Get rid of DWARFFormValue::getFixedFormSizes(uint8_t AddrSize, uint16_t Version).

[clayborg]

Summary

In preparation for a follow on patch that improves DWARF parsing speed, clean up DWARFFormValue so that we have can get the fixed byte size of a form value given a DWARFUnit or given the version, address byte size and dwarf32/64.

This patch cleans up code so that everyone is using one of the new DWARFFormValue functions:

static Optional<uint8_t> DWARFFormValue::getFixedByteSize(dwarf::Form Form, const DWARFUnit *U = nullptr);
static Optional<uint8_t> DWARFFormValue::getFixedByteSize(dwarf::Form Form, uint16_t Version, uint8_t AddrSize, bool Dwarf32);

This patch changes DWARFFormValue::skipValue() to rely on the output of DWARFFormValue::getFixedByteSize(...) instead of duplicating the numbers here. This will reduce the number of changes we need to make to DWARF to fewer places in DWARFFormValue when we add support for new form.

This patch also starts to support DWARF64 so that we can get correct byte sizes for forms that vary according the DWARF 32/64.

[dblaikie]

Just a few drive-by comments.

include/llvm/DebugInfo/DWARF/DWARFFormValue.h

149

Might be worth having an enum for this, for readability.

lib/DebugInfo/DWARF/DWARFFormValue.cpp

130

prefer llvm_unreachable over assert(false) (& there's no need for the return afterwards)

459–460

It's pretty common in LLVM to roll a variable declaration into the condition where possible:

if (Optional<uint8_t> FixedByteSize = getFixedByteSize(form, U)) {
  ...
}

(similar feedback elsewhere)

[aprantl]

include/llvm/DebugInfo/DWARF/DWARFFormValue.h

128

Nitpick: \param
(http://llvm.org/docs/CodingStandards.html#doxygen-use-in-documentation-comments)

135

///

140

Could you use the opportunity and document what the bool return value indicates? It's not clear to me from reading the function.

lib/DebugInfo/DWARF/DWARFFormValue.cpp

57

Would it make sense / be possible to encode this tabular data (at least for the majority of values) in the macros in Dwarf.def?

361–363

I know it was like this in the original source, but while you are editing it, we could use the opportunity fix the variable names to start with uppercase (http://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly). Up to you.

421

Should this really be an assertion, in the sense of that it indicates a programming error in the caller? Or can this situation be triggered by malformed DWARF input and should we return an Error instead?

tools/llvm-dwp/llvm-dwp.cpp

138

Comment what this magic indicator does?

[clayborg]

Will update according to comments.

lib/DebugInfo/DWARF/DWARFFormValue.cpp

57

Not sure how you would do this since many don't have a byte size. Zero is a valid DW_FORM size, so you can't use zero for items that don't have byte sizes. I wouldn't want to have a signed integer as the size either where -1 would indicate that there is no size. So since there is no easy way to encode an optional value with no value into the table, I would vote against this. Agains, this is the only function that needs to be updated.

361–363

I desire to keep this patch small so the extra diffs don't cloud our ability to see changes. I will do a follow up patch where I do nothing but case changing.

421

This function is internal to this file and is designed to only be called for variable length forms. The callers of this function both do:

if (Optional<uint8_t> FixedByteSize = getFixedByteSize(form, U)) {
  *offset_ptr += *FixedByteSize;
  return true;
}
return skipVariableLengthValue(...);

I turned this into an llvm_unreachable().

[clayborg]

Addresses issues from dblaike and aprantl

[clayborg]

Marked things as Done.

[dblaikie]

lib/DebugInfo/DWARF/DWARFFormValue.cpp

419–420

This is still a branche-to-unreachable, which should probably just be an assert instead:

assert(indirect && "Form must ...");
if (Optional<uint8_t> ...) {
  ...
}

[aprantl]

include/llvm/DebugInfo/DWARF/DWARFUnit.h

195

I think the llvm:: namespace is redundant here.

207

Up to taste, maybe write this as:

switch (getFormat()) {
  case dwarf::DwarfFormat::Dwarf32: return 4;
  case dwarf::DwarfFormat::Dwarf64: return 8;
}

include/llvm/Support/Dwarf.h

438

Maybe call this DWARF32 and DWARF64 since that is the correct spelling?

lib/DebugInfo/DWARF/DWARFFormValue.cpp

363

Should we make this const DWARFUnit *U = nullptr since you're using it that way later on?

