Debug Info: Fix a bug in the DW_AT_bit_offset calculation that would

result in negative offsets and attempt a better job at documenting
the algorithm.

rdar://21082998

llvm-svn: 240853

1 files changed, 28 insertions, 13 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index f95f79d903c..44de3407369 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1345,7 +1345,7 @@ void DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
       // Handle bitfield, assume bytes are 8 bits.
       addUInt(MemberDie, dwarf::DW_AT_byte_size, None, FieldSize/8);
       addUInt(MemberDie, dwarf::DW_AT_bit_size, None, Size);
-
+      //
       // The DWARF 2 DW_AT_bit_offset is counting the bits between
       // the high end of the aligned storage unit containing the bit
       // field to the high end of the bit field.
@@ -1353,21 +1353,36 @@ void DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
       // FIXME: DWARF 4 states that DW_AT_data_bit_offset (which
       // counts from the beginning, regardless of endianness) should
       // be used instead.
+      //
+      //
+      // Struct      Align       Align       Align
+      // v           v           v           v
+      // +-----------+-----*-----+-----*-----+--
+      // | ...             |b1|b2|b3|b4|
+      // +-----------+-----*-----+-----*-----+--
+      // |           |     |<-- Size ->|     |
+      // |<---- Offset --->|           |<--->|
+      // |           |     |              \_ DW_AT_bit_offset (little endian)
+      // |           |<--->|
+      // |<--------->|  \_ StartBitOffset = DW_AT_bit_offset (big endian)
+      //     \                            = DW_AT_data_bit_offset (biendian)
+      //      \_ OffsetInBytes
       uint64_t Offset = DT->getOffsetInBits();
-      uint64_t AlignMask = ~(DT->getAlignInBits() - 1);
-      uint64_t HiMark = (Offset + FieldSize) & AlignMask;
-      uint64_t FieldOffset = (HiMark - FieldSize);
-      Offset -= FieldOffset;
-
-      // Maybe we need to work from the other end.
-      if (Asm->getDataLayout().isLittleEndian())
-        Offset = FieldSize - (Offset + Size);
-      addUInt(MemberDie, dwarf::DW_AT_bit_offset, None, Offset);
-
-      OffsetInBytes = FieldOffset >> 3;
+      uint64_t Align = DT->getAlignInBits() ? DT->getAlignInBits() : FieldSize;
+      uint64_t AlignMask = ~(Align - 1);
+      // The bits from the start of the storage unit to the start of the field.
+      uint64_t StartBitOffset = Offset - (Offset & AlignMask);
+      // The endian-dependent DWARF 2 offset.
+      uint64_t DwarfBitOffset = Asm->getDataLayout().isLittleEndian()
+        ? OffsetToAlignment(Offset + Size, Align)
+        : StartBitOffset;
+
+      // The byte offset of the field's aligned storage unit inside the struct.
+      OffsetInBytes = (Offset - StartBitOffset) / 8;
+      addUInt(MemberDie, dwarf::DW_AT_bit_offset, None, DwarfBitOffset);
     } else
       // This is not a bitfield.
-      OffsetInBytes = DT->getOffsetInBits() >> 3;
+      OffsetInBytes = DT->getOffsetInBits() / 8;
 
     if (DD->getDwarfVersion() <= 2) {
       DIELoc *MemLocationDie = new (DIEValueAllocator) DIELoc;