[Lldb-commits] [lldb] [LLDB] Respect the DW_AT_alignment attribute. (PR #73307)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lldb

Author: Haojian Wu (hokein)

<details>
<summary>Changes</summary>

Part of fixes for #<!-- -->72913.

clang emits `DW_AT_alignment` attribute, however LLDB didn't respect it, resulting in incorrect RecordDecls built by lldb.

This only fixes non-inheritance cases. The inheritance case will be handled in a follow-up patch.

---
Full diff: https://github.com/llvm/llvm-project/pull/73307.diff


4 Files Affected:

- (modified) lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp (+10-2) 
- (modified) lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h (+1) 
- (modified) lldb/test/API/lang/cpp/alignas_base_class/TestAlignAsBaseClass.py (+4) 
- (modified) lldb/test/API/lang/cpp/alignas_base_class/main.cpp (+2) 


``````````diff
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
index abe3c673e2cce69..a55ca0bf0f0fc1a 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
@@ -355,6 +355,10 @@ ParsedDWARFTypeAttributes::ParsedDWARFTypeAttributes(const DWARFDIE &die) {
       byte_size = form_value.Unsigned();
       break;
 
+    case DW_AT_alignment:
+      alignment = form_value.Unsigned();
+      break;
+
     case DW_AT_byte_stride:
       byte_stride = form_value.Unsigned();
       break;
@@ -1926,12 +1930,13 @@ DWARFASTParserClang::ParseStructureLikeDIE(const SymbolContext &sc,
       // TypeSystemClang is always in C++ mode, but some compilers such as
       // GCC and Clang give empty structs a size of 0 in C mode (in contrast to
       // the size of 1 for empty structs that would be computed in C++ mode).
-      if (attrs.byte_size) {
+      if (attrs.byte_size || attrs.alignment) {
         clang::RecordDecl *record_decl =
             TypeSystemClang::GetAsRecordDecl(clang_type);
         if (record_decl) {
           ClangASTImporter::LayoutInfo layout;
-          layout.bit_size = *attrs.byte_size * 8;
+          layout.bit_size = attrs.byte_size.value_or(0) * 8;
+          layout.alignment = attrs.alignment.value_or(0) * 8;
           GetClangASTImporter().SetRecordLayout(record_decl, layout);
         }
       }
@@ -2270,6 +2275,9 @@ bool DWARFASTParserClang::CompleteRecordType(const DWARFDIE &die,
     if (layout_info.bit_size == 0)
       layout_info.bit_size =
           die.GetAttributeValueAsUnsigned(DW_AT_byte_size, 0) * 8;
+    if (layout_info.alignment == 0)
+      layout_info.alignment =
+          die.GetAttributeValueAsUnsigned(llvm::dwarf::DW_AT_alignment, 0) * 8;
 
     clang::CXXRecordDecl *record_decl =
         m_ast.GetAsCXXRecordDecl(clang_type.GetOpaqueQualType());
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
index 0247783217008e8..81b705a036189eb 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
@@ -456,6 +456,7 @@ struct ParsedDWARFTypeAttributes {
   lldb_private::plugin::dwarf::DWARFFormValue type;
   lldb::LanguageType class_language = lldb::eLanguageTypeUnknown;
   std::optional<uint64_t> byte_size;
+  std::optional<uint64_t> alignment;
   size_t calling_convention = llvm::dwarf::DW_CC_normal;
   uint32_t bit_stride = 0;
   uint32_t byte_stride = 0;
diff --git a/lldb/test/API/lang/cpp/alignas_base_class/TestAlignAsBaseClass.py b/lldb/test/API/lang/cpp/alignas_base_class/TestAlignAsBaseClass.py
index c7a88987733e176..7d97b0c42b7e166 100644
--- a/lldb/test/API/lang/cpp/alignas_base_class/TestAlignAsBaseClass.py
+++ b/lldb/test/API/lang/cpp/alignas_base_class/TestAlignAsBaseClass.py
@@ -12,3 +12,7 @@ def test(self):
 
         # The offset of f2 should be 8 because of `alignas(8)`.
         self.expect_expr("(intptr_t)&d3g.f2 - (intptr_t)&d3g", result_value="8")
+
+        # Verify specified class alignments.
+        self.expect_expr("alignof(B2)", result_value="8")
+        self.expect_expr("alignof(EmptyClassAlign8)", result_value="8")
diff --git a/lldb/test/API/lang/cpp/alignas_base_class/main.cpp b/lldb/test/API/lang/cpp/alignas_base_class/main.cpp
index 8dfced6c784e102..cf727e808017bcc 100644
--- a/lldb/test/API/lang/cpp/alignas_base_class/main.cpp
+++ b/lldb/test/API/lang/cpp/alignas_base_class/main.cpp
@@ -10,4 +10,6 @@ struct D : B1, B2 {};
 
 D d3g;
 
+struct alignas(8) EmptyClassAlign8 {} t;
+
 int main() {}

``````````

</details>


https://github.com/llvm/llvm-project/pull/73307