[SCEV] Document a gotcha; NFC

We should re-consider the design decision that led to this gotcah, but
for now just document it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282367 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/Analysis/ScalarEvolution.h b/include/llvm/Analysis/ScalarEvolution.h
index 64c0378ec6f3f26626f8ef0927a12f06190f9db0..830334db6915b460d585191f46fac3d3b667f319 100644 (file)
--- a/include/llvm/Analysis/ScalarEvolution.h
+++ b/include/llvm/Analysis/ScalarEvolution.h
@@ -368,6 +368,9 @@ public:
 /// This class represents a composition of other SCEV predicates, and is the
 /// class that most clients will interact with.  This is equivalent to a
 /// logical "AND" of all the predicates in the union.
+///
+/// NB! Unlike other SCEVPredicate sub-classes this class does not live in the
+/// ScalarEvolution::Preds folding set.  This is why the \c add function is sound.
 class SCEVUnionPredicate final : public SCEVPredicate {
 private:
   typedef DenseMap<const SCEV *, SmallVector<const SCEVPredicate *, 4>>