[Prototype] Introduce attribute for ignoring C++ ABI

[aaron.ballman]

Some more background information on the idea would be helpful -- this strikes me as something that should probably start out as an RFC on cfe-dev (and maybe libcxx-dev?) to discuss the high-level goals and approaches, before doing much prototype work. Note: some related attributes are AbiTag and TrivialABI.

[aeubanks]

I'm first testing out if there are even noticeable benefits to doing this, then if there are I'll put something together to post to cfe-dev. https://crbug.com/1028731 for some background. I'm aware of trivial_abi and will consider wrapping this up into that, although we may want to separate the two.

(I didn't mean to add you as a reviewer yet, Phab automatically did that, you can ignore this for now if you'd like)

[aaron.ballman]

In D101561#2730201, @aeubanks wrote:

I'm first testing out if there are even noticeable benefits to doing this, then if there are I'll put something together to post to cfe-dev. https://crbug.com/1028731 for some background. I'm aware of trivial_abi and will consider wrapping this up into that, although we may want to separate the two.

(I didn't mean to add you as a reviewer yet, Phab automatically did that, you can ignore this for now if you'd like)

Ah, thanks for the info!

[dblaikie]

In D101561#2730201, @aeubanks wrote:

I'm first testing out if there are even noticeable benefits to doing this, then if there are I'll put something together to post to cfe-dev. https://crbug.com/1028731 for some background. I'm aware of trivial_abi and will consider wrapping this up into that, although we may want to separate the two.

(I didn't mean to add you as a reviewer yet, Phab automatically did that, you can ignore this for now if you'd like)

FWIW, if you weren't ready for this to be seen by others/reviewed, you can make a draft review (I haven't tested - maybe this is still /accessible/ so you can share a link in an RFC, etc) - discussed here: https://reviews.llvm.org/D99650#2671915

[aeubanks]

ah I wasn't aware of that, thanks! sorry for the extra noise

[aeubanks]

turns out this didn't have a good enough impact to pursue

clang/include/clang/Basic/Attr.td

def TrivialABI : InheritableAttr {
   // CPlusPlus language option.
   let Spellings = [Clang<"trivial_abi", 0>];
   let Subjects = SubjectList<[CXXRecord]>;
   let Documentation = [TrivialABIDocs];
   let LangOpts = [CPlusPlus];
   let SimpleHandler = 1;
 }
 
+def UnstableABI : InheritableAttr {
+  // This attribute does not have a C [[]] spelling because it requires the
+  // CPlusPlus language option.
+  let Spellings = [Clang<"unstable_abi", 0>];
+  let Subjects = SubjectList<[CXXRecord]>;
+  let Documentation = [UnstableABIDocs];
+  let LangOpts = [CPlusPlus];
+  let SimpleHandler = 1;
+}
+
 def MaxFieldAlignment : InheritableAttr {
   // This attribute has no spellings as it is only ever created implicitly.
   let Spellings = [];
   let Args = [UnsignedArgument<"Alignment">];
   let SemaHandler = 0;
   let Documentation = [Undocumented];
 }
 

clang/include/clang/Basic/AttrDocs.td

- The class has a non-static data member whose type is non-trivial for the
   purposes of calls, which includes:
 
   - classes that are non-trivial for the purposes of calls
   - __weak-qualified types in Objective-C++
   - arrays of any of the above
   }];
 }
 
+def UnstableABIDocs : Documentation {
+  let Category = DocCatDecl;
+  let Content = [{
+  }];
+}
+
 def MSInheritanceDocs : Documentation {
   let Category = DocCatDecl;
   let Heading = "__single_inhertiance, __multiple_inheritance, __virtual_inheritance";
   let Content = [{
 This collection of keywords is enabled under ``-fms-extensions`` and controls
 the pointer-to-member representation used on ``*-*-win32`` targets.
 
 The ``*-*-win32`` targets utilize a pointer-to-member representation which

clang/lib/CodeGen/TargetInfo.cpp

if (RT) {
     if (!IsReturnType) {
       if (CGCXXABI::RecordArgABI RAA = getRecordArgABI(RT, getCXXABI()))
         return getNaturalAlignIndirect(Ty, RAA == CGCXXABI::RAA_DirectInMemory);
     }
 
     if (RT->getDecl()->hasFlexibleArrayMember())
       return getNaturalAlignIndirect(Ty, /*ByVal=*/false);
 
+    if (RT->getDecl()->hasAttr<UnstableABIAttr>())
+      return ABIArgInfo::getDirect();
   }
 
   const Type *Base = nullptr;
   uint64_t NumElts = 0;
   // vectorcall adds the concept of a homogenous vector aggregate, similar to
   // other targets.
   if ((IsVectorCall || IsRegCall) &&
       isHomogeneousAggregate(Ty, Base, NumElts)) {