Non constant size and offset in DWARF (#141106)

In Ada, a record type can have a non-constant size, and a field can
appear at a non-constant bit offset in a record.

To support this, this patch changes DIType to record the size and offset
using metadata, rather than plain integers. In addition to a constant
offset, both DIVariable and DIExpression are now supported here.

One thing of note in this patch is the choice of how exactly to
represent a non-constant bit offset, with the difficulty being that
DWARF 5 does not support this. DWARF 3 did have a way to support a
non-constant byte offset, combined with a constant bit offset within the
byte, but this was deprecated in DWARF 4 and removed from DWARF 5.

This patch takes a simple approach: a DWARF extension allowing the use
of an expression with DW_AT_data_bit_offset. There is a corresponding
DWARF issue, see https://dwarfstd.org/issues/250501.1.html. The main
reason for this approach is that it keeps API simplicity: just a single
value is needed, rather than having separate data describing the byte
offset and the bit within the byte.

Author: tromey

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -841,8 +841,8 @@ llvm::DIType *CGDebugInfo::CreateType(const BuiltinType *BT) {
     auto *ISATy = DBuilder.createPointerType(ClassTy, Size);
 
     ObjTy = DBuilder.createStructType(TheCU, "objc_object", TheCU->getFile(), 0,
-                                      0, 0, llvm::DINode::FlagZero, nullptr,
-                                      llvm::DINodeArray());
+                                      (uint64_t)0, 0, llvm::DINode::FlagZero,
+                                      nullptr, llvm::DINodeArray());
 
     DBuilder.replaceArrays(
         ObjTy, DBuilder.getOrCreateArray(&*DBuilder.createMemberType(

llvm/include/llvm/IR/DIBuilder.h

@@ -367,6 +367,22 @@ namespace llvm {
                                               uint32_t VBPtrOffset,
                                               DINode::DIFlags Flags);
 
+    /// Create debugging information entry for a member.
+    /// \param Scope        Member scope.
+    /// \param Name         Member name.
+    /// \param File         File where this member is defined.
+    /// \param LineNo       Line number.
+    /// \param SizeInBits   Member size.
+    /// \param AlignInBits  Member alignment.
+    /// \param OffsetInBits Member offset.
+    /// \param Flags        Flags to encode member attribute, e.g. private
+    /// \param Ty           Parent type.
+    /// \param Annotations  Member annotations.
+    LLVM_ABI DIDerivedType *createMemberType(
+        DIScope *Scope, StringRef Name, DIFile *File, unsigned LineNo,
+        Metadata *SizeInBits, uint32_t AlignInBits, Metadata *OffsetInBits,
+        DINode::DIFlags Flags, DIType *Ty, DINodeArray Annotations = nullptr);
+
     /// Create debugging information entry for a member.
     /// \param Scope        Member scope.
     /// \param Name         Member name.
@@ -419,6 +435,23 @@ namespace llvm {
                                                     Constant *Discriminant,
                                                     DIType *Ty);
 
+    /// Create debugging information entry for a bit field member.
+    /// \param Scope               Member scope.
+    /// \param Name                Member name.
+    /// \param File                File where this member is defined.
+    /// \param LineNo              Line number.
+    /// \param SizeInBits          Member size.
+    /// \param OffsetInBits        Member offset.
+    /// \param StorageOffsetInBits Member storage offset.
+    /// \param Flags               Flags to encode member attribute.
+    /// \param Ty                  Parent type.
+    /// \param Annotations         Member annotations.
+    LLVM_ABI DIDerivedType *createBitFieldMemberType(
+        DIScope *Scope, StringRef Name, DIFile *File, unsigned LineNo,
+        Metadata *SizeInBits, Metadata *OffsetInBits,
+        uint64_t StorageOffsetInBits, DINode::DIFlags Flags, DIType *Ty,
+        DINodeArray Annotations = nullptr);
+
     /// Create debugging information entry for a bit field member.
     /// \param Scope               Member scope.
     /// \param Name                Member name.
@@ -510,6 +543,29 @@ namespace llvm {
         unsigned RunTimeLang = 0, DIType *VTableHolder = nullptr,
         MDNode *TemplateParms = nullptr, StringRef UniqueIdentifier = "");
 
+    /// Create debugging information entry for a struct.
+    /// \param Scope        Scope in which this struct is defined.
+    /// \param Name         Struct name.
+    /// \param File         File where this member is defined.
+    /// \param LineNumber   Line number.
+    /// \param SizeInBits   Member size.
+    /// \param AlignInBits  Member alignment.
+    /// \param Flags        Flags to encode member attribute, e.g. private
+    /// \param Elements     Struct elements.
+    /// \param RunTimeLang  Optional parameter, Objective-C runtime version.
+    /// \param UniqueIdentifier A unique identifier for the struct.
+    /// \param Specification The type that this type completes. This is used by
+    /// Swift to represent generic types.
+    /// \param NumExtraInhabitants The number of extra inhabitants of the type.
+    /// An extra inhabitant is a bit pattern that does not represent a valid
+    /// value for instances of a given type. This is used by the Swift language.
+    LLVM_ABI DICompositeType *createStructType(
+        DIScope *Scope, StringRef Name, DIFile *File, unsigned LineNumber,
+        Metadata *SizeInBits, uint32_t AlignInBits, DINode::DIFlags Flags,
+        DIType *DerivedFrom, DINodeArray Elements, unsigned RunTimeLang = 0,
+        DIType *VTableHolder = nullptr, StringRef UniqueIdentifier = "",
+        DIType *Specification = nullptr, uint32_t NumExtraInhabitants = 0);
+
     /// Create debugging information entry for a struct.
     /// \param Scope        Scope in which this struct is defined.
     /// \param Name         Struct name.