SmallPtrSet: Provide a more efficient implementation of swap than the default triple-copy std::swap.

This currently assumes that both sets have the same SmallSize to keep the implementation simple,
a limitation that can be lifted if someone cares.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152143 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/SmallPtrSet.h

@@ -137,6 +137,10 @@ private:
 
   void operator=(const SmallPtrSetImpl &RHS);  // DO NOT IMPLEMENT.
 protected:
+  /// swap - Swaps the elements of two sets.
+  /// Note: This method assumes that both sets have the same small size.
+  void swap(SmallPtrSetImpl &RHS);
+
   void CopyFrom(const SmallPtrSetImpl &RHS);
 };
 
@@ -287,8 +291,20 @@ public:
     return *this;
   }
 
+  /// swap - Swaps the elements of two sets.
+  void swap(SmallPtrSet<PtrType, SmallSize> &RHS) {
+    SmallPtrSetImpl::swap(RHS);
+  }
 };
 
 }
 
+namespace std {
+  /// Implement std::swap in terms of SmallPtrSet swap.
+  template<class T, unsigned N>
+  inline void swap(llvm::SmallPtrSet<T, N> &LHS, llvm::SmallPtrSet<T, N> &RHS) {
+    LHS.swap(RHS);
+  }
+}
+
 #endif

lib/Support/SmallPtrSet.cpp

@@ -14,6 +14,7 @@
 
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Support/MathExtras.h"
+#include <algorithm>
 #include <cstdlib>
 
 using namespace llvm;
@@ -223,6 +224,55 @@ void SmallPtrSetImpl::CopyFrom(const SmallPtrSetImpl &RHS) {
   NumTombstones = RHS.NumTombstones;
 }
 
+void SmallPtrSetImpl::swap(SmallPtrSetImpl &RHS) {
+  if (this == &RHS) return;
+
+  // We can only avoid copying elements if neither set is small.
+  if (!this->isSmall() && !RHS.isSmall()) {
+    std::swap(this->CurArray, RHS.CurArray);
+    std::swap(this->CurArraySize, RHS.CurArraySize);
+    std::swap(this->NumElements, RHS.NumElements);
+    std::swap(this->NumTombstones, RHS.NumTombstones);
+    return;
+  }
+
+  // FIXME: From here on we assume that both sets have the same small size.
+
+  // If only RHS is small, copy the small elements into LHS and move the pointer
+  // from LHS to RHS.
+  if (!this->isSmall() && RHS.isSmall()) {
+    std::copy(RHS.SmallArray, RHS.SmallArray+RHS.NumElements, this->SmallArray);
+    std::swap(this->NumElements, RHS.NumElements);
+    std::swap(this->CurArraySize, RHS.CurArraySize);
+    RHS.CurArray = this->CurArray;
+    RHS.NumTombstones = this->NumTombstones;
+    this->CurArray = this->SmallArray;
+    this->NumTombstones = 0;
+    return;
+  }
+
+  // If only LHS is small, copy the small elements into RHS and move the pointer
+  // from RHS to LHS.
+  if (this->isSmall() && !RHS.isSmall()) {
+    std::copy(this->SmallArray, this->SmallArray+this->NumElements,
+              RHS.SmallArray);
+    std::swap(RHS.NumElements, this->NumElements);
+    std::swap(RHS.CurArraySize, this->CurArraySize);
+    this->CurArray = RHS.CurArray;
+    this->NumTombstones = RHS.NumTombstones;
+    RHS.CurArray = RHS.SmallArray;
+    RHS.NumTombstones = 0;
+    return;
+  }
+
+  // Both a small, just swap the small elements.
+  assert(this->isSmall() && RHS.isSmall());
+  assert(this->CurArraySize == RHS.CurArraySize);
+  unsigned MaxElems = std::max(this->NumElements, RHS.NumElements);
+  std::swap_ranges(this->SmallArray, this->SmallArray+MaxElems, RHS.SmallArray);
+  std::swap(this->NumElements, RHS.NumElements);
+}
+
 SmallPtrSetImpl::~SmallPtrSetImpl() {
   if (!isSmall())
     free(CurArray);

unittests/ADT/SmallPtrSetTest.cpp

@@ -0,0 +1,72 @@
+//===- llvm/unittest/ADT/SmallPtrSetTest.cpp ------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// SmallPtrSet unit tests.
+//
+//===----------------------------------------------------------------------===//
+
+#include "gtest/gtest.h"
+#include "llvm/ADT/SmallPtrSet.h"
+
+using namespace llvm;
+
+// SmallPtrSet swapping test.
+TEST(SmallPtrSetTest, SwapTest) {
+  int buf[10];
+
+  SmallPtrSet<int *, 2> a;
+  SmallPtrSet<int *, 2> b;
+
+  a.insert(&buf[0]);
+  a.insert(&buf[1]);
+  b.insert(&buf[2]);
+
+  std::swap(a, b);
+
+  EXPECT_EQ(1U, a.size());
+  EXPECT_EQ(2U, b.size());
+  EXPECT_TRUE(a.count(&buf[2]));
+  EXPECT_TRUE(b.count(&buf[0]));
+  EXPECT_TRUE(b.count(&buf[1]));
+
+  b.insert(&buf[3]);
+  std::swap(a, b);
+
+  EXPECT_EQ(3U, a.size());
+  EXPECT_EQ(1U, b.size());
+  EXPECT_TRUE(a.count(&buf[0]));
+  EXPECT_TRUE(a.count(&buf[1]));
+  EXPECT_TRUE(a.count(&buf[3]));
+  EXPECT_TRUE(b.count(&buf[2]));
+
+  std::swap(a, b);
+
+  EXPECT_EQ(1U, a.size());
+  EXPECT_EQ(3U, b.size());
+  EXPECT_TRUE(a.count(&buf[2]));
+  EXPECT_TRUE(b.count(&buf[0]));
+  EXPECT_TRUE(b.count(&buf[1]));
+  EXPECT_TRUE(b.count(&buf[3]));
+
+  a.insert(&buf[4]);
+  a.insert(&buf[5]);
+  a.insert(&buf[6]);
+
+  std::swap(b, a);
+
+  EXPECT_EQ(3U, a.size());
+  EXPECT_EQ(4U, b.size());
+  EXPECT_TRUE(b.count(&buf[2]));
+  EXPECT_TRUE(b.count(&buf[4]));
+  EXPECT_TRUE(b.count(&buf[5]));
+  EXPECT_TRUE(b.count(&buf[6]));
+  EXPECT_TRUE(a.count(&buf[0]));
+  EXPECT_TRUE(a.count(&buf[1]));
+  EXPECT_TRUE(a.count(&buf[3]));
+}