[NFC][IR] add comments and a unit test for User::replaceUsesOfWith

[ChuanqiXu]

Summary

The origin comment in User::replaceUsesOfWith writes:

if (getOperand(i) == From) {  // Is This operand is pointing to oldval?
      // The side effects of this setOperand call include linking to
      // "To", adding "this" to the uses list of To, and
      // most importantly, removing "this" from the use list of "From".
      setOperand(i, To);// fix it now
    }

The line fix it now looks like we need a patch here to achieve the effects described above. However, the effects had been implemented. So I think it is more clear to delete this comment.

[MaskRay]

For NFC changes please tag so (you can find plenty of examples in the history). Please upload the full diff (arc diff or git diff 'HEAD^' -U9999).

---

When you commit, please drop Reviewers: Subscribers: Tags: and the text Summary: from the git commit with the following script:

arcfilter () {
        arc amend
        git log -1 --pretty=%B | awk '/Reviewers:|Subscribers:/{p=1} /Reviewed By:|Differential Revision:/{p=0} !p && !/^Summary:$/ {sub(/^Summary: /,"");print}' | git commit --amend --date=now -F -
}

Reviewed By: is considered important by some people. Please keep the tag. (--date=now is my personal preference (author dates are usually not useful. Using committer dates can make log almost monotonic in time))

llvm/utils/git/pre-push.py can validate the message does not include unneeded tags.

[ChuanqiXu]

In D84135#2162478, @MaskRay wrote:

For NFC changes please tag so (you can find plenty of examples in the history). Please upload the full diff (arc diff or git diff 'HEAD^' -U9999).

---

When you commit, please drop Reviewers: Subscribers: Tags: and the text Summary: from the git commit with the following script:

arcfilter () {
        arc amend
        git log -1 --pretty=%B | awk '/Reviewers:|Subscribers:/{p=1} /Reviewed By:|Differential Revision:/{p=0} !p && !/^Summary:$/ {sub(/^Summary: /,"");print}' | git commit --amend --date=now -F -
}

Reviewed By: is considered important by some people. Please keep the tag. (--date=now is my personal preference (author dates are usually not useful. Using committer dates can make log almost monotonic in time))

llvm/utils/git/pre-push.py can validate the message does not include unneeded tags.

Hi， thank you! I am a newbie here. And I don't know much rule here. Thanks for reminding me this.

[junparser]

LGTM

llvm/lib/IR/User.cpp

void User::replaceUsesOfWith(Value *From, Value *To) {
   assert((!isa<Constant>(this) || isa<GlobalValue>(this)) &&
          "Cannot call User::replaceUsesOfWith on a constant!");
 
   for (unsigned i = 0, E = getNumOperands(); i != E; ++i)
     if (getOperand(i) == From) {  // Is This operand is pointing to oldval?
       // The side effects of this setOperand call include linking to
       // "To", adding "this" to the uses list of To, and
       // most importantly, removing "this" from the use list of "From".
-      setOperand(i, To); // Fix it now...
+      setOperand(i, To);
     }
 }
 
 //===----------------------------------------------------------------------===//
 //                         User allocHungoffUses Implementation
 //===----------------------------------------------------------------------===//
 
 void User::allocHungoffUses(unsigned N, bool IsPhi) {

llvm/unittests/IR/UserTest.cpp

TEST(UserTest, ValueOpIteration) {
   CI += 3;
   EXPECT_EQ(std::next(IP->value_op_begin(), 3), CI);
   EXPECT_EQ(IP->getOperand(3), *CI);
   CI++;
   EXPECT_EQ(IP->getOperand(6), CI[2]);
   EXPECT_EQ(IP->value_op_end(), (CI - 2) + 8);
 }
 
+TEST(UserTest, replaceUseOfWith) {
+  LLVMContext C;
+
+  const char *ModuleString = "define void @f(i32 %x) {\n"
+                             "entry:\n"
+                             "  %v0 = add i32 1, 1\n"
+                             "  %v1 = add i32 %x, 2\n"
+                             "  ret void\n"
+                             "}\n";
+  SMDiagnostic Err;
+  std::unique_ptr<Module> M = parseAssemblyString(ModuleString, Err, C);
+  Function *F = M->getFunction("f");
+  EXPECT_TRUE(F);
+  EXPECT_TRUE(F->arg_begin() != F->arg_end());
+  BasicBlock& entryBB = F->front();
+  Instruction& I0 = *(entryBB.begin());
+  Instruction& I1 = *(++(entryBB.begin()));
+
+  Argument &X = *F->arg_begin();
+  EXPECT_EQ("x", X.getName());
+  EXPECT_NE(X.user_begin() ,X.user_end());
+  EXPECT_EQ(I0.user_begin() ,I0.user_end());
+
+
+  auto XUser = find(X.users(), &(I1));
+  EXPECT_NE(XUser, X.user_end());
+  
+  XUser->replaceUsesOfWith(&X, &I0);
+  EXPECT_EQ(X.user_begin() ,X.user_end());
+  EXPECT_NE(I0.user_begin() ,I0.user_end());
+}
+
 TEST(UserTest, PersonalityUser) {
   LLVMContext Context;
   Module M("", Context);
   FunctionType *RetVoidTy = FunctionType::get(Type::getVoidTy(Context), false);
   Function *PersonalityF = Function::Create(
       RetVoidTy, GlobalValue::ExternalLinkage, "PersonalityFn", &M);
   Function *TestF =
       Function::Create(RetVoidTy, GlobalValue::ExternalLinkage, "TestFn", &M);