[DebugInfo] Use the 'getTypeAlignIfRequired' function to get DW_AT_alignment correct when attribute((__aligned__)) is present.

[MaggieYi]

Summary

The detailed description of the issue could be found in
https://bugs.llvm.org/show_bug.cgi?id=51277.

In the original code, the 'getDeclAlignIfRequired' function is used.
The 'getDeclAlignIfRequired' function will return the max alignment
of all aligned attributes if the type has aligned attributes. The
function doesn’t consider the type at all.

The 'getTypeAlignIfRequired' function uses the type’s alignment value,
which also used by the 'alignof' function. I think we should use the
function of 'getTypeAlignIfRequired'.

[dblaikie]

Could you include some static_asserts(+alignof) in the test case to demonstrate these are the alignments that the language is computing too?

[MaggieYi]

Thanks David, I have modified the test following your advice.

[dblaikie]

Looks good, thanks!

clang/test/CodeGenCXX/debug-info-struct-align.cpp

12

You can drop the "struct" here and from other references to these types (in mt1/mt2 and the alignof calls)

[MaggieYi]

clang/test/CodeGenCXX/debug-info-struct-align.cpp

12

Thanks David. All unnecessary structs have been removed.

clang/lib/CodeGen/CGDebugInfo.cpp

llvm::DICompositeType *CGDebugInfo::CreateLimitedType(const RecordType *Ty) {
 
   // If this is just a forward or incomplete declaration, construct an
   // appropriately marked node and just return it.
   const RecordDecl *D = RD->getDefinition();
   if (!D || !D->isCompleteDefinition())
     return getOrCreateRecordFwdDecl(Ty, RDContext);
 
   uint64_t Size = CGM.getContext().getTypeSize(Ty);
-  auto Align = getDeclAlignIfRequired(D, CGM.getContext());
+  // __attribute__((aligned)) can increase or decrease alignment *except* on a
+  // struct or struct member, where it only increases  alignment unless 'packed'
+  // is also specified. To handle this case, the `getTypeAlignIfRequired` needs
+  // to be used.
+  auto Align = getTypeAlignIfRequired(Ty, CGM.getContext());
 
   SmallString<256> Identifier = getTypeIdentifier(Ty, CGM, TheCU);
 
   // Explicitly record the calling convention and export symbols for C++
   // records.
   auto Flags = llvm::DINode::FlagZero;
   if (auto CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
     if (CGM.getCXXABI().getRecordArgABI(CXXRD) == CGCXXABI::RAA_Indirect)

clang/test/CodeGenCXX/debug-info-struct-align.cpp

+//  Test for debug info related to DW_AT_alignment attribute in the struct type.
+// RUN: %clang_cc1 -dwarf-version=5 -debug-info-kind=standalone -S -emit-llvm %s -o - | FileCheck %s
+
+// CHECK-DAG: DICompositeType(tag: DW_TAG_structure_type, name: "MyType", {{.*}}, align: 32
+// CHECK-DAG: DICompositeType(tag: DW_TAG_structure_type, name: "MyType1", {{.*}}, align: 8
+// CHECK-DAG: DICompositeType(tag: DW_TAG_structure_type, name: "MyType2", {{.*}}, align: 8
+
+struct MyType {
+  int m;
+} __attribute__((aligned(1)));
+MyType mt;
+
+static_assert(alignof(MyType) == 4, "alignof MyType is wrong");
+
+struct MyType1 {
+  int m;
+} __attribute__((packed, aligned(1)));
+MyType1 mt1;
+
+static_assert(alignof(MyType1) == 1, "alignof MyType1 is wrong");
+
+struct MyType2 {
+  __attribute__((packed)) int m;
+} __attribute__((aligned(1)));
+MyType2 mt2;
+
+static_assert(alignof(MyType2) == 1, "alignof MyType2 is wrong");