Debug Info: Clarify the documentation for bitfields emission.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240835 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index 51a268a5f7ed5338f00cde7c70f48fd3afe26177..f95f79d903cfa7f433fbf00f3c3fb58c08060298 100644 (file)
--- a/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1346,6 +1346,13 @@ void DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
       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.
+      //
+      // FIXME: DWARF 4 states that DW_AT_data_bit_offset (which
+      // counts from the beginning, regardless of endianness) should
+      // be used instead.
       uint64_t Offset = DT->getOffsetInBits();
       uint64_t AlignMask = ~(DT->getAlignInBits() - 1);
       uint64_t HiMark = (Offset + FieldSize) & AlignMask;
@@ -1357,8 +1364,6 @@ void DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
         Offset = FieldSize - (Offset + Size);
       addUInt(MemberDie, dwarf::DW_AT_bit_offset, None, Offset);
 
-      // Here DW_AT_data_member_location points to the anonymous
-      // field that includes this bit field.
       OffsetInBytes = FieldOffset >> 3;
     } else
       // This is not a bitfield.