[clayborg]

Looking as some of the comments made me think if a nicer way to fix some of the redundant code in DWARFFormValue.cpp and also clear up some of the questions that Adrian had about the fixed sized forms having the assertion in skipVariableLengthValue() in the cases where the fixed sized forms were happening. I am going to abandon this revision and open another otherwise all of the inlined comments won't make sense since a lot will have changed, but for the better.

include/llvm/DebugInfo/DWARF/DWARFFormValue.h

public:
   /// DWARFFormValue has form class is suitable for representing Foo.
   Optional<uint64_t> getAsReference() const;
   Optional<uint64_t> getAsUnsignedConstant() const;
   Optional<int64_t> getAsSignedConstant() const;
   Optional<const char *> getAsCString() const;
   Optional<uint64_t> getAsAddress() const;
   Optional<uint64_t> getAsSectionOffset() const;
   Optional<ArrayRef<uint8_t>> getAsBlock() const;
+  /// Get the fixed byte size for a given form.
+  ///
+  /// If the form always has a fixed valid byte size that doesn't depend on a
+  /// DWARFUnit, then an Optional with a value will be returned. If the form
+  /// can vary in size depending on the DWARFUnit (DWARF version, address byte
+  /// size, or DWARF 32/64) and the DWARFUnit is valid, then an Optional with a
+  /// valid value is returned. If the form is always encoded using a variable
+  /// length storage format (ULEB or SLEB numbers or blocks) or the size
+  /// depends on a DWARFUnit and the DWARFUnit is NULL, then None will be
+  /// returned.
+  /// \param Form The DWARF form to get the fixed byte size for
+  /// \param U The DWARFUnit that can be used to help determine the byte size.
+  ///
+  /// \returns Optional<uint8_t> value with the fixed byte size or None if
+  /// \p Form doesn't have a fixed byte size or a DWARFUnit wasn't supplied
+  /// and was needed to calculate the byte size.
+  static Optional<uint8_t> getFixedByteSize(dwarf::Form Form,
+                                            const DWARFUnit *U = nullptr);
+  /// Get the fixed byte size for a given form.
+  ///
+  /// If the form has a fixed byte size given a valid DWARF version and address
+  /// byte size, then an Optional with a valid value is returned. If the form
+  /// is always encoded using a variable length storage format (ULEB or SLEB
+  /// numbers or blocks) then None will be returned.
+  ///
+  /// \param Form DWARF form to get the fixed byte size for
+  /// \param Version DWARF version number.
+  /// \param AddrSize size of an address in bytes.
+  /// \param Format enum value from llvm::dwarf::DwarfFormat.
+  /// \returns Optional<uint8_t> value with the fixed byte size or None if
+  /// \p Form doesn't have a fixed byte size.
+  static Optional<uint8_t> getFixedByteSize(dwarf::Form Form, uint16_t Version,
+                                            uint8_t AddrSize,
+                                            llvm::dwarf::DwarfFormat Format);
 
+  /// Skip a form in \p debug_info_data at offset specified by \p offset_ptr.
+  ///
+  /// Skips the bytes for this form in the debug info and updates the offset.
+  ///
+  /// \param debug_info_data the .debug_info data to use to skip the value.
+  /// \param offset_ptr a reference to the offset that will be updated.
+  /// \param U the DWARFUnit to use when skipping the form in case the form
+  /// size differs according to data in the DWARFUnit.
+  /// \returns true on success, false if the form was not skipped.
   bool skipValue(DataExtractor debug_info_data, uint32_t *offset_ptr,
                  const DWARFUnit *U) const;
+  /// Skip a form in \p debug_info_data at offset specified by \p offset_ptr.
+  ///
+  /// Skips the bytes for this form in the debug info and updates the offset.
+  ///
+  /// \param form the DW_FORM enumeration that indicates the form to skip.
+  /// \param debug_info_data the .debug_info data to use to skip the value.
+  /// \param offset_ptr a reference to the offset that will be updated.
+  /// \param U the DWARFUnit to use when skipping the form in case the form
+  /// size differs according to data in the DWARFUnit.
+  /// \returns true on success, false if the form was not skipped.
   static bool skipValue(dwarf::Form form, DataExtractor debug_info_data,
-                        uint32_t *offset_ptr, const DWARFUnit *u);
+                        uint32_t *offset_ptr, const DWARFUnit *U);
+  /// Skip a form in \p debug_info_data at offset specified by \p offset_ptr.
+  ///
+  /// Skips the bytes for this form in the debug info and updates the offset.
+  ///
+  /// \param form the DW_FORM enumeration that indicates the form to skip.
+  /// \param debug_info_data the .debug_info data to use to skip the value.
+  /// \param offset_ptr a reference to the offset that will be updated.
+  /// \param Version DWARF version number.
+  /// \param AddrSize size of an address in bytes.
+  /// \param Format enum value from llvm::dwarf::DwarfFormat.
+  /// \returns true on success, false if the form was not skipped.
   static bool skipValue(dwarf::Form form, DataExtractor debug_info_data,
                         uint32_t *offset_ptr, uint16_t Version,
-                        uint8_t AddrSize);
-
-  static ArrayRef<uint8_t> getFixedFormSizes(uint8_t AddrSize,
-                                             uint16_t Version);
+                        uint8_t AddrSize, llvm::dwarf::DwarfFormat Format);
 private:
   void dumpString(raw_ostream &OS) const;
 };
 
