the third case, we might just find another tie breaker rule, such
as a NumOps comparison afterwards or the order in which LoopInfo finds
them (this might be indeterministic). However, it might also be ok to
just return 0, meaning both are equally complex.

Michael

Hi,

Thanks for the input. In my ptr_test.O1.ll, the two loops should fall into the last case due to loop rotations. And I think we should handle that case.
I propose to check the dominance relation between loop predecessor blocks of the two loops if there is no dominance relation between their loop headers just like case three. The only thing I’m not pretty sure is that is there guarantee to exist dominance relation on their loop predecessor blocks. That is, either LPred dominates RPred or RPred dominate LPred?

B.R.

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The SCEVAA issue has come up before; see https://reviews.llvm.org/D41689 . I didn’t really continue pursuing it because SCEVAA is off by default… and the discussion there concluded it’s likely not actually a bug in SCEVAA. Instead, -aa-eval and -basicaa shouldn’t be making those queries.

I think it doesn’t make sense to create a SCEV expression where the components don’t have a dominance relationship. It would be the same as constructing an “Instruction” where the definitions of the operands don’t dominate the instruction; it doesn’t have any meaning given the definition of SSA form.

If you do try to pursue changing CompareSCEVComplexity, it’s more complicated than this discussion indicates. It’s used as a sort comparator, so it needs to be a strict weak ordering, so you’d need a strict ordering for domtree nodes. That’s possible (using getDFSNumIn() or something like that), but I’m not sure how that interacts with domtree updates; I don’t think domtree updates keep the children in a consistent order.

-Eli

Hi,

Thanks for bringing up D41689, it’s really helpful. I think Hal had made a pretty neat summarized of this problem in that review: we definitely need a better contract on the alias query interface regarding instruction dominance.
I think it benefit not just on SCEVAA buy also BasicAA as well as AAEval, where the last one is usually heavily used during prototyping and it’s crucial for it to deliver correct and precise alias information.

B.R.

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