Split DWARFFormValue::getReference into four functions

The result of the function cannot be correctly interpreted without
knowing the precise form type (a type signature needs to be looked up
very differently from a supplementary debug info reference). The
function sort of worked because the two reference types (unit-relative
and section-relative) that can be handled uniformly are also the most
common types of references, but this setup made it easy to write code
which does not support other kinds of reference (and if one tried to
support them, the result didn't look pretty --
https://github.com/llvm/llvm-project/pull/97423/files#r1676217081).

The split is based on the reference type classification from DWARFv5
(Section 7.5.5 Classes and Forms), and it should enable uniform (if
slightly more verbose) hadling. Note that this only affects users which
want more control of how (or if) the references are resolved. Users
which just want to access the referenced DIE can use the higher level
API (DWARFDie::GetAttributeValueAsReferencedDie) which returns (or will
return after llvm#97423 is merged) the correct die for all reference types
(except for supplementary references, which we don't support right now).

llvm/include/llvm/DebugInfo/DWARF/DWARFFormValue.h

@@ -107,12 +107,10 @@ class DWARFFormValue {
 
   /// getAsFoo functions below return the extracted value as Foo if only
   /// DWARFFormValue has form class is suitable for representing Foo.
-  std::optional<uint64_t> getAsReference() const;
-  struct UnitOffset {
-    DWARFUnit *Unit;
-    uint64_t Offset;
-  };
-  std::optional<UnitOffset> getAsRelativeReference() const;
+  std::optional<uint64_t> getAsRelativeReference() const;
+  std::optional<uint64_t> getAsDebugInfoReference() const;
+  std::optional<uint64_t> getAsSignatureReference() const;
+  std::optional<uint64_t> getAsSupplementaryReference() const;
   std::optional<uint64_t> getAsUnsignedConstant() const;
   std::optional<int64_t> getAsSignedConstant() const;
   Expected<const char *> getAsCString() const;
@@ -242,27 +240,102 @@ inline uint64_t toUnsigned(const std::optional<DWARFFormValue> &V,
   return toUnsigned(V).value_or(Default);
 }
 
-/// Take an optional DWARFFormValue and try to extract an reference.
+/// Take an optional DWARFFormValue and try to extract a relative offset
+/// reference.
 ///
-/// \param V and optional DWARFFormValue to attempt to extract the value from.
+/// \param V an optional DWARFFormValue to attempt to extract the value from.
 /// \returns an optional value that contains a value if the form value
-/// was valid and has a reference form.
+/// was valid and has a relative reference form.
 inline std::optional<uint64_t>
-toReference(const std::optional<DWARFFormValue> &V) {
+toRelativeReference(const std::optional<DWARFFormValue> &V) {
   if (V)
-    return V->getAsReference();
+    return V->getAsRelativeReference();
   return std::nullopt;
 }
 
