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@llvm/pr-subscribers-clang

Author: Grigory Pastukhov (grigorypas)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 6e940a318b61d..f47d1447d5b60 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -12417,6 +12417,10 @@ def warn_zero_as_null_pointer_constant : Warning<
   "zero as null pointer constant">,
   InGroup<DiagGroup<"zero-as-null-pointer-constant">>, DefaultIgnore;
 
+def warn_not_eliding_copy_on_return : Warning<
+  "not eliding copy on return">, 
+  InGroup<DiagGroup<"nrvo">>, DefaultIgnore;
+
 def err_nullability_cs_multilevel : Error<
   "nullability keyword %0 cannot be applied to multi-level pointer type %1">;
 def note_nullability_type_specifier : Note<
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index a7d59ec232b64..6dae243b520f0 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -16093,8 +16093,11 @@ void Sema::computeNRVO(Stmt *Body, FunctionScopeInfo *Scope) {
 
   for (unsigned I = 0, E = Scope->Returns.size(); I != E; ++I) {
     if (const VarDecl *NRVOCandidate = Returns[I]->getNRVOCandidate()) {
-      if (!NRVOCandidate->isNRVOVariable())
+      if (!NRVOCandidate->isNRVOVariable()) {
+        Diag(Returns[I]->getRetValue()->getExprLoc(),
+             diag::warn_not_eliding_copy_on_return);
         Returns[I]->setNRVOCandidate(nullptr);
+      }
     }
   }
 }
diff --git a/clang/test/SemaCXX/warn-nrvo.cpp b/clang/test/SemaCXX/warn-nrvo.cpp
new file mode 100644
index 0000000000000..4552891fd4ab6
--- /dev/null
+++ b/clang/test/SemaCXX/warn-nrvo.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fsyntax-only -Wnrvo -verify %s
+struct MyClass {
+    int value;
+    int c;
+    MyClass(int v) : value(v), c(0) {}
+    MyClass(const MyClass& other) : value(other.value) { c++; }
+};
+
+MyClass create_object(bool condition) {
+    MyClass obj1(1);
+    MyClass obj2(2);
+    if (condition) {
+        return obj1; // expected-warning{{not eliding copy on return}}
+    }
+    return obj2; // expected-warning{{not eliding copy on return}}
+}

Thanks. This needs a few more tests.

• A few tests to make sure we don't produce false positives.
• Tests with function templates with placeholder return types.
  We should never be performing copy elision for these, due to implementation limitations.

Thank you for your feedback. I added more tests. By "function templates with placeholder return types", do you mean something similar to create_object4 in my test case?

Thank you for your feedback. I added more tests. By "function templates with placeholder return types", do you mean something similar to create_object4 in my test case?

Yep. That's what I meant.

Thanks!

You should make sure to include a release note before merging.

Also, perhaps these should be remarks instead of warnings.
Seems like a better fit to me, it would be similar to remarks we provide about function inlining.
Did you consider that?

FYI @AaronBallman

Thanks!

You should make sure to include a release note before merging.

Also, perhaps these should be remarks instead of warnings. Seems like a better fit to me, it would be similar to remarks we provide about function inlining. Did you consider that?

FYI @AaronBallman

Remarks are used when there's something to report but it's not about the code being dubious. NRVO issues straddle the line between dubious code and not dubious code (code could rely on NRVO for correctness). But because GCC already has -Wnrvo and this aims to match their behavior, I think going with the warning flag makes more sense.

Can you also include a few tests which show whether we suppress the warning on disabled branches of an 'if constexpr'?

Beware we also have bugs and odd cases in this area:

• It should be possible to NRVO a variable which you don't actually return, by retuning it only on disabled branches.
  It would be interesting to see what the warning does in this case.
• You can generate bad code by returning a variable with the wrong type from a disabled branch.

I added several tests with dead branches. I noticed that Clang missed NRVO opportunity in create_object5 test. I wonder if it is a know issue or not. GCC seems to handle it.

@mizvekov Could you please merge if it is good to go? I don't have write access.

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