release/21.x: [SCEV] Don't perform implication checks with many predicates (#158652) #158798
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@fhahn What do you think about merging this PR to the release branch? |
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@llvm/pr-subscribers-llvm-analysis Author: None (llvmbot) ChangesBackport 7af659d Requested by: @nikic Full diff: https://github.com/llvm/llvm-project/pull/158798.diff 1 Files Affected:
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 24adfa346c642..221468a2d1a84 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -15093,15 +15093,20 @@ void SCEVUnionPredicate::add(const SCEVPredicate *N, ScalarEvolution &SE) {
return;
}
+ // Implication checks are quadratic in the number of predicates. Stop doing
+ // them if there are many predicates, as they should be too expensive to use
+ // anyway at that point.
+ bool CheckImplies = Preds.size() < 16;
+
// Only add predicate if it is not already implied by this union predicate.
- if (implies(N, SE))
+ if (CheckImplies && implies(N, SE))
return;
// Build a new vector containing the current predicates, except the ones that
// are implied by the new predicate N.
SmallVector<const SCEVPredicate *> PrunedPreds;
for (auto *P : Preds) {
- if (N->implies(P, SE))
+ if (CheckImplies && N->implies(P, SE))
continue;
PrunedPreds.push_back(P);
}
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ping @fhahn |
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…8652) When adding a new predicate to a union, we currently do a bidirectional implication for all the contained predicates. This means that the number of implication checks is quadratic in the number of total predicates (if they don't end up being eliminated). Fix this by not checking for implication if the number of predicates grows too large. The expectation is that if there is a large number of predicates, we should be discarding them later anyway, as expanding them would be too expensive. Fixes llvm#156114. (cherry picked from commit 7af659d)
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@nikic (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR. |
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Backport 7af659d
Requested by: @nikic