-/// Take an optional DWARFFormValue and extract a reference.
+/// Take an optional DWARFFormValue and extract a relative offset reference.
 ///
-/// \param V and optional DWARFFormValue to attempt to extract the value from.
+/// \param V an optional DWARFFormValue to attempt to extract the value from.
+/// \param Default the default value to return in case of failure.
+/// \returns the extracted reference value or Default if the V doesn't have a
+/// value or the form value's encoding wasn't a relative offset reference form.
+inline uint64_t toRelativeReference(const std::optional<DWARFFormValue> &V,
+                            uint64_t Default) {
+  return toRelativeReference(V).value_or(Default);
+}
+
+/// Take an optional DWARFFormValue and try to extract an absolute debug info
+/// offset reference.
+///
+/// \param V an optional DWARFFormValue to attempt to extract the value from.
+/// \returns an optional value that contains a value if the form value
+/// was valid and has an (absolute) debug info offset reference form.
+inline std::optional<uint64_t>
+toDebugInfoReference(const std::optional<DWARFFormValue> &V) {
+  if (V)
+    return V->getAsDebugInfoReference();
+  return std::nullopt;
+}
+
+/// Take an optional DWARFFormValue and extract an absolute debug info offset
+/// reference.
+///
+/// \param V an optional DWARFFormValue to attempt to extract the value from.
+/// \param Default the default value to return in case of failure.
+/// \returns the extracted reference value or Default if the V doesn't have a
+/// value or the form value's encoding wasn't an absolute debug info offset
+/// reference form.
+inline uint64_t toDebugInfoReference(const std::optional<DWARFFormValue> &V,
+                            uint64_t Default) {
+  return toDebugInfoReference(V).value_or(Default);
+}
+
+/// Take an optional DWARFFormValue and try to extract a signature reference.
+///
+/// \param V an optional DWARFFormValue to attempt to extract the value from.
+/// \returns an optional value that contains a value if the form value
+/// was valid and has a signature reference form.
+inline std::optional<uint64_t>
+toSignatureReference(const std::optional<DWARFFormValue> &V) {
+  if (V)
+    return V->getAsSignatureReference();
+  return std::nullopt;
+}
+
+/// Take an optional DWARFFormValue and extract a signature reference.
+///
+/// \param V an optional DWARFFormValue to attempt to extract the value from.
+/// \param Default the default value to return in case of failure.
+/// \returns the extracted reference value or Default if the V doesn't have a
+/// value or the form value's encoding wasn't a signature reference form.
+inline uint64_t toSignatureReference(const std::optional<DWARFFormValue> &V,
+                            uint64_t Default) {
+  return toSignatureReference(V).value_or(Default);
+}
+
+/// Take an optional DWARFFormValue and try to extract a supplementary debug
+/// info reference.
+///
+/// \param V an optional DWARFFormValue to attempt to extract the value from.
+/// \returns an optional value that contains a value if the form value
+/// was valid and has a supplementary reference form.
+inline std::optional<uint64_t>
+toSupplementaryReference(const std::optional<DWARFFormValue> &V) {
+  if (V)
+    return V->getAsSupplementaryReference();
+  return std::nullopt;
+}
+
+/// Take an optional DWARFFormValue and extract a supplementary debug info
+/// reference.
+///
+/// \param V an optional DWARFFormValue to attempt to extract the value from.
 /// \param Default the default value to return in case of failure.
 /// \returns the extracted reference value or Default if the V doesn't have a
-/// value or the form value's encoding wasn't a reference form.
-inline uint64_t toReference(const std::optional<DWARFFormValue> &V,
+/// value or the form value's encoding wasn't a supplementary reference form.
+inline uint64_t toSupplementaryReference(const std::optional<DWARFFormValue> &V,
                             uint64_t Default) {
-  return toReference(V).value_or(Default);
+  return toSupplementaryReference(V).value_or(Default);
 }
 
 /// Take an optional DWARFFormValue and try to extract an signed constant.

llvm/lib/DWARFLinker/Classic/DWARFLinker.cpp

@@ -77,7 +77,15 @@ DWARFDie DWARFLinker::resolveDIEReference(const DWARFFile &File,
                                           const DWARFDie &DIE,
                                           CompileUnit *&RefCU) {
   assert(RefValue.isFormClass(DWARFFormValue::FC_Reference));
-  uint64_t RefOffset = *RefValue.getAsReference();
+  uint64_t RefOffset;
+  if (std::optional<uint64_t> Off = RefValue.getAsRelativeReference()) {
+    RefOffset = RefValue.getUnit()->getOffset() + *Off;
+  } else if (Off = RefValue.getAsDebugInfoReference(); Off) {
+    RefOffset = *Off;
+  } else {
+    reportWarning("Unsupported reference type", File, &DIE);
+    return DWARFDie();
+  }
   if ((RefCU = getUnitForOffset(Units, RefOffset)))
     if (const auto RefDie = RefCU->getOrigUnit().getDIEForOffset(RefOffset)) {
       // In a file with broken references, an attribute might point to a NULL
@@ -1073,7 +1081,13 @@ unsigned DWARFLinker::DIECloner::cloneDieReferenceAttribute(
     unsigned AttrSize, const DWARFFormValue &Val, const DWARFFile &File,
     CompileUnit &Unit) {
   const DWARFUnit &U = Unit.getOrigUnit();
-  uint64_t Ref = *Val.getAsReference();
+  uint64_t Ref;
+  if (std::optional<uint64_t> Off = Val.getAsRelativeReference()) 
+    Ref = Val.getUnit()->getOffset() + *Off;
+  else if (Off = Val.getAsDebugInfoReference(); Off)
+    Ref = *Off;
+  else
+    return 0;
 
   DIE *NewRefDie = nullptr;
   CompileUnit *RefUnit = nullptr;