 }
 
 #endif

include/llvm/DebugInfo/DWARF/DWARFUnit.h

public:
   /// Requires that compile unit is already extracted.
   bool extractRangeList(uint32_t RangeListOffset,
                         DWARFDebugRangeList &RangeList) const;
   void clear();
   uint32_t getOffset() const { return Offset; }
   uint32_t getNextUnitOffset() const { return Offset + Length + 4; }
   uint32_t getLength() const { return Length; }
   uint16_t getVersion() const { return Version; }
+  llvm::dwarf::DwarfFormat getFormat() const {
+    return llvm::dwarf::DwarfFormat::Dwarf32; // FIXME: Support DWARF64.
+  }
   const DWARFAbbreviationDeclarationSet *getAbbreviations() const {
     return Abbrevs;
   }
   uint8_t getAddressByteSize() const { return AddrSize; }
   uint8_t getRefAddrByteSize() const {
-    // FIXME: Support DWARF64.
-    return (Version == 2) ? AddrSize : 4;
+    if (Version == 2)
+      return AddrSize;
+    return getDwarfOffsetByteSize();
+  }
+  uint8_t getDwarfOffsetByteSize() const {
+    if (getFormat() == llvm::dwarf::DwarfFormat::Dwarf32)
+      return 4;
+    return 8;
   }
   uint64_t getBaseAddress() const { return BaseAddr; }
 
   void setBaseAddress(uint64_t base_addr) {
     BaseAddr = base_addr;
   }
 
   const DWARFDebugInfoEntryMinimal *getUnitDIE(bool ExtractUnitDIEOnly = true) {

include/llvm/Support/Dwarf.h

private:
   enum {
     KIND_OFFSET = 4,
     KIND_MASK = 7 << KIND_OFFSET,
     LINKAGE_OFFSET = 7,
     LINKAGE_MASK = 1 << LINKAGE_OFFSET
   };
 };
 
+// Constants that define the DWARF format as 32 or 64 bit.
+enum DwarfFormat {
+  Dwarf32,
+  Dwarf64
+};
+
 } // End of namespace dwarf
 
 } // End of namespace llvm
 
 #endif

lib/DebugInfo/DWARF/DWARFDebugInfoEntry.cpp

if (0 == AbbrCode) {
     return true;
   }
   AbbrevDecl = U->getAbbreviations()->getAbbreviationDeclaration(AbbrCode);
   if (nullptr == AbbrevDecl) {
     // Restore the original offset.
     *OffsetPtr = Offset;
     return false;
   }
-  ArrayRef<uint8_t> FixedFormSizes = DWARFFormValue::getFixedFormSizes(
-      U->getAddressByteSize(), U->getVersion());
-  assert(FixedFormSizes.size() > 0);
 
   // Skip all data in the .debug_info for the attributes
   for (const auto &AttrSpec : AbbrevDecl->attributes()) {
     auto Form = AttrSpec.Form;
 
-    uint8_t FixedFormSize =
-        (Form < FixedFormSizes.size()) ? FixedFormSizes[Form] : 0;
-    if (FixedFormSize)
-      *OffsetPtr += FixedFormSize;
+    if (Optional<uint8_t> FixedSize = DWARFFormValue::getFixedByteSize(Form, U))
+      *OffsetPtr += *FixedSize;
     else if (!DWARFFormValue::skipValue(Form, DebugInfoData, OffsetPtr, U)) {
       // Restore the original offset.
       *OffsetPtr = Offset;
       return false;
     }
   }
   return true;
 }

lib/DebugInfo/DWARF/DWARFFormValue.cpp

 #include "llvm/Support/Format.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <limits>
 using namespace llvm;
 using namespace dwarf;
 using namespace syntax;
 
