[PATCH] D155895: Anonymous unions should be transparent wrt `[[clang::trivial_abi]]`.

[Dmitri Gribenko via Phabricator via cfe-commits]

gribozavr2 added inline comments.


================
Comment at: clang/lib/Sema/SemaDeclCXX.cpp:10325
 
-  for (const auto *FD : RD.fields()) {
-    // Ill-formed if the field is an ObjectiveC pointer or of a type that is
-    // non-trivial for the purpose of calls.
-    QualType FT = FD->getType();
-    if (FT.getObjCLifetime() == Qualifiers::OCL_Weak) {
-      PrintDiagAndRemoveAttr(4);
-      return;
-    }
-
-    if (const auto *RT = FT->getBaseElementTypeUnsafe()->getAs<RecordType>())
-      if (!RT->isDependentType() &&
-          !cast<CXXRecordDecl>(RT->getDecl())->canPassInRegisters()) {
-        PrintDiagAndRemoveAttr(5);
+  std::queue<const RecordDecl *> RecordsWithFieldsToCheck;
+  RecordsWithFieldsToCheck.push(&RD);
----------------
It looks like we don't need to visit the fields in a specific order. If that's the case, please use SmallVector and pop_back_val. (See AnalyzeImplicitConversions for an example of a typical worklist algorithm.)


================
Comment at: clang/lib/Sema/SemaDeclCXX.cpp:10330
+      // Ill-formed if the field is an ObjectiveC pointer or of a type that is
+      // non-trivial for the purpose of calls.
+      QualType FT = FD->getType();
----------------
Please split off the part of the comment that talks about "non-trivial for the purpose of calls", move it to the newly added if statement below, and enhance the comment to explain the new rule about anonymous unions.


================
Comment at: clang/test/SemaCXX/attr-trivial-abi.cpp:201-202
+
+  // `S2` is like `S1`, but `field` is wrapped in an anonymous union (and it
+  // seems that trivial relocatability of `S1` and `S2` should be the same).
+  //
----------------



================
Comment at: clang/test/SemaCXX/attr-trivial-abi.cpp:232
+  // `S2` and `S3` examples above show that `[[clang::trivial_abi]]`
+  // *implicitly* propagates into anonymous union and structs.  The example
+  // below shows that it is still *not* okay to *explicitly* apply
----------------
That's not exactly true, we don't propagate the trivial_abi attribute onto the union type. After this patch, the type of the union (which is impossible to get ahold of) is still non-trivial.

What we do though, is that we ignore the trivial relocatability of the union itself, and instead look at the members.


================
Comment at: clang/test/SemaCXX/attr-trivial-abi.cpp:251
+  // Like `S2`, but the field of the anonymous union is *not* trivial.
+  struct [[clang::trivial_abi]] S5 { // expected-warning {{'trivial_abi' cannot be applied to 'S5'}} expected-note {{trivial_abi' is disallowed on 'S5' because it has a field of a non-trivial class type}}
+    S5(S5&& other) {}
----------------
Could you replace the numbers in S1-S5 with more meaningful phrases?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D155895/new/

https://reviews.llvm.org/D155895