llvm/lib/DWARFLinker/Parallel/DWARFLinkerCompileUnit.cpp

@@ -381,38 +381,36 @@ void CompileUnit::updateDieRefPatchesWithClonedOffsets() {
 std::optional<UnitEntryPairTy> CompileUnit::resolveDIEReference(
     const DWARFFormValue &RefValue,
     ResolveInterCUReferencesMode CanResolveInterCUReferences) {
-  if (std::optional<DWARFFormValue::UnitOffset> Ref =
-          *RefValue.getAsRelativeReference()) {
-    if (Ref->Unit == OrigUnit) {
-      // Referenced DIE is in current compile unit.
-      if (std::optional<uint32_t> RefDieIdx =
-              getDIEIndexForOffset(OrigUnit->getOffset() + Ref->Offset))
-        return UnitEntryPairTy{this, OrigUnit->getDebugInfoEntry(*RefDieIdx)};
-    }
-    uint64_t RefDIEOffset =
-        Ref->Unit ? Ref->Unit->getOffset() + Ref->Offset : Ref->Offset;
-    if (CompileUnit *RefCU = getUnitFromOffset(RefDIEOffset)) {
-      if (RefCU == this) {
-        // Referenced DIE is in current compile unit.
-        if (std::optional<uint32_t> RefDieIdx =
-                getDIEIndexForOffset(RefDIEOffset))
-          return UnitEntryPairTy{this, getDebugInfoEntry(*RefDieIdx)};
-      } else if (CanResolveInterCUReferences) {
-        // Referenced DIE is in other compile unit.
-
-        // Check whether DIEs are loaded for that compile unit.
-        enum Stage ReferredCUStage = RefCU->getStage();
-        if (ReferredCUStage < Stage::Loaded || ReferredCUStage > Stage::Cloned)
-          return UnitEntryPairTy{RefCU, nullptr};
-
-        if (std::optional<uint32_t> RefDieIdx =
-                RefCU->getDIEIndexForOffset(RefDIEOffset))
-          return UnitEntryPairTy{RefCU, RefCU->getDebugInfoEntry(*RefDieIdx)};
-      } else
-        return UnitEntryPairTy{RefCU, nullptr};
-    }
+  CompileUnit *RefCU;
+  uint64_t RefDIEOffset;
+  if (std::optional<uint64_t> Offset = RefValue.getAsRelativeReference()) {
+    RefCU = this;
+    RefDIEOffset = RefValue.getUnit()->getOffset() + *Offset;
+  } else if (Offset = RefValue.getAsDebugInfoReference(); Offset) {
+    RefCU = getUnitFromOffset(*Offset);
+    RefDIEOffset = *Offset;
+  } else {
+    return std::nullopt;
   }
 
+  if (RefCU == this) {
+    // Referenced DIE is in current compile unit.
+    if (std::optional<uint32_t> RefDieIdx = getDIEIndexForOffset(RefDIEOffset))
+      return UnitEntryPairTy{this, getDebugInfoEntry(*RefDieIdx)};
+  } else if (RefCU && CanResolveInterCUReferences) {
+    // Referenced DIE is in other compile unit.
+
+    // Check whether DIEs are loaded for that compile unit.
+    enum Stage ReferredCUStage = RefCU->getStage();
+    if (ReferredCUStage < Stage::Loaded || ReferredCUStage > Stage::Cloned)
+      return UnitEntryPairTy{RefCU, nullptr};
+
+    if (std::optional<uint32_t> RefDieIdx =
+            RefCU->getDIEIndexForOffset(RefDIEOffset))
+      return UnitEntryPairTy{RefCU, RefCU->getDebugInfoEntry(*RefDieIdx)};
+  } else {
+    return UnitEntryPairTy{RefCU, nullptr};
+  }
   return std::nullopt;
 }
 

llvm/lib/DebugInfo/DWARF/DWARFDie.cpp

@@ -313,13 +313,12 @@ DWARFDie::getAttributeValueAsReferencedDie(dwarf::Attribute Attr) const {
 DWARFDie
 DWARFDie::getAttributeValueAsReferencedDie(const DWARFFormValue &V) const {
   DWARFDie Result;
-  if (auto SpecRef = V.getAsRelativeReference()) {
-    if (SpecRef->Unit)
-      Result = SpecRef->Unit->getDIEForOffset(SpecRef->Unit->getOffset() +
-                                              SpecRef->Offset);
-    else if (auto SpecUnit =
-                 U->getUnitVector().getUnitForOffset(SpecRef->Offset))
-      Result = SpecUnit->getDIEForOffset(SpecRef->Offset);
+  if (std::optional<uint64_t> Offset = V.getAsRelativeReference()) {
+    Result = const_cast<DWARFUnit *>(V.getUnit())
+                 ->getDIEForOffset(V.getUnit()->getOffset() + *Offset);
+  } else if (Offset = V.getAsDebugInfoReference(); Offset) {
+    if (DWARFUnit *SpecUnit = U->getUnitVector().getUnitForOffset(*Offset))
+      Result = SpecUnit->getDIEForOffset(*Offset);
   }
   return Result;
 }

llvm/lib/DebugInfo/DWARF/DWARFFormValue.cpp

@@ -665,16 +665,7 @@ DWARFFormValue::getAsSectionedAddress() const {
   return getAsSectionedAddress(Value, Form, U);
 }
 
