Index: clang/lib/CodeGen/TargetInfo.cpp
===================================================================
--- clang/lib/CodeGen/TargetInfo.cpp
+++ clang/lib/CodeGen/TargetInfo.cpp
@@ -499,11 +499,15 @@
 
   // Constant arrays of empty records count as empty, strip them off.
   // Constant arrays of zero length always count as empty.
+  bool WasArray = false;
   if (AllowArrays)
     while (const ConstantArrayType *AT = Context.getAsConstantArrayType(FT)) {
       if (AT->getSize() == 0)
         return true;
       FT = AT->getElementType();
+      // The [[no_unique_address]] special case below does not apply to
+      // arrays of C++ empty records, so we need to remember this fact.
+      WasArray = true;
     }
 
   const RecordType *RT = FT->getAs<RecordType>();
@@ -514,7 +518,14 @@
   //
   // FIXME: We should use a predicate for whether this behavior is true in the
   // current ABI.
-  if (isa<CXXRecordDecl>(RT->getDecl()))
+  //
+  // The exception to the above rule are fields marked with the
+  // [[no_unique_address]] attribute (since C++20).  Those do count as empty
+  // according to the Itanium ABI.  The exception applies only to records,
+  // not arrays of records, so we must also check whether we stripped off an
+  // array type above.
+  if (isa<CXXRecordDecl>(RT->getDecl()) &&
+      (WasArray || !FD->hasAttr<NoUniqueAddressAttr>()))
     return false;
 
   return isEmptyRecord(Context, FT, AllowArrays);
@@ -7236,6 +7247,10 @@
       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.
Index: clang/test/CodeGen/systemz-abi.cpp
===================================================================
--- clang/test/CodeGen/systemz-abi.cpp
+++ clang/test/CodeGen/systemz-abi.cpp
@@ -23,3 +23,37 @@
 // CHECK-LABEL: define void @_Z18pass_agg_float_cpp13agg_float_cpp(%struct.agg_float_cpp* noalias sret align 4 %{{.*}}, float %{{.*}})
 // SOFT-FLOAT-LABEL:  define void @_Z18pass_agg_float_cpp13agg_float_cpp(%struct.agg_float_cpp* noalias sret 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; }
+// CHECK-LABEL: define void @_Z22pass_agg_nofloat_empty17agg_nofloat_empty(%struct.agg_nofloat_empty* noalias sret align 4 %{{.*}}, i64 %{{.*}})
+// SOFT-FLOAT-LABEL:  define void @_Z22pass_agg_nofloat_empty17agg_nofloat_empty(%struct.agg_nofloat_empty* noalias sret align 4 %{{.*}}, i64 %{{.*}})
+struct agg_float_empty { float a; [[no_unique_address]] empty dummy; };
+struct agg_float_empty pass_agg_float_empty(struct agg_float_empty arg) { return arg; }
+// CHECK-LABEL: define void @_Z20pass_agg_float_empty15agg_float_empty(%struct.agg_float_empty* noalias sret align 4 %{{.*}}, float %{{.*}})
+// SOFT-FLOAT-LABEL:  define void @_Z20pass_agg_float_empty15agg_float_empty(%struct.agg_float_empty* noalias sret align 4 %{{.*}}, i32 %{{.*}})
+struct agg_nofloat_emptyarray { float a; [[no_unique_address]] empty dummy[3]; };
+struct agg_nofloat_emptyarray pass_agg_nofloat_emptyarray(struct agg_nofloat_emptyarray arg) { return arg; }
+// CHECK-LABEL: define void @_Z27pass_agg_nofloat_emptyarray22agg_nofloat_emptyarray(%struct.agg_nofloat_emptyarray* noalias sret align 4 %{{.*}}, i64 %{{.*}})
+// SOFT-FLOAT-LABEL:  define void @_Z27pass_agg_nofloat_emptyarray22agg_nofloat_emptyarray(%struct.agg_nofloat_emptyarray* noalias sret align 4 %{{.*}}, i64 %{{.*}})
+
+// And likewise for members of base classes.
+struct noemptybase { empty dummy; };
+struct agg_nofloat_emptybase : noemptybase { float a; };
+struct agg_nofloat_emptybase pass_agg_nofloat_emptybase(struct agg_nofloat_emptybase arg) { return arg; }
+// CHECK-LABEL: define void @_Z26pass_agg_nofloat_emptybase21agg_nofloat_emptybase(%struct.agg_nofloat_emptybase* noalias sret align 4 %{{.*}}, i64 %{{.*}})
+// SOFT-FLOAT-LABEL:  define void @_Z26pass_agg_nofloat_emptybase21agg_nofloat_emptybase(%struct.agg_nofloat_emptybase* noalias sret align 4 %{{.*}}, i64 %{{.*}})
+struct emptybase { [[no_unique_address]] empty dummy; };
+struct agg_float_emptybase : emptybase { float a; };
+struct agg_float_emptybase pass_agg_float_emptybase(struct agg_float_emptybase arg) { return arg; }
+// CHECK-LABEL: define void @_Z24pass_agg_float_emptybase19agg_float_emptybase(%struct.agg_float_emptybase* noalias sret align 4 %{{.*}}, float %{{.*}})
+// SOFT-FLOAT-LABEL:  define void @_Z24pass_agg_float_emptybase19agg_float_emptybase(%struct.agg_float_emptybase* noalias sret align 4 %{{.*}}, i32 %{{.*}})
+struct noemptybasearray { [[no_unique_address]] empty dummy[3]; };
+struct agg_nofloat_emptybasearray : noemptybasearray { float a; };
+struct agg_nofloat_emptybasearray pass_agg_nofloat_emptybasearray(struct agg_nofloat_emptybasearray arg) { return arg; }
+// CHECK-LABEL: define void @_Z31pass_agg_nofloat_emptybasearray26agg_nofloat_emptybasearray(%struct.agg_nofloat_emptybasearray* noalias sret align 4 %{{.*}}, i64 %{{.*}})
+// SOFT-FLOAT-LABEL:  define void @_Z31pass_agg_nofloat_emptybasearray26agg_nofloat_emptybasearray(%struct.agg_nofloat_emptybasearray* noalias sret align 4 %{{.*}}, i64 %{{.*}})
+