-namespace {
-uint8_t getRefAddrSize(uint8_t AddrSize, uint16_t Version) {
-  // FIXME: Support DWARF64.
-  return (Version == 2) ? AddrSize : 4;
-}
-
-template <uint8_t AddrSize, uint8_t RefAddrSize>
-ArrayRef<uint8_t> makeFixedFormSizesArrayRef() {
-  static const uint8_t sizes[] = {
-    0,           // 0x00 unused
-    AddrSize,    // 0x01 DW_FORM_addr
-    0,           // 0x02 unused
-    0,           // 0x03 DW_FORM_block2
-    0,           // 0x04 DW_FORM_block4
-    2,           // 0x05 DW_FORM_data2
-    4,           // 0x06 DW_FORM_data4
-    8,           // 0x07 DW_FORM_data8
-    0,           // 0x08 DW_FORM_string
-    0,           // 0x09 DW_FORM_block
-    0,           // 0x0a DW_FORM_block1
-    1,           // 0x0b DW_FORM_data1
-    1,           // 0x0c DW_FORM_flag
-    0,           // 0x0d DW_FORM_sdata
-    4,           // 0x0e DW_FORM_strp
-    0,           // 0x0f DW_FORM_udata
-    RefAddrSize, // 0x10 DW_FORM_ref_addr
-    1,           // 0x11 DW_FORM_ref1
-    2,           // 0x12 DW_FORM_ref2
-    4,           // 0x13 DW_FORM_ref4
-    8,           // 0x14 DW_FORM_ref8
-    0,           // 0x15 DW_FORM_ref_udata
-    0,           // 0x16 DW_FORM_indirect
-    4,           // 0x17 DW_FORM_sec_offset
-    0,           // 0x18 DW_FORM_exprloc
-    0,           // 0x19 DW_FORM_flag_present
-  };
-  return makeArrayRef(sizes);
-}
-}
-
-ArrayRef<uint8_t> DWARFFormValue::getFixedFormSizes(uint8_t AddrSize,
-                                                    uint16_t Version) {
-  uint8_t RefAddrSize = getRefAddrSize(AddrSize, Version);
-  if (AddrSize == 4 && RefAddrSize == 4)
-    return makeFixedFormSizesArrayRef<4, 4>();
-  if (AddrSize == 4 && RefAddrSize == 8)
-    return makeFixedFormSizesArrayRef<4, 8>();
-  if (AddrSize == 8 && RefAddrSize == 4)
-    return makeFixedFormSizesArrayRef<8, 4>();
-  if (AddrSize == 8 && RefAddrSize == 8)
-    return makeFixedFormSizesArrayRef<8, 8>();
-  return None;
-}
-
 static const DWARFFormValue::FormClass DWARF4FormClasses[] = {
   DWARFFormValue::FC_Unknown,       // 0x0
   DWARFFormValue::FC_Address,       // 0x01 DW_FORM_addr
   DWARFFormValue::FC_Unknown,       // 0x02 unused
   DWARFFormValue::FC_Block,         // 0x03 DW_FORM_block2
   DWARFFormValue::FC_Block,         // 0x04 DW_FORM_block4
   DWARFFormValue::FC_Constant,      // 0x05 DW_FORM_data2
   // --- These can be FC_SectionOffset in DWARF3 and below:
static const DWARFFormValue::FormClass DWARF4FormClasses[] = {
   DWARFFormValue::FC_Reference,     // 0x14 DW_FORM_ref8
   DWARFFormValue::FC_Reference,     // 0x15 DW_FORM_ref_udata
   DWARFFormValue::FC_Indirect,      // 0x16 DW_FORM_indirect
   DWARFFormValue::FC_SectionOffset, // 0x17 DW_FORM_sec_offset
   DWARFFormValue::FC_Exprloc,       // 0x18 DW_FORM_exprloc
   DWARFFormValue::FC_Flag,          // 0x19 DW_FORM_flag_present
 };
 