-std::optional<uint64_t> DWARFFormValue::getAsReference() const {
-  if (auto R = getAsRelativeReference())
-    return R->Unit ? R->Unit->getOffset() + R->Offset : R->Offset;
-  return std::nullopt;
-}
-
-std::optional<DWARFFormValue::UnitOffset>
-DWARFFormValue::getAsRelativeReference() const {
-  if (!isFormClass(FC_Reference))
-    return std::nullopt;
+std::optional<uint64_t> DWARFFormValue::getAsRelativeReference() const {
   switch (Form) {
   case DW_FORM_ref1:
   case DW_FORM_ref2:
@@ -683,11 +674,31 @@ DWARFFormValue::getAsRelativeReference() const {
   case DW_FORM_ref_udata:
     if (!U)
       return std::nullopt;
-    return UnitOffset{const_cast<DWARFUnit*>(U), Value.uval};
-  case DW_FORM_ref_addr:
-  case DW_FORM_ref_sig8:
+    return Value.uval;
+  default:
+    return std::nullopt;
+  }
+}
+
+std::optional<uint64_t> DWARFFormValue::getAsDebugInfoReference() const {
+  if (Form == DW_FORM_ref_addr)
+    return Value.uval;
+  return std::nullopt;
+}
+
+
+std::optional<uint64_t> DWARFFormValue::getAsSignatureReference() const {
+  if (Form == DW_FORM_ref_sig8)
+    return Value.uval;
+  return std::nullopt;
+}
+
+std::optional<uint64_t> DWARFFormValue::getAsSupplementaryReference() const {
+  switch (Form) {
   case DW_FORM_GNU_ref_alt:
-    return UnitOffset{nullptr, Value.uval};
+  case DW_FORM_ref_sup4:
+  case DW_FORM_ref_sup8:
+    return Value.uval;
   default:
     return std::nullopt;
   }

llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp

@@ -836,7 +836,7 @@ unsigned DWARFVerifier::verifyDebugInfoForm(const DWARFDie &Die,
   case DW_FORM_ref8:
   case DW_FORM_ref_udata: {
     // Verify all CU relative references are valid CU offsets.
-    std::optional<uint64_t> RefVal = AttrValue.Value.getAsReference();
+    std::optional<uint64_t> RefVal = AttrValue.Value.getAsRelativeReference();
     assert(RefVal);
     if (RefVal) {
       auto CUSize = DieCU->getNextUnitOffset() - DieCU->getOffset();
@@ -854,15 +854,16 @@ unsigned DWARFVerifier::verifyDebugInfoForm(const DWARFDie &Die,
       } else {
         // Valid reference, but we will verify it points to an actual
         // DIE later.
-        LocalReferences[*RefVal].insert(Die.getOffset());
+        LocalReferences[AttrValue.Value.getUnit()->getOffset() + *RefVal]
+            .insert(Die.getOffset());
       }
     }
     break;
   }
   case DW_FORM_ref_addr: {
     // Verify all absolute DIE references have valid offsets in the
     // .debug_info section.
-    std::optional<uint64_t> RefVal = AttrValue.Value.getAsReference();
+    std::optional<uint64_t> RefVal = AttrValue.Value.getAsDebugInfoReference();
     assert(RefVal);
     if (RefVal) {
       if (*RefVal >= DieCU->getInfoSection().Data.size()) {

llvm/lib/DebugInfo/LogicalView/Readers/LVDWARFReader.cpp

@@ -1082,21 +1082,28 @@ void LVDWARFReader::updateReference(dwarf::Attribute Attr,
   // FIXME: We are assuming that at most one Reference (DW_AT_specification,
   // DW_AT_abstract_origin, ...) and at most one Type (DW_AT_import, DW_AT_type)
   // appear in any single DIE, but this may not be true.
-  uint64_t Reference = *FormValue.getAsReference();
+  uint64_t Offset;
+  if (std::optional<uint64_t> Off = FormValue.getAsRelativeReference())
+    Offset = FormValue.getUnit()->getOffset() + *Off;
+  else if (Off = FormValue.getAsDebugInfoReference(); Off)
+    Offset = *Off;
+  else
+    llvm_unreachable("Unsupported reference type");
+
   // Get target for the given reference, if already created.
   LVElement *Target = getElementForOffset(
-      Reference, CurrentElement,
+      Offset, CurrentElement,
       /*IsType=*/Attr == dwarf::DW_AT_import || Attr == dwarf::DW_AT_type);
   // Check if we are dealing with cross CU references.
   if (FormValue.getForm() == dwarf::DW_FORM_ref_addr) {
     if (Target) {
       // The global reference is ready. Mark it as global.
       Target->setIsGlobalReference();
       // Remove global reference from the unseen list.
-      removeGlobalOffset(Reference);
+      removeGlobalOffset(Offset);
     } else
       // Record the unseen cross CU reference.
-      addGlobalOffset(Reference);
+      addGlobalOffset(Offset);
   }
 
   // At this point, 'Target' can be null, in the case of the target element