[DebugInfo] Change handling of structured bindings of bitfields (#94632)

Currently we use DW_OP_plus_uconst to handle the bitfield offset and
handle the bitfield size by choosing a type size that matches, but this
doesn't work if either offset or size aren't byte-aligned. Extracting
the bits using DW_OP_LLVM_extract_bits means we can handle any kind of
offset or size.

Author: john-brawn-arm

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -4874,40 +4874,6 @@ llvm::DILocalVariable *CGDebugInfo::EmitDeclare(const VarDecl *VD,
   return D;
 }
 
-llvm::DIType *CGDebugInfo::CreateBindingDeclType(const BindingDecl *BD) {
-  llvm::DIFile *Unit = getOrCreateFile(BD->getLocation());
-
-  // If the declaration is bound to a bitfield struct field, its type may have a
-  // size that is different from its deduced declaration type's.
-  if (const MemberExpr *ME = dyn_cast<MemberExpr>(BD->getBinding())) {
-    if (const FieldDecl *FD = dyn_cast<FieldDecl>(ME->getMemberDecl())) {
-      if (FD->isBitField()) {
-        ASTContext &Context = CGM.getContext();
-        const CGRecordLayout &RL =
-            CGM.getTypes().getCGRecordLayout(FD->getParent());
-        const CGBitFieldInfo &Info = RL.getBitFieldInfo(FD);
-
-        // Find an integer type with the same bitwidth as the bitfield size. If
-        // no suitable type is present in the target, give up on producing debug
-        // information as it would be wrong. It is certainly possible to produce
-        // correct debug info, but the logic isn't currently implemented.
-        uint64_t BitfieldSizeInBits = Info.Size;
-        QualType IntTy =
-            Context.getIntTypeForBitwidth(BitfieldSizeInBits, Info.IsSigned);
-        if (IntTy.isNull())
-          return nullptr;
-        Qualifiers Quals = BD->getType().getQualifiers();
-        QualType FinalTy = Context.getQualifiedType(IntTy, Quals);
-        llvm::DIType *Ty = getOrCreateType(FinalTy, Unit);
-        assert(Ty);
-        return Ty;
-      }
-    }
-  }
-
-  return getOrCreateType(BD->getType(), Unit);
-}
-
 llvm::DILocalVariable *CGDebugInfo::EmitDeclare(const BindingDecl *BD,
                                                 llvm::Value *Storage,
                                                 std::optional<unsigned> ArgNo,
@@ -4922,7 +4888,8 @@ llvm::DILocalVariable *CGDebugInfo::EmitDeclare(const BindingDecl *BD,
   if (isa<DeclRefExpr>(BD->getBinding()))
     return nullptr;
 
-  llvm::DIType *Ty = CreateBindingDeclType(BD);
+  llvm::DIFile *Unit = getOrCreateFile(BD->getLocation());
+  llvm::DIType *Ty = getOrCreateType(BD->getType(), Unit);
 
   // If there is no debug info for this type then do not emit debug info
   // for this variable.
@@ -4948,7 +4915,6 @@ llvm::DILocalVariable *CGDebugInfo::EmitDeclare(const BindingDecl *BD,
   unsigned Column = getColumnNumber(BD->getLocation());
   StringRef Name = BD->getName();
   auto *Scope = cast<llvm::DIScope>(LexicalBlockStack.back());
-  llvm::DIFile *Unit = getOrCreateFile(BD->getLocation());
   // Create the descriptor for the variable.
   llvm::DILocalVariable *D = DBuilder.createAutoVariable(
       Scope, Name, Unit, Line, Ty, CGM.getLangOpts().Optimize,
@@ -4962,13 +4928,29 @@ llvm::DILocalVariable *CGDebugInfo::EmitDeclare(const BindingDecl *BD,
       const ASTRecordLayout &layout =
           CGM.getContext().getASTRecordLayout(parent);
       const uint64_t fieldOffset = layout.getFieldOffset(fieldIndex);
-
-      if (fieldOffset != 0) {
-        // Currently if the field offset is not a multiple of byte, the produced
-        // location would not be accurate. Therefore give up.
-        if (fieldOffset % CGM.getContext().getCharWidth() != 0)
-          return nullptr;
-
+      if (FD->isBitField()) {
+        const CGRecordLayout &RL =
+            CGM.getTypes().getCGRecordLayout(FD->getParent());
+        const CGBitFieldInfo &Info = RL.getBitFieldInfo(FD);
+        // Use DW_OP_plus_uconst to adjust to the start of the bitfield
+        // storage.
+        if (!Info.StorageOffset.isZero()) {
+          Expr.push_back(llvm::dwarf::DW_OP_plus_uconst);
+          Expr.push_back(Info.StorageOffset.getQuantity());
+        }
+        // Use LLVM_extract_bits to extract the appropriate bits from this
+        // bitfield.
+        Expr.push_back(Info.IsSigned
+                           ? llvm::dwarf::DW_OP_LLVM_extract_bits_sext
+                           : llvm::dwarf::DW_OP_LLVM_extract_bits_zext);
+        Expr.push_back(Info.Offset);
+        // If we have an oversized bitfield then the value won't be more than
+        // the size of the type.
+        const uint64_t TypeSize = CGM.getContext().getTypeSize(BD->getType());
+        Expr.push_back(std::min((uint64_t)Info.Size, TypeSize));
+      } else if (fieldOffset != 0) {
+        assert(fieldOffset % CGM.getContext().getCharWidth() == 0 &&
+               "Unexpected non-bitfield with non-byte-aligned offset");
         Expr.push_back(llvm::dwarf::DW_OP_plus_uconst);
         Expr.push_back(
             CGM.getContext().toCharUnitsFromBits(fieldOffset).getQuantity());

clang/lib/CodeGen/CGDebugInfo.h

@@ -338,9 +338,6 @@ class CGDebugInfo {
                                           llvm::DIScope *RecordTy,
                                           const RecordDecl *RD);
 
-  /// Create type for binding declarations.
-  llvm::DIType *CreateBindingDeclType(const BindingDecl *BD);
-
   /// Create an anonnymous zero-size separator for bit-field-decl if needed on
   /// the target.
   llvm::DIDerivedType *createBitFieldSeparatorIfNeeded(