+Optional<uint8_t> DWARFFormValue::getFixedByteSize(dwarf::Form form,
+                                                   const DWARFUnit *U) {
+  switch (form) {
+  case DW_FORM_addr:
+    if (U)
+      return U->getAddressByteSize();
+    return None;
+
+  case DW_FORM_block:          // ULEB128 length L followed by L bytes.
+  case DW_FORM_block1:         // 1 byte length L followed by L bytes.
+  case DW_FORM_block2:         // 2 byte length L followed by L bytes.
+  case DW_FORM_block4:         // 4 byte length L followed by L bytes.
+  case DW_FORM_string:         // C-string with null terminator.
+  case DW_FORM_sdata:          // SLEB128.
+  case DW_FORM_udata:          // ULEB128.
+  case DW_FORM_ref_udata:      // ULEB128.
+  case DW_FORM_indirect:       // ULEB128.
+  case DW_FORM_exprloc:        // ULEB128 length L followed by L bytes.
+  case DW_FORM_strx:           // ULEB128.
+  case DW_FORM_addrx:          // ULEB128.
+  case DW_FORM_loclistx:       // ULEB128.
+  case DW_FORM_rnglistx:       // ULEB128.
+  case DW_FORM_GNU_addr_index: // ULEB128.
+  case DW_FORM_GNU_str_index:  // ULEB128.
+    return None;
+
+  case DW_FORM_ref_addr:
+    if (U)
+      return U->getRefAddrByteSize();
+    return None;
+
+  case DW_FORM_flag:
+  case DW_FORM_data1:
+  case DW_FORM_ref1:
+    return 1;
+
+  case DW_FORM_data2:
+  case DW_FORM_ref2:
+    return 2;
+
+  case DW_FORM_data4:
+  case DW_FORM_ref4:
+  case DW_FORM_strp:
+    return 4;
+
+  case DW_FORM_GNU_ref_alt:
+  case DW_FORM_GNU_strp_alt:
+  case DW_FORM_line_strp:
+  case DW_FORM_sec_offset:
+  case DW_FORM_strp_sup:
+  case DW_FORM_ref_sup:
+    // 4 bytes in DWARF 32, 8 bytes in DWARF 64.
+    if (U)
+      return U->getDwarfOffsetByteSize();
+    return None;
+
+  case DW_FORM_data8:
+  case DW_FORM_ref8:
+  case DW_FORM_ref_sig8:
+    return 8;
+
+  case DW_FORM_flag_present:
+    return 0;
+
+  case DW_FORM_data16:
+    return 16;
+
+  case DW_FORM_implicit_const:
+    // The implicit value is stored in the abbreviation as a ULEB128, any
+    // there no data in debug info.
+    return 0;
+
+  default:
+    llvm_unreachable("Handle this form in this switch statement");
+    return None;
+  }
+  return None;
+}
+
+Optional<uint8_t> DWARFFormValue::getFixedByteSize(dwarf::Form form, uint16_t Version, uint8_t AddrSize, llvm::dwarf::DwarfFormat Format) {
+  switch (form) {
+  case DW_FORM_addr:
+    return AddrSize;
+
+  case DW_FORM_block:          // ULEB128 length L followed by L bytes.
+  case DW_FORM_block1:         // 1 byte length L followed by L bytes.
+  case DW_FORM_block2:         // 2 byte length L followed by L bytes.
+  case DW_FORM_block4:         // 4 byte length L followed by L bytes.
+  case DW_FORM_string:         // C-string with null terminator.
+  case DW_FORM_sdata:          // SLEB128.
+  case DW_FORM_udata:          // ULEB128.
+  case DW_FORM_ref_udata:      // ULEB128.
+  case DW_FORM_indirect:       // ULEB128.
+  case DW_FORM_exprloc:        // ULEB128 length L followed by L bytes.
+  case DW_FORM_strx:           // ULEB128.
+  case DW_FORM_addrx:          // ULEB128.
+  case DW_FORM_loclistx:       // ULEB128.
+  case DW_FORM_rnglistx:       // ULEB128.
+  case DW_FORM_GNU_addr_index: // ULEB128.
+  case DW_FORM_GNU_str_index:  // ULEB128.
+    return None;
+
+  case DW_FORM_ref_addr:
+    if (Version == 2)
+      return AddrSize;
+    if (Format == Dwarf32)
+      return 4;
+    return 8;
+
+  case DW_FORM_flag:
+  case DW_FORM_data1:
+  case DW_FORM_ref1:
+    return 1;
+
+  case DW_FORM_data2:
+  case DW_FORM_ref2:
+    return 2;
+
+  case DW_FORM_data4:
+  case DW_FORM_ref4:
+  case DW_FORM_strp:
+    return 4;
+
+  case DW_FORM_GNU_ref_alt:
+  case DW_FORM_GNU_strp_alt:
+  case DW_FORM_line_strp:
+  case DW_FORM_sec_offset:
+  case DW_FORM_strp_sup:
+  case DW_FORM_ref_sup:
+    if (Format == Dwarf32)
+      return 4;
+    return 8;
+
+  case DW_FORM_data8:
+  case DW_FORM_ref8:
+  case DW_FORM_ref_sig8:
+    return 8;
+
+  case DW_FORM_flag_present:
+    return 0;
+
+  case DW_FORM_data16:
+    return 16;
+
+  case DW_FORM_implicit_const:
+    // The implicit value is stored in the abbreviation as a ULEB128, any
+    // there no data in debug info.
+    return 0;
+
+  default:
+    llvm_unreachable("Handle this form in this switch statement");
+    return None;
+  }
+  return None;
+}
+
 bool DWARFFormValue::isFormClass(DWARFFormValue::FormClass FC) const {
   // First, check DWARF4 form classes.
   if (Form < makeArrayRef(DWARF4FormClasses).size() &&
       DWARF4FormClasses[Form] == FC)
     return true;
   // Check more forms from DWARF4 and DWARF5 proposals.
   switch (Form) {
   case DW_FORM_ref_sig8:
if (!str.empty()) {
       Value.data = reinterpret_cast<const uint8_t *>(str.data());
       *offset_ptr += Value.uval;
     }
   }
 
