[X86][ABI] Keep empty class argument passing by value compatible with GCC.

[hliao]

ping for review

[hliao]

PING

clang/lib/CodeGen/TargetInfo.cpp

ABIArgInfo X86_32ABIInfo::classifyArgumentType(QualType Ty,
 
   if (isAggregateTypeForABI(Ty)) {
     // Structures with flexible arrays are always indirect.
     // FIXME: This should not be byval!
     if (RT && RT->getDecl()->hasFlexibleArrayMember())
       return getIndirectResult(Ty, true, State);
 
     // Ignore empty structs/unions on non-Windows.
-    if (!IsWin32StructABI && isEmptyRecord(getContext(), Ty, true))
+    if (!IsWin32StructABI && isEmptyRecord(getContext(), Ty, true)) {
+      // For compatibility with GCC, treat it like a struct with a single char.
+      if (getContext().getLangOpts().CPlusPlus)
+        return getIndirectResult(Ty, /*ByVal=*/true, State);
       return ABIArgInfo::getIgnore();
+    }
 
     llvm::LLVMContext &LLVMContext = getVMContext();
     llvm::IntegerType *Int32 = llvm::Type::getInt32Ty(LLVMContext);
     bool NeedsPadding = false;
     bool InReg;
     if (shouldAggregateUseDirect(Ty, State, InReg, NeedsPadding)) {
       unsigned SizeInRegs = (getContext().getTypeSize(Ty) + 31) / 32;
       SmallVector<llvm::Type*, 3> Elements(SizeInRegs, Int32);
if (getRecordArgABI(RT, getCXXABI()))
       return;
 
     const RecordDecl *RD = RT->getDecl();
 
     // Assume variable sized types are passed in memory.
     if (RD->hasFlexibleArrayMember())
       return;
 
+    // According to the resolution to A-5 in
+    // https://itanium-cxx-abi.github.io/cxx-abi/cxx-open.html, empty class by
+    // value should be treated as a struct containing a single character.
+    // That's aligned with record layout in AST and compatible with gcc.
+    // Check https://godbolt.org/z/D1u2MV for the difference in codegen.
+    if (getContext().getLangOpts().CPlusPlus)
+      if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD))
+        if (CXXRD->isEmpty())
+          return;
+
     const ASTRecordLayout &Layout = getContext().getASTRecordLayout(RD);
 
     // Reset Lo class, this will be recomputed.
     Current = NoClass;
 
     // If this is a C++ record, classify the bases first.
     if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
       for (const auto &I : CXXRD->bases()) {

clang/test/CodeGen/empty-class.cpp

+// RUN: %clang_cc1 %s -emit-llvm -o - -triple=x86_64 | FileCheck --check-prefix=X64 %s
+// RUN: %clang_cc1 %s -emit-llvm -o - -triple=i686 | FileCheck --check-prefix=X32 %s
+
+class Empty {};
+
+void bar(Empty *);
+
+// X64-LABEL: _Z3foo5Empty
+// X64-SAME: %class.Empty* byval(%class.Empty) align 8 %e
+// X64-NOT: alloca
+// X32-LABEL: _Z3foo5Empty
+// X32-SAME: %class.Empty* byval(%class.Empty) align 4 %e
+// X32-NOT: alloca
+void foo(Empty e) {
+  bar(&e);
+}