[llvm-dev] [RFC][SCEV] Behavior of AddRec in CompareSCEVComplexity

[Michael Kruse via llvm-dev]

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


Am Di., 11. Juni 2019 um 14:21 Uhr schrieb Min-Yih Hsu via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> 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.
> - Min
>
>
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