   return true;
 }
 
-bool
-DWARFFormValue::skipValue(DataExtractor debug_info_data, uint32_t* offset_ptr,
-                          const DWARFUnit *U) const {
-  return DWARFFormValue::skipValue(Form, debug_info_data, offset_ptr, U);
-}
-
-bool
-DWARFFormValue::skipValue(dwarf::Form form, DataExtractor debug_info_data,
-                          uint32_t *offset_ptr, const DWARFUnit *cu) {
-  return skipValue(form, debug_info_data, offset_ptr, cu->getVersion(),
-                   cu->getAddressByteSize());
-}
-bool 
-DWARFFormValue::skipValue(dwarf::Form form, DataExtractor debug_info_data,
-                          uint32_t *offset_ptr, uint16_t Version,
-                          uint8_t AddrSize) {
+bool skipVariableLengthValue(dwarf::Form form,
+                             const DataExtractor &debug_info_data,
+                             uint32_t *offset_ptr, const DWARFUnit *U) {
   bool indirect = false;
   do {
     switch (form) {
     // Blocks if inlined data that have a length field and the data bytes
     // inlined in the .debug_info
     case DW_FORM_exprloc:
     case DW_FORM_block: {
       uint64_t size = debug_info_data.getULEB128(offset_ptr);
case DW_FORM_block4: {
       return true;
     }
 
     // Inlined NULL terminated C-strings
     case DW_FORM_string:
       debug_info_data.getCStr(offset_ptr);
       return true;
 
-    // Compile unit address sized values
     case DW_FORM_addr:
-      *offset_ptr += AddrSize;
-      return true;
     case DW_FORM_ref_addr:
-      *offset_ptr += getRefAddrSize(AddrSize, Version);
-      return true;
-
-    // 0 byte values - implied from the form.
     case DW_FORM_flag_present:
-      return true;
-
-    // 1 byte values
     case DW_FORM_data1:
     case DW_FORM_flag:
     case DW_FORM_ref1:
-      *offset_ptr += 1;
-      return true;
-
-    // 2 byte values
     case DW_FORM_data2:
     case DW_FORM_ref2:
-      *offset_ptr += 2;
-      return true;
-
-    // 4 byte values
     case DW_FORM_data4:
     case DW_FORM_ref4:
-      *offset_ptr += 4;
-      return true;
-
-    // 8 byte values
     case DW_FORM_data8:
     case DW_FORM_ref8:
     case DW_FORM_ref_sig8:
-      *offset_ptr += 8;
+    case DW_FORM_sec_offset:
+    case DW_FORM_strp:
+    case DW_FORM_GNU_ref_alt:
+    case DW_FORM_GNU_strp_alt:
+      if (indirect) {
+        if (Optional<uint8_t> FixedSize =
+            DWARFFormValue::getFixedByteSize(form, U)) {
+          *offset_ptr += *FixedSize;
           return true;
+        }
+      } else {
+        llvm_unreachable("Form must be variable length");
+      }
+      return false;
 
