[DWARF] Get rid of DWARFDebugNames::HeaderPOD. NFC.

[ikudrin]

Summary

This structure was used to get the size of the fixed-size part of a Name Index header for 32-bit DWARF. It is unsuitable for 64-bit DWARF because the size of the unit length field is different.

[JDevlieghere]

Is there any way we can test this?

llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp

394

How does this solve the issue exactly? Aren't we still checking the DWARF32 case only? I would expect something like

HeaderSize = dwarf::getUnitLengthFieldByteSize(IsDWARF64 ? dwarf::DWARF64 : dwarf::DWARF32) + CommonHeaderSize;

but maybe I'm misunderstanding what's going on here?

[ikudrin]

llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp

394

This is an extract from D72900 where all DWARF64 stuff is added. I made a set of patches so that each of them is more focused. This particular patch merely removes using of sizeof because we have to change the type of the Length to support DWARF64, after which the in-memory structure will anyway not reflect the in-file one.

As for your code sample, we check the remaining length of the section before reading fields, but know the format only after reading the first field, Length.

[probinson]

LGTM.
@JDevlieghere this would be an NFC refactor IMO, so not expecting a test here. The idea is to make D72900 easier to review.

llvm/include/llvm/DebugInfo/DWARF/DWARFAcceleratorTable.h

 /// comes after the name table. Unlike the apple tables, which have a uniform
 /// entry structure described in the header, each .debug_names entry may have
 /// different index attributes (DW_IDX_???) attached to it.
 ///
 /// The last segment consists of a list of entries, which is a 0-terminated list
 /// referenced by the name table and interpreted with the help of the
 /// abbreviation table.
 class DWARFDebugNames : public DWARFAcceleratorTable {
-  /// The fixed-size part of a DWARF v5 Name Index header
-  struct HeaderPOD {
+public:
+  class NameIndex;
+  class NameIterator;
+  class ValueIterator;
+
+  /// DWARF v5 Name Index header.
+  struct Header {
     uint32_t UnitLength;
     uint16_t Version;
     uint16_t Padding;
     uint32_t CompUnitCount;
     uint32_t LocalTypeUnitCount;
     uint32_t ForeignTypeUnitCount;
     uint32_t BucketCount;
     uint32_t NameCount;
     uint32_t AbbrevTableSize;
     uint32_t AugmentationStringSize;
-  };
-
-public:
-  class NameIndex;
-  class NameIterator;
-  class ValueIterator;
-
-  /// DWARF v5 Name Index header.
-  struct Header : public HeaderPOD {
     SmallString<8> AugmentationString;
 
     Error extract(const DWARFDataExtractor &AS, uint64_t *Offset);
     void dump(ScopedPrinter &W) const;
   };
 
   /// Index attribute and its encoding.
   struct AttributeEncoding {

llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp

void DWARFDebugNames::Header::dump(ScopedPrinter &W) const {
   W.printNumber("Bucket count", BucketCount);
   W.printNumber("Name count", NameCount);
   W.printHex("Abbreviations table size", AbbrevTableSize);
   W.startLine() << "Augmentation: '" << AugmentationString << "'\n";
 }
 
 Error DWARFDebugNames::Header::extract(const DWARFDataExtractor &AS,
                                              uint64_t *Offset) {
+  // These fields are the same for 32-bit and 64-bit DWARF formats.
+  constexpr unsigned CommonHeaderSize = 2 + // Version
+                                        2 + // Padding
+                                        4 + // CU count
+                                        4 + // Local TU count
+                                        4 + // Foreign TU count
+                                        4 + // Bucket count
+                                        4 + // Name count
+                                        4 + // Abbreviations table size
+                                        4;  // Augmentation string size
+  constexpr unsigned DWARF32HeaderFixedPartSize =
+      dwarf::getUnitLengthFieldByteSize(dwarf::DWARF32) + CommonHeaderSize;
   // Check that we can read the fixed-size part.
-  if (!AS.isValidOffset(*Offset + sizeof(HeaderPOD) - 1))
+  if (!AS.isValidOffsetForDataOfSize(*Offset, DWARF32HeaderFixedPartSize))
     return createStringError(errc::illegal_byte_sequence,
                              "Section too small: cannot read header.");
 
   UnitLength = AS.getU32(Offset);
   Version = AS.getU16(Offset);
   Padding = AS.getU16(Offset);
   CompUnitCount = AS.getU32(Offset);
   LocalTypeUnitCount = AS.getU32(Offset);