[ADT] Make set_subtract more efficient when subtrahend is larger (NFC)

llvm/include/llvm/ADT/SetOperations.h

@@ -27,6 +27,15 @@ using check_has_member_remove_if_t =
 template <typename Set, typename Fn>
 static constexpr bool HasMemberRemoveIf =
     is_detected<check_has_member_remove_if_t, Set, Fn>::value;
+
+template <typename Set>
+using check_has_member_erase_iter_t =
+    decltype(std::declval<Set>().erase(std::declval<Set>().begin()));
+
+template <typename Set>
+static constexpr bool HasMemberEraseIter =
+    is_detected<check_has_member_erase_iter_t, Set>::value;
+
 } // namespace detail
 
 /// set_union(A, B) - Compute A := A u B, return whether A changed.
@@ -94,7 +103,35 @@ S1Ty set_difference(const S1Ty &S1, const S2Ty &S2) {
 
 /// set_subtract(A, B) - Compute A := A - B
 ///
+/// Selects the set to iterate based on the relative sizes of A and B for better
+/// efficiency.
+///
 template <class S1Ty, class S2Ty> void set_subtract(S1Ty &S1, const S2Ty &S2) {
+  // If S1 is smaller than S2, iterate on S1 provided that S2 supports efficient
+  // lookups via contains().  Note that a couple callers pass a vector for S2,
+  // which doesn't support contains(), and wouldn't be efficient if it did.
+  using ElemTy = decltype(*S1.begin());
+  if constexpr (detail::HasMemberContains<S2Ty, ElemTy>) {
+    auto Pred = [&S2](const auto &E) { return S2.contains(E); };
+    if constexpr (detail::HasMemberRemoveIf<S1Ty, decltype(Pred)>) {
+      if (S1.size() < S2.size()) {
+        S1.remove_if(Pred);
+        return;
+      }
+    } else if constexpr (detail::HasMemberEraseIter<S1Ty>) {
+      if (S1.size() < S2.size()) {
+        typename S1Ty::iterator Next;
+        for (typename S1Ty::iterator SI = S1.begin(), SE = S1.end(); SI != SE;
+             SI = Next) {
+          Next = std::next(SI);
+          if (S2.contains(*SI))
+            S1.erase(SI);
+        }
+        return;
+      }
+    }
+  }
+
   for (typename S2Ty::const_iterator SI = S2.begin(), SE = S2.end(); SI != SE;
        ++SI)
     S1.erase(*SI);