[DWARF] Eliminate the DWARFDebugNames::Header::Padding field.

[ikudrin]

Summary

The padding field is reserved for DWARF and does not contain any useful information. No need to read, store and report it.

[aprantl]

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

241

It looks like the rest of the struct doesn't follow the on-disk memory layout either, so I guess this is ok.

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

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;
     SmallString<8> AugmentationString;
 
     Error extract(const DWARFDataExtractor &AS, uint64_t *Offset);
     void dump(ScopedPrinter &W) const;
   };
 
   /// Index attribute and its encoding.
   struct AttributeEncoding {
     dwarf::Index Index;

llvm/lib/DebugInfo/DWARF/DWARFAcceleratorTable.cpp

AppleAcceleratorTable::equal_range(StringRef Key) const {
   }
   return make_range(ValueIterator(), ValueIterator());
 }
 
 void DWARFDebugNames::Header::dump(ScopedPrinter &W) const {
   DictScope HeaderScope(W, "Header");
   W.printHex("Length", UnitLength);
   W.printNumber("Version", Version);
-  W.printHex("Padding", Padding);
   W.printNumber("CU count", CompUnitCount);
   W.printNumber("Local TU count", LocalTypeUnitCount);
   W.printNumber("Foreign TU count", ForeignTypeUnitCount);
   W.printNumber("Bucket count", BucketCount);
   W.printNumber("Name count", NameCount);
   W.printHex("Abbreviations table size", AbbrevTableSize);
   W.startLine() << "Augmentation: '" << AugmentationString << "'\n";
 }
static const unsigned DWARF32HeaderFixedPartSize =
       dwarf::getUnitLengthFieldByteSize(dwarf::DWARF32) + CommonHeaderSize;
   // Check that we can read the fixed-size part.
   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);
+  // Skip padding
+  *Offset += 2;
   CompUnitCount = AS.getU32(Offset);
   LocalTypeUnitCount = AS.getU32(Offset);
   ForeignTypeUnitCount = AS.getU32(Offset);
   BucketCount = AS.getU32(Offset);
   NameCount = AS.getU32(Offset);
   AbbrevTableSize = AS.getU32(Offset);
   AugmentationStringSize = alignTo(AS.getU32(Offset), 4);
 

llvm/test/DebugInfo/X86/dwarfdump-debug-names.s

.Lnames2:
   .long 0                       # End of list: bar
   .p2align  2
 .Lnames_end1:
 # CHECK: .debug_names contents:
 # CHECK-NEXT: Name Index @ 0x0 {
 # CHECK-NEXT:   Header {
 # CHECK-NEXT:     Length: 0x60
 # CHECK-NEXT:     Version: 5
-# CHECK-NEXT:     Padding: 0x0
 # CHECK-NEXT:     CU count: 1
 # CHECK-NEXT:     Local TU count: 0
 # CHECK-NEXT:     Foreign TU count: 0
 # CHECK-NEXT:     Bucket count: 2
 # CHECK-NEXT:     Name count: 2
 # CHECK-NEXT:     Abbreviations table size: 0x7
 # CHECK-NEXT:     Augmentation: ''
 # CHECK-NEXT:   }

 # CHECK-NEXT:       }
 # CHECK-NEXT:     }
 # CHECK-NEXT:   ]
 # CHECK-NEXT: }
 # CHECK-NEXT: Name Index @ 0x64 {
 # CHECK-NEXT:   Header {
 # CHECK-NEXT:     Length: 0x44
 # CHECK-NEXT:     Version: 5
-# CHECK-NEXT:     Padding: 0x0
 # CHECK-NEXT:     CU count: 1
 # CHECK-NEXT:     Local TU count: 0
 # CHECK-NEXT:     Foreign TU count: 0
 # CHECK-NEXT:     Bucket count: 1
 # CHECK-NEXT:     Name count: 1
 # CHECK-NEXT:     Abbreviations table size: 0x7
 # CHECK-NEXT:     Augmentation: ''
 # CHECK-NEXT:   }