IR: Add Value::reverseUseList()

I'm going to use this to improve `verify-uselistorder`.  Part of PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214594 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Value.h

@@ -462,6 +462,9 @@ public:
   /// expected to compare two \a Use references.
   template <class Compare> void sortUseList(Compare Cmp);
 
+  /// \brief Reverse the use-list.
+  void reverseUseList();
+
 private:
   /// \brief Merge two lists together.
   ///

lib/IR/Value.cpp

@@ -614,6 +614,25 @@ Value *Value::DoPHITranslation(const BasicBlock *CurBB,
 
 LLVMContext &Value::getContext() const { return VTy->getContext(); }
 
+void Value::reverseUseList() {
+  if (!UseList || !UseList->Next)
+    // No need to reverse 0 or 1 uses.
+    return;
+
+  Use *Head = UseList;
+  Use *Current = UseList->Next;
+  Head->Next = nullptr;
+  while (Current) {
+    Use *Next = Current->Next;
+    Current->Next = Head;
+    Head->setPrev(&Current->Next);
+    Head = Current;
+    Current = Next;
+  }
+  UseList = Head;
+  Head->setPrev(&UseList);
+}
+
 //===----------------------------------------------------------------------===//
 //                             ValueHandleBase Class
 //===----------------------------------------------------------------------===//

unittests/IR/UseTest.cpp

@@ -66,4 +66,47 @@ TEST(UseTest, sort) {
   ASSERT_EQ(8u, I);
 }
 
+TEST(UseTest, reverse) {
+  LLVMContext C;
+
+  const char *ModuleString = "define void @f(i32 %x) {\n"
+                             "entry:\n"
+                             "  %v0 = add i32 %x, 0\n"
+                             "  %v2 = add i32 %x, 2\n"
+                             "  %v5 = add i32 %x, 5\n"
+                             "  %v1 = add i32 %x, 1\n"
+                             "  %v3 = add i32 %x, 3\n"
+                             "  %v7 = add i32 %x, 7\n"
+                             "  %v6 = add i32 %x, 6\n"
+                             "  %v4 = add i32 %x, 4\n"
+                             "  ret void\n"
+                             "}\n";
+  SMDiagnostic Err;
+  char vnbuf[8];
+  Module *M = ParseAssemblyString(ModuleString, nullptr, Err, C);
+  Function *F = M->getFunction("f");
+  ASSERT_TRUE(F);
+  ASSERT_TRUE(F->arg_begin() != F->arg_end());
+  Argument &X = *F->arg_begin();
+  ASSERT_EQ("x", X.getName());
+
+  X.sortUseList([](const Use &L, const Use &R) {
+    return L.getUser()->getName() < R.getUser()->getName();
+  });
+  unsigned I = 0;
+  for (User *U : X.users()) {
+    snprintf(vnbuf, sizeof(vnbuf), "v%u", I++);
+    EXPECT_EQ(vnbuf, U->getName());
+  }
+  ASSERT_EQ(8u, I);
+
+  X.reverseUseList();
+  I = 0;
+  for (User *U : X.users()) {
+    snprintf(vnbuf, sizeof(vnbuf), "v%u", (7 - I++));
+    EXPECT_EQ(vnbuf, U->getName());
+  }
+  ASSERT_EQ(8u, I);
+}
+
 } // end anonymous namespace