[SCEV] Don't perform implication checks with many predicates

[nikic]

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.

(Alternatively we could also try to prevent the addition of many predicates in the first place, but I figured this is better as it's centralized in one place.)

Fixes #156114.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Author: Nikita Popov (nikic)

Changes

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.

(Alternatively we could also try to prevent the addition of many predicates in the first place, but I figured this is better as it's centralized in one place.)

Fixes #156114.

---

Full diff: https://github.com/llvm/llvm-project/pull/158652.diff

1 Files Affected:

• (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+7-2)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index a1703a270952e..9698ed80730ee 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -15176,15 +15176,20 @@ 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);
   }

[fhahn]

LGTM, thanks