[SystemZ] Fix C++ ABI for passing args of structs containing zero width bitfield.

[jonpa]

Summary

A struct like { float a; int :0; } should per the SystemZ ABI be passed in a GPR, but to match a bug in GCC it has been passed in an FPR (see 759449c).

GCC has now corrected the C++ ABI for this case, and this patch for clang follows suit.

Not yet sure about what diagnostic should be emitted and when.

[uweigand]

Not yet sure about what diagnostic should be emitted and when.

Looks like clang doesn't usually emit this type of diagnostic, so I think we don't need it in this special case either.

Patch LGTM.

clang/lib/CodeGen/TargetInfo.cpp

if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD))
 
         if (!Found.isNull())
           return Ty;
         Found = GetSingleElementType(Base);
       }
 
     // Check the fields.
     for (const auto *FD : RD->fields()) {
-      // For compatibility with GCC, ignore empty bitfields in C++ mode.
       // Unlike isSingleElementStruct(), empty structure and array fields
       // do count.  So do anonymous bitfields that aren't zero-sized.
-      if (getContext().getLangOpts().CPlusPlus &&
-          FD->isZeroLengthBitField(getContext()))
-        continue;
+
       // Like isSingleElementStruct(), ignore C++20 empty data members.
       if (FD->hasAttr<NoUniqueAddressAttr>() &&
           isEmptyRecord(getContext(), FD->getType(), true))
         continue;
 
       // Unlike isSingleElementStruct(), arrays do not count.
       // Nested structures still do though.
       if (!Found.isNull())

clang/test/CodeGen/SystemZ/systemz-abi.cpp

 // SOFT-FLOAT-LABEL: define{{.*}} void @_Z20pass_agg_float_class15agg_float_class(%class.agg_float_class* noalias sret(%class.agg_float_class) align 4 %{{.*}}, i32 %{{.*}})
 
 class agg_double_class { double a; };
 class agg_double_class pass_agg_double_class(class agg_double_class arg) { return arg; }
 // CHECK-LABEL: define{{.*}} void @_Z21pass_agg_double_class16agg_double_class(%class.agg_double_class* noalias sret(%class.agg_double_class) align 8 %{{.*}}, double %{{.*}})
 // SOFT-FLOAT-LABEL: define{{.*}} void @_Z21pass_agg_double_class16agg_double_class(%class.agg_double_class* noalias sret(%class.agg_double_class) align 8 %{{.*}}, i64 %{{.*}})
 
 
-// For compatibility with GCC, this structure is passed in an FPR in C++,
-// but passed in a GPR in C (checked in systemz-abi.c).
-
+// This structure is passed in a GPR in C++ (and C, checked in systemz-abi.c).
 struct agg_float_cpp { float a; int : 0; };
 struct agg_float_cpp pass_agg_float_cpp(struct agg_float_cpp arg) { return arg; }
-// CHECK-LABEL: define{{.*}} void @_Z18pass_agg_float_cpp13agg_float_cpp(%struct.agg_float_cpp* noalias sret(%struct.agg_float_cpp) align 4 %{{.*}}, float %{{.*}})
+// CHECK-LABEL: define{{.*}} void @_Z18pass_agg_float_cpp13agg_float_cpp(%struct.agg_float_cpp* noalias sret(%struct.agg_float_cpp) align 4 %{{.*}}, i32 %{{.*}})
 // SOFT-FLOAT-LABEL:  define{{.*}} void @_Z18pass_agg_float_cpp13agg_float_cpp(%struct.agg_float_cpp* noalias sret(%struct.agg_float_cpp) align 4 %{{.*}}, i32 %{{.*}})
 
 
 // A field member of empty class type in C++ makes the record nonhomogeneous,
 // unless it is marked as [[no_unique_address]].  This does not apply to arrays.
 struct empty { };
 struct agg_nofloat_empty { float a; empty dummy; };
 struct agg_nofloat_empty pass_agg_nofloat_empty(struct agg_nofloat_empty arg) { return arg; }