     // signed or unsigned LEB 128 values
-    //  case DW_FORM_APPLE_db_str:
     case DW_FORM_sdata:
+      debug_info_data.getSLEB128(offset_ptr);
+      return true;
+
     case DW_FORM_udata:
     case DW_FORM_ref_udata:
-    case DW_FORM_GNU_str_index:
+    case DW_FORM_strx:
+    case DW_FORM_addrx:
+    case DW_FORM_loclistx:
+    case DW_FORM_rnglistx:
     case DW_FORM_GNU_addr_index:
+    case DW_FORM_GNU_str_index:
       debug_info_data.getULEB128(offset_ptr);
       return true;
 
     case DW_FORM_indirect:
       indirect = true;
       form = static_cast<dwarf::Form>(debug_info_data.getULEB128(offset_ptr));
       break;
 
-    // FIXME: 4 for DWARF32, 8 for DWARF64.
-    case DW_FORM_sec_offset:
-    case DW_FORM_strp:
-    case DW_FORM_GNU_ref_alt:
-    case DW_FORM_GNU_strp_alt:
-      *offset_ptr += 4;
-      return true;
-
     default:
       return false;
     }
   } while (indirect);
   return true;
 }
 
+bool DWARFFormValue::skipValue(DataExtractor debug_info_data,
+                               uint32_t *offset_ptr, const DWARFUnit *U) const {
+  return DWARFFormValue::skipValue(Form, debug_info_data, offset_ptr, U);
+}
+
+bool DWARFFormValue::skipValue(dwarf::Form form, DataExtractor debug_info_data,
+                               uint32_t *offset_ptr, const DWARFUnit *U) {
+  if (Optional<uint8_t> FixedSize = getFixedByteSize(form, U)) {
+    *offset_ptr += *FixedSize;
+    return true;
+  }
+  return skipVariableLengthValue(form, debug_info_data, offset_ptr, U);
+}
+
+bool DWARFFormValue::skipValue(dwarf::Form form, DataExtractor debug_info_data,
+                               uint32_t *offset_ptr, uint16_t Version,
+                               uint8_t AddrSize,
+                               llvm::dwarf::DwarfFormat Format) {
+  if (Optional<uint8_t> FixedSize = getFixedByteSize(form, Version, AddrSize,
+                                                     Format)) {
+    *offset_ptr += *FixedSize;
+    return true;
+  }
+  return skipVariableLengthValue(form, debug_info_data, offset_ptr, nullptr);
+}
+
 void
 DWARFFormValue::dump(raw_ostream &OS) const {
   uint64_t uvalue = Value.uval;
   bool cu_relative_offset = false;
 
   switch (Form) {
   case DW_FORM_addr:      OS << format("0x%016" PRIx64, uvalue); break;
   case DW_FORM_GNU_addr_index: {
Optional<uint64_t> DWARFFormValue::getAsReference() const {
   case DW_FORM_ref2:
   case DW_FORM_ref4:
   case DW_FORM_ref8:
   case DW_FORM_ref_udata:
     if (!U)
       return None;
     return Value.uval + U->getOffset();
   case DW_FORM_ref_addr:
+  case DW_FORM_ref_sig8:
+  case DW_FORM_GNU_ref_alt:
     return Value.uval;
-  // FIXME: Add proper support for DW_FORM_ref_sig8 and DW_FORM_GNU_ref_alt.
   default:
     return None;
   }
 }
 
 Optional<uint64_t> DWARFFormValue::getAsSectionOffset() const {
   if (!isFormClass(FC_SectionOffset))
     return None;

tools/llvm-dwp/llvm-dwp.cpp

 }
 
 static Expected<CompileUnitIdentifiers> getCUIdentifiers(StringRef Abbrev,
                                                          StringRef Info,
                                                          StringRef StrOffsets,
                                                          StringRef Str) {
   uint32_t Offset = 0;
   DataExtractor InfoData(Info, true, 0);
-  InfoData.getU32(&Offset); // Length
+  llvm::dwarf::DwarfFormat Format = llvm::dwarf::DwarfFormat::Dwarf32;
+  uint64_t Length = InfoData.getU32(&Offset);
+  // If the length is 0xffffffff, then this indictes that this is a DWARF 64
+  // stream and the length is actually encoded into a 64 bit value that follows.
+  if (Length == 0xffffffffU) {
+    Format = llvm::dwarf::DwarfFormat::Dwarf64;
+    Length = InfoData.getU64(&Offset);
+  }
   uint16_t Version = InfoData.getU16(&Offset);
   InfoData.getU32(&Offset); // Abbrev offset (should be zero)
   uint8_t AddrSize = InfoData.getU8(&Offset);
 
   uint32_t AbbrCode = InfoData.getULEB128(&Offset);
 
