From 60c620272db2b3cb71b973c9ae45a639aa492164 Mon Sep 17 00:00:00 2001
From: Teresa Johnson <tejohnson@google.com>
Date: Thu, 20 Feb 2025 08:02:13 -0800
Subject: [PATCH] [ADT] Add set_intersects to check if there is any
 intersection (#127907)

Add a facility to check if there is any intersection between 2 sets.
This will be used in some follow on changes to MemProf.
---
 llvm/include/llvm/ADT/SetOperations.h    | 20 ++++++++++++++++++++
 llvm/unittests/ADT/SetOperationsTest.cpp | 17 +++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/llvm/include/llvm/ADT/SetOperations.h b/llvm/include/llvm/ADT/SetOperations.h
index 86a27b683ebc..4d4ff4045f81 100644
--- a/llvm/include/llvm/ADT/SetOperations.h
+++ b/llvm/include/llvm/ADT/SetOperations.h
@@ -157,6 +157,26 @@ bool set_is_subset(const S1Ty &S1, const S2Ty &S2) {
   return true;
 }
 
+namespace detail {
+
+template <class S1Ty, class S2Ty>
+bool set_intersects_impl(const S1Ty &S1, const S2Ty &S2) {
+  for (const auto &E : S1)
+    if (S2.count(E))
+      return true;
+  return false;
+}
+
+} // namespace detail
+
+/// set_intersects(A, B) - Return true iff A ^ B is non empty
+template <class S1Ty, class S2Ty>
+bool set_intersects(const S1Ty &S1, const S2Ty &S2) {
+  if (S1.size() < S2.size())
+    return detail::set_intersects_impl(S1, S2);
+  return detail::set_intersects_impl(S2, S1);
+}
+
 } // namespace llvm
 
 #endif
diff --git a/llvm/unittests/ADT/SetOperationsTest.cpp b/llvm/unittests/ADT/SetOperationsTest.cpp
index f99f5c9b2af1..6d191160f774 100644
--- a/llvm/unittests/ADT/SetOperationsTest.cpp
+++ b/llvm/unittests/ADT/SetOperationsTest.cpp
@@ -273,4 +273,21 @@ TEST(SetOperationsTest, SetIsSubset) {
   EXPECT_FALSE(set_is_subset(Set1, Set2));
 }
 
+TEST(SetOperationsTest, SetIntersects) {
+  std::set<int> Set1 = {1, 2, 3, 4};
+  std::set<int> Set2 = {3, 4, 5};
+  EXPECT_TRUE(set_intersects(Set1, Set2));
+  EXPECT_TRUE(set_intersects(Set2, Set1));
+
+  Set2 = {5, 6, 7};
+  EXPECT_FALSE(set_intersects(Set1, Set2));
+  EXPECT_FALSE(set_intersects(Set2, Set1));
+
+  // Check that intersecting with a null set returns false.
+  Set1.clear();
+  EXPECT_FALSE(set_intersects(Set1, Set2));
+  EXPECT_FALSE(set_intersects(Set2, Set1));
+  EXPECT_FALSE(set_intersects(Set1, Set1));
+}
+
 } // namespace