   DataExtractor AbbrevData(Abbrev, true, 0);
   uint32_t AbbrevOffset = getCUAbbrev(Abbrev, AbbrCode);
case dwarf::DW_AT_GNU_dwo_name: {
         return EName.takeError();
       ID.DWOName = *EName;
       break;
     }
     case dwarf::DW_AT_GNU_dwo_id:
       ID.Signature = InfoData.getU64(&Offset);
       break;
     default:
-      DWARFFormValue::skipValue(Form, InfoData, &Offset, Version, AddrSize);
+      DWARFFormValue::skipValue(Form, InfoData, &Offset, Version, AddrSize,
+                                Format);
     }
   }
   return ID;
 }
 
 struct UnitIndexEntry {
   DWARFUnitIndex::Entry::SectionContribution Contributions[8];
   std::string Name;

unittests/DebugInfo/DWARF/DWARFFormValueTest.cpp

 #include <climits>
 using namespace llvm;
 using namespace dwarf;
 
 namespace {
 
 TEST(DWARFFormValue, FixedFormSizes) {
   // Size of DW_FORM_addr and DW_FORM_ref_addr are equal in DWARF2,
-  // DW_FORM_ref_addr is always 4 bytes in DWARF32 starting from DWARF3.
-  ArrayRef<uint8_t> sizes = DWARFFormValue::getFixedFormSizes(4, 2);
-  EXPECT_EQ(sizes[DW_FORM_addr], sizes[DW_FORM_ref_addr]);
-  sizes = DWARFFormValue::getFixedFormSizes(8, 2);
-  EXPECT_EQ(sizes[DW_FORM_addr], sizes[DW_FORM_ref_addr]);
-  sizes = DWARFFormValue::getFixedFormSizes(8, 3);
-  EXPECT_EQ(4, sizes[DW_FORM_ref_addr]);
-  // Check that we don't have fixed form sizes for weird address sizes.
-  EXPECT_EQ(0U, DWARFFormValue::getFixedFormSizes(16, 2).size());
+  Optional<uint8_t> RefSize;
+  Optional<uint8_t> AddrSize;
+  // Test 32 bit DWARF version 2 with 4 byte addresses
+  RefSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 2, 4, Dwarf32);
+  AddrSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 2, 4, Dwarf32);
+  EXPECT_TRUE(RefSize.hasValue());
+  EXPECT_TRUE(AddrSize.hasValue());
+  EXPECT_EQ(*RefSize, *AddrSize);
+
+  // Test 32 bit DWARF version 2 with 8 byte addresses
+  RefSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 2, 8, Dwarf32);
+  AddrSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 2, 8, Dwarf32);
+  EXPECT_TRUE(RefSize.hasValue());
+  EXPECT_TRUE(AddrSize.hasValue());
+  EXPECT_EQ(*RefSize, *AddrSize);
+
+  // DW_FORM_ref_addr is 4 bytes in DWARF 32 in DWARF version 3 and beyond.
+  RefSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 3, 4, Dwarf32);
+  EXPECT_TRUE(RefSize.hasValue());
+  EXPECT_EQ(*RefSize, 4);
+
+  RefSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 4, 4, Dwarf32);
+  EXPECT_TRUE(RefSize.hasValue());
+  EXPECT_EQ(*RefSize, 4);
+
+  RefSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 5, 4, Dwarf32);
+  EXPECT_TRUE(RefSize.hasValue());
+  EXPECT_EQ(*RefSize, 4);
+
+  // DW_FORM_ref_addr is 8 bytes in DWARF 64 in DWARF version 3 and beyond.
+  RefSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 3, 8, Dwarf64);
+  EXPECT_TRUE(RefSize.hasValue());
+  EXPECT_EQ(*RefSize, 8);
+
+  RefSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 4, 8, Dwarf64);
+  EXPECT_TRUE(RefSize.hasValue());
+  EXPECT_EQ(*RefSize, 8);
+
+  RefSize = DWARFFormValue::getFixedByteSize(DW_FORM_ref_addr, 5, 8, Dwarf64);
+  EXPECT_TRUE(RefSize.hasValue());
+  EXPECT_EQ(*RefSize, 8);
 }
 
 bool isFormClass(dwarf::Form Form, DWARFFormValue::FormClass FC) {
   return DWARFFormValue(Form).isFormClass(FC);
 }
 
 TEST(DWARFFormValue, FormClass) {
   EXPECT_TRUE(isFormClass(DW_FORM_addr, DWARFFormValue::FC_Address));